CA3051292C

CA3051292C - Multistage solvent stimulation for heavy oil or bitumen recovery process

Info

Publication number: CA3051292C
Application number: CA3051292A
Authority: CA
Inventors: Tair Ibatullin; Hossein Aghabarati
Original assignee: Suncor Energy Inc
Current assignee: Suncor Energy Inc
Priority date: 2019-08-07
Filing date: 2019-08-07
Publication date: 2021-12-07
Anticipated expiration: 2039-08-07
Also published as: CA3051292A1

Abstract

There is provided a process for injecting a solvent into a bitumen containing underground reservoir including at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process including injecting the solvent in both the first and second horizontal wells in a multistage mode comprising sequentially injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the interwell region. In some implementations, the horizontal injection zones of the first well are offset in a horizontal direction relative to the horizontal injection zones of the second well to provide a staggered injection pattern.

Description

MULTISTAGE SOLVENT STIMULATION FOR HEAVY OIL OR
BITUMEN RECOVERY PROCESS
TECHNICAL FIELD
[1] The technical field generally relates to heavy oil or bitumen recovery processes from an underground reservoir. More particularly, the technical field relates to a multistage solvent stimulation process for mobilizing heavy oil or bitumen in a well region of the underground reservoir.
BACKGROUND

[2] Many bitumen recovery processes, such as steam assisted gravity drainage (SAGD), expanding solvent steam assisted gravity drainage (ES-SAGD), or solvent only assisted gravity drainage, require establishing initial hydraulic communication between an injection well and a production well prior to commencing the bitumen recovery process.
Establishing hydraulic communication between the well pair can be referred to as the start-up phase of the process. This start-up is typically achieved by hot fluid circulation, most commonly steam, or by deploying electrical heaters to warm-up the bitumen to the point that the bitumen becomes mobile in the region of the reservoir between the well pair. Other less common start-up methods include solvent circulation, or variations of bullheading the well with steam or solvent or a combination of steam and solvent. In the early phases of hot fluid circulation, bitumen viscosity reduction can be achieved through conductive heating, whereas at the end of circulation convective heat transfer contribution increases substantially.

[3] One issue encountered during circulation or bullheading is non-uniform heating conformance of the production well, due to reservoir heterogeneity, initial mobility, variations in well trajectories and/or other factors. Controlling the temperature conformance during conventional circulation or bullheading operations can be challenging, and "hot spots" can be difficult to heal even after transitioning into the main recovery process stage.

[4] Another issue with conventional start-up operations is that it can take significant time to establish sufficient hydraulic communication between the injection well and the production well. Typically, the circulation phase can last between two to four months, depending on the reservoir properties and operational strategy. Moreover, to initiate circulation or bullheading, the entire surface infrastructure must be ready, and the wells have to be connected to supply and return lines for hot fluid circulation.
Also, in steam-short situations where steam is required for various operating well pairs within an overall in situ recovery facility, steam circulation for start-up can divert valuable steam volumes from the producing well pairs to the circulation.

[5] Furthermore, in some bitumen or heavy oil recovery processes, certain conformance problems can be observed even after a certain period of production. For instance, conformance issues can be encountered at the production wells that have notable cold spots. Conformance issues can also be due to the uncontrolled leakage of the circulation fluid into the formation.

[6] Various challenges exist in terms of technologies for establishing fluid communication between wells in an underground reservoir and/or for improving conformance in a wellbore region.
SUMMARY

[7] According to one aspect, there is provided a process for injecting a solvent into a bitumen containing underground reservoir including at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process including:
injecting the solvent in both the first and second horizontal wells in a multistage mode including injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the intenNell region.

[8] In some implementations, the process includes:
(i) isolating a first horizontal injection zone along the first horizontal well and a second horizontal injection zone along the second horizontal well;
(ii) injecting the solvent into both horizontal injection zones of the first and second horizontal wells;

(iii) halting injection of the solvent into the horizontal injection zones;
and (iv) isolating additional injection horizontal zones one-by-one along each one of the first and second horizontal wells and repeating steps (ii) and (iii) for each of the additional horizontal injection zones.

[9] In some implementations, the solvent is injected simultaneously at least into the first and second horizontal injection zones.

[10] In some implementations, the solvent is sequentially injected into the reservoir in consecutive horizontal injection zones along each of the first and second horizontal wells.

[11] In some implementations, the process can include:
(i) providing a first packer assembly for isolating horizontal injection zones in a targeted zone of the first horizontal well and providing a second packer assembly for isolating horizontal injection zones in a targeted zone of the second horizontal well;
(ii) sequentially injecting solvent into each horizontal injection zones of the targeted zones;
(iii) halting injection of the solvent into the horizontal injection zones;
(iv) moving the first packer assembly along the first horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the first horizontal well and moving the second packer assembly along the second horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the second horizontal well; and (v) repeating steps (ii) and (iii) for each of the further horizontal injection zones isolated in step (iv).

[12] In some implementations, each of the horizontal injection zones of the first horizontal well is vertically aligned with a corresponding horizontal injection zone of the second horizontal well to provide an aligned injection pattern.

[13] In some implementations, the horizontal injection zones of the first horizontal well are offset in a horizontal direction relative to the horizontal injection zones of the second horizontal well to provide a staggered injection pattern.

[14] According to a second aspect, there is provided a process for establishing fluid communication between at least one pair of wells in a bitumen containing underground reservoir, the well pair including a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process including:
(i) isolating a first horizontal injection zone along the first horizontal well and isolating a second horizontal injection zone along the second horizontal well;
(ii) injecting a solvent into the first and second isolated horizontal injection zones;
(iii) halting injection of the solvent into the isolated horizontal injection zones; and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second wells and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones;
wherein the horizontal injection zones of the first well are offset in a horizontal direction relative to the horizontal injection zones of the second well to provide a staggered injection pattern; and wherein injecting the solvent results in mobilization of the bitumen by diffusion of the solvent in the interwell region.

[15] In some implementations, in step (ii) of the process according to the second aspect, the solvent is simultaneously injected in the first horizontal injection zone and the second horizontal injection well.

[16] In some implementations, in step (iv) of the process according to the second aspect, the solvent is simultaneously injected in the additional horizontal zones along each one of the first and second wells.

[17] In some implementations, the process according to any one of the first and second aspect can include a soaking period during which the solvent is allowed to further diffuse into the bitumen once injection into each of the horizontal zones is terminated.

[18] In some implementations of the process according to the first or second aspects, the solvent diffusion can result in mobilizing the bitumen in the interwell region and establishing communication between the first and second horizontal wells as part of a well pair start-up method. In some implementations, the solvent diffusion can result in mobilizing the bitumen in the interwell region and improve conformance of the interwell region.

[19] According to a third aspect, there is provided a process for recovering bitumen from an underground reservoir provided with at least one well pair including a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process including:
establishing communication between the first horizontal well and the second horizontal well by injecting at least one solvent according to any implementation of the process according to the second aspect;
once fluid communication is established, injecting a production fluid into the first horizontal well; and recovering a produced fluid including the bitumen and the solvent from the second horizontal well.

[20] In some implementations, the process according to the third aspect, further includes separating the solvent from the produced fluid for re-use in establishing communication between the horizontal wells of another well pair.

[21] In some implementations, in the process according to any one of the first, second and third aspect, the solvent is injected in the first horizontal well in liquid phase. In some implementations, the solvent is injected in the second horizontal well in liquid phase. In some implementations, the solvent is injected in the second horizontal well in gas phase.
In some implementations, the solvent is injected in the second horizontal well in both liquid and gas phase.

[22] In some implementations, the process according to any one of the first, second and third aspects can include monitoring fluid communication establishment between the horizontal wells once injection into each of the horizontal zones is terminated, and if fluid communication is incomplete, implementing a soaking period during which the solvent is allowed to further diffuse into the bitumen until fluid communication is established.

[23] In some implementations, in the process according to any one of the first, second and third aspect, the first and second horizontal wells are configured as a well pair with the wells operated under gravity dominated control.

[24] According to a fourth aspect, there is provided a process for injecting at least one solvent in a bitumen containing underground reservoir provided with at least one horizontal well pair, the process including:
(i) isolating a horizontal injection zone along the horizontal well;
(ii) injecting at least one solvent into the isolated horizontal injection zone;
(iii) halting injection of the solvent into the isolated horizontal injection zone;
(iv) isolating additional injection horizontal zones one-by-one along the horizontal well and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones; and (v) allowing the solvent to soak for a soaking period;
wherein upon injection and soaking, the solvent diffuses in the underground reservoir and mobilizes the bitumen in a region surrounding the horizontal well.

[25] In some implementations of the process according to the fourth aspect, the horizontal well is a production well of a SAGD-type well pair.

[26] In some implementations of the process according to the fourth aspect, the horizontal well is an infill well positioned between adjacent SAGD-type well pairs.

[27] In some implementations of the process according to the fourth aspect, the solvent is injected in horizontal injection zones in a cold region of the reservoir.

[28] In some implementations of the process according to the fourth aspect, the solvent diffusion allows improving conformance of the horizontal well.

[29] In some implementations of the process according to the fourth aspect, the process is implemented following of a pump failure within the horizontal well.

[30] According to a fifth aspect, there is provided a process for improving conformance of a horizontal well in a bitumen containing underground reservoir, wherein the horizontal well is in production mode, the process including:
(i) stopping production from the horizontal well;
(ii) isolating a horizontal injection zone along the horizontal well;
(iii) injecting at least one solvent into the isolated horizontal injection zone;
(iv) halting injection of the solvent into the isolated horizontal injection zone;
(v) isolating additional horizontal injection zones one-by-one along the horizontal well and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones;
wherein upon injection, the solvent diffuses in the underground reservoir and mobilizes the bitumen in a region surrounding the horizontal well where conformance needs to be improved.

[31] In some implementations of the process according to the fifth aspect, the process further includes a step wherein the solvent is allowed to soak for a soaking period in the region surrounding the horizontal well.

[32] In some implementations of the process according to the fifth aspect, the solvent is injected in horizontal injection zones in a cold region of the reservoir.

[33] In some implementations of the process according to the fifth aspect, production is stopped following of a pump failure within the horizontal well.

[34] In some implementations of the process according to the fifth aspect, the horizontal well is a production well of a SAGD-type well pair.

[35] In some implementations of the process according to the fourth and fifth aspects, the solvent is injected in the horizontal well in liquid phase.

[36] In some implementations of the process according to the fourth and fifth aspects, the solvent is injected in the horizontal well in gas phase.

[37] In some implementations of the process according to the fourth and fifth aspects, the solvent is injected in the horizontal well in both liquid and gas phase.

[38] The following implementations can be applied to the process according to any one of the above-mentioned aspects.

[39] In some implementations, the isolating steps can include inserting at least one isolating device within the horizontal well(s) and activating the isolating device downhole.

[40] In some implementations, the isolating device can include packers, diverters or balls combined with sliding sleeves.

[41] In some implementations, the isolating device include packers.

[42] In some implementations, the process further includes removing the isolating device from the horizontal well(s) once injection into each of the horizontal zones is terminated.

[43] In some implementations, the solvent is selected to limit or avoid asphaltene deposition.

[44] In some implementations, the solvent is selected to have a diffusion coefficient in bitumen of at least 10-7cm2/s, when measured at 20 C and averaged over a concentration range of 10 to 90% of solvent in bitumen.

[45] In some implementations, the solvent includes a water-soluble solvent, an oil-soluble solvent and/or a water-oil soluble solvent.

[46] In some implementations, the solvent is a mono-component solvent or multicomponent solvent.

[47] In some implementations, the solvent includes at least one of an ether, an aromatic, a diluent, a light oil, a refining product or a condensate.

[48] In some implementations, the solvent includes an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha; or diesel.
In some implementations, the aromatic is selected from benzene, toluene, m-xylene, o-xylene and p-xylene. In some implementations, the alkane is selected from cyclohexane and C2-C8 alkanes.

[49] In some implementations, the solvent includes:
- dimethyl ether;
- toluene and cyclohexane;
- toluene and C2-C8 alkanes;
- toluene and naphtha;
- m-xylene, o-xylene, p-xylene and cyclohexane;
- toluene, m-xylene, o-xylene, p-xylene, benzene and C2-C8 alkanes; or - diesel.

[50] In some implementations, the solvent includes dimethylether.

[51] In some implementations, the solvent can be heated before injection.
In some implementations, the solvent is heated to a temperature of up to 350 C above initial reservoir temperature. In some implementations, the solvent is heated to a temperature of from 5 C up to 150 C, above the initial reservoir temperature. In some implementations, the solvent is heated to a temperature of from 150 C up to 250 C, above the initial reservoir temperature. In some implementations, the solvent is heated to a temperature of from 250 C up to 350 C, above the initial reservoir temperature.

[52] In some implementations, the solvent is injected at a pressure of 0.1 to 20 MPa above an original reservoir pressure.

[53] In some implementations, the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 1 to 200 %.

[54] In some implementations, the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 2 to 100%.

[55] In some implementations, each of the horizontal injection zones has a length ranging from about 10 m to about 800 m. In some implementations, each of the horizontal injection zones has a length ranging from about 10 m to about 500 m. In other implementations, each of the horizontal injection zones has a length ranging from about m to about 200 m. In another implementation, each of the horizontal injection zones has a length ranging from about 10 m to about 100 m.

[56] In some implementations, the process according to any one of the above-mentioned aspects, can include providing heat to the horizontal wells during the soaking period to enhance the solvent diffusion. In some implementations, heat is provided using one of: downhole electric resistive heaters, radio frequency heaters, microwave heaters, or other types of downhole heaters. In some implementations, heat is provided through circulation of at least one of steam and hot solvent.
[56a] According to a further aspect, there is provided a process for injecting a solvent into a bitumen containing underground reservoir including at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process including:
injecting the solvent in both the first and second horizontal wells in a multistage mode including injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the interwell region; and wherein the horizontal injection zones of the first horizontal well are offset in a horizontal direction relative to the horizontal injection zones of the second horizontal well to provide a staggered injection pattern.
[56b] In some implementations, the process according to paragraph [56a] can further include any of the optional features disclosed herein.
Date Recue/Date Received 2021-06-08 10a [56c] According to a further aspect, there is provided a process for injecting a solvent into a bitumen containing underground reservoir comprising at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process comprising:
injecting the solvent simultaneously in both the first and second horizontal wells in a multistage mode comprising injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the interwell region.
[56d] In some implementations, the process according to paragraph [56c] can further include any of the optional features disclosed herein.
[56e] In some implementations, the process according to paragraph [56c] can further include:
(i) isolating a first horizontal injection zone along the first horizontal well and a second horizontal injection zone along the second horizontal well;
(ii) simultaneously injecting the solvent into both the first horizontal injection zone and the second horizontal injection zone;
(iii) halting injection of the solvent into the horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second horizontal wells and repeating steps (ii) and (iii) for each of the additional horizontal injection zones.
[56f] According to a further aspect, there is provided a process for establishing fluid communication between at least one pair of wells in a bitumen containing underground reservoir, the well pair comprising a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process comprising:
(i) isolating a first horizontal injection zone along the first horizontal well and isolating a second horizontal injection zone along the second horizontal well;
Date Recue/Date Received 2021-06-08 1 Ob (ii) injecting a solvent simultaneously into the first isolated horizontal injection zone and the second isolated horizontal injection zone;
(iii) halting injection of the solvent into the isolated horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second wells and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones;
wherein injecting the solvent results in mobilization of the bitumen by diffusion of the solvent in the interwell region.
[56g] In some implementations, the process according to paragraph [56f] can further include any of the optional features disclosed herein.

[57] It should also be noted that various aspects, implementations, features or steps described or illustrated herein can be combined with other aspects, implementations, features or steps.
BRIEF DESCRIPTION OF THE DRAWINGS

[58] Fig. 1 represents a side plan view of a SAG D-type well pair showing a horizontal injection zone configuration along the injection and production wells according to one implementation.

[59] Fig. 2 represents a side plan view of a SAG D-type well pair showing a horizontal injection zone configuration along the injection and production wells according to another implementation.

[60] Fig. 3 is a side plan view of a SAGD-type well pair after solvent injection in a linear multistage mode using the horizontal injection zone configuration depicted in Fig. 1.
Date Recue/Date Received 2021-06-08

[61] Fig. 4 is a side plan view of a SAGD-type well pair after solvent injection in a staggered multistage mode using the horizontal injection zone configuration depicted in Fig. 2.

[62] Fig. 5 is a side plan view of a SAGD-type well pair after solvent injection followed by a soaking period in a staggered multistage mode using the horizontal injection zone configuration depicted in Fig. 2.

[63] Fig. 6 is a schematic representation of a cylinder drawn around a stimulated well, used to express how to calculate the pore volume percentage of solvent that can be injected in a given horizontal injection zone.
DETAILED DESCRIPTION

[64] Techniques described herein relate to processes for establishing fluid communication between wells in an underground reservoir and/or for improving conformance of wellbores. More particularly, the techniques involve injecting at least one solvent in a multistage mode as will be described in more detail below, for mobilizing heavy oil or bitumen in a well region of the underground reservoir.

[65] "Bitumen" as used herein can refer to hydrocarbon material extracted from bituminous formations, such as oil sands formations, the density of which is typically around 1000 kg/rn3 and the American Petroleum Industry's (API) gravity is around 8 .
Bitumen can be recovered from a bitumen-containing reservoir using in situ recovery processes. The bitumen can include various non-hydrocarbon compounds (e.g., sulfur, metals, etc.) that are often found in bitumen and can be associated with certain hydrocarbon components (e.g., asphaltenes). Examples of bitumen include bitumen extracted from the Athabasca and Cold Lake regions, in Alberta, Canada.

[66] "Heavy oil", which can also be referred to as "heavy crude oil", can refer to any liquid petroleum with an American Petroleum Industry's (API) gravity of less than 22.3 and a density of about 920 to about 1000 kg/m3, although the density could be higher.
Heavy oil usually contains asphaltenes and resins.

[67] For ease of reading, the term "bitumen" will be used in the following description of some implementations. However, one will understand that the described processes can be used in underground reservoirs containing either bitumen or heavy oil.

[68] As mentioned above, the techniques described herein involve multistage solvent stimulation that can be used to establish communication between horizontal injection and production wells of a well pair, such as a.SAGD-type well pair for instance, in subsurface bitumen containing reservoirs. These techniques could substantially shorten the start-up phase of bitumen recovery processes, such as SAGD, ES-SAGD or solvent only assisted gravity drainage process, to name a few examples. Stimulation can start right after drilling and completion of the wells, without requiring full surface infrastructure or tie-ins to be in place.

[69] Some of the multistage solvent stimulation techniques are not limited to establishing communication between SAGD well pairs and can also be used for mobilizing bitumen along a single wellbore, such as a horizontal infill well positioned between adjacent SAGD well pairs, while improving conformance of the infill well.

[70] In addition, the present techniques can also be used to improve the conformance of injector-producer communication even after a certain period of production.

[71] A SAGD-type well pair 10 that can be used for implementing the multistage solvent stimulation process according to some implementations is represented in Figs.
1 to 5. The well pair 10 includes a first horizontal well 12, also referred to as an injection well, and a second horizontal well 14, also referred to as a production well, downwardly spaced away from the injection well 12. A bitumen containing space 16 is defined between the two horizontal wells and can be referred to as the interwell region or interwell spacing 16. As can be seen in Figs. 1 to 5, each of the injection and production wells also include a vertical portion from which extends the horizontal portion of the well. It should be understood that when referring to SAGD-type horizontal wells, one refers to the well in its entirety including both the vertical and horizontal portions thereof. In other words, a SAGD
horizontal well does not mean that the entire well from the surface to the toe of the well must be oriented horizontally. Multiple well pairs are generally arranged in parallel to one another in the reservoir, with an array of well pairs extending from a well pad 18 at surface. The well pairs 10 can be connected to above-ground equipment on the well pad 18.

[72] In one implementation, the multistage solvent injection can be performed by injecting at least one solvent 20 in both the injection and production wells 12, 14. Referring to Fig. 1, the solvent 20 can be provided via a solvent module 22 or a piping configuration for supplying solvent into the injection well 12 and production well 14. The multistage solvent injection can be implemented by injecting solvent into horizontal zones isolated along the length of the injection well and production well. In Figs. 1 and 2, horizontal injection zones 24a, 24b, 24c, 24d, 24e and 24f can be isolated in the injection well 12 and horizontal injection zones 26a, 26b, 26c, 26d, 26e and 261 can be isolated in the production well 14. One will understand that even if Figs. 1 and 2 show that the wells can be each divided into six isolated zones, the number of isolated zones in each well can be more or less than six.

[73] The solvent injection techniques described herein can be performed at the isolated zones one at a time. In some implementations, the solvent can thus be sequentially injected in the isolated zones. In this regard, it should be noted that the formation rock and bitumen in the well pair region, including in the interwell region 16, is not homogeneous or constant along the length of the in situ well pair, which can have horizontal portions about 1000 m long. Certain intervals along the length of the well pair have higher permeability while certain regions have higher bitumen content with lower effective permeability to the solvent. The isolated zone-based approach can improve the solvent diffusion within the bitumen in the well region including in the interwell region.

[74] The isolated zone-based approach for injecting the solvent within the reservoir can have a number of advantages. The isolated zone-based approach can help to ensure solvent penetration at each internal along the wells, which in turn can improve solvent assisted fluid communication of the interwell region and/or conformance along the length of well pair. Due to variations along the length of the wells running from the heel to the toe of the well pair, solvent injection along the entire length of the wells can tend to have increased penetration at high permeability locations in the well region. Such high permeability locations can consist of low bitumen saturation zones or locations with naturally occurring higher permeability sand. If the solvent is injected along the entire length of the wells, it can quickly establish fluid communication between the wells at a high permeability location, after which the injected solvent can tend to preferentially flow through the interwell region at this location, which can prematurely short-circuit the process for establishing fluid communication between the wells. The present isolated zone-based approach can mitigate this problem by ensuring that the solvent is injected into the interwell region at multiple zones along the length of the well pair and thus reducing breakthrough issues.

[75] The isolating of the horizontal injection zones can be done in a number of ways.
For example, one method uses frac-balls in combination with frac-ports and sliding sleeves, the frac-balls being delivered into the injection well to block or close off portions of the well. Each ball is smaller than the opening of all of the previous sleeves, but larger than the sleeve it is intended to open. Another method can use cup packers to isolate the horizontal injection zones. A further method can use a diverter to block the flow through the annulus between the formation and the exterior surface of the liner which can improve the efficiency of the process improving the containment in the selected horizontal injection zones 24a, 24b, 24c, 24d, 24e, 24f, 26a, 26b, 26c, 26d, 26e and/or 26f.

[76] It should be noted that any other methods can be used to create the horizontal injection zones. The isolating methods can employ various isolation devices such as packer assemblies, plug assemblies, sleeve assemblies, etc. Typically, for a given well, the zones will be isolated and treated with solvent one at a time starting at the toe and moving toward the heel of the well. After each solvent treatment, an isolation device can be provided to isolate the treated interval from the subsequent upstream interval to be treated. It is worth mentioning that there is no strict requirement that the horizontal injection zones be perfectly isolated as long as there is enough flux of solvent into the formation to mobilize the bitumen.

[77] In some implementations, only one horizontal injection zone can be provided in at least one of the horizontal wells, i.e. only one injection zone is used for the entire well.
However, in some implementations, many horizontal injection zones can be provided. For instance, the horizontal well could be segmented into up to 100 horizontal injection zones, if desired. Upon injecting the solvent into the horizontal injection zones, one increases the viscous force and promotes the diffusion of the solvent into the horizontal well.

[78] The horizontal injection zones can also present many different lengths.
As mentioned above, the injection zone can have the entire well length. In some implementations, the horizontal injection zones can each have a length ranging from about m to about 800 m. For example, the length of the horizontal injection zones can range from about 10 m to about 500 m, or from about 10 m to about 200 m, or from about 10 m to about 100 m. A horizontal injection zone can be about 40 m long. In some other implementations, the length of the horizontal injection zones can range from about 100 m to about 200 m, or from about 200 m to about 500 m, or from about 500 m to about 800 m. The horizontal injection zones can each have identical lengths or can have different lengths from each other if desired. Depending on the method used to create the isolated zones, the order of solvent injection can be from toe to heel, heel to toe, or another order.
When a string of packers is used, the order can be from toe to heel of the injection well and production well, that is the injection would start at the toe end zone 24f and/or 26f and work its way toward the heel end zone 24a and/or 26a. In some implementations, the isolation method can be performed in order that subsequent isolated zones to be activated are as far away as possible from the previously activated zone along the well length. In such a case, the horizontal injection zones can be alternately activated at the heel and toe ends of each well and work its way toward the middle of the wells. In some implementations, a packer assembly can be used to isolate a few horizontal zones in a first targeted zone along at least one well, e.g. starting from the toe of the well, and then, upon completion of the solvent stimulation in the first targeted zone, the whole packer assembly can be pulled back to the next targeted zone which can then be stimulated. In some implementations, the packer assembly can successively be pulled back to isolate further targeted zones to be stimulated until the whole length of the well is stimulated.
Using such "sliding" packer assembly can allow stimulating multiple targeted zones, at various locations along the well length, without requiring retrieving the whole assembly.

[79] Various solvent injection patterns and modes can be implemented using the isolated zone-based approach described above. The multistage solvent injection implementation represented in Figs. 1 and 3 can use a "linear" or "aligned"
pattern in which each of the horizontal injection zones 24a, 24b, 24c, 24d, 24e, 24f of the injection well is substantially vertically aligned with each corresponding horizontal injection zones 26a, 26b, 26c, 26d, 26e, 26f of the production well. In other words, injection zone 24a is substantially vertically aligned with injection zone 26a, injection zone 24b is substantially vertically aligned with injection zone 26b, injection zone 24c is substantially vertically aligned with injection zone 26c, injection zone 24d is substantially vertically aligned with injection zone 26d, injection zone 24e is substantially vertically aligned with injection zone 26e, and injection zone 24f is substantially vertically aligned with injection zone 26f, and so on.

[80] In this aligned mode, at least the central axis of corresponding injection zones extending laterally out from the injection and production wells should be substantially aligned, but the length of each injection zone can be different or similar.
Fig. 3 represents the impact of the multistage stimulation after solvent injection according to the above described linear pattern. The solvent that is injected into each of the horizontal zones 24a, 24b, 24c, 24d, 24e, 24f and 26a, 26b, 26c, 26d, 26e, 26f, can penetrate and diffuse into the reservoir in the region surrounding each of the injection and production wells, including in the interwell region 16, resulting in stimulated zones 28. In this aligned pattern, the stimulated zones 28 along the length of the injection well are substantially vertically aligned with the corresponding stimulated zones 28 along the length of the production well.
Therefore, according to this implementation, stimulated zone 28 resulting from solvent injection into horizontal zones 24a is vertically aligned with stimulated zone 28 resulting from solvent injection into horizontal zone 26a, stimulated zone 28 resulting from solvent injection into horizontal zones 24b is vertically aligned with stimulated zone 28 resulting from solvent injection into horizontal zone 26b, and so on along the well. At each stimulated zone 28, the solvent can dissolve into the bitumen present in the stimulated zone which, in turn, reduces bitumen viscosity and enhances its mobility in the corresponding zone. This aligned injection pattern can enhance establishing fluid communication between the injection and production wells as the corresponding vertically developed stimulated zones can connect between the wells more rapidly.

[81] The multistage solvent injection implementation represented in Figs. 2 and 4 can use a "staggered" pattern in which the injection zones 24a, 24b, 24c, 24d, 24e, 24f of the injection well are staggered or offset relative to the injection zones 26a, 26b, 26c, 26d, 26e, 26f of the production well in a horizontal direction. In other words, injection zone 24a is horizontally offset relative to injection zone 26a, injection zone 24b is horizontally offset relative to injection zone 26b, injection zone 24c is horizontally offset relative to injection zone 26c, injection zone 24d is horizontally offset relative to injection zone 26d, injection zone 24e is horizontally offset relative to injection zone 26e, injection zone 24f is horizontally offset relative to injection zone 26f.

[82] In some implementations, this staggered multistage solvent injection can involve injecting the solvent in one injection zone or several injection zones simultaneously.
Moreover, injection can be performed interchangeably in the injection and production wells 12, 14.

[83] Fig. 4 represents the development of the multistage stimulation after solvent injection according to the staggered pattern. In this case, the stimulated zones 28 resulting from solvent injection into the horizontal zones of the injection well are horizontally offset relative to the stimulated zones 28 resulting from solvent injection into the horizontal zones of the production well. This particular injection pattern can thus form staggered stimulated zones 28 along the wells, allowing penetration and diffusion of the solvent at more locations in the interwell region. The stimulated zones 28 can become connected both horizontally and vertically, which can enhance establishing fluid communication between the injection and production wells while improving conformance along the well length. The staggered pattern can have various spacing configurations, e.g., regularly spaced along the wells, variable spacings along the well, or various different spacing arrangements that could be based on geology, well completion, and other factors.

[84] In some implementations, injection of the solvent can be performed simultaneously into a horizontal injection zone of the injection well and into a horizontal injection zone of the production well, independently of which injection pattern is used. In some implementations, the multistage solvent injection process uses an aligned pattern as described above with simultaneous injection of the solvent into a horizontal injection zone of each of the injection and production wells, optionally where solvent injection is done simultaneously into opposing aligned zones of the two wells.

[85] In some implementations, the solvent is sequentially injected into the reservoir in consecutive horizontal injection zones along each of the injection and production wells.
Sequential solvent injection can be performed from toe to heel or from heel to toe in each one of the injection well and production well, either using a linear or staggered injection pattern. In one example, the entire length of one of the wells is treated by solvent injection, followed by treating the other well by solvent injection. In another example, solvent is injected into one injection zone of one of the wells, and then solvent is injected into an injection zone of the other well; and the zones of the two wells can be treated in an alternating fashion along the wells.

[86] In some implementations, solvent is injected into each isolated interval in a fashion adapted to each interval. By adapting solvent injection for each isolated zone (e.g., by controlling injection pressure, rate, time and total quantity of injected solvent), each injection interval can receive a tailored treatment suited for instance to its geological properties. For example, for intervals with higher permeability, the solvent can be injected at lower pressure or shorter times to achieve a certain solvent penetration;
while intervals with lower permeability may receive solvent injection at higher pressures or longer injection times. The injection for each stage or interval can be monitored to detect downhole properties and predict development of the solvent-diluted zones, and thus adapt the solvent injection accordingly.

[87] In some implementations, the solvent to be used in the multistage solvent stimulation process can be a mono-component solvent or a multicomponent solvent, and it can be selected to obtain a fairly rapid bitumen viscosity reduction upon injection and diffusion in the reservoir. In some implementations, the solvent is selected to avoid inducing asphaltene precipitation. The solvent can include a water-soluble solvent, an oil-soluble solvent and/or a water-oil soluble solvent. Solvents that are soluble in both oil and water can be advantageously used, especially in reservoirs with existing water mobility.

[88] In some implementations, the selected solvent or mixture of solvents can have a diffusion coefficient in bitumen of at least 10-7 cm2/s, when measured at 20 C
and averaged over a concentration range of 10 to 90% of solvent in bitumen.

[89] In some implementations, the solvents that can be used in the multistage solvent stimulation process can include an ether, an aromatic, a diluent, a light oil, a refining product or a condensate. Alternatively, the solvent can include an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha;
or diesel. In some implementations, the aromatics can be selected from benzene, toluene, m-xylene, o-xylene and p-xylene. The alkanes can be selected from cyclohexane and C2-C8 alkanes, i.e., ethane, propane, butane, pentane, hexane, heptane, octane or any mixtures thereof.

[90] In some other implementations, the solvent can be any one of the following solvent or mixture of solvents: dimethylether (DME); toluene and cyclohexane; toluene and C2-C8 alkanes; toluene and naphtha; m-xylene, o-xylene, p-xylene and cyclohexane;
toluene, m-xylene, o-xylene, p-xylene, benzene and C2-C8 alkanes; or diesel. The use of solvents including aromatics can allow limiting or avoiding asphaltene precipitation.
Hence, using a combination of aromatics and alkanes at suitable ratios can allow dissolving the bitumen while avoiding asphaltene deposition. Solvents such as diesel or DME are also good candidates for avoiding asphaltene precipitation. Moreover, DME is soluble in water in addition to being able to dissolve bitumen. Hence, DME can be recovered through both the aqueous and oil phases. Using DME as solvent can be economically beneficial as DME is not expensive and can be easily recovered from the reservoir, while still being efficient to produce bitumen.

[91] Injection of the solvent or mixture of solvents can be preferably performed in liquid phase in the injection well; while injection via the production well can be performed in liquid phase, in gas phase or in a mixture of liquid and gas phases. In some implementations, the solvent(s) can be heated before injection, for instance to a temperature of up to about 350 C above initial reservoir temperature. Injection of heated solvent can improve penetration and diffusion thereof into the reservoir, which in turn can enhance bitumen mobilization. In some implementations, the solvent can be heated to a temperature of up to 250 C, or up to 150 C above initial reservoir temperature. The solvent heating temperature can depend on the nature of the solvent and the reservoir conditions. In other implementations, the solvent can be heated to a temperature of from 5 C up to 150 C, or from 150 C up to 250 C, or 250 C up to 350 C, above the initial reservoir temperature.

[92] In some implementations, solvent injection can be carried out at a pressure of up to 20 MPa above the original reservoir pressure at the location of the stimulated well(s), for instance from 0.1 to 20 MPa above the original reservoir pressure. Solvent injection pressure can be selected depending on the reservoir conditions. For example, a higher pressure injection can be required for deeper reservoir, as the original reservoir pressure is high. Moreover, a lower injection pressure can be required in reservoirs with existing mobility compared to reservoirs without initial mobility.

[93] The volume of injected solvent can be adapted to the reservoir conditions. In some implementations, the volume of solvent to be injected into each horizontal injection zone can be represented as a percentage of pore volume, confined by a cylinder drawn around the stimulated wells 12 and 14, and that has a radius r equal to the interwell spacing 16 and a length L equal to the length of the horizontal injection zone (Fig. 6).
The pore volume confined by the cylinder can be calculated using the following formula:
V=Trxr2xLx0 where 0 is an average porosity within the cylinder.

[94] In some implementations, the percentage of pore volume injected can range from 1 to 200%, or from 2 to 100% in other implementations.

[95] The above-described multistage solvent stimulation processes can allow delivering controlled volumes of solvent into the specified locations (horizontal zones) along the wellbore, so that solvent would not tend to develop preferential paths and hot spots that would negatively impact conformance and complicate subsequent bitumen recovery process.

[96] In addition, the present multistage solvent stimulation technology does not require creating a pressure sink in the production well to promote pressure drive of the solvent from the injection well toward the production well to mobilize bitumen in the interwell region, as required in certain known processes. This multistage stimulation technology also does not require producing any fluid during the stimulation and does not require systems to recover produced fluids from the reservoir. Indeed, in some implementations, little or no fluids are produced from the wells during the start-up process, which is focused on solvent injection into isolated zones along the wells and the injected solvent is left in the surrounding reservoir regions to mobilize the bitumen.

[97] Depending on the reservoir conditions, the selected solvent and the injection implementation mode (e.g., the number of stimulated zones, the solvent temperature and volume, etc.), a period of solvent soaking can be performed following injection to allow solvent to diffuse deeper into the bitumen and potentially connect stimulated zones from the injection and production wells. In some implementations, systems can be provided to monitor fluid communication between the injection and production, to determine if a soaking period is desirable. For example, the degree of communication can be inferred through pressure communication.

[98] Fig. 5 represents the impact of implementing an additional solvent soaking period after the multistage solvent injection on the stimulated zones 28. In Fig. 5, the soaking period is applied after multistage solvent stimulation using the staggered injection pattern described above. The stimulated zones 28 can coalesce at least in the interwell region, both in vertical and horizontal directions. In addition, although this is not represented in the figures, the soaking period could be prolonged to allow horizontal coalescence of the stimulated zones 28 above the injection well and/or below the production well to further improve bitumen mobilization in the well region.

[99] Prior to the solvent injection or during the soaking period, if desired, additional heat can be supplied to the well pair to further improve the solvent diffusion and bitumen mobilization. Heat can be provided to the well pair using various systems, such as downhole electric resistive heaters, radio frequency-based heaters, microwave heaters, and other types of downhole heaters. In some other implementations, steam circulation or hot solvent circulation or mixture or those can be performed to provide additional heat to the well pair. The hot solvent that can be circulated for providing additional heat can be the same solvent as the one injected in the present multistage stimulation process or a different solvent. A natural opportunity for implementing solvent soaking can be the period between completion of the well after stimulation, and the time when all of the surface tie-ins and infrastructure are ready to start production.

[100] After termination of the multistage stimulation, the isolating devices that have been used for isolating the horizontal zones along the well length can be removed from the wells before starting the production phase. Once the isolating devices (e.g. a packer assembly) is pulled out from the wells, injection tubing can be inserted in the injection well and a pump provided in the production well for starting bitumen recovery. In some particular implementations however, the isolating devices can be left in the injection well for a very limited period of time in case further stimulation is required. For instance, some solvent can further be injected into the injection well using the multistage approach if required, during the transition phase before starting production.

[101] After termination of the multistage stimulation, the well pair can be ready to be ramped-up to the preferred bitumen recovery process (e.g., SAGD, ES-SAGD, Solvent+, etc.). The selected mobilizing fluid, such as steam, solvent (which can be the same or different from the solvent used for the multistage stimulation) or a mixture of steam and solvent, can be injected via the injection well and a produced fluid is recovered via the production well. The produced fluid can include at least bitumen and the solvent used in the multistage stimulation, particularly at the beginning of the production process when soaked solvent is being recovered. In some implementations, the multistage stimulation solvent can be separated from the produced fluid at the surface and re-used in a different well pair or directed to the central processing facility (CPF) along with produced bitumen.

The well pair can also be operated in a cyclical matter, if desired, where at least the lower production well is operated cyclically as an injector and then as a producer.

[102] Although the above description particularly focuses on well stimulation of a SAGD-type well pair before starting the bitumen recovery process, application of the present multistage solvent stimulation technology is not limited to SAGD-type wells nor to the fluid communication establishment phase preceding the bitumen recovery process.
Indeed, the technology can also be used to stimulate an infill well or improve conformance in a well pair or infill well after a certain period of production. For instance, the multistage solvent stimulation can be employed to start up an infill well positioned in between two adjacent SAGO well pairs. It can be advantageous to stimulate multiple horizontal zones along the length of the infill well using solvent injection according to the above described technology to promote bitumen mobilization in the region surrounding the infill well and enhance fluid communication between the infill well and adjacent horizontal wells (e.g. SAGD
wells or another infill well) and/or previously developed steam/solvent chambers.

[103] In some other implementations, the multistage solvent stimulation can be applied to a SAGD-type well pair or an infill well pair after a certain period of production. Indeed, it can be advantageous to stimulate horizontal zones at specific locations along the length of the well pair using solvent injection according to the above described technology. For instance, the stimulation can be advantageously carried out in dedicated zones where bitumen mobilization has not occurred or is limited, such as at cold spots in the reservoir, resulting in conformance improvement. Thus, underperforming wells can be stimulated by implementing the multistage solvent stimulation process.

[104] In order to implement the multistage solvent stimulation process to stimulate a production well that has already been in production, one needs to interrupt production first, meaning that the submersible pump provided in the production well must be switched off.
In some implementations, a good opportunity to perform the multistage solvent stimulation process once production has started, can follow a pump failure in the production well.
Typical production wells are equipped with submersible pumps, which can fail many times within the lifetime of the producer due to the harsh recovery process conditions. When a pump fails, the pump and the production tubing and some instrumentation need to be pulled out of the well. Hence, this represents an opportunity to conduct the multistage solvent stimulation, rather than interrupting production while the pump is running.

[105] In some other implementations, the multistage stimulation techniques as described can be used with other well configurations, such as vertical or slanted infill or step-out wells, SAGD variants such as solvent-SAGD operations, and/or infill or step-out wells that are provided in between or adjacent to steam chambers or mobilized zones other than SAGD steam chambers.

[106] Other variants, embodiment and aspects can also be used under the present technology.

Claims

24

1- A process for injecting a solvent into a bitumen containing underground reservoir comprising at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process comprising:
injecting the solvent in both the first and second horizontal wells in a multistage mode comprising injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the interwell region; and wherein the horizontal injection zones of the first horizontal well are offset in a horizontal direction relative to the horizontal injection zones of the second horizontal well to provide a staggered injection pattern.

2- The process of claim 1, comprising:
(i) isolating a first horizontal injection zone along the first horizontal well and a second horizontal injection zone along the second horizontal well;
(ii) injecting the solvent into both horizontal injection zones of the first and second horizontal wells;
(iii) halting injection of the solvent into the horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second horizontal wells and repeating steps (ii) and (iii) for each of the additional horizontal injection zones.

3- The process of claim 2, wherein isolating comprises inserting at least one isolating device within each of the first and second horizontal wells and activating the isolating device downhole.

4- The process of claim 3, wherein the isolating device is selected from packers, diverters, and balls combined with sliding sleeves.

5- The process of claim 3, wherein the isolating device comprises packers.

6- The process of any one of claims 3 to 5, further comprising removing the isolating device from the first and second horizontal wells once injection into each of the horizontal zones is terminated.

7- The process of any one of claims 1 to 6, wherein the solvent is injected simultaneously at least into the first and second horizontal injection zones.

8- The process of any one of claims 1 to 7, wherein the solvent is sequentially injected into the reservoir in consecutive horizontal injection zones along each of the first and second horizontal wells.

9- The process of claim 1, comprising:
(i) providing a first packer assembly for isolating horizontal injection zones in a targeted zone of the first horizontal well and providing a second packer assembly for isolating horizontal injection zones in a targeted zone of the second horizontal well;
(ii) sequentially injecting solvent into each horizontal injection zone of the targeted zones;
(iii) halting injection of the solvent into the horizontal injection zones;
(iv) moving the first packer assembly along the first horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the first horizontal well and moving the second packer assembly along the second horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the second horizontal well; and (v) repeating steps (ii) and (iii) for each of the further horizontal injection zones isolated in step (iv).

10- The process of any one of claims 1 to 9, wherein the solvent is selected to limit or avoid asphaltene deposition.

11- The process of any one of claims 1 to 10, wherein the solvent is selected to have a diffusion coefficient in bitumen of at least 10-7 cm2/s, when measured at 20 C
and averaged over a concentration range of 10 to 90% of solvent in bitumen.

12- The process of any one of claims 1 to 11, wherein the solvent is selected from a water-soluble solvent, an oil-soluble solvent, a water-oil soluble solvent, and any mixture thereof.

13- The process of any one of claims 1 to 12, wherein the solvent is a mono-component solvent or multicomponent solvent.

14- The process of any one of claims 1 to 13, wherein the solvent is selected from an ether, an aromatic, a diluent, a light oil, a refining product, a condensate, and any mixture thereof.

15- The process of any one of claims 1 to 14, wherein the solvent is selected from an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha; and diesel.

16- The process of claim 14 or 15, wherein the aromatic is selected from benzene, toluene, m-xylene, o-xylene, and p-xylene.

17- The process of claim 15, wherein the alkane is selected from cyclohexane and 02-08 alkanes.

18- The process of any one of claims 1 to 14, wherein the solvent is selected from:
d im ethyl ether;
toluene and cyclohexane;
toluene and 02-08 alkanes;
toluene and naphtha;
m-xylene, o-xylene, p-xylene and cyclohexane;
toluene, m-xylene, o-xylene, p-xylene, benzene and 02-08 alkanes; and diesel.

19- The process of any one of claims 1 to 14, wherein the solvent comprises dimethylether.

20- The process of any one of claims 1 to 19, wherein the solvent is injected in the first horizontal well in liquid phase.

21- The process of any one of claims 1 to 20, wherein the solvent is injected in the second horizontal well in liquid phase.

22- The process of any one of claims 1 to 20, wherein the solvent is injected in the second horizontal well in gas phase.

23- The process of any one of claims 1 to 20, wherein the solvent is injected in the second horizontal well in both liquid and gas phase.

24- The process of any one of claims 1 to 23, further comprising heating the solvent before injection.

25- The process of claim 24 wherein the solvent is heated to a temperature of up to 350 C above initial reservoir temperature.

26- The process of claim 24, wherein the solvent is heated to a temperature of from 5 C up to 150 C, above the initial reservoir temperature.

27- The process of claim 24, wherein the solvent is heated to a temperature of from 150 C up to 250 C, above the initial reservoir temperature.

28- The process of claim 24, wherein the solvent is heated to a temperature of from 250 C up to 350 C, above the initial reservoir temperature.

29- The process of any one of claims 1 to 28, wherein the solvent is injected at a pressure of 0.1 to 20 MPa above an original reservoir pressure.

30- The process of any one of claims 1 to 29, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 1 to 200 %.

31- The process of any one of claims 1 to 29, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 2 to 100%.

32- The process of any one of claims 1 to 31, further comprising a soaking period during which the solvent is allowed to further diffuse into the bitumen once injection into each of the horizontal zones is terminated.

33- The process of claim 32, further comprising providing heat to the horizontal wells during the soaking period to enhance the solvent diffusion.

34- The process of claim 33, wherein heat is provided using one of:
downhole electric resistive heaters, radio frequency heaters, microwave heaters, and other types of downhole heaters.

35- The process of claim 33, wherein heat is provided through circulation of at least one of steam and hot solvent.

36- The process of any one of claims 1 to 35, wherein the solvent diffusion results in mobilizing the bitumen in the interwell region and establishing communication between the first and second horizontal wells as part of a well pair start-up method.

37- The process of any one of claims 1 to 36, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 800 m.

38- The process of any one of claims 1 to 36, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 500 m.

39- The process of any one of claims 1 to 36, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 200 m.

40- The process of any one of claims 1 to 36, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 100 m.

41- The process of any one of claims 1 to 40, wherein the first and second horizontal wells are configured as a well pair with the wells operated under gravity dominated control.

42- A process for establishing fluid communication between at least one pair of wells in a bitumen containing underground reservoir, the well pair comprising a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process comprising:

(i) isolating a first horizontal injection zone along the first horizontal well and isolating a second horizontal injection zone along the second horizontal well;
(ii) injecting a solvent into the first and second isolated horizontal injection zones;
(iii) halting injection of the solvent into the isolated horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second wells and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones;
wherein the horizontal injection zones of the first well are offset in a horizontal direction relative to the horizontal injection zones of the second well to provide a staggered injection pattern; and wherein injecting the solvent results in mobilization of the bitumen by diffusion of the solvent in the interwell region.

43- The process of claim 42, wherein isolating comprises inserting at least one isolating device within each of the first and second horizontal wells and activating the isolating device downhole.

44- The process of claim 43, wherein the isolating device is selected from packers, diverters, and balls combined with sliding sleeves.

45- The process of claim 43, wherein the isolating device comprises packers.

46- The process of any one of claims 43 to 45, further comprising removing the isolating device from the first and second horizontal wells once injection into each of the horizontal zones is terminated.

47- The process of any one of claims 42 to 44, wherein in step (ii) the solvent is simultaneously injected in the first horizontal injection zone and the second horizontal injection well.

48- The process of any one of claims 42 to 45, wherein in step (iv) the solvent is simultaneously injected in the additional horizontal zones along each one of the first and second wells.

49- The process of any one of claims 42 to 48 wherein the solvent is selected to limit or avoid asphaltene deposition.

50- The process of any one of claims 42 to 49, wherein the solvent is selected to have a diffusion coefficient in bitumen of at least 10-7 cm2/s, when measured at 20 C
and averaged over a concentration range of 10 to 90% of solvent in bitumen.

51- The process of any one of claims 42 to 50, wherein the solvent is selected from a water-soluble solvent, an oil-soluble solvent, a water-oil soluble solvent, and any mixture thereof.

52- The process of any one of claims 42 to 51, wherein the solvent is a mono-component solvent or multicomponent solvent.

53- The process of any one of claims 42 to 52, wherein the solvent is selected from an ether, an aromatic, a diluent, a light oil, a refining product, a condensate, and any mixture thereof.

54- The process of any one of claims 42 to 52, wherein the solvent is selected from an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha; and diesel.

55- The process of claim 53 or 54, wherein the aromatic is selected from benzene, toluene, m-xylene, o-xylene, and p-xylene.

56- The process of claim 54, wherein the alkane is selected from cyclohexane and C2-C8 alkanes.

57- The process of any one of claims 42 to 52, wherein the solvent is selected from:
d im ethyl ether;
toluene and cyclohexane;

toluene and 02-08 alkanes;
toluene and naphtha;
m-xylene, o-xylene, p-xylene and cyclohexane;
toluene, m-xylene, o-xylene, p-xylene, benzene and 02-08 alkanes; and diesel.

58- The process of any one of claims 42 to 52, wherein the solvent comprises dimethylether.

59- The process of any one of claims 42 to 58, wherein the solvent is injected in the first horizontal well in liquid phase.

60- The process of any one of claims 42 to 59, wherein the solvent is injected in the second horizontal well in liquid phase.

61- The process of any one of claims 42 to 59, wherein the solvent is injected in the second horizontal well in gas phase.

62- The process of any one of claims 42 to 59, wherein the solvent is injected in the second horizontal well in both liquid and gas phase.

63- The process of any one of claims 42 to 62, further comprising heating the solvent before injection.

64- The process of claim 63, wherein the solvent is heated to a temperature of up to 350 C above initial reservoir temperature.

65- The process of claim 64, wherein the solvent is heated to a temperature of from 5 C up to 150 C, above the initial reservoir temperature.

66- The process of claim 64, wherein the solvent is heated to a temperature of from 150 C up to 250 C, above the initial reservoir temperature.

67- The process of claim 64, wherein the solvent is heated to a temperature of from 250 C up to 350 C, above the initial reservoir temperature.

68- The process of any one of claims 42 to 67, wherein the solvent is injected at a pressure of 0.1 to 20 MPa above an original reservoir pressure.

69- The process of any one of claims 42 to 68, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 1 to 200 %.

70- The process of any one of claims 42 to 69, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 2 to 100 %.

71- The process of any one of claims 42 to 70, further comprising a soaking period during which the solvent is allowed to further diffuse into the bitumen once injection into each of the horizontal zones is terminated.

72- The process of any one of claims 42 to 70, further comprising monitoring fluid communication establishment between the horizontal wells once injection into each of the horizontal zones is terminated, determining that fluid communication is incomplete, and implementing a soaking period during which the solvent is allowed to further diffuse into the bitumen until fluid communication is established.

73- The process of claim 71 or 72, further comprising providing heat to the horizontal wells during the soaking period to enhance the solvent diffusion.

74- The process of claim 73, wherein heat is provided using one of:
downhole electric resistive heaters, radio frequency heaters, microwave heaters, and other types of downhole heaters.

75- The process of claim 73, wherein heat is provided through circulation of at least one of steam and hot solvent.

76- The process of any one of claims 42 to 75, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 800 m.

77- The process of any one of claims 42 to 75, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 500 m.

78- The process of any one of claims 42 to 75, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 200 m.

79- The process of any one of claims 42 to 75, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 100 m.

80- The process of any one of claims 42 to 79, wherein the first and second horizontal wells are configured as a well pair with the wells operated under gravity dominated control.

81- A process for recovering bitumen from an underground reservoir provided with at least one well pair comprising a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process comprising:
establishing communication between the first horizontal well and the second horizontal well by injecting at least one solvent according to the process of any one of claims 42 to 80;
once fluid communication is established, injecting a production fluid into the first horizontal well; and recovering a produced fluid comprising the bitumen and the solvent from the second horizontal well.

82- The process of claim 81, further comprising separating the solvent from the produced fluid for re-use in establishing communication between the horizontal wells of another well pair.

83- A process for injecting a solvent into a bitumen containing underground reservoir comprising at least a first horizontal well and a second horizontal well positioned below the first horizontal well, the wells being separated by an interwell region, the process comprising:
injecting the solvent simultaneously in both the first and second horizontal wells in a multistage mode comprising injecting the solvent in horizontal injection zones arranged along each of the first and second horizontal wells, wherein after injection the solvent diffuses in the interwell region.

84- The process of claim 83, comprising:
(i) isolating a first horizontal injection zone along the first horizontal well and a second horizontal injection zone along the second horizontal well;
(ii) simultaneously injecting the solvent into both the first horizontal injection zone and the second horizontal injection zone;
(iii) halting injection of the solvent into the horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second horizontal wells and repeating steps (ii) and (iii) for each of the additional horizontal injection zones.

85- The process of claim 84, wherein isolating comprises inserting at least one isolating device within each of the first and second horizontal wells and activating the isolating device downhole.

86- The process of claim 85, wherein the isolating device is selected from packers, diverters, and balls combined with sliding sleeves.

87- The process of claim 85, wherein the isolating device comprises packers.

88- The process of any one of claims 85 to 87, further comprising removing the isolating device from the first and second horizontal wells once injection into each of the horizontal zones is terminated.

89- The process of any one of claims 83 to 88, wherein the solvent is sequentially injected into the reservoir in consecutive horizontal injection zones along each of the first and second horizontal wells.

90- The process of claim 83, comprising:
(i) providing a first packer assembly for isolating horizontal injection zones in a targeted zone of the first horizontal well and providing a second packer assembly for isolating horizontal injection zones in a targeted zone of the second horizontal well;
(ii) sequentially injecting solvent into each horizontal injection zone of the targeted zones;
(iii) halting injection of the solvent into the horizontal injection zones;
(iv) moving the first packer assembly along the first horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the first horizontal well and moving the second packer assembly along the second horizontal well for isolating further horizontal injection zones in at least one other targeted zone of the second horizontal well; and (v) repeating steps (ii) and (iii) for each of the further horizontal injection zones isolated in step (iv).

91- The process of any one of claims 83 to 90, wherein the solvent is selected to limit or avoid asphaltene deposition.

92- The process of any one of claims 83 to 91, wherein the solvent is selected to have a diffusion coefficient in bitumen of at least 10-7 cm2/s, when measured at 20 C
and averaged over a concentration range of 10 to 90% of solvent in bitumen.

93- The process of any one of claims 83 to 92, wherein the solvent is selected from a water-soluble solvent, an oil-soluble solvent, a water-oil soluble solvent and any mixture thereof.

94- The process of any one of claims 83 to 93, wherein the solvent is a mono-component solvent or multicomponent solvent.

95- The process of any one of claims 83 to 94, wherein the solvent is selected from an ether, an aromatic, a diluent, a light oil, a refining product, a condensate, and any mixture thereof.

96- The process of any one of claims 83 to 94, wherein the solvent is selected from an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha; and diesel.

97- The process of claim 95 or 96, wherein the aromatic is selected from benzene, toluene, m-xylene, o-xylene, and p-xylene.

98- The process of claim 96, wherein the alkane is selected from cyclohexane and 02-08 alkanes.

99- The process of any one of claims 83 to 94, wherein the solvent is selected from:
d im ethyl ether;
toluene and cyclohexane;
toluene and 02-08 alkanes;
toluene and naphtha;
m-xylene, o-xylene, p-xylene and cyclohexane;
toluene, m-xylene, o-xylene, p-xylene, benzene and 02-08 alkanes; and diesel.

100- The process of any one of claims 83 to 94, wherein the solvent comprises dimethylether.

101- The process of any one of claims 83 to 100, wherein the solvent is injected in the first horizontal well in liquid phase.

102- The process of any one of claims 83 to 101, wherein the solvent is injected in the second horizontal well in liquid phase.

103- The process of any one of claims 83 to 101, wherein the solvent is injected in the second horizontal well in gas phase.

104- The process of any one of claims 83 to 101, wherein the solvent is injected in the second horizontal well in both liquid and gas phase.

105- The process of any one of claims 83 to 104, further comprising heating the solvent before injection.

106- The process of claim 105, wherein the solvent is heated to a temperature of up to 350 C above initial reservoir temperature.

107- The process of claim 105, wherein the solvent is heated to a temperature of from C up to 150 C, above the initial reservoir temperature.

108- The process of claim 105, wherein the solvent is heated to a temperature of from 150 C up to 250 C, above the initial reservoir temperature.

109- The process of claim 105, wherein the solvent is heated to a temperature of from 250 C up to 350 C, above the initial reservoir temperature.

110- The process of any one of claims 83 to 109, wherein the solvent is injected at a pressure of 0.1 to 20 MPa above an original reservoir pressure.

111- The process of any one of claims 83 to 110, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 1 to 200 %.

112- The process of any one of claims 83 to 110, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 2 to 100%.

113- The process of any one of claims 83 to 112, further comprising a soaking period during which the solvent is allowed to further diffuse into the bitumen once injection into each of the horizontal zones is terminated.

114- The process of claim 113, further comprising providing heat to the horizontal wells during the soaking period to enhance the solvent diffusion.

115- The process of claim 114, wherein heat is provided using one of: downhole electric resistive heaters, radio frequency heaters, microwave heaters, and other types of downhole heaters.

116- The process of claim 115, wherein heat is provided through circulation of at least one of steam and hot solvent.

117- The process of any one of claims 83 to 116, wherein the solvent diffusion results in mobilizing the bitumen in the interwell region and establishing communication between the first and second horizontal wells as part of a well pair start-up method.

118- The process of any one of claims 83 to 117, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 800 m.

119- The process of any one of claims 83 to 117, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 500 m.

120- The process of any one of claims 83 to 117, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 200 m.

121- The process of any one of claims 83 to 117, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 100 m.

122- The process of any one of claims 83 to 121, wherein the first and second horizontal wells are configured as a well pair with the wells operated under gravity dominated control.

123- A process for establishing fluid communication between at least one pair of wells in a bitumen containing underground reservoir, the well pair comprising a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process comprising:
(i) isolating a first horizontal injection zone along the first horizontal well and isolating a second horizontal injection zone along the second horizontal well;
(ii) injecting a solvent simultaneously into the first isolated horizontal injection zone and the second isolated horizontal injection zone;
(iii) halting injection of the solvent into the isolated horizontal injection zones;
and (iv) isolating additional horizontal injection zones one-by-one along each one of the first and second wells and repeating steps (ii) and (iii) for each of the additional isolated horizontal injection zones;

wherein injecting the solvent results in mobilization of the bitumen by diffusion of the solvent in the interwell region.

124- The process of claim 123, wherein isolating comprises inserting at least one isolating device within each of the first and second horizontal wells and activating the isolating device downhole.

125- The process of claim 124, wherein the isolating device is selected from packers, diverters, and balls combined with sliding sleeves.

126- The process of claim 124, wherein the isolating device comprises packers.

127- The process of any one of claims 124 to 126, further comprising removing the isolating device from the first and second horizontal wells once injection into each of the horizontal zones is terminated.

128- The process of any one of claims 123 to 127, wherein the solvent is selected to limit or avoid asphaltene deposition.

129- The process of any one of claims 123 to 128, wherein the solvent is selected to have a diffusion coefficient in bitumen of at least 10-7 cm2/s, when measured at 20 C and averaged over a concentration range of 10 to 90% of solvent in bitumen.

130- The process of any one of claims 123 to 129, wherein the solvent is selected from a water-soluble solvent, an oil-soluble solvent, a water-oil soluble solvent, and any mixture thereof.

131- The process of any one of claims 123 to 130, wherein the solvent is a mono-component solvent or multicomponent solvent.

132- The process of any one of claims 123 to 131, wherein the solvent is selected from an ether, an aromatic, a diluent, a light oil, a refining product, a condensate, and any mixture thereof.

133- The process of any one of claims 123 to 131, wherein the solvent is selected from an ether; a mixture of at least one aromatic and at least one alkane; a mixture of at least one aromatic and naphtha; and diesel.

134- The process of claim 132 or 133, wherein the aromatic is selected from benzene, toluene, m-xylene, o-xylene, and p-xylene.

135- The process of claim 133, wherein the alkane is selected from cyclohexane and C2-C8 alkanes.

136- The process of any one of claims 123 to 131, wherein the solvent is selected from:
dimethyl ether;
toluene and cyclohexane;
toluene and C2-C8 alkanes;
toluene and naphtha;
m-xylene, o-xylene, p-xylene and cyclohexane;
toluene, m-xylene, o-xylene, p-xylene, benzene and C2-C8 alkanes; and diesel.

137- The process of any one of claims 123 to 131, wherein the solvent comprises dimethylether.

138- The process of any one of claims 123 to 137, wherein the solvent is injected in the first horizontal well in liquid phase.

139- The process of any one of claims 123 to 138, wherein the solvent is injected in the second horizontal well in liquid phase.

140- The process of any one of claims 123 to 138, wherein the solvent is injected in the second horizontal well in gas phase.

141- The process of any one of claims 123 to 138, wherein the solvent is injected in the second horizontal well in both liquid and gas phase.

142- The process of any one of claims 123 to 141, further comprising heating the solvent before injection.

143- The process of claim 142, wherein the solvent is heated to a temperature of up to 350 °C above initial reservoir temperature.

144- The process of claim 142, wherein the solvent is heated to a temperature of from °C up to 150°C, above the initial reservoir temperature.

145- The process of claim 142, wherein the solvent is heated to a temperature of from 150 °C up to 250 °C, above the initial reservoir temperature.

146- The process of claim 142, wherein the solvent is heated to a temperature of from 250 °C up to 350 °C, above the initial reservoir temperature.

147- The process of any one of claims 123 to 146, wherein the solvent is injected at a pressure of 0.1 to 20 MPa above an original reservoir pressure.

148- The process of any one of claims 123 to 147, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 1 to 200 %.

149- The process of any one of claims 123 to 148, wherein the solvent is injected into each of the horizontal injection zones at a percentage of pore volume ranging from 2 to 100 %.

150- The process of any one of claims 123 to 149, further comprising a soaking period during which the solvent is allowed to further diffuse into the bitumen once injection into each of the horizontal zones is terminated.

151- The process of any one of claims 123 to 150, further comprising monitoring fluid communication establishment between the horizontal wells once injection into each of the horizontal zones is terminated, determining that fluid communication is incomplete, and implementing a soaking period during which the solvent is allowed to further diffuse into the bitumen until fluid communication is established.

152- The process of claim 150 or 151, further comprising providing heat to the horizontal wells during the soaking period to enhance the solvent diffusion.

153- The process of claim 152, wherein heat is provided using one of: downhole electric resistive heaters, radio frequency heaters, microwave heaters, and other types of downhole heaters.

154- The process of claim 152, wherein heat is provided through circulation of at least one of steam and hot solvent.

155- The process of any one of claims 123 to 154, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 800 m.

156- The process of any one of claims 123 to 154, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 500 m.

157- The process of any one of claims 123 to 154, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 200 m.

158- The process of any one of claims 123 to 154, wherein each of the horizontal injection zones has a length ranging from about 10 m to about 100 m.

159- The process of any one of claims 123 to 158, wherein the first and second horizontal wells are configured as a well pair with the wells operated under gravity dominated control.

160- A process for recovering bitumen from an underground reservoir provided with at least one well pair comprising a first horizontal well and a second horizontal well located below the first horizontal well, the wells being separated by an interwell region, the process comprising:
establishing communication between the first horizontal well and the second horizontal well by injecting at least one solvent according to the process of any one of claims 123 to 159;
once fluid communication is established, injecting a production fluid into the first horizontal well; and recovering a produced fluid comprising the bitumen and the solvent from the second horizontal well.

161- The process of claim 160, further comprising separating the solvent from the produced fluid for re-use in establishing communication between the horizontal wells of another well pair.