WO2014179833A1

WO2014179833A1 - Controlled burn back method

Info

Publication number: WO2014179833A1
Application number: PCT/AU2014/000503
Authority: WO
Inventors: Kuldeep Varma; Casper Jan Hendrik BURGER
Original assignee: Linc Energy Ltd
Priority date: 2013-05-10
Filing date: 2014-05-09
Publication date: 2014-11-13

Abstract

The invention provides a method for carrying out underground coal gasification that includes burn back of a casing inserted into a wellbore that links an injection well and a production well in an underground coal gasifier.

Description

CONTROLLED BURN BACK METHOD

TECHNICAL FIELD

[0001 ] This invention relates to underground coal gasification (UCG). In particular, UCG methods that include burn back of a casing inserted into a wellbore that links an injection well and a production well in an underground coal gasifier are disclosed.

BACKGROUND ART

[0002] Underground coal gasification is a process by which product gas is produced from a subterranean coal seam by combusting and gasifying the coal in situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or syngas and can be used as a feedstock for various applications, including clean fuels production, chemical production and electricity generation.

[0003] Wells are drilled into the coal seam to allow for oxidant injection and product gas extraction. The wells are often connected to facilitate oxidant injection, cavity development, and product gas flow. The well allowing the injection of oxidant is called an injection well. The well from which product gas emerges is called a production well. Both substantially horizontal and substantially vertical well regions can be used for injection and production. Underground coal gasification can also utilise one or more generally vertical wells (service wells) located between the injection and production wells.

[0004] A coal seam having injection and production wells is typically referred to as an underground coal gasifier. The gasifier will have a combustion zone within which coal is combusted in the presence of an oxidant, a gasification zone located downstream of the combustion zone in which coal is gasified and partially oxidized to produce product gas, and a downstream pyrolysis zone in which pyrolysis of coal occurs. Hot product gas flows downstream from the gasification zone and exits the ground from a well head of the production well. As coal is consumed or gasified, a gasifier (gasification) cavity within the coal seam develops and grows in size. [0005] The product gas (raw syngas) generated by UCG typically includes syngas as well as other components, and the constituency will depend on various factors including the type of oxidant used for UCG (air or other oxidant, such as oxygen or oxygen-enriched air), water presence (both ground water and exogenous water), coal quality, and UCG operating temperature and pressure.

[0006] An ongoing problem in UCG is the need to re-ignite a coal seam one or more times following initial ignition of the seam.

SUMMARY OF INVENTION

[0007] An object of the present invention is to provide a method for UCG that minimises one or more of the problems of the prior art.

[0008] In one aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a sectional casing into the wellbore, the casing including one or more perforated sections, b) positioning an air injection device in the wellbore and providing air to the injection device, thereby injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) monitoring burn back of the casing inserted into the wellbore, and f) retracting the air injection device positioned in the wellbore in the direction of the injection well at a rate sufficient to achieve a desired rate of burn back of the casing inserted into the wellbore.

[0009] In one embodiment, the casing includes alternating perforated and non- perforated sections.

[0010] In another embodiment, the casing consists exclusively of perforated sections.

[0011] In yet another embodiment, the method further includes the steps of stopping the injection of air into the wellbore for a period of time sufficient to allow backflow of UCG product gas from the gasifier cavity into the wellbore and subsequently resuming air injection into the wellbore.

[0012] In another aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a sectional casing into the wellbore, the casing including one or more perforated sections, b) injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) stopping the injection of air into the wellbore for a period of time sufficient to allow backflow of UCG product gas from the gasifier cavity into the wellbore, f) resuming air injection into the wellbore so as to burn back the casing inserted into the wellbore, and g) monitoring burn back of the casing inserted into the wellbore.

[0013] In one embodiment, injecting air into the wellbore includes injecting air through the injection well into the wellbore.

[00 4] In another embodiment, injecting air into the wellbore includes positioning an air injection device in the wellbore and providing air to the injection device.

[0015] Suitably, steps (e) to (g) can be repeated at least once after the practice thereof to further burn back the casing inserted into the wellbore.

[0016] In a further aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a perforated casing into the wellbore, b) positioning an air injection device in the wellbore and providing air to the injection device, thereby injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) monitoring burn back of the casing inserted into the wellbore, and f) retracting the air injection device positioned in the wellbore in the direction of the injection well at a rate sufficient to achieve a desired rate of burn back of the casing inserted into the wellbore.

[0017] In some embodiments of the methods disclosed herein, the casing further includes a combustible sheath that covers at least some of the perforations in the casing.

[0018] In other embodiments of the methods disclosed herein, one or more additional active gasifier cavities are established in the coal seam.

[00 9] In order that the invention may be more readily understood and put into practice, one or more preferred embodiments thereof will now be described, by way of example only.

DESCRIPTION OF EMBODIMENTS

[0020] The present invention relates to methods of UCG that include wellbore casing burn back to eliminate the need for re-ignition of an underground coal seam following initial ignition of the coal seam.

[0021] Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to mean the inclusion of a stated integer, group of integers, step, or steps, but not the exclusion of any other integer, group of integers, step, or steps.

[0022] In one aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a sectional casing into the wellbore, the casing including one or more perforated sections, b) positioning an air injection device in the wellbore and providing air to the injection device, thereby injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) monitoring burn back of the casing inserted into the wellbore, and f) retracting the air injection device positioned in the wellbore in the direction of the injection well at a rate sufficient to achieve a desired rate of burn back of the casing inserted into the wellbore.

[0023] In another aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a sectional casing into the wellbore, the casing including one or more perforated sections, b) injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) stopping the injection of air into the wellbore for a period of time sufficient to allow backflow of UCG product gas from the gasifier cavity into the wellbore, f) resuming air injection into the wellbore so as to burn back the casing inserted into the wellbore, and g) monitoring burn back of the casing inserted into the wellbore.

[0024] In a further aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of a) inserting a perforated casing into the wellbore, b) positioning an air injection device in the wellbore and providing air to the injection device, thereby injecting air into the wellbore, c) igniting the coal seam, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) monitoring burn back of the casing inserted into the wellbore, and f) retracting the air injection device positioned in the wellbore in the direction of the injection well at a rate sufficient to achieve a desired rate of burn back of the casing inserted into the wellbore.

[0025] As will be understood by one of ordinary skill in the art, the UCG methods disclosed herein can include the use of any injection/production well (including linking wellbore) design, any arrangement of wells and any gasifier layout, including a knife edge layout or a linear layout.

[0026] The casing (also referred to as a "well liner") inserted into the wellbore can be of any suitable size, shape and construction, and can be made of any suitable material or combination of materials. Exemplary materials suitable for the casing include, but are not limited to, metal (including steel, such as carbon steel, and aluminium), fibreglass, carbon fibre, and plastic. As will be understood by one of ordinary skill in the art, selection of the casing material (and wall thickness of the casing) influences casing ignition and burn back rate in a coal seam when practicing the disclosed UCG methods. Preferably, the casing has a round cross-section to provide an annular passage.

[0027] The casing can be of any suitable diameter and length. Typically, the casing will have an outside diameter of anywhere between about 5 to 10 inches, more preferably about 5 to 8 inches, and even more preferably about 5.5 to 7 inches.

[0028] The casing can be of unitary construction or can include a plurality of connectable units (i.e., sections/segments). The casing or sections can be of any suitable length, including, metres, tens of metres, hundreds of metres, and

kilometres. Accordingly, casing sections can be connected together to form a full- length casing being tens of metres long, hundreds of metres long, or even several kilometres in length, depending on the length of the wellbore. Each casing section can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length.

[0029] Casing sections can be connected together in any suitable way. For example, the ends of each section can be threaded, and the full-length casing can include one or more threaded collars for connecting the ends of adjacent casing sections together.

[0030] Casing perforations can be of any suitable size, shape and arrangement as required to achieve one or more desired outcomes. For example, perforations allow the ignition of a coal seam from within a wellbore using an ignition tool located within the casing to ignite the surrounding coal seam. As will be understood by one of ordinary skill in the art, the size, shape and arrangement of casing perforations influences casing ignition and burn back rate in a coal seam when practicing the disclosed UCG methods. In some embodiments, casing perforations are grouped together in one or more regions along the length of a casing section, alternating with non-perforated regions. In other embodiments, the entire casing section is perforated. Perforations in the casing also allow oxidant and water/steam to pass through the casing and contact the coal while preventing coal and ash from blocking the path of production gases.

[0031] Casing perforations can be in periodic symmetry in both circumferential and axial directions. The perforations can be in the form of circular or other shaped holes (e.g., hexagonal or octagonal), or slots. The perforations can be, for example, circular having a diameter of about 10 mm to about 25 mm. The perforations can be in a rectangular or diamond-shaped grid pattern, or both, for example. Preferably, the perforations are in a staggered arrangement (diamond spacing) as this provides the casing with the greatest structural integrity.

[0032] The casing can include a firebreak material connected to the casing or forming part of the casing (e.g., a casing section constructed of the firebreak material). Exemplary firebreak materials include, but are not limited to, the Inconel ® (predominantly nickel-chromium alloys), Monel ® (predominantly nickel-copper alloys), and Hastelloy ® (predominantly nickel-containing alloys) families of high- performance alloys. The firebreak material and/or section can be positioned adjacent a heel of the injection well.

[0033] Preferably, the casing extends from adjacent the heel of the injection well to adjacent a heel of the production well.

[0034] The casing can further include a combustible sheath configured to cover some or all of the casing perforations. Similar to the casing, the sheath can be of unitary construction or can include a plurality of units (i.e., sections/segments). For example, sheath sections can be utilised to cover perforated sections of the casing, leaving intervening non-perforated sections of casing uncovered. The sheath or sections can be of any suitable length as required by the casing (e.g., perforated sections or parts thereof), including, metres, tens of metres, hundreds of metres, and kilometres. Each sheath section can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length. [0035] The combustible sheath (including sections thereof) can be of any suitable size, shape, and construction, and can be made of any suitable material or materials, including, for example, aluminium, fibreglass, carbon fibre, plastic, and combinations thereof. Preferably, the combustible sheath is made of high density polyethylene (HDPE). The combustible sheath can be about 1 mm to about 20 mm thick, including about 2 mm, about 5 mm, about 10 mm, or about 15 mm thick.

[0036] The combustible sheath can be, for example, a membrane, sheet, or film that wraps around the outside or the inside of the casing at least once and covers some or all of the casing perforations (including in one or more perforated sections) of the casing.

[0037] The combustible sheath can be attached to or associated with the casing in any suitable way. For example, fasteners, such as screws, ring clamps, or straps (including stainless steel straps), can be used to bind the sheath to the casing.

Additionally, the sheath can be integrated into the casing (e.g., sandwiched into a double-layer casing).

[0038] Alternatively, where the combustible sheath is made of plastic, it can be hot-rolled in place around the outside or the inside of the casing, particularly in association with some or all of the perforations (including in one or more perforated sections) of the casing. That is, the sheath covers the perforations.

[0039] The step of injecting air into the wellbore can be achieved in any suitable manner. In one embodiment, injecting air into the wellbore includes injecting air through the injection well (or a service well) into the wellbore. For example, a source of air (e.g., an air compressor) can be connected directly or indirectly to a well head of the injection well (or service well), such that the air is injected/introduced into the wellbore via the injection well (or service well).

[0040] In another embodiment, injecting air into the wellbore includes positioning an air injection device in the wellbore and providing air to the injection device. As will be understood by one of ordinary skill in the art, the air injection device can provide directed/projected air injection to the underground coal seam. In one embodiment, the air injection device includes an associated sacrificial ignition tool that is consumed during the ignition/gasification process. Preferably, the air injection device is maintained in the wellbore upstream of the active UCG cavity. The air injection device can be positionable and retractable.

[0041] Positioning of the air injection device can be achieved utilising coiled tubing connected to the device and extendible within the wellbore to position the device at a desired location within the wellbore. The coiled tubing can be of any suitable size, shape and construction and can be made of any suitable material or materials. More particularly, the coiled tubing can be of any suitable length and diameter. Preferably, the coiled tubing is made of metal, such as stainless steel, carbon steel, or copper. The coiled tubing can be of unitary construction or can include two or more connectable tube pieces. The coiled tubing can be maintained on a spool from which the coiled tubing is unspooled.

[0042] Accordingly, coiled tubing can include at least one inner tube (inner line) extending within an outer tube (outer line), wherein one or both of the inner and outer tubes are connected to an air injection device with associated sacrificial ignition tool (e.g., with one tube supplying an ignition fuel and the other tube facilitating injection of air into the coal seam). That is, the coiled tubing can include at least one inner tube and an outer tube that extend concentrically relative to one another. More than one inner tube may extend within the same outer tube.

[0043] An associated controller operable to control the release/injection of air can be used when injecting air into the wellbore. The controller can include a pipe manifold in fluid communication with a well head/coiled tubing and a source of air. The controller can be operable remotely from the air injection device. The controller can include trim, non-return, and isolation valves, flow measuring devices, and pressure relief devices. Such operating devices allow for air injection rate

measurement and control. Air injection rate and quantity can be adjusted using flow controlling devices, such devices being either pneumatically actuated, manually choked, quarter-turn types, or electrically actuated. The controller can include pressure safety devices, filtration devices, and flow metering devices, in addition to isolation valves. Control logic can allow the air to flow as per required settings.

[0044] Air can be injected into the wellbore at any suitable injection rate and quantity. The injection rate can be chosen with respect to various criteria, including ensuring that downhole temperatures in the vicinity of an air injection device are maintained within a desired range, controlling casing burn back, and/or the quality and/or composition of the product gas (e.g., the calorific value and/or the H o CO ratio) emerging from the production well are as desired. One of ordinary skill in the art will be able to formulate the rate and quantity of air injection necessary to achieve desired outcomes.

[0045] As will be understood by one of ordinary skill in the art, the step of igniting the coal seam preferably includes using an ignition tool (including sacrificial ignition tool), whereby an ignition tool that includes ignition means is inserted into the coal seam via the injection well, a service well, and/or the production well. Once introduced into the coal seam, the ignition tool is used to ignite the coal seam and establish a combustion zone. The ignition tool can be positionable and retractable. As with the air injection device, positioning of the ignition tool can be achieved utilising coiled tubing connected to the ignition tool and extendible within the wellbore to position the ignition tool at a desired location within the wellbore.

[0046] As will be understood by one of ordinary skill in the art, ignition means includes an ignition fuel (e.g., hydrocarbon gases, such as methane, propane, butane, and mixtures thereof), an electrical spark generator (e.g., a spark plug), an electrical heat resistor (e.g., a glow plug), an ignition chemical, such as thermite (i.e., a pyrotechnic composition of a metal powder fuel and a metal oxide), a pyrophoric substance (e.g., a liquid, such as triethylboron (TEB), a gas, such as silane, a solid, such as phosphorus or an alkali metal), a pyrophoric substance and a hydrocarbon mixture, such as TEB vaporised in methane, or a pyrophoric substance and an inert gas, such as TEB and nitrogen, and combinations thereof.

[0047] Once the coal seam has been ignited, the ignition tool (or air injection device in the case of a sacrificial ignition tool) is retracted a safe distance while continuing the injection of air into the wellbore to fuel/maintain combustion of the coal seam. Alternatively, the ignition tool can be withdrawn from the coal seam following successful ignition of the seam.

[0048] As will be understood by one of ordinary skill in the art, establishment of an active gasifier cavity in the coal seam and the production of UCG product gas can be assessed in a number of ways, including monitoring the quality and/or

composition of the product gas (e.g., the calorific value and/or the H₂ to CO ratio) emerging from the production well.

[0049] As discussed herein, the air injection device is positionable and

retractable. In order to continue the UCG process through a coal seam, it will be necessary to reposition the air injection device periodically to progress a combustion zone along the coal seam and provide optimum resource recovery of the

underground coal resource. Thus, to support particularly the movement of the combustion zone and casing burn back, the coiled tubing and air injection device can be drawn along the cased wellbore that extends through the coal seam. A preferred method is utilising the controlled retracting injection point (CRIP) concept, as will be understood by one of ordinary skill in the art. Retraction of the air injection device can be practiced in such a way as to favour either continuous bum back of the casing inserted into the wellbore (including at a variable rate) or intermittent discontinuous burn back of the casing.

[0050] According to an important aspect of the present invention, stopping the injection of air into the wellbore (after establishing an active gasifier cavity in the coal seam) for a period of time sufficient to allow backflow of UCG product gas from the gasifier cavity into the wellbore enhances the ignition/auto-ignition of the coal seam and ignition and burn back of the casing inserted into the wellbore once air injection is resumed, and eliminates the need for re-ignition of the coal seam with an ignition tool to continue the UCG process. The period of time necessary to allow sufficient backflow of UCG product gas from the gasifier cavity into the wellbore for ignition and burn back of coal and the casing inserted into the wellbore can be determined empirically by one of ordinary skill in the art, taking into account factors relating to product gas composition and characteristics of the casing (e.g., material, thickness and perforations).

[0051] The step of monitoring burn back of the casing inserted into the wellbore can be achieved using instrumentation associated with the casing. Any suitable type of instrumentation can be used. For example, suitable instrumentation can include one or more conducting wires and/or thermocouples attached to the casing, to facilitate continuous length measurements and/or assess the temperature of the casing. Additionally, suitable instrumentation can include one or more optical fibres attached to the casing to facilitate optical time domain reflectometry (OTDR) for continuous length measurements of the casing. As will be understood by one of ordinary skill in the art, continuous length measurements and/or temperature information/data collected from the casing can be used to control the operating parameters of the gasifier, including, for example, air injection rate and quantity.

[0052] The associated instrumentation can be connected to the casing in any suitable way. For example, the perforations (including in the one or more perforated sections) of the casing can be used in the connection of the associated

instrumentation.

[0053] Suitably, the steps of stopping the injection of air into the wellbore for a period of time sufficient to allow backflow of UCG product gas from the gasifier cavity into the wellbore and subsequently resuming air injection into the wellbore can be repeated as needed to further burn back the casing inserted into the wellbore.

Additionally, as will be understood by one of ordinary skill in the art, these steps can be automated to facilitate their repetition.

[0054] In some embodiments of the methods disclosed herein, the casing inserted into the wellbore is cemented into place in the wellbore. As will be understood by one of ordinary skill in the art, cementing the casing into the wellbore influences progress of a combustion zone along the coal seam (including casing ignition and burn back rate) during UCG. Suitably, when cementing the casing into the wellbore the cement can be inserted into the space between the wellbore and the casing by pouring and/or pumping the cement into the desired space. [0055] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics can be combined in any suitable manner in one or more

combinations.

[0056] Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will therefore be appreciated by those of skill in the art that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.

Claims

1. A method of underground coal gasification (UCG) in a coal seam provided with an injection well, a production well and a substantially horizontal wellbore linking the injection well and the production well, including the steps of:

a. inserting a sectional casing into the wellbore, the casing comprising one or more perforated sections;

b. positioning an air injection device in the wellbore and providing air to the injection device, thereby injecting air into the wellbore;

c. igniting the coal seam;

d. establishing an active gasifier cavity in the coal seam so as to produce UCG product gas;

e. monitoring burn back of the casing inserted into the wellbore; and f. retracting the air injection device positioned in the wellbore in the direction of the injection well at a rate sufficient to achieve a desired rate of burn back of the casing inserted into the wellbore.

2. The method of claim 1 , wherein the casing comprises alternating perforated and non-perforated sections.

3. The method of claim 1 , wherein the casing consists of perforated sections.

4. The method of any one of claims 1 to 3, wherein the casing further comprises a combustible sheath that covers at least some of the perforations in the casing.

5. The method of claim 4, wherein the combustible sheath is made of aluminium.

6. The method of claim 4, wherein the combustible sheath is made of high density polyethylene.

7. The method of claim 1 , wherein the casing is made of metal and the casing further comprises a combustible sheath that covers at least some of the perforations in the casing.

8. The method of claim 7, wherein the combustible sheath is made of high density polyethylene.

9. The method of any one of claims 1 to 8, further including the steps of stopping the injection of air into the wellbore for a period of time sufficient to allow backflow of UCG product gas from the active gasifier cavity into the wellbore and subsequently resuming air injection into the wellbore. 0. The method of any one of claims 1 to 9, wherein one or more additional active gasifier cavities are established in the coal seam.