AU2015100794A4 - Oxidant Injection Method For Underground Coal Gasification - Google Patents

Abstract

The invention provides a method for carrying out underground coal gasification (UCG). Illo oii I 0i 11111C> co liiO 1111co LOii I I co

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Linc Energy Ltd Invention Title: OXIDANT INJECTION METHOD FOR UNDERGROUND COAL GASIFICATION The invention is described in the following statement: 1 OXIDANT INJECTION METHOD FOR UNDERGROUND COAL GASIFICATION TECHNICAL FIELD [0001] This invention relates to a method for carrying out underground coal gasification (UCG). In particular, a method for multipoint oxidant injection for UCG is disclosed. BACKGROUND ART [0002] Underground coal gasification is a process by which product gas is produced from a 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 linked or extended to form a substantially horizontal wellbore (also referred to as an in-seam well channel/linkage channel) 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 horizontal and vertical well regions can be used for injection and production. Underground coal gasification can also utilise one or more vertical wells (service wells) located between the injection and production wells. [0004] A coal seam having an injection well and a production well, with a well channel linking the two 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 2 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 comprises 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] Major challenges of UCG include controlled combustion and gasification of the coal seam and controlling the composition of the product gas. 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, an ignition well, a production well, and a substantially horizontal wellbore linking the injection well and the production well, the wellbore having a device operably inserted therein, the device including a pipe having an inlet end configured to receive oxidant from above ground, a plurality of discharge ports aligned along its length for discharging oxidant along a flow axis and a capped terminal end, the method including the steps of: a) igniting the coal seam using an ignition tool located within the ignition well, b) providing oxidant to the device for injection into the coal seam to support combustion and gasification of the coal seam at multiple points along the length of the wellbore linking the injection well and the production well, and c) withdrawing product gas from the production well. [0009] In one embodiment, the method further includes the step of injecting water into the production well to cool the product gas.

3 [0010] In another embodiment, the coal seam is further provided with one or more quench wells and the method further includes the step of injecting water and/or

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2 into the one or more quench wells to cool the product gas. [0011] 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, with reference to the accompanying figure. BRIEF DESCRIPTION OF DRAWING [0012] Figure 1 is a side section view of a portion of an underground coal gasifier illustrating certain aspects of the present invention. DESCRIPTION OF EMBODIMENTS [0013] The present invention relates to a method for multipoint oxidant injection for UCG. [0014] 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. [0015] In one aspect, the invention provides a method of underground coal gasification in a coal seam provided with an injection well, an ignition well, a production well, and a substantially horizontal wellbore linking the injection well and the production well, the wellbore having a device operably inserted therein, the device including a pipe having an inlet end configured to receive oxidant from above ground (i.e., an oxidant-conveying pipe), a plurality of discharge ports aligned along its length for discharging oxidant along a flow axis and a capped terminal end, the method including the steps of: a) igniting the coal seam using an ignition tool located within the ignition well, b) providing oxidant to the device for injection into the coal seam to support combustion and gasification of the coal seam at multiple points 4 along the length of the wellbore linking the injection well and the production well, and c) withdrawing product gas from the production well. [0016] The oxidant-conveying pipe can be of any suitable size, shape and construction, and can be made of any suitable material or materials. The pipe can be manufactured in shapes and sizes to suit the specific application. Preferably, the pipe has a round cross-section to provide an annular passage, although other cross section shapes are possible, as will be understood by one of ordinary skill in the art. [0017] The size, shape and construction of the oxidant-conveying pipe is selected to ensure that it can be extended through an underground coal seam, including, for example, being inserted into an in-seam well channel of an underground coal gasifier, and remains intact during service (i.e., it is able to supply oxidant to the coal seam/gasifier). Preferably, the pipe is strong enough to be inserted into the wellbore using traditional drilling service equipment, as will be well known to one of ordinary skill in the art. The pipe can be flexible such that it can be fed into the wellbore from a spool, like coil tubing. [0018] The oxidant-conveying pipe can be of unitary construction or can include a plurality of connectable units (i.e., segments). The pipe or segments can be of any suitable length, including, metres, tens of metres, hundreds of metres, and kilometres. Accordingly, pipe segments can be connected together to form a full length oxidant-conveying pipe being tens of metres long, hundreds of metres long, or even several kilometres in length, depending on the length of the in-seam well channel. Each pipe segment can be, for example, about 1 to 10 metres in length, including about 2, 3, 4, 5, 6, 7, 8, or 9 metres in length. [0019] The pipe segments can be connected together in any suitable way to form a full-length oxidant-conveying pipe. For example, the ends of each segment can be threaded, and the full-length pipe can include one or more threaded collars for connecting the ends of adjacent segments together. Alternatively, adjacent segments can be welded together to form a full-length pipe.

5 [0020] The oxidant-conveying pipe will have an outside diameter (or width) appropriate for the well channel into which it is being extended/inserted. Typically, the pipe will have an outside diameter of anywhere between about 2 and about 8 inches, including about 2.5, 3, 3.5, 4, 5, 6, and 7 inches. [0021] As will be understood by one of ordinary skill in the art, the oxidant conveying pipe (including pipe segments and threaded collars) can be made of material that is resistant to high temperatures and corrosion, and/or undergoes controlled expansion at elevated temperatures, such as those found in an active underground coal gasifier (e.g., in the range of 1,200 0 C). [0022] Exemplary metal, metal alloys, and ceramics suitable for the oxidant conveying pipe (including pipe segments and threaded collars) include, but are not limited to, stainless steel (and alloys thereof), chromium-nickel alloys (including those containing silicon, cobalt, tungsten, molybdenum, and microalloying elements such as nitrogen, and rare earth metals such as cesium),the Inconel @ (predominantly nickel-chromium alloys), Monel @ (predominantly nickel-copper alloys), and Hastelloy @ (predominantly nickel-containing alloys) families of high-performance alloys, zirconia toughened alumina, yttrium stabilised zirconia, zirconia di-oxide, and silicon carbide. [0023] Additionally, the oxidant-conveying pipe (including pipe segments and threaded collars) can be coated (e.g., via plasma coating) with a protective coating, including, for example, ceramic coatings, zirconia (zirconium oxide) coatings, alumina-zirconia coatings, and carbon composite coatings. [0024] In one embodiment, the oxidant-conveying pipe extends substantially the entire length of the wellbore linking the injection well and the production well. [0025] The inlet end of the oxidant-conveying pipe is configured to receive oxidant from above ground. For example, the device optionally further includes well casing, such as well casing in an injection well, connected to the inlet end of the pipe. Alternatively, the device optionally further includes tubing (e.g., coiled tubing) 6 connected to the inlet end of the pipe. The tubing can be extended downhole into an underground coal gasifier via a well (e.g., an injection well). [0026] The well casing can be of any suitable size, shape and construction and can be made of any suitable material or materials (including material that is resistant to high temperatures and corrosion, and/or is coated with a protective coating, as discussed herein). More particularly, the well casing can be of any suitable length and diameter. Preferably, the well casing is made of metal, such as carbon steel, stainless steel, copper, or aluminium. The well casing can be of unitary construction or can include two or more connectable casing segments/pieces. Typically, the well 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. [0027] The tubing can be of any suitable size, shape and construction and can be made of any suitable material or materials. More particularly, the tubing can be of any suitable length and diameter. Preferably, the tubing is made of metal, such as stainless steel, carbon steel, or copper. The tubing can be of unitary construction or can include two or more connectable tube segments/pieces. A preferred outside diameter for the tubing is 2.0 to 3.5 inches. [0028] The oxidant-conveying pipe can be connected to the well casing/tubing in any suitable way, so long as a fluid-tight connection is formed between the pipe and the well casing/tubing. The pipe can be releasably connected or permanently connected to the well casing/tubing. Preferably, the pipe is connected to an end of the well casing/tubing by way of a screw thread or weld. [0029] The oxidant is preferably a gas such as air (approximately 20% oxygen), oxygen-enriched air (greater than 20% oxygen), or a gas/gas mixture (e.g., carbon dioxide, steam, and/or nitrogen in any desired ratio) enriched with oxygen (greater than 20% oxygen), or substantially pure oxygen. The oxidant source can include an air compressor, a tank/cylinder of compressed air or oxygen, an air separation unit, or a tank/cylinder of liquid oxygen, for example. The source of the oxidant can be connected directly or indirectly to well casing/tubing associated with the oxidant- 7 conveying pipe in a fluid-tight manner, for introduction of the oxidant into the wellbore linking the injection well and the production well via the oxidant-conveying pipe. [0030] The oxidant-conveying pipe includes a plurality of discharge ports aligned along its length for discharging oxidant along a flow axis. [0031] The plurality of discharge ports aligned along the length of the oxidant conveying pipe can be of any suitable size and shape for distributing oxidant along the entire length of the wellbore linking the injection well and the production well in a gasifier, and can be located at any angular position around the circumference of the pipe. Preferably, the discharge ports are sized so that oxidant passing through the ports has an exit velocity sufficient to prevent a flame from contacting the pipe and prevent blockage by ash. [0032] In one embodiment, the discharge ports are located on the top of the oxidant-conveying pipe. In another embodiment, the discharge ports are located on one or both sides of the pipe. In a further embodiment, the discharge ports are located on the top and one or both sides of the pipe. [0033] According to an important aspect of the present invention, selection of the orientation of the discharge ports aligned along the length of the oxidant-conveying pipe and the the oxidant exit velocity can be used to direct underground gasifier cavity growth in either horizontal or vertical directions. That is, gasifier cavity growth in a coal seam can be tailored by adjusting the axial and radial distribution of the discharge ports along the length of the pipe, as well as the oxidant exit velocity. This can be used to promote lateral growth of the cavity to increase resource recovery, particularly in thin coal seams. [0034] In one embodiment, the discharge ports include holes. In another embodiment, the discharge ports include slots. In a further embodiment, the discharge ports include nozzles.

8 [0035] The holes can be in the form of circular or other shaped holes (e.g., hexagonal or octagonal). The holes can be, for example, circular having a diameter of about 2 mm to 30 mm, including about 5, 10, 12, 15, 20, 22, 25, and 27 mm. [0036] Where the discharge ports include nozzles, the nozzles can be made of material that is resistant to high temperatures and corrosion, and/or are coated with a protective coating, as discussed herein. [0037] The oxidant-conveying pipe includes a capped terminal end to maintain oxidant pressure in the pipe and prevent oxidant bypass. As will be understood by one of ordinary skill in the art, the terminal end can be capped (or plugged) in any number of ways, so long as the terminal end is closed off and oxidant is unable to exit the terminal end of the pipe. [0038] In one embodiment, the substantially horizontal wellbore linking the injection well and the production well is cased with a perforated liner. [0039] The perforated liner can be of any suitable size, shape and construction, and can be made of any suitable material or materials. The size, shape and construction of the liner are selected to ensure that it can be installed into an in-seam well channel of an underground coal gasifier and remains intact during service (i.e., it keeps the wellbore linking the injection well and the production well open). Preferably, the liner is strong enough to be inserted into the wellbore using traditional drilling service equipment, as will be known to one of ordinary skill in the art. [0040] The perforated liner can be of unitary construction or can include a plurality of connectable units (i.e., segments). The liner or segments can be of any suitable length, including, metres, tens of metres, hundreds of metres, and kilometres. Accordingly, liner segments can be connected together to form a full length perforated liner being tens of metres long, hundreds of metres long, or even several kilometres in length, depending on the length of the wellbore. Each liner segment can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length.

9 [0041] The liner segments can be connected together in any suitable way to form a full-length perforated liner. For example, the ends of each segment can be threaded, and the full-length liner can include one or more threaded collars for connecting the ends of adjacent liner segments together. [0042] The perforated liner will have an outside diameter (or width) appropriate for the wellbore into which it is being inserted. Typically, the liner 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. [0043] The perforated liner is preferably resistant to chemical attack from the products of coal gasification and pyrolysis (e.g., sulfur), as well as attack from the syngas itself (including, CO, H 2 , CO 2 , and H 2 0) which may be corrosive. [0044] The perforated liner can be made of any suitable metal, including, for example, steel, such as carbon steel and stainless steel, and aluminium. [0045] The perforated liner (including segments thereof) can be manufactured in shapes and sizes to suit the specific application. Preferably, the liner has a round cross-section to provide an annular passage, although other cross-section shapes are possible, as will be understood by one of ordinary skill in the art. Generally, the cross-section of the liner will match that of the oxidant-conveying pipe. [0046] The liner perforations can be of any suitable size, shape and arrangement as required to mitigate a number of technical and operational issues associated with a non-lined well. For example, perforations in the liner allow oxidant to pass through the liner and contact the coal while preventing coal and ash from blocking the path of production gases. [0047] The 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 10 perforations are in a staggered arrangement (diamond spacing) as this provides the liner with the greatest structural integrity. [0048] The perforated liner can have anywhere between about 10% to about 60% of its surface area in an open configuration (i.e., perforated), provided that the structural integrity of the liner meets operational, in-seam requirements. Although the perforations can include about 10%, about 15%, about 20%, about 25%, about 30%, about 35%, about 40%, about 45%, about 50%, about 55%, or about 60% of the liner's surface area, about 10% to about 50% appears to be optimal, as adequate structural integrity is retained by the liner. [0049] The perforated liner (and/or oxidant-conveying pipe) can include associated instrumentation such as one or more sensors for sensing and reporting conditions in the liner, the oxidant-conveying pipe, the in-seam linkage channel, and/or the surrounding coal seam. Any suitable type of sensor can be used. For example, the sensor can be a thermocouple for sensing the temperature, a gas sensor for sensing the nature of the product gas, a pressure sensor for sensing pressure, or an optical sensor for viewing the liner, the oxidant-conveying pipe, and/or the linkage channel. [0050] 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, whereby an ignition tool that includes ignition means is inserted into the coal seam via the ignition well. Once introduced into the coal seam, the ignition tool is used to ignite the coal seam. [0051] The ignition well will typically be a vertical ignition well that intercepts the substantially horizontal wellbore linking the injection well and the production well at the injection-end of the wellbore, near the inlet end of the oxidant-conveying pipe. As will be understood by one of ordinary skill in the art, locating the ignition well near the inlet end of the oxidant-conveying pipe (i.e., in close proximity to the first discharge ports) facilitates efficient ignition of the underground coal seam and subsequent combustion of a portion of the coal. Where necessary (e.g., for re-ignition), one or more additional ignition wells can be provided.

11 [0052] According to an important aspect of the present invention, the distribution of oxidant along an extended length of the substantially horizontal wellbore linking the injection well and the production well via the oxidant-conveying pipe that extends within the linkage channel allows the initiation of combustion and gasification of the coal seam at multiple points along the length of the pipe. A high temperature zone is thus created along an extended length of the gasifier (i.e., multiple active combustion/gasification zones), which enables rapid gasification of the surrounding coal, promotes formation of H 2 and CO, prevents water-gas shift reaction (CO + H 2 0 -> CO 2 + H 2 ), prevents CH 4 formation, and gasifies liquid hydrocarbons released from pyrolysis of the coal. Underground coal gasification product gas flows along the annulus between the in-seam well channel (and/or perforated liner) and the pipe. [0053] The ignition tool can ignite the coal seam in any suitable way. For example, the ignition tool can directly ignite the coal seam or ignite a combustible fuel (i.e., an ignition fuel) supplied to the ignition well (e.g., supplied as a gas, liquid, or solid). Suitable ignition fuels include, but are not limited to, hydrocarbon gases, for example, methane, propane, butane, and mixtures thereof. [0054] 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. [0055] Once the coal seam has been ignited (or re-ignited), oxidant is injected into the substantially horizontal wellbore via the oxidant-conveying pipe that extends within the wellbore to fuel/maintain combustion of the coal seam. As discussed herein, the oxidant is preferably a gas such as air (approximately 20% oxygen), oxygen-enriched air (greater than 20% oxygen), or a gas/gas mixture (e.g., carbon 12 dioxide, steam, and/or nitrogen in any desired ratio) enriched with oxygen (greater than 20% oxygen), a mixture of oxygen and water, or substantially pure oxygen. [0056] In one embodiment, the method further includes the step of injecting a quenching fluid into the production well to cool the product gas. Suitably, the quenching fluid can be injected directly near the base of the production well using a quenching fluid delivery system to deliver the quenching fluid. The quenching fluid delivery system can include tubing for conveying the quenching fluid, a circulation pump and fluid reservoir connected to the tubing for pumping the quenching fluid into the production well, and, optionally, a nozzle or pig tail fitted to a lower end of the tubing for spraying the quenching fluid into the product gas stream. This type of delivery system can be extended to a desired location in the production well. The tubing for conveying the quenching fluid can be flexible, such that it can be unwound from a spool. [0057] In another embodiment, the coal seam is further provided with one or more quench wells for conveying quenching fluid to the product gas stream and the method further includes the step of injecting a quenching fluid into the one or more quench wells to cool the product gas. [0058] Suitably, the quench well is located downstream of the oxidant-conveying pipe and upstream of the production well. The positioning of the quench well for quenching fluid injection can be chosen with respect to various design criteria, including sufficient distance from the injection location to the production well so as to ensure good mixing between the injected quenching fluid and hot product gas stream, and appropriate temperature reduction of the product gas stream. Preferably, the quench well is a vertical well. [0059] The quenching fluid can be injected into the quench well in any suitable way. For example, the quenching fluid can be delivered using a quenching fluid delivery system to deliver the quenching fluid and this can be of any suitable size, shape and construction. The delivery system can include a circulation pump and fluid reservoir connected to a well head of a quench well for pumping the quenching fluid into the well. Alternatively, the delivery system can further include tubing for 13 conveying the quenching fluid and, optionally, a nozzle or pig tail fitted to a lower end of the tubing for spraying the quenching fluid into the product gas stream. As discussed herein, this type of delivery system can be extended to a desired location in the quench well. [0060] Any suitable type of quenching fluid can be used. The quenching fluid can be a liquid or a gas (including a liquid or a gas with particulates suspended therein), or any combination thereof. The quenching fluid can consist of more than one type of liquid or gas. The choice of quenching fluid will depend on the desired outcome. For example, the quenching fluid can be used to lower the temperature of a product gas stream such that less damage is caused to mechanical components used in underground coal gasification, particularly the production well. Alternatively (and/or additionally), the quenching fluid can be used to alter the chemical composition of the product gas stream prior to it reaching or leaving the production well. [0061] In one embodiment, the quenching fluid is a liquid. The liquid can be water. The water can be obtained from a naturally occurring water source, such as surface water or ground water. The water can be either fresh water or brine. The water can be treated water, such as demineralised water or raw water separated from product gas. [0062] In another embodiment, the quenching fluid is a gas, such as any available gas at surface. The gas can be treated syngas. The syngas can be cooled syngas from the same or another underground coal gasifier. [0063] As will be understood by one of ordinary skill in the art, the quenching fluid can be injected at any suitable injection rate and quantity. The injection rate can be chosen with respect to various design criteria, including ensuring that injection of the quenching fluid via the production/quench well is sufficient to lower the product gas stream temperature to a desired lower temperature prior to it reaching the production well head. One of ordinary skill in the art will be able to formulate the rate and quantity of quenching fluid injection necessary to achieve desired outcomes.

14 [0064] Reducing the temperature of the product gas via a quenching fluid may require that the temperature of the product gas in-seam and/or within a well (e.g., the production well) be monitored, and the injection rate and quantity of quenching fluid be regulated according to the temperature reading. To that end, the quenching fluid delivery system can include at least one thermocouple (located in-seam or within a well) electrically connected to a computer-operable valve for regulating flow of the quenching fluid. [0065] Referring to Figure 1, there is generally depicted an underground coal gasifier 10 illustrating certain aspects of the invention. A coal seam 12 is located under overburden 15, and includes a substantially horizontal wellbore 17 linking an injection well 20 and a production well 22. The underground coal gasifier 10 also includes an ignition well 25 and, optionally, a quench well 27. Although not illustrated, the injection well 20, production well 22, ignition well 25, and/or optional quench well 27 can include well casing, while wellbore 17 can include a perforated liner, as discussed herein. [0066] An oxidant-conveying pipe 30 inserted into the substantially horizontal wellbore 17 extends through the coal seam 12. The pipe 30 is connected at its inlet end to tubing 32 (alternatively, as discussed herein, the pipe 30 is connected at its inlet end to injection well casing). The pipe 30 includes a plurality of discharge ports 35, aligned along its length for discharging oxidant along a flow axis and is capped at its terminal end 37. For illustrative purposes, the discharge ports 35 are shown on one side of the pipe 30, and are not to scale. [0067] In use, the oxidant-conveying pipe 30 is placed downhole in the substantially horizontal wellbore 17 having an outside diameter of about 5 to 10 inches. The wellbore 17 is dried by injecting air into the wellbore 17 via the pipe 30, and the coal seam 12 is then ignited using an ignition tool (not shown) inserted into the coal seam 12 via the ignition well 25. Hot product gas 40 propagates along the annular passage between the pipe 30 and the wellbore 17, and is re-combusted at the discharge ports 35 of the pipe 30, resulting in ignition of the coal seam 12 at multiple points along the length of the wellbore 17.

15 [0068] An extended section of the gasifier 10 is simultaneously consumed by the distributed discharge ports 35 of the oxidant-conveying pipe 30, causing multiple combustion and gasification zones (not shown), and hot product gas 40 flows from the gasification zones to the downstream production well 22. Quenching fluid (e.g., water) 42 is injected into the optional quench well 27. As injected quenching fluid 42 travels down the optional quench well 27 into the wellbore 17, it vaporises as it mixes with hot product gas 40 moving downstream to the production well 22. In this way the hot product gas 40 is cooled prior to it reaching the production well 22. Alternatively, as discussed herein, quenching fluid 42 can be introduced at the base of the production well 22 through a pipe or tubing inserted from the surface through the production well 22. [0069] 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. [0070] 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 (9)

1. A method of underground coal gasification in a coal seam provided with an injection well, an ignition well, a production well, and a substantially horizontal wellbore linking the injection well and the production well, the wellbore having a device operably inserted therein, the device comprising: a pipe having an inlet end configured to receive oxidant from above ground, a plurality of discharge ports aligned along its length for discharging oxidant along a flow axis and a capped terminal end, the method including the steps of: a. igniting the coal seam using an ignition tool located within the ignition well; b. providing oxidant to the device for injection into the coal seam to support combustion and gasification of the coal seam at multiple points along the length of the wellbore linking the injection well and the production well; and c. withdrawing product gas from the production well.

2. The method of claim 1, wherein the pipe extends substantially the entire length of the wellbore linking the injection well and the production well.

3. The method of claim 1 or claim 2, wherein the substantially horizontal wellbore linking the injection well and the production well is cased with a perforated liner.

4. The method of any one of claims 1 to 3, wherein the discharge ports comprise nozzles.

5. The method of any one of claims 1 to 4, wherein the discharge ports are located on: i) the top of the pipe; ii) one side of the pipe; iii) both sides of the pipe; and iv) any combination thereof. 17

6. The method of any one of claims 1 to 5, wherein the discharge ports are sized so that oxidant passing through the ports has an exit velocity sufficient to prevent a flame from contacting the pipe.

7. The method of any one of claims 1 to 6, further including the step of injecting water into the production well to cool the product gas.

8. The method of any one of claims 1 to 7, wherein the coal seam is further provided with one or more quench wells.

9. The method of claim 8, further including the step of injecting water and/or CO 2 into the one or more quench wells to cool the product gas.