AU2012101716A4 - Underground coal gasification in thick coal seams - Google Patents

Abstract

The invention provides a method of carrying out underground coal gasification (UCG) in a thick coal seam. I * I >

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Line Energy Ltd Invention Title: UNDERGROUND COAL GASIFICATION IN THICK COAL SEAMS The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 UNDERGROUND COAL GASIFICATION IN THICK COAL SEAMS TECHNICAL FIELD [0001] This invention relates to a method of carrying out underground coal gasification (UCG) of a coal seam. In particular, the invention concerns carrying out UCG in a thick coal seam. 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 an in-seam well 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 production well. As coal is consumed or gasified, a gasifier (gasification) cavity within the coal seam develops and grows in size. [0005] Typically, UCG product gas will contain: (1) main syngas components (e.g., CO,

H

2 , C0 2 , N 2 , and CH 4 ); (2) solid particles/particulates (e.g., soot, ash, and coal particles); (3) 2 water; (4) minor components such as C 2

-C

6 hydrocarbons, oxygen, argon, sulphur containing components (e.g., H 2 S, COS, CS 2 , mercaptans, and thiophenes), nitrogen based components (e.g., NH 3 and HCN), hydrocarbon components (e.g., coal condensate, BTEX (benzene, toluene, ethylbenzene and xylenes), and PAHs (polycyclic aromatic hydrocarbons)); and (5) trace components such as heavy metals (arsenic and mercury) and chlorides. [0006] To date most UCG trials have been conducted in coal seam panels having a thickness (i.e., the distance from the bottom of the coal seam to the top of the coal seam) of no more than about 10 metres. [0007] In modern UCG practices, especially those for coal seams located at great depths, horizontal well drilling and completions are preferred. In these practices the horizontal wells can be several kilometers in length. [0008] In UCG of a coal seam using horizontal wells, the volume of coal converted (V) by the in situ gasification process can be estimated as: V = horizontal length (L) x height (H) x width (W) x efficiency [0009] Horizontal length relates to the x-dimension and is dictated by the well drilling technology capability. Height relates to the z-dimension and is estimated by the height of the coal seam, minus a small distance where the well is located, as previous UCG trials have shown that the coal below a horizontal well is not gasified. Thus, H is approximately the height of the coal seam. Width relates to the y-dimension and is based on the gasification process. It can be estimated from theoretical studies and previous field trials of UCG. Typically, W is thought to be a function of the height of the coal seam, whereby: W = alpha x H [0010] The value of alpha is typically expected to be between 3 and 5. [0011] Based on earlier trials, contact efficiency is estimated to be between about 70 and 80%. Therefore: 3 V = L x alpha x efficiency x H 2 [0012] In order to maximise V, it is therefore desirable to apply the UCG process to thick coal seams. For example, increasing the coal seam height from 5 metres to 10 metres quadruples the volume of coal converted according to the above formulae. [0013] However, in order to successfully convert large volumes in thick coal seams other criteria and design considerations must be taken into account. [0014] It is desirable that the reaction contact area and residence time of both solids and gases inside the reaction zone is relatively constant throughout the process. This ensures that the gas quality will remain constant. Techniques such as the linear controlled retracting injection point (CRIP) method lead to an expanding reaction volume and surface area as the process proceeds, thereby leading to variability in gas quality with time. [0015] Similarly, the geotechnical aspects of removing the coal from within the formation need to be considered in order to avoid generation of fracture systems that can lead to unwanted connection between the reaction zone and overlying formations, some of which can have high quality groundwater. This is unwanted in order to avoid both product gas losses from the process and potential contamination of the overlying formations. [0016] Removal of large volumes of coal can also impact on overburden stability, and in situations where the design is inappropriate, lead to collapse of the overburden formations into the reaction zone. This is unwanted as this will affect the established flow paths and lead to quenching of the process. In a worst case scenario, the collapse could smother the process and cause the gasification process to be stopped altogether. [0017] Thus, there is a need for improved methods of carrying out UCG in thick coal seams. SUMMARY OF INVENTION [0018] It is an object of the present invention to provide an improved method for carrying out underground coal gasification of a coal seam, particularly a thick coal seam.

4 [0019] In one aspect, the invention provides a method of carrying out underground coal gasification of a coal seam having a thickness of greater than 10 metres, the method including the steps of forming at least one primary injection well having a substantially horizontally extending section for injecting oxidant along and adjacent the bottom of the coal seam and an inclined section that extends from the horizontally extending section to ground surface, and forming a least one primary production well having a substantially horizontally extending section extending along and adjacent the top of the coal seam and an inclined section that extends from the horizontally extending section to ground surface, wherein product gas that is produced by UCG is recoverable at ground surface via the production well. [0020] Preferably, the method is used for carrying out UCG of a coal seam having a thickness of greater than about 15 metres, including, for example, 20 metres, 25 metres, 30 metres, 35 metres, 40 metres, 45 metres, 50 metres, or more. [0021] In one embodiment, the substantially horizontal sections of the injection and production wells are offset in the z-dimension, the offset distance in the z-dimension being proportional to the coal seam thickness. [0022] The primary injection well can be formed in any suitable way and can be of any suitable size, shape and construction. The primary injection well can include the substantially horizontally extending section that extends adjacent to the bottom of the coal seam and an inclined section that extends from the horizontally extending section to ground surface. Preferably, the primary injection well is formed by way of directional drilling technology. [0023] The primary production well can be formed in any suitable way and can be of any suitable size, shape and construction. The primary production well can include the substantially horizontally extending section that extends adjacent to the top of the coal seam and an inclined section that extends from the horizontally extending section to ground surface. Preferably, the primary production well is formed by way of directional drilling technology. [0024] As disclosed herein, the orientations of the substantially horizontally extending sections of the primary injection and production wells can be offset with respect to the z dimension.

5 [0025] By offsetting the horizontal sections of the injection and production wells in the z dimension the stability of the overlying formations can be addressed, and the effective linear distance between the respective horizontal injection well and production well sections extended. The offset distance in the z-dimension can be proportional to the coal seam height. Therefore, the linear distance can be 1.414 x H, which will further increase V. [0026] The method can include the step of introducing a well liner into the horizontally extending section of the primary injection well and the well liner can extend from adjacent a heel of the inclined section. [0027] The method can also include the step of introducing a well liner into the substantially horizontally extending section of the primary production well and the well liner can extend from adjacent a heel of the inclined section. [0028] The well liner can be of any suitable size, shape and construction, and can be made of any suitable material, including, for example, metal (including steel, such as carbon steel, and aluminium), fibreglass, carbon fibre, plastic, and combinations thereof [0029] Typically, each well liner will include a gas-conveying pipe for keeping the substantially horizontally extending sections of the primary injection and production wells from collapsing or blocking. The inclined sections of the primary injection and production wells can be concrete-encased pipes, as is known to one of ordinary skill in the art. The inclined sections of the primary injection and production wells can each have a well head. [0030] The well liner that extends within the substantially horizontally extending section of the primary injection well can include perforations to allow the injected oxidant to have intimate contact with the coal formation close to the desired injection location. [0031] The well liner that extends within the substantially horizontally extending section of the primary production well can be used to collect the product gas and convey it to the surface. The well liner that extends within the substantially horizontally extending section of the primary production well can also include perforations. The well liner perforations can be of any suitable size, shape and arrangement as required. An advantage of a perforated well liner is that hot product gas can enter the horizontally extending section of the primary 6 production well at any location, and this makes blockage of the primary production well unlikely. [0032] The perforations are preferably 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 well liner with the greatest structural integrity. Preferably, the perforations in the well liner are grouped together in one or more regions along the length of the liner, alternating with non-perforated sections of the liner. [0033] The hot product gas can be produced through the well liner of the substantially horizontally extending section of the production well. In one embodiment, coiled tubing can be used in the method for production (tubing string) and be located as desired. This can aid in eliminating short-circuiting of some of the oxidant and/or product gas, thereby aiding in ensuring that the product gas is of the highest quality. [0034] Preferably, the method includes the step of forming at least one secondary injection well having a substantially horizontally extending section for injecting oxidant along and adjacent the bottom of the coal seam. The secondary injection well can be formed in any suitable way and can be of any suitable size, shape and construction. The secondary injection well can further comprise an inclined section that extends from the horizontally extending section to ground surface or a linking section that links it to the inclined section of the primary injection well such that these sections are in fluid communication with one another. Preferably, the substantially horizontally extending section is substantially the same as the substantially horizontally extending section of the primary injection well. Preferably, the secondary injection well is formed by way of directional drilling technology. [0035] Preferably, the method includes the step of forming at least one secondary production well having a substantially horizontally extending section extending along and adjacent the top of the coal seam. The secondary production well can further comprise an inclined section that extends from the horizontally extending section to ground surface or a 7 linking section that links it to the inclined section of the primary production well such that these sections are in fluid communication with one another. Preferably, the substantially horizontally extending section is substantially the same as the substantially horizontally extending section of the primary production well. Preferably, the secondary production well is formed by way of directional drilling technology. [0036] The orientations of the substantially horizontally extending sections of the secondary injection and production wells can be offset with respect to the z-dimension, as described herein for the primary wells. [0037] Preferably, each substantially horizontally extending section of the secondary injection well and each substantially horizontally extending section of the secondary production well is fitted with a perforated well liner as described herein. [0038] The method can include the steps of igniting the coal seam, injection of the oxidant in the substantially horizontally extending section of each injection well, and combustion/gasification of the coal in a predominately vertical direction. The method intends that a vertical retort be established whereby hot combustion gases move vertically up through coal that is heated, pyrolysed, gasified, and combusted as it moves downwards. This is similar to a moving bed gasifier design, except that the ash remains behind in the reactor and after all the coal in a specified zone has been exhausted the ignition and injection location are moved back towards the heel of the inclined section of the injection well (i.e., injection point). [0039] The gasification theory for the present method is based on a moving bed process, (e.g., Lurgi fixed bed gasifiers or BGL gasifiers). However, for the present method, it is not expected that the oxygen/steam ratio needs to keep the maximum temperature below the ash fusion temperature as in dry bottom gasifiers. [0040] By the nature of the reaction temperature being lower than in some other designs, the product gas will have significant pyrolysis products, such as tars, ethane, propane, butane, and methane. Phenols may also be present. This improves thermal efficiency, but does mean that product gas handling needs to be appropriately catered for at the surface.

8 [0041] Any suitable type of oxidant can be used. For example, air, oxygen-enriched air, or substantially pure oxygen can be used. Alternatively, the oxidant can be a combination of oxygen, carbon dioxide, water, and nitrogen in any desired ratio. The oxidant can be injected in any suitable way. [0042] In one embodiment, oxidant injection occurs using an oxidant injection system including coil tubing connected to a pressurised source of oxidant and an injection nozzle located at an end of the coil tubing. The injection system can be used to inject the oxidant into the coal seam at any desired location along the substantially horizontal section of an injection well. [0043] In another embodiment oxidant (e.g., oxygen) injection occurs using an oxidant injection system including coil tubing connected to a pressurised source of oxidant and a manifold connected to an end of the coil tubing so that the oxidant can be distributed over a large surface area in-seam (i.e., to the substantially horizontally extending sections of numerous injection wells at the same time). By heating a large coal volume to a moderate temperature of less than about 700 'C, substantial pyrolysis of the coal takes place, thereby leading to high combustible gas yields per unit of oxygen injected. For downstream chemical synthesis applications this can be very beneficial, as the product gas can be further converted into carbon monoxide and hydrogen using methods such as secondary reforming, auto thermal reforming, and/or partial oxidation, thereby producing product gas with predominately carbon monoxide and hydrogen, and having a low to moderate oxygen consumption for the hydrocarbons produced. Heat loss to overburden is controlled, as hot gases will preferentially migrate up the production well. [0044] The method can include the step of using an ignition device to ignite the coal and establish a combustion zone. The ignition device can be can be positionable and retractable, so that ignition (including re-ignition) of the coal seam and the formation of combustion zones can be established in sequence. A preferred method is utilising the CRIP concept. [0045] Positioning of the ignition device can be achieved utilising coiled tubing connected to the ignition device and extendible within the substantially horizontally extending section of an injection well (primary and/or secondary) to position the ignition device at a desired location within the substantially horizontally extending section of the injection well.

9 [0046] As a flame front is periodically moved towards the heel (or linking section) of each well, there is no risk of blockage of either the injection well or production well. In front of the flame front, the removal of coal can lead to subsidence and so forth, especially of the production well. However, this potential problem can be avoided by use of a perforated well liner, which keeps the well channel open, and in any case the production area has moved to a new zone where the impact of material removal is much less. [0047] In order to facilitate the initial ignition, the substantially horizontal sections of the injection and production wells can intersect each other. Preferably, the substantially horizontal section of the production well is drilled to intersect the injection well and then increase in distance to the top of the coal seam, thereby improving resource recovery. This method provides the initial linking of the injection and production wells. [0048] 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 figures. BRIEF DESCRIPTION OF DRAWINGS [0049] Figure 1 depicts in side elevation an underground coal seam gasifier including a coal seam having a thickness of greater than 10 metres, primary and secondary injection wells, each having a substantially horizontally extending section for injecting oxidant along and adjacent the bottom of the coal seam, and primary and secondary production wells, each having a substantially horizontally extending section extending along and adjacent the top of the coal seam, according to an embodiment of the present invention. [0050] Figure 2 depicts the gasifier of Figure 1 (transverse cross section) and further depicts which coal zones may be combusted and gasified, according to an embodiment of the present invention. DESCRIPTION OF EMBODIMENTS [0051] In the figures, like reference numerals refer to like features.

10 [0052] Referring to Figure 1, there is generally depicted an underground coal gasifier 10 illustrating certain aspects of the invention. A preferred method concerns drilling a number of horizontal well pairs, including injection wells 20 and production wells 30 into a coal seam 40 to be converted into product gas 45. Coal seam 40 is surrounded by overburden 50 and underburden 55. [0053] As depicted in Figure 1, a first well of a first well pair is a primary injection well 20 having a substantially horizontally extending section 23a that extends adjacent to the bottom 60 of the coal seam 40 and an inclined section 25 that extends from the substantially horizontally extending section 23a to ground surface 70. [0054] A second well of the first well pair is a primary production well 30 having a substantially horizontally extending section 33a extending along and adjacent the top 65 of the coal seam 40 and an inclined section 35 that extends from the substantially horizontally extending section 33a to ground surface 70. [0055] Both the substantially horizontally extending section 23a of the injection well 20 and the substantially horizontally extending section 33a of the production well 30 are fitted with perforated well liners (illustrated as dashed lines). [0056] As depicted in Figure 2, a first well of a second well pair is a secondary injection well having a substantially horizontally extending section 23b that extends adjacent to the bottom 60 of the coal seam 40 and a linking section (not shown) that links it to the inclined section 25 of the primary injection well 20 such that these sections are in fluid communication with one another. [0057] A first well of a third well pair is a secondary injection well having a substantially horizontally extending section 23c that extends adjacent to the bottom 60 of the coal seam 40 and a linking section (not shown) that links it to the inclined section 25 of the primary injection well 20 such that these sections are in fluid communication with one another. [0058] Therefore, the inclined section 25 of the primary injection well 20 can be used to inject oxidant 75 into the substantially horizontally extending sections 23a, 23b, and 23c.

11 [0059] A second well of the second well pair is a secondary production well having a substantially horizontally extending section 33b that extends adjacent to the top 65 of the coal seam 40 and a linking section (not shown) that links it to the inclined section 35 of the primary production well 30 such that these sections are in fluid communication with one another. [0060] A second well of the third well pair is a secondary production well having a substantially horizontally extending section 33c that extends adjacent to the top 65 of the coal seam 40 and a linking section (not shown) that links it to the inclined section 35 of the primary production well 30 such that these sections are in fluid communication with one another. [0061] Therefore, the inclined section 35 of the primary production well 30 can be used to convey product gas 45 to ground surface 70 from the substantially horizontally extending sections 33a, 33b, and 33c. [0062] The actual configuration of well pairs 20 and 30 is dependent upon the coal seam 40 thickness. In coal seams having a thickness of greater than 10 metres, the injection well 20 is drilled as close as possible to the bottom 60 of the coal seam 40, and the production well 30 is drilled as close as possible to the top 65 of the coal seam 40. This maximises the conversion of coal to product gas 45. [0063] The substantially horizontally extending injection sections 23a, 23b, and 23c and production sections 33a, 33b, and 33c can be positioned directly above one another, or can be offset to further increase the linear distance between the substantially horizontally extending injection sections 23a, 23b, and 23c and production sections 33a, 33b, and 33c. An offset arrangement is shown in Figure 2, where y is equal to or less than % x. Each combustion and gasification zone is labeled 80. [0064] The injection wells 20 are used to inject oxidant 75. The oxidant can be a combination of oxygen, carbon dioxide, water, and nitrogen in any desired ratio. Alternatively, the oxidant can be air or oxygen.

12 [0065] In one embodiment, an oxidant injection system including coil tubing connected to a pressurized source of oxidant and an injection nozzle located at an end of the coil tubing is used. The injection system is used to inject the oxidant 75 into the coal seam 40 at any desired location along the substantially horizontally extending section 23a of injection well 20. The well liner located in the substantially horizontally extending section 23a of injection well 20 includes perforations that allow the injected oxidant 75 to come into intimate contact with the coal seam 40 close to the desired injection location. [0066] In another embodiment, oxidant injection occurs using an oxidant injection system including coil tubing connected to a pressurized source of oxidant and a manifold connected to an end of the coil tubing so that the oxidant 75 can be distributed over a large surface area in-seam (i.e., that is, to the substantially horizontally extending sections 23a, 23b, and 23c of injection well 20 at the same time). [0067] The substantially horizontally extending section 23a of injection well 20 can be designed to enable a coil tubing ignition system to be used to ignite the coal seam 40 at any point along the substantially horizontally extending section 23a of the injection well 20 as desired. An ignition system can be used at least once during the start up of the process in each well pair. [0068] As illustrated in Figure 1, the ignition system can include an ignition device 90. Once ignition is established, oxidant 75 is injected into the coal seam 40 and temperatures above about 1000 'C are established to provide sufficient temperature to convert the coal into product gas 45 containing high proportions of CO, C0 2 , H 2 , CH 4 , and H 2 0, and smaller quantities of higher hydrocarbons, such as methane, ethane, propane, butane, and so forth. Such an ignition establishes the underground coal gasification (gasifier) process. [0069] Due to the pressure profile in the gasifier 10, the injected oxidant 75 has intimate contact with the coal seam 40. It is heated by hot ash that is left as a residue after conversion of in situ coal. The high temperature of the oxidant 75 and the intimate contact with the coal creates a good environment for the production of high quality product gas 45. In the region of the ignition device 90, a high temperature combustion zone 95 is created. The primary combustion products CO 2 and H 2 0 have intimate contact with hot coal and this also facilitates production of high quality product gas 45. The product gas 45 moves upwards through the 13 coal seam 40 at the combustion zone 95 having further intimate contact with pyrolysed coal. Overall, the process sets up a type of "moving bed" gasifier at the combustion zone 95, whereby hot oxidant 75 and product gas 45 flow counter-currently with coal which is pyrolysed, gasified, and then combusted as it falls towards the bottom of the coal seam 40. [0070] In order to optimise the process, the residence time of the gases in the gasifier 10 are controlled within limits to ensure high quality product gas 45 and the highest process efficiency. The residence time is manipulated by designing the injection flow of the oxidant 75 and the distance between the substantially horizontally extending injection sections 23a, 23b, and 23c and production sections 33a, 33b, and 33c. The target residence time is dependent on the coal reactivity at process conditions (i.e., temperature and pressure) as is well known to one of ordinary skill in the art. [0071] The product gas 45 enters the production well 30 in the substantially horizontally extending section 33a in line with the injection location (as illustrated by the arrows in combustion zone 95). [0072] The substantially horizontally extending section 33a of production well 30, fitted with a perforated well liner, has sufficient porosity to enable good gas collection into the well 30 over a distance at least equal to (and preferably greater than) the height of the coal seam 40. An advantage of a perforated well liner is that hot product gas 45 can enter the substantially horizontally extending section 33a of the production well 30 at any location, and this makes blockage of the production well 30 unlikely. It also means that as the combustion zone 95 is moved through the coal seam 40, the path from oxidant injection to the production well 30 is constant. [0073] Within the production well 30, water cooling can be applied to ensure that product gas 45 temperatures at a well head (not shown) of the production well 30 do not exceed material limits. [0074] In summary, the method according to the present invention enables: (1) the successful conversion of large volumes of coal in thick coal seams; (2) the reaction contact area and residence time of both solids and gases inside a reaction zone to remain relatively constant throughout the UCG process, thereby ensuring that the gas quality will remain 14 constant; (3) removal of coal from within the coal seam formation without generation of fracture systems that can lead to unwanted connection between the reaction zone and overlying formations; and (4) removal of coal with little impact on overburden stability, thereby minimising subsidence. [0075] 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. [0076] 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 (5)

1. A method of carrying out underground coal gasification (UCG) of a coal seam having a thickness of greater than 10 metres, the method including the steps of: a. forming at least one primary injection well comprising a substantially horizontally extending section for injecting oxidant along and adjacent the bottom of the coal seam and an inclined section that extends from the horizontally extending section to ground surface; and b. forming a least one primary production well comprising a substantially horizontally extending section extending along and adjacent the top of the coal seam and an inclined section that extends from the horizontally extending section to ground surface, wherein product gas that is produced by UCG is recoverable at ground surface via the production well.

2. The method of claim 1, further including the step of forming at least one secondary injection well comprising a substantially horizontally extending section for injecting oxidant along and adjacent the bottom of the coal seam and a linking section that links the secondary injection well to the inclined section of the primary injection well such that these sections are in fluid communication with one another.

3. The method of claim 1 or claim 2, further including the step of forming at least one secondary production well comprising a substantially horizontally extending section extending along and adjacent the top of the coal seam and a linking section that links the secondary production well to the inclined section of the primary production well such that these sections are in fluid communication with one another.

4. The method of any one of claims 1 to 3, wherein the substantially horizontal sections of the injection and production wells intersect each other.

5. The method of any one of claims 1 to 4, wherein the substantially horizontal sections of the injection and production wells are offset in the z-dimension, the offset distance in the z dimension being proportional to the coal thickness.