[0001] This invention relates to underground coal gasification (UCG). In particular, a method for controlling and correcting asymmetric growth of a gasification cavity in an underground coal gasifier 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 an in-seam well channel (also referred to as a "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 pyro lysis 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] Steam stripping is the main mechanism through which coal is liberated from a coal seam into the gas phase during the underground gasification process. The natural ingress of water from the coal seam and from the surrounding formation into a gasification cavity, together with the heat of the gasification process, creates steam that drives conversion of the coal into UCG product gas.

[0006] However, the rate and distribution of water ingress into a gasification cavity from a coal seam and/or from the surrounding formation will be uneven. Therefore, the amount of steam generated by the naturally occurring water will be greater in some areas of the gasification cavity than in others. Thus, coal consumption in the gasification cavity will be uneven, causing the cavity to grow in an asymmetric manner.

[0007] Asymmetric growth of a gasification cavity in an underground coal gasifier is undesirable for a number of reasons, including poor resource recovery and concomitant lower synthesis gas production, as well as generation of an inefficient gas flow pattern through the gasifier. The less symmetrical the gasification cavity (particularly around fixed points, such as near injection or production points), the poorer the gas flow pattern. This leads to large areas of convective mixing (preventing UCG product gas from easily exiting the gasification cavity), long residence times (which can lead to undesirable re-combustion of the synthesis gas), and/or "dead" zones within the gasification cavity where oxidant supply is poor or limited (which can lead to reduced reaction rates).

SUMMARY OF INVENTION

[0008] 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.

[0009] 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.

[0010] Relying on natural water inflow into a gasification cavity is inherently limiting, can result in sub-optimal gasifier performance, and can create restrictions or impediments to optimal UCG operation. For example, natural water ingress is intrinsically linked to the hydrogeology and geology (e.g., porosity, permeability, saturation, hydrostatic pressure, location of faults, etc.) of the coal seam formation and its surrounds. Furthermore, water ingress may not be uniformly distributed over the gasifier cavity, and this can result in asymmetric growth of the cavity, causing poor resource recovery and/or a poor product gas flow through the gasifier. These factors can reduce the volume and/or quality of the synthesis gas produced. [0011 ] Thus, in one aspect, the invention provides a method for correcting asymmetric growth of a gasification cavity in an underground coal gasifier, including injecting an aqueous fluid into a targeted region of the cavity to thereby promote steam stripping of the coal in the targeted region of the cavity and symmetric growth of the cavity.

[0012] In one embodiment, the aqueous fluid is water. In another embodiment, the aqueous fluid is steam.

[0013] 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 UCG product gas.

[0014] As will be understood by one of ordinary skill in the art, water that is injected into a gasification cavity in an underground coal gasifier will be converted to steam by heat in or associated with the gasifier. The aqueous fluid can be injected alone, or together with or as part of an oxidant (or a flow of oxidant) into the underground gasifier. In either case, the aqueous fluid is preferably injected in a controlled manner.

[0015] The aqueous fluid can optionally include one or more chemicals for conditioning UCG product gas produced in a gasifier prior to the product gas reaching or leaving a production well of the gasifier.

[0016] The aqueous fluid can be injected or delivered to a targeted region in or around a gasification cavity using any suitable means or equipment. For example, a delivery system can be used which includes an existing well (or the casing thereof) for conveying other fluids (e.g., oxidant) to the product gas stream. In another example, a circulation pump connected to a water reservoir can be used.

[0017] In a further example, a compressor connected to a well (or well head) can be used to inject steam or water vapour in a compressed state to a targeted region of a gasification cavity. In a further example, steam (or water) can be injected or delivered via a pipe and, optionally, a nozzle or pig tail fitted to a lower end of the pipe for injecting the steam (or water). The pipe can be flexible to enable it to be wound and unwound from a spool.

[0018] A pipe-based delivery system can further include a circulation pump and fluid reservoir connected to an upper end of the pipe. Alternatively, the pipe-based delivery system can further include a gas compressor connected to an upper end of the pipe. The pipe can have a diameter of anywhere between about 0.5 and 4 inches, and preferably between about 1 and 2 inches. The pipe can be made of steel, including carbon steel and stainless steel, for example.

[0019] The location or locations at which aqueous fluid is injected (whether by itself or with an oxidant) into an underground coal gasifier can be chosen to improve resource recovery. For example, the location(s) can be chosen or determined so as to promote symmetrical (or more symmetrical) growth of one or more gasifier cavities.

[0020] In embodiments where an aqueous fluid is injected with an oxidant, the introduction of the aqueous fluid can increase the rate of coal consumption within the underground gasifier, thereby increasing the rate of synthesis gas production.

[0021 ] In some embodiments, the aqueous fluid can be injected (with or without oxidant) at one or more predetermined fixed points through one or more vertical service wells. This can help to increase resource recovery in the area(s) surrounding the aqueous fluid injection point(s).

[0022] For example, in embodiments where an underground coal gasifier has a so-called knife edge CRIP (controlled retracting injection point) configuration, aqueous fluid can be injected via a service well located upstream of the apex point of the knife edge, and this can enhance resource recovery around the apex. One of ordinary skill in the art will readily appreciate that injection of aqueous fluid via a service well can also provide improved resource recovery and/or production enhancements in underground coal gasifiers of other configurations.

[0023] According to an important aspect of the present invention, where aqueous fluid is injected at predetermined fixed point(s) (whether via service well(s) or by some other means), location(s) or point(s) for aqueous fluid injection can be chosen to coincide with area(s)/region(s) of a gasification cavity away from the part (or parts) of the cavity that receive greater natural water ingress (i.e., natural ingress or flow of water from the coal seam itself and/or surrounding formation). In other words, those parts of the gasification cavity that contain less natural water or receive less natural inflow of water can be subjected to targeted aqueous fluid injection(s). This ameliorates the negative effects associated with uneven water distribution within the gasification cavity, such as, for example, asymmetrical cavity growth.

[0024] The method can also include monitoring the extent and direction of the growth of a gasification cavity. Data obtained through such monitoring can be used to vary and/or optimise how much and/or where aqueous fluid is injected. Any suitable means for performing this monitoring can be used, including, for example, seismic measurements (e.g., electrical resistivity tomography (ERT)) and subsurface instrumentation (e.g., using one or more sensors). In some embodiments, an online and/or real-time monitoring system can be used.

[0025] As will be understood by one of ordinary skill in the art, an appropriate sensor for monitoring the extent and direction of the growth of a gasification cavity includes, but is not limited to, a thermocouple for sensing the temperature (e.g., in the gasification cavity, a well channel, and/or a production well), a gas sensor for sensing the nature of the UCG product gas, and a pressure sensor for sensing pressure.

[0026] The location(s) at which aqueous fluid is injected into a gasifier, and/or the amount of aqueous fluid that is injected, can also be determined and/or regulated to enhance and/or control the water-gas shift reaction (CO + H ₂ 0 <→ H ₂ + C0 ₂ ). This can be of particular benefit in applications where, in the production of the synthesis gas, it is desired or important to control or optimise the production or concentration of carbon dioxide and/or hydrogen gas.

[0027] In some embodiments, liquid and/or gas produced from a UCG process can be used in a steam generating system to generate steam for injection as described herein. Heated liquid and/or gas from the UCG process can be recovered and fed into the steam generating system, optionally before being used for further purposes. Utilising liquid and/or gas produced in the UCG process for steam generation can help to reduce costs (e.g., by reducing/eliminating the requirement for external fuel and/or electricity sources for steam generation).

[0028] 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 an in-seam well channel linking the injection well and the production well, including the steps of: a) establishing a gasifier cavity in the coal seam so as to produce UCG product gas, b) monitoring the gasifier cavity for asymmetric cavity growth and identifying the same, c) drilling a vertical service well into the gasifier cavity in the vicinity of the identified asymmetric cavity growth, and d) injecting an aqueous fluid into the vertical service well to promote steam stripping of the coal in the region of the gasifier cavity identified as exhibiting asymmetric cavity growth.

[0029] In one embodiment, the aqueous fluid is water. In another embodiment, the aqueous fluid is steam. [0030] 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

[0031 ] Figure 1 is a schematic representation of a conventional underground coal gasifier, illustrating asymmetric growth of a gasification cavity.

[0032] Figure 2 is a schematic representation of an underground coal gasifier in which use is made of the methods of the present invention.

[0033] Figure 3 is a diagrammatic representation of asymmetric growth of a gasification cavity.

[0034] Figure 4 is a diagrammatic representation of the more symmetric growth of a gasification cavity that can be achieved using the methods of the present invention.

DESCRIPTION OF EMBODIMENTS

[0035] In the figures, like reference numerals refer to like features.

[0036] Referring to Figure 1, there is generally depicted an underground coal gasifier 10. A coal seam 12 is located underground and covered by overburden 14, and includes a generally horizontally-extending well channel 16 linking an injection well 18 and a production well 20. The underground coal gasifier 10 also includes a first service well 22 and a gasification cavity 24.

[0037] In use, gasification cavity 24 and additional upstream gasification cavities (not shown) are produced by retracting an ignition point (not shown) away from the production well 20. Ideally, gasification cavity 24 grows and ultimately reaches the overburden 14 (at which point most of the coal in the gasification cavity 24 has been consumed), and a new ignition point will be created upstream in the well channel 16, thereby commencing the formation of a new gasification cavity. However, as illustrated in Figure 1, cavity growth of gasification cavity 24 can be asymmetric due to the uneven rate and distribution of water ingress into the gasification cavity 24 from the coal seam 12 and/or the overburden 14, leading to an asymmetrically protruding region 26 of the gasification cavity 24.

[0038] Referring to Figure 2, there is generally depicted an underground coal gasifier 10 illustrating certain aspects of the present invention. A coal seam 12 is located underground and covered by overburden 14, and includes a generally horizontally-extending well channel 16 linking an injection well 18 and a production well 20. The underground coal gasifier 10 also includes a first service well 22, a gasification cavity 24, and a second service well 28.

[0039] In use, second service well 28 is drilled at a position specifically intended to facilitate injection of an aqueous fluid 30 (e.g., steam or water) into region 32 of the gasification cavity 24, where the natural amount or inflow of water (and concomitant amount of naturally occurring steam) is less. By injecting/creating additional steam in region 32 of the gasification cavity 24 via second service well 28, the amount of steam in region 32 is increased. This increases steam stripping in region 32 of the gasification cavity 24, thereby promoting symmetric growth of the gasification cavity 24 and enhanced high-quality UCG product gas recovery.

[0040] Figures 3 and 4 provide a further illustration of the principles of operation of the present invention. Figure 3 is a plan view diagram of an underground coal gasifier 10, which includes a generally horizontally-extending well channel 16 linking an injection well 18 and a production well 20, a first service well 22, and a gasification cavity 24. An asymmetrically protruding region 26 of the gasification cavity 24 is the result of uneven ingress of water into the gasification cavity 24 from the coal seam 12 and/or overburden (not shown) (the direction of water ingress is illustrated by the set of four arrows).

[0041 ] Figure 4 is a plan view diagram of an underground coal gasifier 10 in which the methods of the present invention are used. A second service well 28 is drilled at a position specifically intended to facilitate injection of an aqueous fluid (e.g., steam or water) into region 32 of the gasification cavity 24, where the natural amount or inflow of water (and concomitant amount of naturally occurring steam) is less, to promote symmetric growth of the gasification cavity 24 and enhanced high-quality UCG product gas recovery (the direction of water ingress is illustrated by the set of four arrows).

[0042] While Figures 1-4 have been used to provide simplified two-dimensional illustrations of the methods of the present invention, one of ordinary skill in the art will readily appreciate how the principles represented in these figures are extended to three dimensions.

[0043] 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.