AU2016100004A4 - Oxygen enriched ucg method - Google Patents

Abstract

The invention provides a method of carrying out oxygen eliriched underground coal gasification.

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Linc Energy Ltd Invention Title: OXYGEN ENRICHED UCG METHOD The invention is described in the following statement: 1 OXYGEN ENRICHED UCG METHOD TECHNICAL FIELD [0001] This invention relates to a method of carrying out underground coal gasification (UCG). In particular, UCG methods that include oxygen enriched burn back of a metal 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 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) to facilitate oxidant injection, cavity development and product gas flow. [0004] 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 may also utilise one or more vertical wells (e.g., a service well and an ignition well) located between the injection and production wells. [0005] A coal seam having injection and production wells with a substantially horizontal wellbore linking the two 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 2 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. [0006] 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. [0007] 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 [0008] An object of the present invention to provide a method for UCG that minimises one or more of the problems of the prior art. [0009] 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 metal casing into the wellbore, b) injecting air into the wellbore, c) igniting the coal seam while continuing the injection of air into the wellbore, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) increasing downhole oxygen concentration to a level sufficient to ignite the metal casing inserted into the wellbore, f) monitoring burn back of the metal casing inserted into the wellbore, and g) decreasing downhole oxygen concentration until a desired rate of burn back of the metal casing inserted into the wellbore is achieved. [0010] Alternatively, for the step of decreasing downhole oxygen concentration, the oxygen concentration can be decreased once the metal casing inserted into the wellbore has been burned back a desired distance within the wellbore.

3 [0011] In one embodiment, the coal seam is further provided with an ignition well. [0012] In another embodiment, increasing downhole oxygen concentration includes injecting substantially pure oxygen through the injection well or a service well into the wellbore while continuing the injection of air into the wellbore through the injection well. [0013] In yet another embodiment, increasing downhole oxygen concentration includes stopping the injection of air into the wellbore and injecting substantially pure oxygen through the injection well or a service well into the wellbore, and wherein carbon dioxide, nitrogen, and/or water are injected with the oxygen as diluent. [0014] In still another embodiment, increasing downhole oxygen concentration includes positioning an oxygen injection device in the wellbore and providing substantially pure oxygen to the injection device while continuing the injection of air into the wellbore through the injection well, wherein the injection device is positioned in the wellbore upstream of the active UCG cavity. [0015] In a further embodiment, increasing downhole oxygen concentration includes positioning an oxygen injection device in the wellbore and providing substantially pure oxygen to the injection device, and replacing the injection of air into the wellbore through the injection well with the injection of carbon dioxide, nitrogen, and/or water, wherein the injection device is positioned in the wellbore upstream of the active UCG cavity. DESCRIPTION OF EMBODIMENTS [0016] The present invention relates to methods of UCG that include oxygen enriched wellbore casing burn back to eliminate the need for re-ignition of a coal seam following initial ignition of the seam. [0017] Throughout this specification, unless the context requires otherwise, the words "comprise"/"include", "comprises"/"includes" and "comprising"/"including" will 4 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. [0018] 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 metal casing into the wellbore, b) injecting air into the wellbore through the injection well, c) igniting the coal seam while continuing the injection of air into the wellbore, d) establishing an active gasifier cavity in the coal seam so as to produce UCG product gas, e) increasing downhole oxygen concentration to a level sufficient to ignite the metal casing inserted into the wellbore, f) monitoring burn back of the metal casing inserted into the wellbore, and g) decreasing downhole oxygen concentration until a desired rate of burn back of the metal casing inserted into the wellbore is achieved. [0019] In another aspect, the step of decreasing downhole oxygen concentration can be practiced once the metal casing inserted into the wellbore has been burned back a desired distance within the wellbore. That is, the step of decreasing downhole oxygen concentration can be practiced in such a way as to favour either continuous burn back of the metal casing inserted into the wellbore (including at a variable rate) or intermittent discontinuous burn back of the casing. [0020] 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. [0021] The size, shape and construction of the casing is selected to ensure that it can be installed into a wellbore of an underground coal gasifier and remains intact during service (i.e., it keeps the wellbore open). Preferably, the casing 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.

5 [0022] The casing (also referred to as a "well liner") can be of any suitable size, shape and construction and can be made of any suitable material or materials. Exemplary metals suitable for the casing include, but are not limited to, carbon steel, copper, or aluminium, and combinations thereof. [0023] As will be understood by one of ordinary skill in the art, selection of the metal casing material, wall thickness of the casing, and/or coating of the casing influences casing burn back, including casing ignition and burn back rate. Accordingly, the casing can be made of material that is resistant to high temperatures. [0024] Exemplary metal and metal alloys resistant to high temperatures and suitable for the casing 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. [0025] Additionally, the casing 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. [0026] 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. [0027] The casing can be of unitary construction or can include a plurality of connectable units (i.e., segments/sections). The casing or segments can be of any suitable length, including, metres, tens of metres, hundreds of metres, and kilometres. Accordingly, casing segments 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 segment 6 can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length. [0028] The casing segments can be connected together in any suitable way. For example, the ends of each segment can be threaded, and the full-length casing can include one or more threaded collars for connecting the ends of adjacent casing segments together. As necessary, threaded collars can be made of metal, metal alloys, and ceramics resistant to high temperatures, and/or coated with a protective coating as discussed herein. [0029] The casing (including segments/sections thereof) can be manufactured in shapes and sizes to suit the specific application. Preferably, the casing 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. [0030] In one embodiment, the metal casing inserted into the wellbore includes one or more perforated sections. The perforations in such perforated sections of the metal casing can be of any suitable size, shape and arrangement as required to mitigate a number of technical and operational issues associated with a fully closed in casing design. For example, the 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. Additionally, perforations in the casing 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] 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 perforations are in a staggered arrangement (diamond spacing) as this provides the casing with the greatest structural integrity.

7 [0032] Preferably, the casing extends from adjacent a heel of the injection well to adjacent a heel of the production well. [0033] 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/service well, such that the air is injected/introduced into the wellbore via the injection well/service well. [0034] 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 an ignition well or the injection well. Once introduced into the coal seam, the ignition tool is used to ignite the coal seam and establish a combustion zone. [0035] Inserting (and positioning) the ignition tool can be achieved utilising coiled tubing connected to the tool and extendible within the ignition/injection well to position the tool at a desired location within the coal seam. [0036] The ignition tool can be connected to the coiled tubing in any suitable way. Preferably, the ignition tool is connected to the coiled tubing in a fluid-tight manner. The ignition tool can be releasably (i.e., non-permanently) connected or permanently connected to the coiled tubing. Preferably, the ignition tool is connected to an end of the coiled tubing by way of a quick connector (such as grapple, torque thru, dimple, etc.), screw thread, or weld. [0037] 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. A preferred outside diameter for the coiled tubing is 1.75 to 3.5 inches. The coiled tubing can be maintained on a spool from which the coiled tubing is unspooled.

8 [0038] The ignition tool can ignite the coal seam in any suitable way. For example, the ignition tool can directly ignite the coal seam at the perforations in the one or more perforated sections of the metal casing inserted into the wellbore, 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. [0039] As will be understood by one of ordinary skill in the art, ignition means includes an electrical spark generator (e.g., a spark plug) or an electrical heat resistor (e.g., a glow plug) and a power supply for generating the spark/electrifying the resistor. Ignition means further include at least one type of ignition chemical. The ignition chemical can be 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. The hydrocarbon or inert gas flow can help transport/vaporise the pyrophoric substance to the ignition tool. [0040] Once the coal seam has been ignited, the ignition tool is retracted a safe distance while continuing the injection of air into the wellbore (e.g., through the injection or a service well 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. In one embodiment, the ignition tool is a sacrificial ignition tool that is consumed during the ignition/gasification process. [0041] 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 H 2 to CO ratio) emerging from the production well. [0042] The step of increasing downhole oxygen concentration (in the wellbore) to a level sufficient to ignite (or combust/oxidize) the metal casing inserted into the 9 wellbore can be achieved in any suitable manner. In one embodiment, increasing downhole oxygen concentration includes injecting substantially pure oxygen through the injection well (or a service well) into the wellbore while continuing the injection of air into the wellbore through the injection well. For example, a source of substantially pure oxygen can be connected directly or indirectly to a well head of the injection well/service well, such that the oxygen is injected/introduced into the wellbore via the injection well/service well. [0043] Alternatively, increasing downhole oxygen concentration can be achieved by stopping the injection of air into the wellbore and injecting substantially pure oxygen through the injection well (or a service well) into the wellbore, with an inert liquid or gas (e.g., carbon dioxide, nitrogen and combinations thereof), and/or water being injected along with the oxygen as a diluent. For example, sources of substantially pure oxygen and a diluent can be connected directly or indirectly to a well head of the injection well/service well, such that the oxygen and diluent are injected/introduced into the wellbore via the injection well/service well. [0044] In another embodiment, increasing downhole oxygen concentration includes positioning an oxygen injection device in the wellbore (via the injection well or a service well) and providing substantially pure oxygen to the injection device for delivery (e.g., injection) to the wellbore while continuing the injection of air into the wellbore through the injection well (or a service well). As will be understood by one of ordinary skill in the art, the oxygen injection device can provide directed/projected oxygen injection to the underground coal seam. In one embodiment, the oxygen injection device includes an associated sacrificial ignition tool that is consumed during the ignition/gasification process (in which case the oxygen injection device is positioned in the wellbore prior to the ignition of the coal seam). Preferably, the oxygen injection device is positioned in the wellbore upstream of the active UCG cavity (or upstream of where an active UCG cavity will be formed). The oxygen injection device can be positionable and retractable. [0045] Alternatively, increasing downhole oxygen concentration can be achieved by positioning the oxygen injection device in the wellbore (via the injection well or a 10 service well) and providing substantially pure oxygen to the injection device for delivery (e.g., injection) to the wellbore as described herein, while replacing the injection of air into the wellbore through the injection well (or a service well) with the injection of an inert liquid or gas (e.g., carbon dioxide, nitrogen and combinations thereof), and/or water. [0046] As with the ignition tool, positioning of the oxygen injection device can be achieved utilising coiled tubing connected to the oxygen injection device and extendible within the wellbore to position the oxygen injection device at a desired location within the wellbore. The source of the substantially pure oxygen can be connected directly or indirectly to the coiled tubing associated with the oxygen injection device in a fluid-tight manner, for introduction of the substantially pure oxygen into the wellbore via the oxygen injection device. [0047] 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 oxygen injection device with associated sacrificial ignition tool (e.g., with one tube supplying an ignition fuel and the other tube facilitating injection of substantially pure oxygen 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. [0048] The substantially pure oxygen for increasing downhole oxygen concentration can be provided via a tank/cylinder of compressed oxygen, an air separation unit, or a tank/cylinder of liquid oxygen, for example. [0049] As discussed herein, the oxygen injection device is positionable and retractable. When used, in order to continue the UCG process through a coal seam, it will be necessary to reposition the oxygen injection device periodically to progress the 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 oxygen injection device can be drawn along the cased wellbore that extends through the coal seam. A 11 preferred method is utilising the controlled retracting injection point (CRIP) concept, as will be understood by one of ordinary skill in the art. [0050] According to an important aspect of the present invention, retracting the oxygen injection device positioned in the wellbore (continuously or intermittently) in the direction of the injection well/service well causes the active UCG cavity to burn towards the oxygen injection device. Accordingly, re-ignition of the coal seam is not necessary to continue the UCG process. [0051] The oxygen injection device can include a controller operable to control the release/injection of substantially pure oxygen. The controller can include a pipe manifold in fluid communication with the coiled tubing and substantially pure oxygen source. The controller can be operated remotely from the oxygen injection device. The controller can include trim, non-return, and isolation valves, flow measuring devices, and pressure relief devices. Such operating devices allow for injection rate measurement and control of substantially pure oxygen. Substantially pure oxygen 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 substantially pure oxygen to flow as per the required settings. [0052] The source of substantially pure oxygen can be connected to a pipe manifold of the controller and further to the coiled tubing and a well head of an injection well (or a service well). A power supply (e.g., generator) can be electrically connected to the controller and further to an electrical cable and sensor cable extending through the coiled tubing. [0053] As will be understood by one of ordinary skill in the art, substantially pure oxygen 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 the rate of casing burn-back and/or the quality and/or composition of the product gas (e.g., the calorific value and/or the H 2 to CO ratio) emerging from the production well are as 12 desired. One of ordinary skill in the art will be able to formulate the rate and quantity of substantially pure oxygen injection necessary to achieve desired outcomes. [0054] Injection of substantially pure oxygen into the wellbore via the injection well and/or the oxygen injection device is carried out to achieve downhole oxygen enrichment in the range of about 30 mol% to about 100 mol%, including about 35 mol%, about 40 mol%, about 45 mol%, about 50 mol%, about 55 mol%, about 60 mol%, about 65 mol%, about 70 mol%, about 75 mol%, about 80 mol%, about 85 mol%, about 90 mol%, and about 95 mol%. [0055] The step of monitoring burn back of the metal 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 outside or the inside of the metal 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 outside or the inside of the metal casing to facilitate optical time domain reflectometry (OTDR) for continuous length measurements of the casing. [0056] Regulating downhole oxygen concentration to achieve a desired rate of burn back of the metal casing inserted into the wellbore can be achieved using instrumentation (e.g., conducting wires, thermocouples, and/or optical fibres) associated with the casing via a feedback loop from the reading of the instrumentation, as will be understood by one of ordinary skill in the art. [0057] Suitably, the steps of increasing downhole oxygen concentration to a level sufficient to ignite the metal casing inserted into the wellbore, monitoring burn back of the metal casing inserted into the wellbore, and decreasing downhole oxygen concentration once the metal casing inserted into the wellbore has been burned back a desired distance within the wellbore can be repeated as needed to further burn back the metal casing inserted into the wellbore.

13 [0058] Additionally, as will be understood by one of ordinary skill in the art, the steps of increasing downhole oxygen concentration to a level sufficient to ignite the metal casing inserted into the wellbore, monitoring burn back of the metal casing inserted into the wellbore, and decreasing downhole oxygen concentration once the metal casing inserted into the wellbore has been burned back a desired distance within the wellbore can be automated to facilitate repetition of these steps. [0059] Alternatively, the step of monitoring burn back of the metal casing inserted into the wellbore can be achieved by monitoring the quality and/or composition of the product gas (e.g., the calorific value and/or the H 2 to CO ratio) emerging from the production well. [0060] The step of decreasing downhole oxygen concentration can include halting the injection of substantially pure oxygen through the injection well/service well or via the oxygen injection device into the wellbore. [0061] In one embodiment of the methods disclosed herein, the metal 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 metal casing into the wellbore influences casing burn back, including casing ignition and burn back rate. [0062] Suitably, when cementing the metal 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. [0063] In another embodiment of the methods disclosed herein, localised downhole water cooling/quenching can be used to influence casing burn back, including casing ignition and burn back rate. Water for injection 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. [0064] As will be understood by one of ordinary skill in the art, water can be injected at any suitable injection rate and quantity. The injection rate can be chosen 14 with respect to various design criteria, including ensuring that injection of water is sufficient to maintain downhole well/wellbore temperatures within a desired range. One of ordinary skill in the art will be able to formulate the rate and quantity of water injection necessary to achieve desired outcomes relating to casing burn back, including casing ignition and burn back rate. [0065] Suitably, downhole water cooling/quenching can be carried out using a water delivery system to deliver the water, and this can be of any suitable size, shape, and construction. For example, the water delivery system can include tubing for conveying the water, and optionally a nozzle or pig tail fitted to a lower end of the tubing for spraying the water into the cased wellbore in the area where cooling/quenching is desired. This type of delivery system can be extended to the desired position within the cased wellbore via a service well. The tubing can be flexible, such that it can be unwound from a spool. The water delivery system can further include a circulation pump and fluid reservoir connected to the upper end of the tubing. [0066] 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. [0067] 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 (12)

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 metal casing into the wellbore; b. injecting air into the wellbore through the injection well; c. igniting the coal seam while continuing the injection of air into the wellbore; d. establishing an active gasifier cavity in the coal seam so as to produce UCG product gas; e. increasing downhole oxygen concentration to a level sufficient to ignite the metal casing inserted into the wellbore; f. monitoring burn back of the metal casing inserted into the wellbore; and g. decreasing downhole oxygen concentration until a desired rate of burn back of the metal casing inserted into the wellbore is achieved.

2. The method of claim 1, wherein downhole oxygen concentration is decreased once the metal casing inserted into the wellbore has been burned back a desired distance within the wellbore.

3. The method of claim 1 or claim 2, wherein increasing downhole oxygen concentration comprises injecting substantially pure oxygen through the injection well into the wellbore while continuing the injection of air into the wellbore through the injection well.

4. The method of claim 1 or claim 2, wherein increasing downhole oxygen concentration comprises stopping the injection of air into the wellbore and injecting substantially pure oxygen through the injection well into the wellbore, and wherein carbon dioxide, nitrogen, and/or water are injected with the oxygen as diluent.

5. The method of claim 1 or claim 2, wherein increasing downhole oxygen concentration comprises positioning an oxygen injection device in the wellbore and 16 providing substantially pure oxygen to the injection device while continuing the injection of air into the wellbore through the injection well, wherein the injection device is positioned in the wellbore upstream of the active UCG cavity.

6. The method of claim 1 or claim 2, wherein increasing downhole oxygen concentration comprises positioning an oxygen injection device in the wellbore and providing substantially pure oxygen to the injection device, and replacing the injection of air into the wellbore through the injection well with the injection of carbon dioxide, nitrogen, and/or water, wherein the injection device is positioned in the wellbore upstream of the active UCG cavity.

7. The method of any one of claims 1 to 6, wherein downhole oxygen concentration is increased to greater than or equal to 35-100 mol%.

8. The method of claim 7, wherein downhole oxygen concentration is increased to about 60 mol%.

9. The method of any one of claims 1 to 8, wherein the metal casing inserted into the wellbore is made of carbon steel, stainless steel, an Inconel @ alloy, a Monel @ alloy, and/or a Hastelloy @ alloy.

10. The method of any one of claims 1 to 9, wherein the metal casing inserted into the wellbore is coated with a protective coating selected from the group consisting of ceramic coatings, zirconia coatings, alumina-zirconia coatings, and carbon composite coatings.

11. The method of any one of claims 1 to 10, wherein the metal casing inserted into the wellbore is cemented in the wellbore.

12. The method of any one of claims 1 to 11, wherein one or more conducting wires and/or thermocouples are attached to the metal casing inserted into the wellbore.