GB2501074A - Underground gasification with conduits disposed in a wellbore - Google Patents

Abstract

An underground coal gasification well includes a delivery conduit 3 for delivering an oxidizing fluid to a coal deposit. A recovery conduit 5 is present to recover syngas from the coal deposit in a gasification process and both the delivery 3 and recovery 5 conduits are disposed in a single wellbore. An ignition conduit may also be disposed in the wellbore to initiate the gasification process in the coal deposit. Further conduits may also be present such as a water conduit 13 and a cooling fluid conduit.

Description

I lmriroved Underground Coal Gasification Methods. Systems and Aooaratus 3 The present invention relates to underground coal gasification, and in particular improved 4 methods, systems and apparatus for performing underground coal gasification processes.

6 Background to the invention

8 Underground coal gasification (UCG) is a process in which virgin coal is partially 9 combusted and reacted with air or oxygen pumped into the coal. This enables energy to be extracted from in-situ underground coal deposits. UCG has been demonstrated and 11 practiced for over a century to exploit coal deposits which are too difficult to mine or too 12 deep for economic recovery using conventional methods.

14 Typically, a UCG installation comprises a pair of wells drilled from the surface down into the coal deposit and connected within the coal deposit. In one approach parallel pairs of 16 wells are separated usually by a distance of 10 to 50 metres. Air or oxygen is pumped into 17 the coal through one of the wells and the gasification product, commonly referred to as 18 synthetic gas or "syngas", is extracted through the other.

1 The main economic use of syngas from the UCG process is the generation of electricity or 2 the manufacture of chemicals or transport fuels on a relatively large scale. However, the 3 output of each gasification chamber is limited so it is necessary to produce syngas from a 4 chamber of larger size or a large number of chambers operating simultaneously.

6 In another UCG approach, vertical or near vertical wells may be separated by a distance 7 of 200 to 700 metres but linked by a common horizontal well extending through the coal 8 seam in a configuration known as "linked vertical well" UCG.

A further variant referred to as "continuously retractable injection point" involves a 11 modification to the means by which an oxidising fluid of air or oxygen or a mixture of these 12 gases and which may contain water or steam, is delivered to the coal. In this variant the 13 well for oxidising fluid delivery is drilled into the coal seam and fitted with a perforated liner.

14 An oxidising fluid is passed down a flexible tubing inserted within the perforated liner, and the flexible tubing is retracted as the coal is consumed by the gasification process to 16 ensure that the oxidising fluid is delivered to the surface of the coal being gasified.

18 Each of these UCG methods requires the drilling of two separate vertical (or inclined) wells 19 from the surface into the coal deposit. A further alternative method requires the drilling of yet another well for the purpose of inserting an ignition device or system to initiate the 21 gasification process.

23 It is therefore an object of at least one embodiment of the present invention to obviate 24 and/or mitigate one or more disadvantages associated with conventional underground coal gasification processes.

1 Summary of the invention

3 According to a first aspect of the invention, there is provided an underground coal 4 gasification well comprising; a delivery conduit configured to deliver an oxidising fluid to a coal deposit associated with the installation; and a recovery conduit configured to recover 6 syngas produced from the coal deposit in a gasification process; wherein the first conduit 7 and the second conduit are disposed within a single wellbore.

9 The skilled person will understand the term fluid to include liquid, gas, liquid-like and gas-like materials. For example, fluid may also refer to gel or particulate material.

12 The invention employs a novel single well approach in which an oxidising fluid is pumped 13 from the surface, to supply a gasification process, via the same wellbore through which 14 gas produced by the gasification process is transported to the surface. Preferably, the wellbore is cased.

17 The oxidising fluid may comprise air, oxygen, or a mixture of these gases.

19 Preferably, the well further comprises an ignition and fuel conduit disposed within the wellbore to supply fuel and ignition service to initiate the gasification process in the coal 21 deposit. Optionally, the ignition and fuel conduit is deployed alongside the delivery 22 conduit. Alternatively, and preferably, the ignition and tuel conduit is disposed within the 23 delivery conduit.

Optionally, the well further comprises a water conduit disposed within the wellbore to 26 deliver water into the delivery conduit. Water may be delivered in the form of steam or by 27 a water spray arrangement.

29 Preferably, the well further comprises at least one cooling fluid conduit disposed within the wellbore to supply cooling fluid to cool the recovery conduit. Preferably, at least one 31 cooling fluid conduit supplies cooling fluid to the interior of the casing. Optionally, the well 32 further comprises cooling means, such as a cooling jacket, mounted on the recovery 33 conduit, to which the cooling fluid is supplied by at least one cooling fluid conduit.

1 Preferably, the well is configured to control a temperature of the recovered syngas by 2 controlling cooling fluid supply through one or more cooling fluid conduits.

4 Preferably, the well further comprises a monitoring conduit disposed within the wellbore and arranged to transport material from the coal deposit. Most preferably, the well further 6 comprises corresponding monitoring means configured to analyse the material. Such 7 analysis may identify contaminants or the like present in the coal deposit. Optionally, the 8 well is configured to adjust, or indeed cease, delivery of oxidising fluid to the coal deposit 9 responsive to the analysis.

11 Most preferably, the well comprises a wellhead, the wellhead comprising a plurality of 12 interfaces corresponding and coupled to the plurality of pipelines disposed within the 13 wellbore.

Preferably, the well further comprises a vent to provide fluid communication with the 16 interior of the wellbore. Preferably, the well further comprises corresponding monitoring 17 means configured to analyse fluid from the interior of the wellbore. Such analysis may 18 reveal casing leaks or leaks from one or more of the conduits disposed within the wellbore.

19 Optionally, the well is configured to adjust, or indeed cease, delivery of oxidising fluid to the coal deposit responsive to the analysis.

22 According to a second aspect of the invention, there is provided an underground coal 23 gasification array comprising; a recovery conduit extending within a coal deposit and 24 configured to recover syngas produced from the coal deposit in a gasification process; and plurality of delivery conduits associated with the recovery conduit, each of the delivery 26 conduits configured to deliver an oxidising fluid to the coal deposit.

28 Most preferably, the conduit array comprises a plurality of recovery conduits, each of the 29 plurality having an associated plurality of delivery conduits. Most preferably, at least two of the delivery conduits are located at different distances from the recovery conduit.

32 Optionally, at least one of the delivery conduits houses a seal. Advantageously the seal 33 may be selected to open the delivery conduit dependent upon temperature. Accordingly, 34 the seal may comprise a eutectic or fusible plug. Alternatively, the seal may be configured to be drilled through to open the delivery conduit.

2 Preferably, the conduits comprise one or more apertures. Most preferably, the conduits 3 comprise perforated liners. Optionally, the conduits comprise one or more slotted guide 4 tubes.

6 Optionally, each recovery conduit and its associated plurality of delivery conduits are 7 disposed substantially in a same plane.

9 Alternatively, the array comprises a plurality of recovery conduits disposed at different vertical heights. Most preferably, the different vertical heights correspond to locations of 11 different coal seams. Optionally, the array comprises a plurality of recovery conduits 12 extending radially from a central location. In an embodiment of the third aspect of the 13 present invention, this provides for coal to be gasified in an arc of up to 360 degrees 14 around the wellbore. A particular advantage stems from the fact that the wellbore can therefore be drilled into the middle of a coal seam or field, rather than at an edge.

17 Preferably, the conduit array comprises one or more monitoring conduits configured to 18 transport material from the coal deposit for analysis. Most preferably, the one or more 19 monitoring conduits are arranged around the periphery of the coal deposit.

21 Optionally, the or each recovery conduit comprises a cooling jacket arranged to cool the 22 syngas within the recovery conduit. Alternatively, the or each recovery conduit comprises 23 one or more perforations through which a cooling fluid is injected directly into the syngas.

Embodiments of the second aspect of the invention may include one or more features 26 corresponding to features of the first aspect of the invention or its embodiments, or vice 27 versa.

29 According to a third aspect of the invention, there is provided an underground coal gasification installation comprising a well according to the first aspect and an array 31 according to the second aspect; wherein the delivery conduit of the well is in fluid 32 communication with the plurality of delivery conduits of the array to deliver oxidising fluid 33 from the surface; and wherein the recovery conduit of the well is in fluid communication 34 with the recovery conduit of the array to recover syngas from the coal deposit.

1 Advantageously, the installation comprises a plurality of arrays according to the second 2 aspect, each of the arrays disposed at a different vertical height. In this way, syngas can 3 be recovered from several vertically separated coal seams again through a single 4 wellbore.

6 Optionally, the installation further comprises an ignition system deployed within the same 7 casing of the well.

9 Embodiments of the third aspect of the invention may include one or more features corresponding to features of the first and/or second aspects of the invention or their 11 embodiments, or vice versa.

13 According to a fourth aspect of the present invention, there is provided a method of 14 performing an underground coal gasification process comprising: delivering an oxidising fluid to an underground coal deposit; 16 gasifying at least a portion of the coal deposit; and 17 recovering syngas produced from the underground coal deposit; 18 wherein the delivery of oxidising fluid and recovery of syngas is performed via a 19 single wellbore.

21 Preferably, the method comprises delivering a fuel to the coal deposit and igniting the fuel.

22 This provides a more furious ignition source for igniting the coal to be gasified.

24 Optionally, the method comprises cooling the syngas as it is recovered.

26 Preferably, the method comprises controlling a temperature of the syngas by controlling 27 the cooling of the syngas as it is recovered.

29 Preferably, the method comprises obtaining a material sample from the coal deposit during the gasification process and analysing the sample.

32 Optionally, the method comprises obtaining a fluid sample from within the wellbore and 33 analysing the fluid sample.

1 The gasification process and/or syngas recovery may be controlled, or indeed halted, 2 dependent on the analysis. This may be achieved by varying the supply of oxygen and/or 3 water (or steam).

Embodiments of the fourth aspect of the invention may include one or more features 6 corresponding to features of the first to third aspects of the invention or their embodiments, 7 or vice versa.

9 According to a fifth aspect of the present invention, there is provided a method of providing an underground coal gasification well comprising: 11 drilling a wellbore; and 12 disposing an oxidising fluid delivery conduit and a syngas recovery conduit within 13 the wellbore.

Optionally, the method comprises casing the wellbore to provide a cased well.

17 Embodiments of the fifth aspect of the invention may include one or more features 18 corresponding to features of the first to fourth aspects of the invention or their 19 embodiments, or vice versa.

21 According to a sixth aspect of the present invention, there is provided a method of 22 providing an underground coal gasification pipeline array comprising; 23 drilling a syngas recovery bore within a coal deposit; and 24 drilling a plurality of oxidising fluid delivery bores associated with the syngas recovery bore within the coal deposit 27 Optionally, the method is repeated at a plurality of depths. Optionally, the plurality of 28 depths correspond with a series of coal deposits.

Preferably, the method comprises lining at least one of the syngas recovery bore and the 31 plurality of oxidising fluid delivery bores. Preferably, the at least one bore is lined with a 32 perforated or slotted liner.

34 Most preferably, the syngas recovery bore and the oxidising fluid delivery bore are drilled from the base of a single associated production well.

2 Embodiments of the sixth aspect of the invention may include one or more features 3 corresponding to features of the first to fifth aspects of the invention or their embodiments, 4 or vice versa.

6 According to a seventh aspect of the present invention, there is provided a method of 7 generating electrical power comprising recovering syngas from a coal deposit according to 8 the method of the fourth aspect and converting the recovered syngas to electrical power.

Embodiments of the seventh aspect of the invention may include one or more features 11 corresponding to features of the first to sixth aspects of the invention or their 12 embodiments, or vice versa.

14 According to an eighth aspect of the present invention, there is provided a syngas or hydrocarbon recovered via an underground coal gasification well according to the first 16 aspect, from an underground coal gasification pipeline array according to the second 17 aspect, by an installation according to the third aspect, using a method according to the 18 fourth aspect, from an underground coal gasification well provided by the method of the 19 fifth aspect or from an underground coal gasification pipeline array provided by the method of the sixth aspect.

22 Embodiments of the eighth aspect of the invention may include one or more features 23 corresponding to features of the first to seventh aspects of the invention or their 24 embodiments, or vice versa.

1 Brief descrirtion of the drawings 3 There will now be described, by way of example only, various embodiments of the 4 invention with reference to the drawings, of which: 6 Figure 1 illustrates in schematic form the top of an underground coal gasification well, in 7 accordance with an embodiment of the present invention; 9 Figure 2 illustrates in schematic form the bottom of an underground coal gasification well, in accordance with an embodiment of the present invention; 12 Figure 3 illustrates in schematic form a well-head associated with the top of the 13 underground coal gasification well illustrated in Figure 1, in accordance with an 14 embodiment of the present invention; 16 Figure 4 illustrates in schematic form the layout of conduits in an underground coal 17 gasification array, in accordance with an embodiment of the present invention; 19 Figure 5 illustrates in schematic form the layout of conduits in an alternative underground coal gasification array, in accordance with an alternative embodiment of the present 21 invention; 23 Figure 6 illustrates in schematic form a multi array installation serviced via a common, 24 single well, in accordance with an alternative embodiment of the present invention; and 26 Figure 7 illustrates in schematic form an installation in which the array comprises a number 27 of delivery conduits and recovery conduits extending radially from a central, single, 28 common well, in accordance with a further alternative embodiment of the present 29 invention.

1 Detailed description of preferred embodiments

3 Figures 1 to 3 illustrate various components of an underground coal gasification 4 installation in accordance with one or more aspects of the invention, which comprises a single vertical or inclined well connected to a wellhead and drilled and completed (i.e. 6 cased) below ground. The well contains multiple conduits that are connected to and in 7 fluid communication with a horizontal array of bores within the plane of a coal seam. One 8 conduit supplies a mixture of an oxidising agent and water (or steam) to a gasification 9 chamber in the coal seam to fuel a gasification process and another conduit provides a means of recovering the syngas produced by the gasification process to the surface. The 11 well also supplies a fuel gas or liquid for initiating the gasification process, supplies water 12 to cool the syngas, and incorporates conduits for monitoring contaminants escaping from 13 the gasification chamber.

Figure 1 illustrates the top of an underground coal gasification well 1 in which, in contrast 16 to conventional underground coal gasification installations, the oxidising agent pipeline 3 17 ("delivery conduit") and the syngas pipelines ("recovery conduit") are deployed within a 18 same single casing 7. Figure 2 illustrates the bottom of the well 1 and the interface with an 19 underground coal gasification pipeline array 51, and Figure 3 provides a top-down view of the corresponding wellhead 23. Note that the wellbore itself need not necessarily be 21 cased to benefit from the invention, provided the pipelines are still deployed within the 22 same wellbore.

24 The delivery pipeline 3 conveys an oxidising agent, with water or steam, under pressure to a coal gasification chamber (not shown) associated with the well 1 for the purposes of 26 initiating and maintaining the gasification process. Once the process is underway, the 27 recovery pipeline 5 conveys the produced syngas, under pressure, from the gasification 28 chamber to the surface for extraction and subsequent conversion, storage, or other syngas 29 application.

31 By housing the delivery pipeline 3 and the recovery pipeline 5 in a single cased well, the 32 requirement to drill pairs of parallel (relatively vertical) wells is negated. One particular 33 advantage stems from the ability to perform underground coal gasification processes in 34 coal seams or deposits which are too deep to justify the drilling of two separate wells but may not be of a thickness or quantity to be commercially attractive for UCG. In addition, it 1 may not be possible to site a second well due to geological or geographical constraints 2 which would make a particular coal seam or deposit inaccessible or commercially 3 attractive to conventional UCO methodologies. Another advantage is that the land area 4 required for the surface installation is significantly decreased by deploying the delivery pipeline and recovery pipeline within a single wellbore (and wellhead) rather than separate 6 wellbores (and their associated wellheads).

7 Surface conductor 29 and well casing 7 provide a barrier between the pipelines and the 8 surrounding rock formation and are fixed in position by cement 31 which fills the void 9 between the well 1 and the drilled hole in the rock formation.

11 Furthermore, such an arrangement provides for local control of all of the parameters of the 12 underground coal gasification process via a single well-head. Additional advantages thus 13 stem from safety implications, such as being able to shut down a UCG operation or 14 installation via a single well-head.

16 Water pipeline 11 may be employed to convey water (or steam) under pressure into the 17 delivery pipeline 3 whereupon the water (or steam) is mixed with the oxidising agent and 18 conveyed to the gasification chamber for the purpose of maintaining the gasification 19 process.

The exemplary single well casing 7 contains several additional pipelines to perform 21 additional functions. For example, cooling fluid pipeline 9 conveys an aqueous or gaseous 22 medium under pressure into the open space within the casing 7 for the purpose of 23 assisting the cooling of syngas within the recovery pipeline 5.

For additional cooling of the syngas within the recovery pipeline 5, a cooling jacket pipeline 26 13 conveys water under pressure to a cooling jacket 17 located around recovery pipeline 5 27 near the base of the well casing (see Figure 2). Supply to the water cooling jacket 17 may 28 be regulated dependent on feedback from a temperature probe 19 mounted on the 29 recovery pipeline 5.

31 In this embodiment, an ignition pipeline 15 is located within the delivery pipeline 3 to 32 convey fuel gas or liquid, under pressure, to the furthest end of the delivery pipeline 3 to 33 assist in combusting the coal in the underground coal deposit to initiate the gasification 34 process.

1 A monitoring pipeline 21 conveys contaminants that may permeate from the gasification 2 chamber through the surrounding coal and rock formations for the purpose of detecting 3 and analysing such contaminants.

The wellhead 23 provides an isolatable termination point at the surface for all the above 6 pipelines, by way of appropriate interfaces, valves etc. as illustrated in schematic form in 7 Figure 3. For example, recovery pipeline interface 27 is an isolatable connection to 8 recovery pipeline 5, for the purpose (for example) of facilitating the cleaning out of solids 9 that may have become deposited inside delivery pipeline 5, or for the occasional sampling or extraction of syngas from the delivery pipeline 5.

12 Casing vent 25 provides access to the interior of the casing 7, for example to convey the 13 flow of aqueous or gaseous medium from said open space in the well casing 7 for 14 monitoring purposes. For example, medium extracted from the interior of the casing 7 via the casing vent 25 may be analysed for the purpose of detecting leakage from any of the 16 pipelines into the well casing 7.

18 Note that while an underground coal gasification well in accordance with the present 19 invention, such as the exemplary embodiment described above, may be employed with a conventional underground coal gasification chamber, particular benefits of the well provide 21 for complementary underground coal gasification chamber arrangements which in turn 22 bring their own additional advantages.

24 One such arrangement will now be described with reference to Figure 4 which illustrates the layout of conduits (or bores or liners) in an underground coal gasification array 51 in 26 accordance with an embodiment of another aspect of the invention (the combination of 27 well and array providing an installation in accordance with an embodiment of yet another 28 aspect of the invention).

Figure 4 shows the extension of pipelines 3, 5, 21 into a coal panel (shown in plan view) to 31 create an array of corresponding conduits 53, 55, 71 within the coal deposit. The conduits 32 53, 55, 71 located in the coal deposit are perforated liners to allow the flow of gases while 33 preventing blockage caused by creeping of the coal or ingress of impurities etc. It is 34 particularly advantageous if the conduits 3, 5, 21 are disposed substantially in the plane of 1 the coal panel; however it will be appreciated that a variety of deployments will provide the 2 same effect.

4 As illustrated, delivery conduit 53 is side-tracked' repeatedly to create multiple delivery conduits 53, 53a substantially (but not necessarily) in parallel to and to either side of the 6 recovery conduits 55. The direction and terminal point of each conduit 53, 53a, 55 is 7 arranged such that the effective gasification chamber extends in a pre-determined manner.

8 While in the illustrated example the effective gasification chamber extends laterally in one 9 direction away from the well, an alternative gasification chamber might extend in all directions; comprise a single or indeed several recovery conduits, or indeed any other 11 equivalent arrangement.

13 In operation, fuel gas or liquid and air or oxygen are supplied to the location where the 14 gasification process is to be initiated via corresponding pipelines and conduits (first in-well then within the coal panel) and ignited for example by a piezoelectric sparking device fitted 16 to the end of an ignition pipeline (or end of an appropriate delivery conduit or conduits).

17 Any appropriate ignition means may of course serve this purpose.

19 The resulting combustion forms a gasification chamber in the coal panel and establishes sufficient temperature for gasification to commence, at which time the supply of fuel gas or 21 liquid is stopped and gasification continues.

23 Delivery conduits 53 which supply the oxidising agent to the gasification are arranged on 24 either side of each of the recovery conduits 55 within the coal seam to provide localised oxidation to assist the gasification where it is required. As the gasification chamber 26 naturally enlarges as coal is consumed, parallel delivery conduits 53a begin to provide the 27 required local oxidation. Accordingly, by providing an array of delivery conduits 53,53a 28 etc. a gasification process that would otherwise die out due to lack of a sufficient oxidising 29 agent supply is able to continue much longer and indeed for as long as there are corresponding delivery conduits.

32 Furthermore, the provision of multiple recovery conduits 55 and associated multiple 33 delivery conduits 53, 53a, means that syngas can be produced from multiple burn-fronts, 34 which allows syngas production per recovery well to be increased significantly in comparison to conventional methods. In the described example, said benefits are enjoyed 1 in addition to the benefits afforded by the above-described single-well installation. The 2 skilled person will also appreciate that the averaging effect provided by multiple burn-fronts 3 results in a dilution effect and balancing of gas conditions in the coal seam which are 4 additional advantages of arrays according to the invention.

6 The subsequent delivery conduits 53a may be provided with drillable plugs 54 which 7 provide a fluid seal until drilled through, for example when the gasification process 8 progresses to a point at which it is desirable to open the relevant pipeline 53a and permit 9 additional oxidising agent to be supplied to the gasification chamber. As the gasification chamber thereby expands both perpendicularly to the delivery conduits 53 and in the 11 direction of flow of the syngas, this ensures that the gasification reaction is sustained and 12 that the dimensions of the gasification chamber are controllable.

14 Figure 5 illustrates an alternative layout to that shown in Figure 4 in which fusible or eutectic plugs 154,1 54a are disposed at the ends of delivery conduits 153,1 53a to initially 16 prevent (and subsequently controllably allow) the flow of oxidising agent directly from the 17 open end of the delivery conduit into the coal panel. Also illustrated is a packer 157 18 arranged at the end of one of the conduits 1 53a which is employed to prevent oxidising 19 agent from bypassing the plug 154a via slots or perforations that might exist in the lining of the conduits 153a. Each conduit may be provided with such a packer.

22 Note that monitoring conduit 21 is side-tracked' to create corresponding coal panel 23 monitoring conduits 71,171 around the periphery of the coal deposit. Accordingly, in-coal 24 panel conditions can be monitored topside by extracting fluids (gases, liquids, vapours etc.) and particulates via the corresponding interface in the wellhead.

27 In the presence of multiple coal seams which may lie within the same field, multiple arrays 28 of recovery and delivery conduits may be provided with appropriate vertical separation(s) 29 between one another so as to service such additional coal seams. It is of course particularly advantageous if all of the arrays are serviced and in communication with a 31 common vertical wellbore. Figure 6 illustrates an exemplary embodiment of such a multi- 32 array installation in which several arrays 202a, b, c, d of different size, orientation and 33 vertical depth are serviced via a common, single well 201.

1 Figure 7 illustrates an alternative embodiment of an installation in which the array 302 2 comprises a number of delivery conduits 353 and recovery conduits 355 extending radially 3 from a central, single, common well 301. In such an embodiment the well 301 may be 4 drilled in the centre of a coal deposit and the array provide 360 degree coverage around the well within the coal deposit -servicing all of the conduits simultaneously or separately 6 according to gas demand or the availability of gasifiable coal. Furthermore, it maximises 7 access to the coal deposit for a given drilling range from the common well.

9 When forming the array (as shown for example in Figuie 4 or Figure 5) within the coal panel, at least one of the delivery conduits which supplies the oxidising agent and at least 11 one of the recovery conduits which extracts the syngas during the gasification process are 12 drilled to intercept at the location where the gasification is initiated. Once a sufficient 13 number of bores have been drilled in the coal seam and perforated liners have been 14 installed, the bores are pressurised and / or pumped out to remove water from the bore. In a similar manner to the optional use of eutectic or fusible plugs, ceramic plugs (such as 16 indicated by reference number 56 in Figure 2 and Figure 4) can be inserted in the recovery 17 pipelines and drilled through when ready for use.

19 Since delivery of oxygen or air to the combustion zone is a specific objective in underground coal gasification processes, any losses of oxygen into the coal seam prior to 21 arrival at the combustion zone is undesirable due to the cost (e.g. compression of air or 22 oxygen production). In highly porous coal types, it is desirable therefore to provide a 23 barrier to such losses. Depending upon the porosity of the coal seam therefore, the 24 delivery pipelines may need to be lined with an impervious barrier to prevent the absorption of oxygen or air into the coal formation.

27 In a first embodiment, a polymeric or cement (or combination thereof) barrier casing' 28 material that is impervious (or substantially impervious) to oxygen is applied during the 29 drilling process onto the wall of the delivery conduit whereby such material adheres to the wall sufficiently to act as a barrier against the flow of oxygen into the coal seam. For this 31 purpose, the drill-head for drilling the delivery conduit is fitted with a nozzle which sprays 32 the casing material onto the face of the coal which has been drilled. A pumping unit on the 33 surface or close to the surface is used to pressurise the barrier material within the drilling 34 stem or within a conduit contained within the drilling stem and provides sufficient flow of the barrier material to case' (or line) the exterior of the drilled-out cavity. Upon completion 1 of the conduit drilling activity, the casing activity is actuated by the pressure of the 2 polymeric or cement material within the drill stem. This treatment to prevent oxygen 3 ingress also serves to act as a cave-in barrier for friable coals which may collapse due to 4 the nature of the coal type and which cave-in may otherwise block the flow of oxygen/steam to the combustion zone.

7 In an alternative embodiment, following extraction of the drilling apparatus from the 8 delivery conduit an oxygen delivery liner pipe manufactured from a polymeric plastic (or 9 like material) is inserted into the conduit and, by means of an internal support or mandrel, progressed to the furthest extent of the bore from the surface. This delivery liner pipe acts 11 both as a cave-in barrier to the coal -in particular where soft or friable coals may present 12 a blockage to the flow of oxygen to the combustion zone and to the ingress of oxygen into 13 the coal seam. The liner pipe is manufactured from a material which melts and/or volatises 14 at a temperature below pyrolysis temperature, ensuring that it's structural integrity meets the desired objectives of supporting the surrounding coal body and preventing the ingress 16 of oxygen into the seam. Upon commencing UCG operations, in the vicinity of the 17 gasification chamber, the liner pipe would therefore (intentionally) melt and/or vaporise 18 when subjected to the heat of gasification (pyrolysis temperature), thereby continuously 19 allowing the free passage of oxygen to the gasification zone. The flow of oxygen or air through the liner pipe during gasification operations would provide a cooling means to 21 prevent heat transfer from the gasification chamber from affecting the structural or 22 mechanical properties of the liner pipe within the coal seam from being degraded.

24 In a further alternate installation method to that above, a drill-bit with a modified open-able tip may be employed which would allow the insertion of the pipeline liner into the drilled 26 pipeline and to retract the drill at completion of the drilling activity leaving the liner pipe 27 behind in the coal seam for the purpose of delivery of oxygen to the combustion zone. As 28 with the embodiment described above, the liner pipe would melt at or about gasification 29 (pyrolysis) temperature in the locality of the gasification chamber.

31 Note that while an underground coal gasification installation comprising the previously 32 described well and the above described underground coal gasification array (or a plurality 33 of arrays) is preferred, it will be understood by the skilled person that such arrays in 34 accordance with the invention may be employed with existing underground coal gasification installations (e.g. comprising one or more pairs of parallel vertical wells) 1 mutatis mutandis. That is, to benefit from the advantages of the array the delivery pipeline 2 and the recovery pipeline need not be located within the same casing; however, as an 3 installation it is particularly advantageous if they are so co-located.

The invention provides an underground gas transport and production system applicable to 6 the process of coal gasification employing a novel single well approach in which air, 7 oxygen or a mixture of these gases together with water (e.g. steam) is pumped from the 8 surface, to supply a gasification process, via the same well casing through which gas 9 produced by the gasification piocess is transported to the surface. A corresponding underground arrangement is also disclosed which complements the single well approach 11 and its own advantages in terms of increased recovery of syngas by use of an array of 12 delivery and recovery conduits. By providing multiple burn-fronts an averaging effect is 13 achieved which results in a dilution effect and balancing of gas conditions in the coal 14 seam.

16 Various modifications may be made within the scope of the invention as herein intended, 17 and embodiments of the invention may include combinations of features other than those 18 expressly claimed.