Combustion cavity control method for coal seam in-situ gasification mining and application of combustion cavity control method

Abstract

The invention discloses a combustion chamber control method for coal seam in-situ gasification mining and application of the combustion chamber control method. The method comprises the steps that whether the injection-production point retreating time is met or not is judged according to the flow of a production well and / or the high calorific value of produced gas; if yes, an injection-production point retreating mode is selected according to the type of a production well pipeline, the retreating distance, a retreating injection point and / or a production point are / is determined according to the thickness of a coal seam and the size of a current combustion cavity, and the injection well and the production well are double horizontal wells for conducting in-situ gasification production on the coal seam. According to the method, stable combustion of the deep coal seam combustion cavity can be achieved, stability of the output gas flow and the heat value is guaranteed, and long-time benefit mining is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of clean utilization of coal, and specifically to related technologies such as coal seam drilling and completion, coal seam in-situ combustion, injection and output component control, and more particularly to a combustion chamber control method for in-situ gasification mining of coal seams and its application. Background Art

[0002] Underground coal gasification technology is a cutting-edge and strategic new energy development technology. The construction of underground coal gasification furnaces is the key to the success of underground coal gasification projects. It is a comprehensive engineering technology involving the cross-integration of multiple disciplines such as drilling, cementing, materials, pipe mechanics, and machinery. my country's underground coal gasification research started late. Although preliminary exploratory tests have been carried out and successful shallow underground coal gasification tests have been carried out in a certain area, large-scale field tests have not yet been carried out.

[0003] Field tests at home and abroad have shown that shallow underground coal gasification technology is feasible, but there are problems such as low calorific value of output gas and environmental protection. At present, underground coal gasification is transitioning from shallow to deep, and deep underground coal gasification has obvious advantages such as high calorific value of output gas. Deep coal mainly refers to coal buried at a depth of more than 2,200 meters, and deep underground coal gasification is still a cutting-edge exploratory technology. There is no field test verification, and the technology needs to be tackled urgently. Summary of the invention

[0004] In order to enrich the method to be applicable to more application scenarios, the present invention proposes a combustion chamber control method for in-situ gasification mining of coal seams and its application.

[0005] In a first aspect, an embodiment of the present invention provides a combustion chamber control method for in-situ gasification mining of coal seams, comprising:

[0006] Determine whether the timing of retreating the injection and production point is met based on the flow rate of the production well and / or the high heating value of the produced gas;

[0007] If so, the injection and production point retreat method is selected according to the type of production well pipeline, and the retreat distance is determined according to the coal seam thickness and the current size of the combustion chamber. The injection point and / or production point are retreated, and the injection well and the production well are double horizontal wells for in-situ gasification and mining of the coal seam.

[0008] In some embodiments, whether the timing of retreating the injection and production point is met is determined based on the flow rate of the production well and / or the high heating value of the produced gas;

[0009] If so, the injection and production point retreat method is selected according to the type of production well pipeline, and the retreat distance is determined according to the coal seam thickness and the current size of the combustion chamber. The injection point and / or production point are retreated, and the injection well and the production well are double horizontal wells for in-situ gasification and mining of the coal seam.

[0010] In some embodiments, judging whether the timing of retreating the injection and production point is met according to the high calorific value of the gas produced by the production well includes:

[0011] Whether the timing of retreating the injection and production point is met is determined based on whether the high heating value HHV of the gas produced by the production well is lower than the set HHV threshold.

[0012] In some embodiments, the step of selecting the injection and production point retreat method according to the type of the production well pipeline includes:

[0013] If the gas production pipe of the production well is an oil pipe in casing, choose the retreat method of keeping the production point unchanged and retreating the injection point;

[0014] If the gas production pipe of the production well is a casing, select the retreat method of keeping the injection point fixed and retreating the production point, or select the retreat method of retreating both the injection point and the production point, or select the retreat method of keeping the production point fixed and retreating the injection point;

[0015] If the horizontal section of the production well is completed using an open-hole screen, select a retreat method in which the injection point retreats and the production point retreats with the combustion surface, or select a retreat method in which the injection point remains stationary and the production point retreats with the combustion surface, or select a retreat method in which the production point remains stationary and the injection point retreats.

[0016] In some embodiments, determining the retreat distance according to the coal seam thickness and the current combustion chamber size includes:

[0017] If the coal seam thickness is less than 10 meters, determine the retreat distance as 10 times the coal seam thickness;

[0018] Otherwise, the retreat distance is determined according to the width of the current combustion chamber along the direction perpendicular to the well trajectory.

[0019] In some embodiments, it also includes:

[0020] If there is a measured coal seam pore pressure, the injection pressure of the injection well is controlled so that the current pressure in the combustion chamber is 70% to 90% of the measured coal seam pore pressure;

[0021] Otherwise, the hydrostatic pressure is determined according to the hydrostatic pressure gradient and the current depth of the combustion chamber, and the injection pressure of the injection well is controlled so that the pressure in the current combustion chamber is 70% to 90% of the hydrostatic pressure.

[0022] In some embodiments, it also includes:

[0023] The injected water-oxygen ratio of the injection well is controlled so that the current maximum temperature in the combustion chamber is not lower than a first set temperature threshold.

[0024] In some embodiments, it also includes:

[0025] When the injection point is retreated, whether the ignition is successful is determined by the temperature change characteristics in the current combustion chamber.

[0026] In some embodiments, it also includes:

[0027] During the combustion process of the first combustion chamber, monitoring whether the bottom hole temperature of the production well is increased to above a second set temperature threshold;

[0028] If so, water is injected into the service well so that the bottom hole temperature of the production well is reduced to below the second set temperature threshold.

[0029] In some embodiments, it also includes:

[0030] When the flameout occurs before the backward injection point, the injection well stops injecting, and water is injected through the service well to reduce the temperature inside the current combustion chamber until the flameout is completed.

[0031] In some embodiments, it also includes:

[0032] The oxygen utilization rate is calculated by the following formula, and the economic feasibility of in-situ gasification of coal seams is determined based on the calculated oxygen utilization rate:

[0033]

[0034] In a second aspect, an embodiment of the present invention provides a coalbed methane mining method, comprising:

[0035] During in-situ gasification mining of coal seams through dual horizontal wells of injection wells and production wells, the combustion chamber is controlled according to any of the above-mentioned combustion chamber control methods for in-situ gasification mining of coal seams.

[0036] The beneficial effects of the above technical solution provided by the embodiment of the present invention include at least:

[0037] (1) The combustion chamber control method for in-situ gasification mining of coal seams provided in the embodiment of the present invention proposes a method for determining the retreat mode, retreat distance and retreat timing, which can effectively ensure the stability of the output gas quality, the efficiency of gasification chamber combustion, and to a certain extent ensure the safety of the production well.

[0038] (2) The combustion chamber control method for in-situ coal seam gasification mining provided in the embodiment of the present invention reasonably controls the injection water-oxygen ratio of the injection well, can effectively ensure the hydrogen yield, and maintain the dynamic balance of the combustion chamber.

[0039] (3) The combustion chamber control method for in-situ gasification mining of coal seams provided in the embodiment of the present invention provides a combustion chamber oxygen utilization rate evaluation method, which can effectively evaluate the injection-output ratio of the entire combustion chamber and provide a reference basis for the selection of combustion chamber working conditions.

[0040] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0041] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0043] Figure 1 This is a flow chart of a combustion chamber control method for in-situ gasification mining of coal seams in Example 1 of the present invention;

[0044] Figure 2 Schematic diagram of underground gasification of deep coal seams with double horizontal wells in Embodiment 2 of the present invention;

[0045] Figure 3 Schematic diagram of the change of HHV and production well flow rate in the second embodiment of the present invention. DETAILED DESCRIPTION

[0046] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0047] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0048] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which the invention belongs. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0049] In the description of the present invention, it should be noted that the terms "include", "including", "have", "contain" and the like are open terms, meaning including but not limited to.

[0050] In order to establish a deep coal seam in-situ gasification combustion control method (the main application scenario is a coal seam with a burial depth greater than 2,200 meters and a thickness of not less than 5 meters), achieve stable combustion in the deep coal seam combustion chamber, ensure the stability of the output gas flow, pressure and calorific value, ensure the stability of the combustion chamber cavity, and achieve stable combustion of the cavity and long-term efficient mining. The embodiment of the present invention provides a combustion chamber control method for in-situ gasification mining of coal seams and its application.

[0051] Embodiment 1

[0052] Embodiment 1 of the present invention provides a combustion chamber control method for in-situ gasification mining of coal seams. The injection and production wells are designed with double horizontal wells of injection wells and production wells. The vertical well service well is used for water injection, flameout and monitoring. The service well is drilled to the coal seam. Figure 1 As shown, the following steps are included:

[0053] Step S11: judging whether it is time to retreat the injection and production point according to the flow rate of the production well and / or the high heating value of the produced gas.

[0054] Specifically, based on the flow rate of the production well, determine whether it is time to retreat the injection and production point, including whether the decline in the flow rate of the production well is greater than a set amplitude threshold, determining whether it is time to retreat the injection and production point; based on the high heating value of the gas produced from the production well, determine whether it is time to retreat the injection and production point, including whether the high heating value HHV of the gas produced from the production well is lower than a set HHV threshold, determining whether it is time to retreat the injection and production point.

[0055] High Heating Value (HHV) refers to the high heating value generated by the combustion of unit volume of output gas per unit time. The unit of HHV is usually MJ / (sm 3 It is generally believed that HHV is lower than 4.5MJ / (sm 3 ) That is, the injection-production point retreat operation should be performed, and the specific critical value should be determined based on the continuous change of HHV during the actual cavity combustion process.

[0056] Whether it is time to retreat the injection and production points can be determined based on the flow rate of the production well or the high calorific value of the produced gas according to the specific circumstances; or, whether it is time to retreat the injection and production points can be determined based on the flow rate of the production well and the high calorific value of the produced gas.

[0057] If the answer in step S11 is yes, execute step S12.

[0058] Step S12: Select the injection and production point retreat method according to the type of production well pipeline, determine the retreat distance according to the coal seam thickness and the current combustion chamber size, and retreat the injection point and / or production point.

[0059] After the cavity burns at one location for too long, the multi-component raw coal gas output will show an obvious staged flow rate drop and gas quality drop, and the injection point needs to be retreated regularly. Especially for drilling-type deep coal seam underground gasification, the bottom hole pressure of the production well generally exceeds 10MPa. The appropriate injection and production point retreat method, retreat timing and retreat distance can improve the flow rate / calorific value stability of the output gas, use the formation to assist the output gas in cooling / depressurizing, and facilitate the anti-corrosion design of the production well.

[0060] 1. Choice of retreat method for injection and production points.

[0061] Injection wells usually use concentric continuous tubing, so the injection point can be retreated or remain stationary. Therefore, the retreat method of the injection and production point is determined according to the type of production well pipeline.

[0062] (1) The gas production pipe of the production well is a casing-in-tubing

[0063] Select the retreat method in which the production point remains stationary and the injection point retreats. During the combustion process, both the injection point and the production point remain stationary. When retreating, the combustible is added to the position where the injection well is to be retreated through the concentric continuous pipe. The original stainless steel tail pipe is melted and burned through by the combustible, completing the retreat and establishing a new injection point.

[0064] (2) The gas production pipe of the production well is casing

[0065] Select the retreat method in which the injection point remains stationary and the production point retreats, or select the retreat method in which both the injection point and the production point retreat, or select the retreat method in which the production point remains stationary and the injection point retreats.

[0066] Taking the retreat method in which the injection point remains stationary and the production point retreats as an example, during the combustion process, the injection point remains stationary. When the production point retreats, a perforating gun is used to perforate the casing at the position where the production well is to be retreated, thus completing the retreat and establishing a new production point.

[0067] Taking the retreat method in which both the injection point and the production point are retreated as an example, during the combustion process, the injection well and the production well remain stationary. When retreating, the combustible agent is added to the position to which the injection well is to be retreated through the concentric continuous pipe, and the original stainless steel tail pipe is melted and burned through by the combustible agent to complete the retreat of the injection point; the perforating gun is used to perforate the casing at the position to which the production well is to be retreated to complete the retreat of the production point.

[0068] (3) The horizontal section of the production well is completed using open hole high-strength and high-temperature resistant screen pipe

[0069] Select the retreat method in which the injection point retreats and the production point retreats with the combustion surface, or select the retreat method in which the injection point remains stationary and the production point retreats with the combustion surface, or select the retreat method in which the production point remains stationary and the injection point retreats.

[0070] 2. Determination of retreat distance.

[0071] If the coal seam thickness is less than 10 meters, the retreat distance is determined to be 10 times the coal seam thickness; otherwise, the retreat distance is determined based on the width of the current combustion chamber in the direction perpendicular to the well trajectory, and the retreat distance should be greater than the width of the current combustion chamber in the direction perpendicular to the well trajectory.

[0072] When performing the retreat, the retreat distance should be determined based on the indicator characteristics of the HHV after retreating.

[0073] The combustion chamber control method for in-situ gasification mining of coal seams provided in Example 1 of the present invention proposes a method for determining the retreat mode, retreat distance and retreat timing, which can effectively ensure the stability of the output gas quality, the efficiency of gasification chamber combustion, and to a certain extent ensure the safety of the production well.

[0074] In some embodiments, in the combustion chamber control process of coal seam in-situ gasification mining, in order to ensure stable combustion in the combustion chamber, the following injection and extraction surface control measures are also included:

[0075] 1. Pressure control of injection wells

[0076] If there is measured coal seam pore pressure, by controlling the injection pressure of the injection well, under the premise of good sealing of the cavity top and bottom plates, the current pressure in the combustion chamber is made to be 70% to 90% of the measured coal seam pore pressure;

[0077] If the coal seam pore pressure is not measured, the hydrostatic pressure is determined based on the hydrostatic pressure gradient and the current depth of the combustion chamber. By controlling the injection pressure of the injection well, the pressure in the current combustion chamber is made 70% to 90% of the hydrostatic pressure, provided that the top and bottom plates of the chamber are well sealed.

[0078] Specifically, the pressure gradient is usually 1 MPa / 100 m, and the ground hydrostatic pressure is 0, so the hydrostatic pressure can be determined based on the hydrostatic pressure gradient and the current depth of the combustion chamber.

[0079] 2. Injection well flow rate and water-oxygen ratio control

[0080] The injected oxygen is pure oxygen, and the injected water will trigger a supercritical state under the pressure conditions generated at this formation depth. The injected water-oxygen ratio of the injection well is controlled so that the current maximum temperature in the combustion chamber is not lower than the first set temperature threshold.

[0081] The first set temperature threshold is usually 700 degrees Celsius, ensuring that the maximum temperature collected by the thermocouple close to the combustion chamber is not less than 700 degrees Celsius to ensure the smooth occurrence of the water-gas reaction. When the maximum temperature collected by the thermocouple is lower than 700 degrees Celsius, the water-oxygen ratio should be reduced, and the water-oxygen ratio should be increased after the temperature rises to above 1000 degrees.

[0082] Generally speaking, the best water-oxygen mass ratio is 1.3:1. The specific best water-oxygen ratio should be determined comprehensively based on specific parameters such as the high calorific value of the generated gas and the hydrogen content during actual combustion.

[0083] Reasonable control of the injection water-oxygen ratio of the injection well and the pressure in the combustion chamber can effectively ensure the hydrogen yield and maintain the dynamic balance of the combustion chamber.

[0084] In some embodiments, the oxygen utilization rate is calculated by the following formula, and the economic feasibility of in-situ gasification mining of coal seams is determined based on the calculated oxygen utilization rate:

[0085]

[0086] In the above formula, the calorific value of the output gas refers to the heat generated by the total combustion of the output gas per unit time.

[0087] When pure oxygen is injected, the calorific value of the mixed output gas is generally between 10 and 11 MJ / Nm 3 , the oxygen utilization rate in this case is about 1200MJ / kmol.

[0088] The combustion chamber control method for in-situ gasification mining of coal seams provided in an embodiment of the present invention provides a combustion chamber oxygen utilization rate evaluation method, which can effectively evaluate the injection-output ratio of the entire combustion chamber and provide a reference basis for the selection of combustion chamber working conditions.

[0089] In some embodiments, when the injection point is retreated, whether the ignition is successful is determined by the temperature change characteristics in the current combustion chamber.

[0090] A specially designed igniter is used underground to ignite the combustible. The pressure applied to the igniter sometimes cuts off the ignition plug, and the temperature change monitored by the thermocouple closest to the igniter in the injection well can be used to determine whether the ignition is successful.

[0091] In some embodiments, it is also included that during the combustion process of the first combustion chamber, the bottom temperature of the production well is monitored to be higher than the second set temperature threshold; if so, water is injected into the service well to reduce the bottom temperature of the production well to below the second set temperature threshold. Otherwise, some instruments will exceed the tolerance temperature and fail.

[0092] The second set temperature threshold is usually 500 degrees.

[0093] Before the next cavity is burned, the previous cavity must be extinguished, otherwise uncontrolled combustion will occur. In some embodiments, when the flame is extinguished before the back injection point, the injection well stops injecting, and water is injected into the service well to reduce the temperature in the current combustion cavity until the flame is extinguished.

[0094] In the above method, when the surface water source of the selected block is insufficient, the water produced by the production well can be recycled and reused. The injected water may not be fresh water, such as industrial wastewater containing chloride ions.

[0095] Embodiment 2

[0096] Embodiment 2 of the present invention provides a specific application of a combustion chamber control method for in-situ gasification mining of coal seams. In this embodiment, see Figure 2 As shown in the figure, the injection well and the production well adopt the double horizontal well docking form, and the two horizontal wells adopt the symmetrical trajectory design. The injection well and the production well are connected at the intersection of the two wells. The horizontal section length is 500 meters. The service well is a vertical well, which is used for water injection, flameout and monitoring. The injection well adopts the concentric coiled tubing design. The inner tube of the coiled tubing is used to inject oxygen and combustibles. The inner tube needs to be kept clean. The combustible can use triethyl borane that can self-ignite; the oxygen injection pipe must have an automatic cut-off system to ensure that the oxygen supply is automatically stopped immediately when there is any abnormality in the well; oxygen is transported by liquid oxygen tanker and injected into the ground after being gasified on the ground; liquid water is injected into the annulus of the inner and outer tubes of the concentric coiled tubing. The bottom of the well adopts a nozzle design to mix the oxygen and water injected by the concentric coiled tubing. The bottom of the production well uses a back pressure valve made of high-temperature and corrosion-resistant nickel-based alloy to control the bottom hole pressure of the production well, and the horizontal section uses a naked hole high-strength and high-temperature resistant screen pipe for completion. Thermocouple monitoring is installed in both injection wells and production wells, with a thermocouple arranged every 20-50 meters in the horizontal section.

[0097] The combustion chamber control process includes:

[0098] Step S1: During the combustion process, the injection well and the production well remain stationary;

[0099] Step S2: See Figure 3 The figure shows the HHV and the flow rate of the production well. When the flow rate and HHV both drop significantly and the HHV is lower than 4.5 MJ / (sm 3), the injection well needs to be retreated 100 meters, and the combustible agent is added to the position where the injection well is to be retreated through the concentric continuous pipe. The original stainless steel tail pipe is melted and burned through by the combustible agent, and the retreat is completed, and a new injection point is established. At the same time, the production point is required to retreat with the combustion surface;

[0100] Step S3: igniting the combustible agent using a specially designed igniter underground;

[0101] Step S4: During the combustion process, the pressure in the combustion chamber is guaranteed to be no higher than 10MPa (1000m deep coal seam) and the minimum temperature is no lower than 700°C. The water-oxygen ratio of the injection well is 1.3:1, and the final flow rate of the production well is 5000sm 3 / hr, the calorific value of the output gas is 11MJ / Nm 3 , oxygen utilization rate is 1100MJ / kmol;

[0102] Step S5: After the combustion is finished, the injection well stops injecting oxygen and water, and water is injected into the combustion chamber through the service well to lower its temperature until the reaction stops.

[0103] Based on the inventive concept of the present invention, an embodiment of the present invention further provides a coal seam in-situ gasification mining method, comprising:

[0104] During the in-situ gasification mining of the coal seam through the double horizontal wells of the injection well and the production well, the combustion of the combustion chamber is controlled according to the combustion chamber control method for in-situ gasification mining of the coal seam.

[0105] It should be understood that the specific order or hierarchy of steps in the disclosed process is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the scope of protection of the present disclosure. The attached method claims present the elements of the various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy described.

[0106] In the above detailed description, various features are grouped together in a single embodiment to simplify the disclosure. This method of disclosure should not be interpreted as reflecting an intention that the embodiments of the claimed subject matter require more features than are clearly stated in each claim. On the contrary, as reflected in the appended claims, the invention is in a state of having less than all the features of the disclosed individual embodiments. Therefore, the appended claims are hereby expressly incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment of the invention.

[0107] The above description includes examples of one or more embodiments. Of course, it is impossible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it should be recognized by those skilled in the art that the various embodiments can be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the word is covered in a manner similar to the term "including", just as "including," explained as a transitional word in the claims. In addition, any term "or" used in the specification of the claims is intended to represent "non-exclusive or". The terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance.

Claims

1. A combustion chamber control method for in-situ coal seam gasification mining, characterized in that: include: Determine whether the timing of retreating the injection and production point is met based on the flow rate of the production well and / or the high heating value of the produced gas; If so, the injection and production point retreat method is selected according to the type of production well pipeline, and the retreat distance is determined according to the coal seam thickness and the current size of the combustion chamber. The injection point and / or production point are retreated, and the injection well and the production well are double horizontal wells for in-situ gasification and mining of the coal seam.

2. The method according to claim 1, characterized in that According to the flow rate of the production well, determine whether it is appropriate to retreat the injection and production point, including: Whether the timing of retreating the injection and production point is met is determined by whether the flow rate decline of the production well is greater than the set amplitude threshold.

3. The method according to claim 1, characterized in that According to the high calorific value of the gas produced by the production well, determine whether it is appropriate to retreat the injection and production point, including: Whether the timing of retreating the injection and production point is met is determined based on whether the high heating value HHV of the gas produced by the production well is lower than the set HHV threshold.

4. The method according to claim 1, characterized in that The method of selecting the injection and production point retreat method according to the type of the production well pipeline includes: If the gas production pipe of the production well is an oil pipe in casing, choose the retreat method of keeping the production point unchanged and retreating the injection point; If the gas production pipe of the production well is a casing, select the retreat method of keeping the injection point fixed and retreating the production point, or select the retreat method of retreating both the injection point and the production point, or select the retreat method of keeping the production point fixed and retreating the injection point; If the horizontal section of the production well is completed using an open-hole screen, select a retreat method in which the injection point retreats and the production point retreats with the combustion surface, or select a retreat method in which the injection point remains stationary and the production point retreats with the combustion surface, or select a retreat method in which the production point remains stationary and the injection point retreats.

5. The method according to claim 1, characterized in that Determining the retreat distance according to the coal seam thickness and the current combustion chamber size includes: If the coal seam thickness is less than 10 meters, determine the retreat distance as 10 times the coal seam thickness; Otherwise, the retreat distance is determined according to the width of the current combustion chamber in a direction perpendicular to the well trajectory.

6. The method according to claim 1, characterized in that Also includes: If there is a measured coal seam pore pressure, the injection pressure of the injection well is controlled so that the current pressure in the combustion chamber is 70% to 90% of the measured coal seam pore pressure; Otherwise, the hydrostatic pressure is determined according to the hydrostatic pressure gradient and the current depth of the combustion chamber, and the injection pressure of the injection well is controlled so that the pressure in the current combustion chamber is 70% to 90% of the hydrostatic pressure.

7. The method according to claim 6, characterized in that Also includes: The injected water-oxygen ratio of the injection well is controlled so that the current maximum temperature in the combustion chamber is not lower than a first set temperature threshold.

8. The method according to claim 1, characterized in that Also includes: When the injection point is retreated, whether the ignition is successful is determined by the temperature change characteristics in the current combustion chamber.

9. The method according to claim 1, characterized in that Also includes: During the combustion process of the first combustion chamber, monitoring whether the bottom hole temperature of the production well is increased to above a second set temperature threshold; If so, water is injected into the service well so that the bottom hole temperature of the production well is reduced to below the second set temperature threshold.

10. The method according to claim 1, characterized in that Also includes: When the flameout occurs before the backward injection point, the injection well stops injecting, and water is injected through the service well to reduce the temperature inside the current combustion chamber until the flameout is completed.

11. The method according to claim 1, characterized in that Also includes: The oxygen utilization rate is calculated by the following formula, and the economic feasibility of in-situ gasification of coal seams is determined based on the calculated oxygen utilization rate:

12. A coal seam in-situ gasification mining method, characterized in that: include: During in-situ gasification mining of coal seams through double horizontal wells of injection well and production well, the combustion chamber is controlled according to the method described in any one of claims 1 to 11.

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