Deep coal supercritical water forward gasification method

Abstract

The present invention discloses a deep coal supercritical water forward gasification method, comprising the following steps: injecting fracturing fluid from an injection well and creating a negative pressure in a production well, so that under the guidance of the negative pressure of the production well, the fracturing fluid fractures the coal seam to form a communication fracture network between the output section of the injection well and the input section of the production well; injecting supercritical water from the injection well into the coal seam to heat up a portion of the coal body of the coal seam; injecting oxygen from the injection well into the coal seam to cause an oxidation reaction between a portion of the coal body of the coal seam and the oxygen, thereby heating the portion of the coal body of the coal seam and the oxygen, and then a redox reaction occurs between a portion of the coal body of the coal seam and the supercritical water and oxygen to form a reaction zone of the coal seam and generate gas; the gas enters the production well through the communication fracture network and is produced through the production well; and alternately injecting supercritical water and oxygen until the reaction zone of the coal seam is gradually advanced from the output section of the injection well through the communication fracture network to the input section of the production well. The present invention can improve the gas capture efficiency, the total gas production volume, and the resource utilization rate of the coal seam.

Description

Technical Field

[0001] The present invention relates to the technical field of clean coal utilization, and in particular to a deep coal supercritical water forward gasification method. Background Art

[0002] Underground coal gasification (UCG) technology is currently an effective means of mining deep coal seams. Its safety and environmental advantages have been initially verified, and it holds great promise for future application. Underground coal gasification (UCG) using supercritical water is a novel coal utilization technology that generates gaseous fuel by reacting supercritical water with coal underground. It offers advantages such as high efficiency and environmental friendliness. Supercritical water (SCW), characterized by temperatures reaching 374.3°C and pressures of 22.05 MPa, is an excellent reaction medium with exceptional mass, heat, and solubility properties. It can efficiently and cleanly gasify a variety of biomass, coal, petroleum coke, and organic waste solids and liquids. This technology leverages the high heat, mass, and solubility properties of supercritical water, as well as the chemical reactions between coal, supercritical water, and oxygen at high temperature and pressure, enabling efficient and clean mining of deep coal seams. Furthermore, by storing CO2 deep underground, it effectively reduces greenhouse gas emissions, offering significant environmental benefits.

[0003] However, in-situ supercritical water underground coal gasification technology is still in the research and development stage and has yet to achieve large-scale application. Due to the relatively complex geological conditions of deep coal deposits and the difficulty in predicting the gasification reaction process, the key bottleneck that needs to be overcome in deep coal in-situ supercritical water gasification technology is how to stably and efficiently produce combustible gas from coal combustion under these complex and changing conditions. Existing technologies using the receding injection point (CRIP) gasification method are prone to problems such as uncontrolled combustion, ground collapse, water inrush into the gasification chamber, and gas escape. Summary of the Invention

[0004] The purpose of the present invention is to provide a deep coal supercritical water forward gasification method to solve the technical problem that the in-situ coal supercritical water underground gasification method cannot stably and efficiently produce combustible gas generated by coal combustion.

[0005] The above-mentioned purpose of the present invention can be achieved by adopting the following technical solutions:

[0006] The present invention provides a method for deep coal supercritical water forward gasification, comprising the following steps: guided fracturing: injecting fracturing fluid from an injection well and forming a negative pressure in a production well, so that under the guidance of the negative pressure of the production well, the fracturing fluid fractures the coal seam to form a communication fracture network between the output section of the injection well and the input section of the production well; supercritical water injection: injecting supercritical water into the coal seam from the injection well to increase the temperature of a portion of the coal body in the coal seam; oxygen injection: injecting oxygen into the coal seam from the injection well to cause an oxidation reaction between a portion of the coal body in the coal seam and the oxygen to increase the temperature, and then a redox reaction occurs between a portion of the coal body in the coal seam, the supercritical water and the oxygen to form a reaction zone in the coal seam and generate gas; gas collection: the gas enters the production well through the communication fracture network and is produced through the production well; and reaction zone advancement: alternately repeating the supercritical water injection step and the oxygen injection step until the reaction zone of the coal seam is gradually advanced from the output section of the injection well through the communication fracture network to the input section of the production well.

[0007] In an embodiment of the present invention, the injection well includes an injection vertical shaft extending from top to bottom to the coal seam and at least one injection branch well extending along the coal seam and connected to the injection vertical shaft, and the production well extends from top to bottom to the coal seam; in the guided fracturing step, the fracturing fluid is injected into the injection branch well through the injection vertical shaft, and then the fracturing fluid is gradually fractured in the coal seam from the inlet of the injection branch well to the bottom of the injection branch well to form cracks, and the cracks gradually extend toward the production well under the guidance of the negative pressure of the production well, thereby forming the communicating fracture network between the injection branch well and the input section of the production well.

[0008] In an embodiment of the present invention, an underground temperature monitoring structure is provided in the output section of the injection well; when the underground temperature monitoring structure monitors that part of the coal body of the coal seam has been heated to the oxidation reaction temperature of the coal body during the supercritical water injection step, the injection of the supercritical water is stopped and the oxygen injection step is started.

[0009] In an embodiment of the present invention, the injection amount of the supercritical water and the injection amount of the oxygen are in a preset ratio; wherein the preset ratio is 1:2, 1:1 or 2:1.

[0010] In an embodiment of the present invention, when the downhole temperature monitoring structure monitors that the temperature of the reaction zone is lower than 600° C. during the gas collection step, the supercritical water injection step is performed again.

[0011] In an embodiment of the present invention, the guided fracturing is guided hydraulic fracturing.

[0012] In an embodiment of the present invention, before the guided fracturing step, the method further includes the following steps: determining the efficient coal production area in the coal seam, and then determining the design position of the injection well within the efficient coal production area; simulating and analyzing the gas enrichment area of ​​the coal seam during the redox reaction process, and then determining the design position of the production well within the gas enrichment area; establishing the injection well at the design position of the injection well, and establishing the production well at the design position of the production well.

[0013] In an embodiment of the present invention, determining the design position of the injection well includes the following steps: conducting a comprehensive evaluation of the coal seam based on the geological exploration and geophysical exploration results of the coal seam to identify the high-efficiency coal production area; determining the design position of the injection well based on the pressure distribution, fluid properties and temperature distribution of the high-efficiency coal production area.

[0014] In an embodiment of the present invention, determining the design position of the production well includes the following steps: establishing a geological model of the coal seam; simulating the flow of multiphase fluid in the coal seam during the redox reaction process based on the design position of the injection well and the geological model of the coal seam, thereby analyzing the gas-enriched area; and determining the design position of the production well within the gas-enriched area.

[0015] In an embodiment of the present invention, before establishing the injection well and the production well, the method further includes: determining the wellbore structure and wellbore material of the injection well and the production well according to the pressure design requirements and temperature design requirements of the injection well and the production well.

[0016] The characteristics and advantages of the present invention are:

[0017] The deep coal supercritical water forward gasification method of the present invention forms a negative pressure in the production well during the fracturing process, and uses the negative pressure of the production well to guide the flow of fracturing fluid in the coal seam, thereby improving the fracturing effect and enabling the communication fracture network formed by the fracturing in the coal seam to achieve more effective communication between the output section of the injection well and the input section of the production well. Furthermore, during the process of alternately injecting supercritical water and oxygen to cause the coal seam to undergo redox reactions with the supercritical water and oxygen, on the one hand, the communication fractures can be used to produce gas from the production well, thereby improving the gas capture efficiency and the total produced gas volume and reducing the unit cost of produced gas. On the other hand, the communication fractures are used to guide the flow of supercritical water and oxygen to achieve relatively stable and gradual advancement of the reaction zone of the coal seam, avoiding the problem of uncontrollable combustion, and having faster heat and mass transfer efficiency, thereby improving the gasification efficiency and synthesis gas yield of the coal seam, thereby improving the resource utilization rate of the coal seam. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0019] Figure 1 Schematic diagram of the distribution of injection wells and production wells in the present invention.

[0020] Figure 2 Schematic diagram of the gasification flow field between the injection well and the production well in the present invention.

[0021] In the picture:

[0022] 1. Injection well; 11. Vertical injection shaft; 12. Branch injection well; 2. Production well; 3. Coal seam; 4. Communication seam network. DETAILED DESCRIPTION

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0024] Combine Figure 1 As shown, the present invention provides a deep coal supercritical water forward gasification method, comprising the following steps: guided fracturing: injecting fracturing fluid from an injection well 1 and creating a negative pressure in a production well 2, so that under the guidance of the negative pressure in the production well 2, the fracturing fluid fractures the coal seam 3 to form a communication fracture network 4 between the output section of the injection well 1 and the input section of the production well 2; supercritical water injection: injecting supercritical water into the coal seam 3 from the injection well 1 to heat a portion of the coal body in the coal seam 3; oxygen injection: injecting oxygen into the coal seam 3 from the injection well 1 to cause an oxidation reaction between a portion of the coal body in the coal seam 3 and the oxygen, thereby heating the portion of the coal body in the coal seam 3, and then undergoing an oxidation-reduction reaction with the supercritical water and oxygen to form a reaction zone in the coal seam 3 and generate gas; gas collection: gas enters the production well 2 through the communication fracture network 4 and is produced through the production well 2; reaction zone advancement: alternately repeating the supercritical water injection step and the oxygen injection step until the reaction zone in the coal seam 3 is gradually advanced from the output section of the injection well 1 through the communication fracture network 4 to the input section of the production well 2.

[0025] The deep coal supercritical water forward gasification method of the present invention forms a negative pressure in the production well 2 during the fracturing process, and uses the negative pressure of the production well 2 to guide the flow of the fracturing fluid in the coal seam 3, thereby improving the fracturing effect and enabling the communication fracture network 4 formed by the fracturing in the coal seam 3 to achieve more effective communication between the output section of the injection well 1 and the input section of the production well 2. Furthermore, during the process of alternately injecting supercritical water and oxygen to cause the coal seam 3 to undergo an oxidation-reduction reaction with the supercritical water and oxygen, on the one hand, the communication fractures can be used to produce gas from the production well 2, thereby improving the gas capture efficiency and the total gas production volume and reducing the unit cost of produced gas. On the other hand, the communication fractures are used to guide the flow of supercritical water and oxygen to achieve relatively stable and gradual advancement of the reaction zone of the coal seam 3, avoiding the problem of uncontrollable combustion, and having faster heat and mass transfer efficiency, thereby improving the gasification efficiency and synthesis gas yield of the coal seam 3, thereby improving the resource utilization rate of the coal seam 3.

[0026] Specifically, the injection well 1 includes an injection shaft 11 extending from top to bottom to the coal seam 3, and at least one injection branch well 12 extending along the coal seam 3 and connected to the injection shaft 11. The production well 2 extends from top to bottom to the coal seam 3. During the guided fracturing step, the fracturing fluid is injected into the injection branch well 12 via the injection shaft 11, thereby fracturing the coal seam 3 to form a communicating fracture network 4 between the injection branch well 12 and the input section of the production well 2. The injection branch well 12 is a horizontal well, while the injection shaft 11 and the production well 2 are both vertical wells. The number of injection branch wells 12 is preferably set to multiple, such as two in the present embodiment, but can also be set to three, four, or more as needed, or of course, only one.

[0027] The CRIP process currently widely used is to drag the reaction zone from the end of the injection branch well 12 (i.e., the bottom end of the injection branch well 12) to the front end (i.e., the inlet end of the injection branch well 12, that is, the bottom end of the injection shaft 11) through a continuous pipe. After the first reaction zone is formed, the continuous pipe is dragged back and ignited to form a second reaction zone. This process requires repeated ignition. After ignition and combustion, the underground combustion situation is uncontrollable, and the continuous pipe is prone to getting stuck during retreat. The gasification process needs to be maintained in a strictly water-free state, and a combustion oxidation-reduction reaction occurs. It is also impossible to fill in the reaction process to support the top plate, resulting in the collapse of the top plate and water gushing into the reaction zone, causing the reaction to terminate. The present invention adopts supercritical water forward in-situ gasification to advance the reaction zone from the inlet end of the injection branch well 12 to the bottom end of the injection branch well 12, that is, from the front end to the end of the injection branch well 12, to extract the gasification products from the underground. The strong oxidizing property of supercritical water is utilized to forcibly oxidize and heat deep coal without ignition, thereby maintaining a water-rich state and allowing high-temperature oxidation-hydrogenation reactions to occur. Furthermore, the reaction can be carried out while filling is performed, ensuring that the roof does not collapse, and allowing heat transfer, mass transfer, and physical and chemical reactions in the reaction zone to proceed in an orderly manner. Therefore, the present invention can effectively circumvent the problems of the above-mentioned CRIP process.

[0028] according to Figure 1 As shown in the fracturing direction A, the fracturing fluid is gradually fractured from the inlet of the injection branch well 12 (that is, the bottom of the injection well 11) to the bottom of the injection branch well 12 to form cracks. The cracks gradually extend toward the production well 2 under the guidance of the negative pressure of the production well 2, thereby forming a connecting fracture network 4. In an embodiment of the present invention, the guided fracturing is guided hydraulic fracturing, wherein the fracturing fluid can be slick water, guar gum fracturing fluid or other hydraulic fracturing fluids in the prior art. The specific composition is the same as that of the prior art and is not described in detail here. By connecting the wellhead of the production well 2 to a vacuum pump, a negative pressure is formed in the production well 2.

[0029] like Figure 1 As shown, the injection branch well 12 constitutes the output section of the injection well 1, that is, the supercritical water and oxygen injected into the injection branch well 12 through the injection shaft 11 can flow out to the communication fracture network 4. The well section of the production well 2 extending into the coal seam 3 constitutes the input section of the production well 2, that is, the gas generated by the reaction can flow into the well section of the production well 2 extending into the coal seam 3 through the communication fracture network 4. Therefore, according to Figure 2 As shown in the gas flow direction B, the gas generated at the inlet of the injection branch well 12 (i.e., the bottom of the injection shaft 11) is the largest, and therefore the temperature is the highest, so that the gas and heat flow from the inlet of the injection branch well 12 to the input section of the production well 2 under the guidance of the injection branch well 12 and the communication fracture network 4. Figure 1As shown, in an embodiment of the present invention, the output section of the injection well 1 is equipped with a downhole temperature monitoring structure. When the downhole temperature monitoring structure detects that the temperature of a portion of the coal in the coal seam 3 has risen to the oxidation reaction temperature during the supercritical water injection step, the supercritical water injection is stopped and the oxygen injection step is initiated. After the supercritical water is injected, the gasification reaction of the supercritical water is utilized to raise the temperature of a portion of the coal to the oxidation reaction temperature. The oxidation reaction temperature of the coal is generally 300°C to 400°C. After the oxygen is injected, the coal can undergo an oxidation reaction with the oxygen, further raising the temperature to the redox reaction temperature of the coal, which is generally above 900°C. This promotes the in-situ redox reaction of the coal with the supercritical water and oxygen to produce gases such as H2 and CO2. The downhole temperature monitoring structure is preferably located at the entrance of the injection branch well 12 (i.e., the bottom of the injection shaft 11). The specific structure and operating principle of the downhole temperature monitoring structure are the same as those in the prior art and will not be described in detail here.

[0030] Specifically, the injection amount of supercritical water and the injection amount of oxygen are in a preset ratio; wherein the preset ratio is 1:2, 1:1, or 2:1. Therefore, after the injection of supercritical water is stopped, the injection amount of oxygen is determined based on the injection amount of supercritical water and the preset ratio, and the injection of oxygen is stopped after the injection amount is reached. At the same time, during the oxygen injection process and after the oxygen injection is stopped, the gas collection step is performed, that is, the generated gas is collected from the wellhead of the production well 2.

[0031] Combine Figure 1 As shown, in the embodiment of the present invention, the temperature range of the exothermic oxidation reaction of the coal body in the reaction zone should be higher than 600°C. Therefore, when the downhole temperature monitoring structure monitors that the temperature of the reaction zone is lower than 600°C in the gas collection step, it means that the current reaction zone in the coal seam 3 has basically stopped reacting, and the supercritical water injection step is performed again in the same manner, and then the oxygen injection step is performed again, so as to achieve the advancement of the reaction zone. This alternation is carried out until the reaction zone of the coal seam 3 is gradually advanced from the output section of the injection well 1 through the communication seam network 4 to the input section of the production well 2.

[0032] In addition, combined Figure 1 and Figure 2 As shown, in the embodiment of the present invention, by optimizing the layout of the injection well 1 and the production well 2, it is beneficial to further improve the resource utilization rate of the coal seam 3 and the gas recovery rate.

[0033] Specifically, before the guided fracturing step, the method further includes the following steps: determining an efficient coal production area in the coal seam 3, and then determining the design location of injection well 1 within the efficient coal production area; simulating and analyzing the gas enrichment area of ​​the coal seam 3 during the redox reaction process, and then determining the design location of production well 2 within the gas enrichment area; establishing injection well 1 at the design location of injection well 1, and establishing production well 2 at the design location of production well 2. The efficient coal production area has the characteristics of thick coal seams, good quality, high permeability, and ease of fracturing and transformation. Therefore, locating injection well 1 within the efficient coal production area is beneficial to improving the fracturing effect and the resource utilization rate of the coal seam; and locating production well 2 within the gas enrichment area is beneficial to improving the gas capture efficiency.

[0034] In an embodiment of the present invention, determining the design position of the injection well 1 includes the following steps: comprehensively evaluating the coal seam 3 based on the geological exploration and geophysical exploration results of the coal seam 3 to identify the coal efficient production area; and determining the design position of the injection well 1 based on the pressure distribution, fluid properties and temperature distribution in the coal efficient production area.

[0035] In an embodiment of the present invention, determining the design location of production well 2 includes the following steps: establishing a geological model of coal seam 3; simulating the flow of multiphase fluids in coal seam 3 during redox reactions based on the design location of injection well 1 and the geological model of coal seam 3, thereby analyzing gas-rich regions; and determining the design location of production well 2 within the gas-rich regions. The migration path of generated gas in the underground space is tracked by combining the pore network, permeability distribution, and fluid pressure field data in the geological model to identify gas-rich regions.

[0036] In an embodiment of the present invention, before establishing the injection well 1 and the production well 2, the method further includes: determining the wellbore structure and wellbore material of the injection well 1 and the production well 2 according to the pressure design requirements and temperature design requirements of the injection well 1 and the production well 2.

[0037] The above descriptions are only several embodiments of the present invention. Those skilled in the art may make various changes or modifications to the embodiments of the present invention based on the contents disclosed in the application documents without departing from the spirit and scope of the present invention.

Claims

1. A deep coal supercritical water forward gasification method, characterized in that: The following steps are involved: Guided fracturing: fracturing fluid is injected from an injection well and negative pressure is formed in the production well. Under the guidance of the negative pressure of the production well, the fracturing fluid fractures the coal seam to form a communication fracture network between the output section of the injection well and the input section of the production well. Supercritical water injection: injecting supercritical water into the coal seam from the injection well to heat up a portion of the coal body of the coal seam; Oxygen injection: injecting oxygen into the coal seam from the injection well, causing a portion of the coal seam to undergo an oxidation reaction with the oxygen and thus increase its temperature, and then a portion of the coal seam to undergo an oxidation-reduction reaction with the supercritical water and the oxygen to form a reaction zone of the coal seam and generate gas; Gas collection: the gas enters the production well through the communication seam network and is produced through the production well; Advancing the reaction zone: alternately repeating the supercritical water injection step and the oxygen injection step until the reaction zone of the coal seam is gradually advanced from the output section of the injection well through the communication fracture network to the input section of the production well; Before the guided fracturing step, the method further comprises the following steps: Determining a high-efficiency coal production area in the coal seam, and then determining a design location of the injection well within the high-efficiency coal production area; Simulating and analyzing the gas enrichment area of ​​the coal seam during the redox reaction process, and then determining the design location of the production well within the gas enrichment area; The injection well is established at the designed location of the injection well, and the production well is established at the designed location of the production well.

2. The deep coal supercritical water forward gasification method according to claim 1, characterized in that: The injection well includes an injection vertical shaft extending from top to bottom to the coal seam and at least one injection branch well extending along the coal seam and connected to the injection vertical shaft, and the production well extends from top to bottom to the coal seam; In the guided fracturing step, the fracturing fluid is injected into the injection branch well through the injection vertical well, and then the fracturing fluid gradually fractures the coal seam from the inlet of the injection branch well to the bottom of the injection branch well to form cracks. The cracks gradually extend toward the production well under the guidance of the negative pressure of the production well, thereby forming the communicating fracture network between the injection branch well and the input section of the production well.

3. The deep coal supercritical water forward gasification method according to claim 2, characterized in that: An underground temperature monitoring structure is provided in the output section of the injection well; when the underground temperature monitoring structure monitors that part of the coal body of the coal seam has risen to the oxidation reaction temperature of the coal body during the supercritical water injection step, the injection of the supercritical water is stopped and the oxygen injection step is started.

4. The deep coal supercritical water forward gasification method according to claim 3, characterized in that: The injection amount of the supercritical water and the injection amount of the oxygen are in a preset proportional relationship; Wherein, the preset ratio relationship is 1:2, 1:1 or 2:

1.

5. The deep coal supercritical water forward gasification method according to claim 3, characterized in that: When the downhole temperature monitoring structure monitors that the temperature of the reaction zone is lower than 600° C. during the gas collection step, the supercritical water injection step is performed again.

6. The deep coal supercritical water forward gasification method according to claim 1, characterized in that: The guided fracturing is guided hydraulic fracturing.

7. The deep coal supercritical water forward gasification method according to claim 1, characterized in that: The step of determining the design position of the injection well comprises the following steps: Conduct a comprehensive evaluation of the coal seam based on the geological exploration and geophysical exploration results of the coal seam to identify the high-efficiency coal production area; The design position of the injection well is determined based on the pressure distribution, fluid properties and temperature distribution in the coal efficient production area.

8. The deep coal supercritical water forward gasification method according to claim 1, characterized in that: The step of determining the design position of the production well comprises the following steps: establishing a geological model of the coal seam; According to the designed location of the injection well and the geological model of the coal seam, the flow of multiphase fluid in the coal seam during the redox reaction process is simulated to thereby analyze the gas enrichment area; The design location of the production well is determined in the gas-rich area.

9. The deep coal supercritical water forward gasification method according to claim 1, characterized in that: Before establishing the injection well and the production well, the method further includes: determining the wellbore structure and wellbore material of the injection well and the production well according to the pressure design requirements and temperature design requirements of the injection well and the production well.

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