Supercritical water advancing type gasification method for deep coal

Abstract

The invention discloses a deep coal supercritical water advancing type gasification method, which comprises the following steps that fracturing fluid is injected from an injection well, and negative pressure is formed in a production well, so that under the guidance of the negative pressure of the production well, the fracturing fluid is fractured in a coal seam to form a communication seam network between an output section of the injection well and an input section of the production well; supercritical water is injected into the coal seam from the injection well, so that part of the coal body of the coal seam is heated; oxygen is injected into the coal seam from the injection well, part of the coal body of the coal seam and the oxygen are subjected to an oxidation reaction to be heated, then part of the coal body of the coal seam, the supercritical water and the oxygen are subjected to an oxidation-reduction reaction to form a reaction area of the coal seam, and gas is generated; gas enters the production well through the communication seam net and is extracted through the production well; and the supercritical water and the oxygen are injected alternately and repeatedly until the reaction area of the coal seam is gradually propelled from the output section of the injection well to the input section of the production well through the communication seam net. According to the invention, the gas trapping efficiency, the total produced gas quantity and the resource utilization rate of the coal seam can be improved.

Description

Technical Field

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

[0002] Deep coal underground gasification (UCG) technology is currently an effective means of mining deep coal. Its advantages in safety and environmental protection have been preliminarily verified, and its application prospects are good. Supercritical water underground coal gasification is a new type of coal utilization technology. It uses supercritical water to react with coal underground to generate gas fuel. It has the advantages of high efficiency and environmental protection. Supercritical water (SCW, with the characteristics of temperature reaching 374.3℃ and pressure of 22.05MPa) is an excellent reaction medium with excellent mass transfer, heat transfer and solubility properties. It can efficiently and cleanly gasify various biomass, coal, petroleum coke and organic waste solids and liquids. This technology makes full use of the high heat transfer, mass transfer and solubility of supercritical water, as well as the heat transfer, mass transfer and solubility of coal and supercritical water, O 2 The chemical reaction characteristics under high temperature and high pressure enable efficient and clean mining of deep coal seams. At the same time, by storing CO 2 , and also effectively reduces greenhouse gas emissions, which has significant environmental protection significance.

[0003] However, the in-situ coal supercritical water underground gasification technology is still in the research and development stage and has not yet been applied on a large scale. Due to the relatively complex geological conditions of deep coal and the difficulty in predicting the gasification reaction process, how to stably and efficiently produce combustible gas generated by coal combustion under complex and changeable conditions has become a key bottleneck that needs to be broken through in the deep coal in-situ supercritical water gasification technology. The existing technology uses the injection point retreat (CRIP) gasification method, which is prone to problems such as loss of control of the ignition surface, ground collapse, water gushing in the gasification chamber, and gas emission. 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 invention provides a deep coal supercritical water forward gasification method, 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 is fracturing 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 part of the coal body of the coal seam; oxygen injection: injecting oxygen into the coal seam from the injection well to increase the temperature of a part of the coal body of the coal seam by oxidation reaction with the oxygen, and then a part of the coal body of the coal seam undergoes 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 fracture network and is produced through the production well; 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 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 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 shaft, and then the fracturing fluid is gradually fractured in the coal seam from the entrance of the injection branch well to the bottom of the injection branch well to form cracks, and the cracks gradually extend toward the direction of the production well under the guidance of the negative pressure of the production well, thereby forming the communication 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 is 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 proportional relationship; wherein the preset proportional relationship 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. in 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 an efficient coal production area in the coal seam, and then determining a design position of the injection well within the efficient coal production area; simulating and analyzing a gas enrichment area of ​​the coal seam during an oxidation-reduction reaction, and then determining a 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: comprehensively evaluating the coal seam based on the geological exploration and geophysical exploration results of the coal seam to identify the efficient coal production area; determining the design position of the injection well based on the pressure distribution, fluid properties and temperature distribution of the efficient 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 according to 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 direction of the 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, and then in the process of alternately injecting supercritical water and oxygen to cause the coal seam to undergo redox reaction with the supercritical water and oxygen, on the one hand, the gas generated by the reaction can be produced from the production well by using the communication fractures, thereby improving the gas capture efficiency and the total gas production volume, and reducing the unit cost of produced gas, and on the other hand, the reaction zone of the coal seam is gradually advanced relatively stably by using the communication fractures to guide the flow of supercritical water and oxygen, thereby 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, and 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 It is a schematic diagram of the distribution of injection wells and production wells in the present invention.

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

[0021] In the figure:

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

[0023] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0024] Combination 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 forming a negative pressure in a production well 2, so that under the guidance of the negative pressure of the production well 2, the fracturing fluid is fractured in a coal seam 3 to form a communication seam 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 increase the temperature of a portion of the coal body of the coal seam 3; oxygen injection: injecting oxygen into the coal seam 3 from the injection well 1, so that a portion of the coal body of the coal seam 3 undergoes an oxidation reaction with oxygen and increases the temperature, and then a portion of the coal body of the coal seam 3 undergoes an oxidation-reduction reaction with supercritical water and oxygen to form a reaction zone of the coal seam 3 and generate gas; gas collection: the gas enters the production well 2 through the communication seam 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 of the coal seam 3 is gradually advanced from the output section of the injection well 1 to the input section of the production well 2 through the communication seam network 4.

[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 direction of the fracturing fluid in the coal seam 3, thereby improving the fracturing effect, and enabling the communication fracture network 4 formed by 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, and then in the process of alternately injecting supercritical water and oxygen to cause the coal seam 3 to undergo redox reaction with the supercritical water and oxygen, on the one hand, the gas generated by the reaction can be produced from the production well 2 by using the communication fractures, thereby improving the gas capture efficiency and the total gas production volume, and reducing the unit cost of produced gas, and on the other hand, the reaction zone of the coal seam 3 is gradually advanced relatively stably by using the communication fractures to guide the flow of supercritical water and oxygen, 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, and 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, and the production well 2 extends from top to bottom to the coal seam 3; in the guided fracturing step, the fracturing fluid is injected into the injection branch well 12 through the injection shaft 11, and then the coal seam 3 is fractured to form a communication fracture network 4 between the injection branch well 12 and the input section of the production well 2. Among them, the injection branch well 12 is a horizontal well, and the injection shaft 11 and the production well 2 are both vertical wells; the number of the injection branch wells 12 is preferably set to multiple, such as two in the present embodiment, and can also be set to three, four or more as needed, and of course, only one can be set.

[0027] The CRIP process, which is widely used at present, uses a continuous pipe 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). 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 get stuck during the retreat process. The gasification process needs to be maintained in a strictly water-free state, and a combustion oxidation-reduction reaction occurs. It is 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, so that the reaction is terminated. The present invention uses 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 of the injection branch well 12 to the end, and the gasification product is mined from underground. The strong oxidizing property of supercritical water is utilized to perform forced oxidation heating on deep coal without ignition, thereby being able to maintain a water-rich state and undergo high-temperature oxidation-hydrogenation interval reactions. Moreover, the reaction can be performed while filling is performed to ensure that the roof does not collapse, so that the heat transfer, mass transfer, and physical and chemical reactions in the reaction zone proceed in an orderly manner. Therefore, the present invention can effectively avoid the problems of the above-mentioned CRIP process.

[0028] according to Figure 1 It can be seen from the fracturing direction A shown in the figure that the fracturing fluid is gradually fractured from the inlet of the injection branch well 12 (that is, the bottom of the injection shaft 11) to the bottom of the injection branch well 12 to form cracks, and the cracks gradually extend toward the production well 2 under the guidance of the negative pressure of the production well 2, thereby forming a communication fracture network 4. In the embodiment of the present invention, the guided fracturing is guided hydraulic fracturing, wherein the fracturing fluid can be slick water, guar fracturing fluid or other hydraulic fracturing fluid in the prior art, and the specific composition is the same as that in the prior art, which will not be described in detail here. By connecting the wellhead of the production well 2 with 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 extending from the production well 2 to 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 extending from the production well 2 to 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 thus 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 the embodiment of the present invention, the output section of the injection well 1 is provided with an underground temperature monitoring structure; when the underground temperature monitoring structure monitors that part of the coal body of the coal seam 3 has been heated to the oxidation reaction temperature of the coal body during the supercritical water injection step, the injection of supercritical water is stopped and the oxygen injection step is started. After the supercritical water is injected, the gasification reaction of the supercritical water is used to heat part of the coal body to the oxidation reaction temperature of the coal body. Among them, the oxidation reaction temperature of the coal body is generally 300°C to 400°C, so that after the oxygen is injected, the coal body can undergo an oxidation reaction with oxygen and further heat up to the redox reaction temperature of the coal body. The redox reaction temperature of the coal body is generally above 900°C, thereby prompting the coal body to undergo an oxidation-reduction reaction with supercritical water and oxygen in the in-situ state to produce H 2 , CO 2 Among them, the downhole temperature monitoring structure is preferably arranged at the entrance of the injection branch well 12 (that is, the bottom of the injection shaft 11). The specific structure and working 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 proportional relationship, wherein the preset proportional relationship is 1:2, 1:1 or 2:1. Therefore, when the injection of supercritical water stops, the injection amount of oxygen is determined according to the injection amount of supercritical water and the preset proportional relationship, so that the injection of oxygen is stopped after the injection amount is reached, and at the same time, during the injection process of oxygen and after the injection of oxygen is stopped, the gas collection step is performed, that is, the generated gas is collected from the wellhead of the production well 2.

[0031] Combination 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 realize the advancement of the reaction zone, and 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 to the input section of the production well 2 through the communication seam network 4.

[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 the coal efficient utilization area in the coal seam 3, and then determining the design position of the injection well 1 in the coal efficient utilization area; simulating and analyzing the gas enrichment area of ​​the coal seam 3 during the redox reaction process, and then determining the design position of the production well 2 in the gas enrichment area; establishing the injection well 1 at the design position of the injection well 1, and establishing the production well 2 at the design position of the production well 2. Among them, the coal efficient utilization area has the characteristics of large coal seam thickness, good quality, high permeability and easy fracturing transformation. Therefore, by setting the injection well 1 in the coal efficient utilization area, it is beneficial to improve the fracturing effect and the resource utilization rate of the coal seam; by setting the production well 2 in the gas enrichment area, it is beneficial to improve 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: conducting a comprehensive evaluation of the coal seam 3 based on the geological survey and geophysical exploration results of the coal seam 3 to identify the efficient coal production area; determining the design position of the injection well 1 based on the pressure distribution, fluid properties and temperature distribution in the efficient coal production area.

[0035] In the embodiment of the present invention, determining the design position of the production well 2 includes the following steps: establishing a geological model of the coal seam 3; simulating the flow of multiphase fluid in the coal seam 3 during the redox reaction according to the design position of the injection well 1 and the geological model of the coal seam 3, thereby analyzing the gas enrichment area; determining the design position of the production well 2 in the gas enrichment area. In particular, the migration path of the generated gas in the underground space is tracked in combination with the pore network, permeability distribution and fluid pressure field data in the geological model, thereby analyzing the gas enrichment area.

[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 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 method for deep coal supercritical water forward gasification, characterized in that: The following steps are involved: Guided fracturing: injecting fracturing fluid from the injection well and causing negative pressure in the production well, so that under the guidance of the negative pressure of the production well, the fracturing fluid fracturs 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 of the coal seam; Oxygen injection: injecting oxygen into the coal seam from the injection well, so that part of the coal body of the coal seam undergoes oxidation reaction with the oxygen to increase the temperature, and then part of the coal body of the coal seam undergoes 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 communicating seam network to the input section 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 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 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, and the cracks gradually extend toward the production well under the guidance of the negative pressure of the production well, thereby forming the communication 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 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.

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. in 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: Prior to the guided fracturing step, the method further comprises the following steps: Determine a high-efficiency coal production area in the coal seam, and then determine a design position 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 in the gas enrichment area; The injection well is established at the designed position of the injection well, and the production well is established at the designed position of the production well.

8. The deep coal supercritical water forward gasification method according to claim 7, characterized in that: The step of determining the design position of the injection well comprises the following steps: Based on the geological survey and geophysical exploration results of the coal seam, a comprehensive evaluation is conducted on the coal seam to identify the efficient 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 utilization area.

9. The deep coal supercritical water forward gasification method according to claim 7, characterized in that: Determining the design position of the production well comprises the following steps: establishing a geological model of the coal seam; According to the designed position 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 is simulated, so as to analyze the gas enrichment area; The design location of the production well is determined in the gas-enriched area.

10. The deep coal supercritical water forward gasification method according to claim 7, 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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