Crushed soft coal seam roof pressing-injection synergistic anti-reflection extraction and carbon sequestration method

Abstract

The invention discloses a broken soft coal seam roof pressure-injection synergetic permeability-increasing extraction and carbon sequestration method, which is characterized in that horizontal long drill holes are arranged in a broken soft coal seam roof, comb-shaped drill hole branches for hydraulic fracturing are arranged at intervals, and the branches face the direction of a coal-rock interface. A 1 # pressure-injection pipe extends into the first drill hole branch, and hydraulic fracturing and gas extraction are conducted in sequence; after the gas extraction concentration in the first drill hole branch is reduced and gradually tends to be stable, the second pressure-injection pipe extends into the second drill hole branch to conduct hydraulic fracturing and gas extraction as well; cO2 is injected into the first drill hole branch through a 1 # pressure-injection pipe to displace coal seam gas, then plugging liquid is used for conducting interface modification on a coal matrix, and the carbon sequestration target that gas is sealed through liquid is achieved; and by parity of reasoning, fracturing, extraction, gas injection and plugging operations are carried out by adopting an alternate method. Based on the method, the gas extraction effect is remarkably improved, and the stability and reliability of carbon sequestration are improved.

Description

Technical Field

[0001] This invention belongs to the field of comprehensive development and utilization of coal mine gas, specifically involving a method for combined permeability enhancement and extraction and carbon sequestration in soft coal seams by pressure-injection synergistic method. Background Technology

[0002] Coalbed methane is a high-quality unconventional natural gas that can serve as an important supplement to my country's energy structure, alleviating the predicament of over-reliance on natural gas imports. However, methane is also the most significant greenhouse gas besides carbon dioxide, with a greenhouse effect dozens of times greater. Controlling coalbed methane emissions is a crucial guideline and pathway for achieving green and low-carbon development in the industry. Furthermore, coalbed methane is a major source of mine disasters, causing coal and gas outbursts and gas explosions, which seriously threaten safe mine production. Therefore, strengthening the efficient extraction of coalbed methane is of profound significance for achieving clean resource utilization and safe mine production.

[0003] As my country's shallow coal seam resources become increasingly depleted and the country gradually enters the stage of deep mining, the low permeability and fragmented nature of these coal seams result in long gas extraction times and poor efficiency. In particular, for high-gas, fragmented, thin coal seams, coal mining is difficult, costly, and carries significant safety risks, leading to a dilemma of "no profit to mine, but a pity to abandon."

[0004] To make fuller use of this resource, existing scholars have proposed using it as an excellent medium for carbon sequestration. By displacing gas in the coal seam with CO2, both the resource utilization of gas and the carbon sequestration are achieved. However, the low permeability and fragmented nature of the coal seam result in low CO2 migration velocity and poor injection efficiency. Even with permeability enhancement measures, problems such as borehole collapse and limited modification range still restrict the rapid and efficient sequestration of CO2. Therefore, there is an urgent need to propose a technical solution for efficient gas extraction and stable carbon sequestration in fragmented and soft coal seams. Summary of the Invention

[0005] The purpose of this invention is to provide a method for combined permeability enhancement and extraction and carbon sequestration in the roof of soft and fractured coal seams by pressure-injection, so as to solve or alleviate the problems existing in the above-mentioned prior art methods.

[0006] To address the above problems, the present invention provides the following technical solution: This solution provides a method for combined permeability enhancement and extraction, as well as carbon sequestration, in the roof of fractured and soft coal seams, comprising the following three steps: S1 performs hydraulic fracturing of the roof to increase the permeability of the soft coal seam; Injecting CO2 into the S2 sealing section to displace coal seam gas and improve gas extraction rate; S3 injection modification and grouting sealing achieve carbon sequestration.

[0007] The hydraulic fracturing of the roof in step S1 includes the following steps: S11 involves drilling long horizontal boreholes within the coal seam roof and arranging comb-shaped borehole branches at intervals. S12 Insert the No. 1 injection pipe into the first borehole branch and seal the borehole using the sealing device; S13 performs hydraulic fracturing of the roof within the first borehole branch. The hydraulic fractures cross the coal-rock interface and enter the coal body, thereby increasing the permeability of the soft coal seam. The CO2 injection into the sealing section in step S2 to displace coal seam gas includes the following steps: S21 monitors the changes in injection pressure in the first borehole branch in real time. After the fracturing operation is completed, the gas extraction operation of the first borehole branch is carried out. The gas in the coal body around the fracture moves towards the first borehole branch. S22 After the gas extraction concentration in the first borehole branch decreases and gradually stabilizes, insert the No. 2 injection pipe into the second borehole branch and seal the borehole using the sealing device. S23 The roof hydraulic fracturing is carried out in the second borehole branch. After the fracturing operation is completed, the gas drainage operation of the second borehole branch is carried out. The gas in the coal body around the fracture moves towards the second borehole branch. S24 injects CO2 into the first borehole branch through the No. 1 injection pipe to displace the coal seam gas. The CO2 moves from the borehole branch to the coal body around the fracture, continuously replacing and displacing the gas on the surface of the coal body pore-fracture, thereby improving the gas extraction rate.

[0008] The liquid injection modification and grouting sealing in step S3 include the following steps: While S31 uses the No. 2 injection pipe to extract gas from the second borehole branch, it observes the change in CO2 concentration in the extraction pipeline. When the CO2 concentration increases and tends to stabilize, it stops injecting CO2 into the first borehole branch. S32 uses the No. 1 injection pipe to inject a sealing fluid containing a hydrophobic surfactant into the first borehole branch. The wettability of the coal hole-fracture surface changes from hydrophilic to weakly hydrophilic. The high concentration of CO2 is sealed in the coal hole-fracture due to the water-locking effect caused by the capillary. S33 adopts a backward fracturing method, extending the No. 1 injection pipe into the third borehole branch adjacent to the second borehole branch, and sequentially carrying out hydraulic fracturing and gas extraction operations; among them, when gas extraction is carried out in the third borehole branch, CO2 is injected into the second borehole branch through the No. 2 injection pipe to displace coal seam gas. S34 When the CO2 concentration in the extraction pipeline in the third borehole branch increases and tends to stabilize, stop injecting CO2 into the second borehole branch, and use the No. 2 pressure-injection pipe to inject a sealing liquid containing hydrophobic surfactant into the second borehole branch to achieve the purpose of "sealing gas with liquid". S35 follows the same pattern, employing alternating fracturing-extraction-gas injection-sealing operations at different locations within the horizontal long borehole. After gas extraction and CO2 sealing have been carried out in all areas, slurry is injected into the entire long borehole for sealing.

[0009] The horizontal long boreholes are arranged in the roof of the soft coal seam to avoid borehole collapse. Comb-shaped borehole branches for hydraulic fracturing are arranged at intervals, with the branches facing the coal-rock interface. Two sets of hydraulic injection pipes are arranged in the borehole, named No. 1 hydraulic injection pipe and No. 2 hydraulic injection pipe respectively. The alternating fracturing, extraction, gas injection and plugging operations can significantly improve construction efficiency.

[0010] Preferably, in step S13, the fracturing fluid used in the hydraulic fracturing of the roof contains a hydrophilic water-locking agent. After the fracturing fluid enters the coal body, the hydrophilic, low surface energy surfactant is adsorbed on the surface of the pores and fractures, which reduces the adsorption capacity of the narrow pore surface and avoids water adsorption, accumulation and blockage of gas / CO2 transport channels in the pores, thereby improving the gas extraction effect.

[0011] Preferably, in step S3, the sealing liquid used in the injection modification process contains a hydrophobic surfactant. The surfactant adheres to the surface of the coal pore-fracture through physical adsorption, changing its surface wettability from hydrophilic to weakly hydrophilic. Under the permeation effect generated by capillary force, water accumulates and fills the pore throat with a smaller pore size, blocking the channel for CO2 migration and forming a water-locking effect. This allows high concentrations of CO2 to be sealed in the coal pore-fracture, achieving the purpose of "sealing gas with liquid".

[0012] Preferably, in step S3, the grouting and sealing is achieved by injecting grout into the entire long borehole, where the grout bonds and adheres to the surrounding wall surface to form a stable and complete cover, thereby effectively preventing CO2 leakage and dispersion and achieving the purpose of carbon sequestration.

[0013] Due to the adoption of the above technical solution, the beneficial effects of this invention patent are: This invention employs an integrated pressure-injection method to enhance permeability and promote gas extraction, while simultaneously achieving effective CO2 sequestration, thus promoting the low-carbon and green development of mines.

[0014] This invention allows for the sequential execution of fracturing, extraction, gas injection, and plugging operations within a single borehole, reducing the workload of re-drilling and sealing, increasing the number of times a borehole can be used, lowering the average cost of a borehole, and achieving the effect of multi-purpose use of a single borehole.

[0015] This invention adds a water-locking agent during the hydraulic fracturing process to promote gas migration, which helps improve gas extraction and CO2 displacement efficiency. During the injection and sealing process, a sealing fluid containing a hydrophobic surfactant is added to utilize the capillary action of the coal-rock pore-fracture structure itself to seal CO2 migration channels, achieving the goal of "sealing gas with liquid" for carbon fixation. Furthermore, the use of a long borehole integral grouting sealing method forms a complete cap, reducing the risk of CO2 leakage and escape caused by discontinuous sealing in stages, and increasing the stability and reliability of carbon sequestration. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process of the present invention; Figure 2 This is a schematic diagram of the first borehole branch gas extraction structure of the present invention; Figure 3 This is a schematic diagram of the second borehole branch gas extraction structure of the present invention; Figure 4 This is a schematic diagram of the third borehole branch gas extraction structure of the present invention; Figure 5 This is a schematic diagram of the long borehole grouting and sealing structure of the present invention; Figure 6 This is a schematic diagram of the gas transport state under the water-lock elimination state of the present invention; Figure 7 This is a schematic diagram of the CO2 blocking structure under water-lock conditions according to the present invention; In the diagram: 1. Roof; 2. Horizontal long borehole; 3. Pressure-injection pipe; 4. Second borehole branch; 5. First borehole branch; 6. Sealing section; 7. Hydraulic fracture; 8. Coal seam; 9. Floor; 10. Slurry; 11. Coal matrix; 12. Water-locking effect caused by capillary action. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. Here, the illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0018] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device, component or structure referred to must have a specific orientation, be constructed or operated in a specific orientation, and should not be construed as a limitation of this invention.

[0019] The specific implementation method of the present invention will be further described below with reference to the accompanying drawings.

[0020] As shown in the figure, this scheme provides a method for combined permeability enhancement and extraction and carbon sequestration in the roof of fractured and soft coal seams, which includes the following three steps: S1 performs hydraulic fracturing of the roof to increase the permeability of the soft coal seam. S2 sealing section 6 internal CO2 injection to displace 8 coal seam gas, improving gas extraction rate; S3 injection modification and grouting sealing achieve carbon sequestration.

[0021] The hydraulic fracturing of the top plate 1 in step S1 includes the following steps: S11 drills a horizontal long borehole 2 in the roof of the coal seam 1, and arranges comb-shaped borehole branches at intervals; S12 Insert the No. 1 injection pipe into the first borehole branch 5 and seal the borehole using the sealing device; S13 performs hydraulic fracturing of the roof 1 within the first borehole branch 5, and the hydraulic fractures 7 cross the coal-rock interface and enter the coal body, thereby increasing the permeability of the soft coal seam 8. The CO2 injection into the sealing section 6 in step S2 to displace the gas in the coal seam 8 includes the following steps: S21 monitors the changes in injection pressure in the first borehole branch 5 in real time. After the fracturing operation is completed, the gas extraction operation of the first borehole branch 5 is carried out. The gas in the coal body around the fracture moves towards the first borehole branch 5. S22 After the gas extraction concentration in the first borehole branch 5 decreases and gradually stabilizes, insert the No. 2 injection pipe into the second borehole branch 4 and seal the borehole using the sealing device. S23 Hydraulic fracturing of the roof 1 is carried out in the second borehole branch 4. After the fracturing operation is completed, gas extraction operation is carried out in the second borehole branch 4. The gas in the coal body around the fracture moves towards the second borehole branch 4. S24 injects CO2 into the first borehole branch 5 through the No. 1 injection pipe to displace the gas in the coal seam 8. The CO2 is transported from the borehole branch to the coal body around the fracture, continuously replacing and displacing the gas on the surface of the coal body pore-fracture, thereby improving the gas extraction rate.

[0022] The liquid injection modification and grouting sealing in step S3 include the following steps: While S31 uses the No. 2 injection pipe to extract gas from the second borehole branch 4, it observes the change in CO2 concentration in the extraction pipeline. When the CO2 concentration increases and tends to stabilize, it stops injecting CO2 into the first borehole branch 5. S32 Using the No. 1 injection pipe, a sealing fluid containing a hydrophobic surfactant is injected into the first borehole branch 5. The wettability of the coal hole-fracture surface changes from hydrophilic to weakly hydrophilic. The high concentration of CO2 is sealed in the coal hole-fracture due to the water-locking effect 12 caused by the capillary. S33 adopts a backward fracturing method, extending the No. 1 injection pipe into the third borehole branch adjacent to the second borehole branch 4, and sequentially carrying out hydraulic fracturing and gas extraction operations; among them, when gas extraction is carried out in the third borehole branch, CO2 is injected into the second borehole branch 4 through the No. 2 injection pipe to displace coal seam gas. S34 When the CO2 concentration in the extraction pipeline in the third borehole branch increases and tends to stabilize, stop injecting CO2 into the second borehole branch 4, and use the No. 2 pressure-injection pipe to inject a sealing liquid containing a hydrophobic surfactant into the second borehole branch 4 to achieve the purpose of "sealing gas with liquid". S35 and so on, using alternating fracturing-extraction-gas injection-sealing operations at different locations within the horizontal long borehole 2. After gas extraction and CO2 sealing have been carried out in all areas, slurry is injected into the entire long borehole for sealing.

[0023] The horizontal long borehole 2 is arranged in the roof of the soft coal seam 1 to avoid borehole collapse. At regular intervals, comb-shaped borehole branches for hydraulic fracturing are arranged, with the branches facing the coal-rock interface. Two sets of hydraulic injection pipes 3 are arranged in the borehole, named No. 1 hydraulic injection pipe and No. 2 hydraulic injection pipe respectively. The alternating fracturing, extraction, gas injection and plugging operations can significantly improve construction efficiency.

[0024] Preferably, in step S13, the fracturing fluid used in the hydraulic fracturing of the roof 1 contains a hydrophilic water-locking agent. After the fracturing fluid enters the coal body, the hydrophilic, low surface energy surfactant is adsorbed on the surface of the pores and fractures, reducing the adsorption capacity of the narrow pore surface and preventing water from adsorbing, accumulating and blocking the gas / CO2 migration channels in the pores, thereby improving the gas extraction effect.

[0025] Preferably, in step S3, the sealing liquid used in the injection modification process contains a hydrophobic surfactant. The surfactant adheres to the surface of the coal pore-fracture through physical adsorption, changing its surface wettability from hydrophilic to weakly hydrophilic. Under the permeation effect generated by capillary force, water accumulates and fills the pore throat with a smaller pore size, blocking the channel for CO2 migration and forming a water-locking effect. This allows high concentrations of CO2 to be sealed in the coal pore-fracture, achieving the purpose of "sealing gas with liquid".

[0026] Preferably, in step S3, the grouting and sealing is achieved by injecting grout 10 into the entire long borehole, whereby the grout 10 bonds and adheres to the surrounding wall surface of the borehole to form a stable and complete cover, thereby effectively preventing CO2 leakage and dispersion and achieving the purpose of carbon sequestration.

[0027] Therefore, those skilled in the art should recognize that although exemplary embodiments of the present invention have been shown and described in detail herein, many other variations or modifications conforming to the principles of the present invention can still be directly determined or derived from the disclosure of the present invention without departing from the spirit and scope of the invention. Thus, the scope of the present invention should be understood and recognized as covering all such other variations or modifications.

Claims

1. A method for combined permeability enhancement and carbon sequestration extraction and carbon fixation in the roof of a soft, fractured coal seam, characterized in that: The method comprises the following three steps: S1: developing roof hydraulic fracturing to increase the permeability of soft and broken coal seam; S2: injecting CO2 into the sealing section to displace the coal seam gas and improve the gas extraction rate; S3: injecting liquid modification and grouting sealing to realize carbon sequestration.

2. The press-injection synergistic permeability extraction and carbon sequestration method for soft coal seam roof according to claim 1, characterized in that, The step S1 of developing roof hydraulic fracturing comprises the following steps: S11: drilling a horizontal long borehole in the coal seam roof, and arranging comb-shaped borehole branches at intervals; S12: extending the 1# pressure-injection pipe into the first borehole branch, and sealing the borehole by using a sealing device; S13: developing roof hydraulic fracturing in the first borehole branch, and the hydraulic fracture acrosses the coal-rock interface into the coal body, thereby achieving the purpose of increasing the permeability of soft and broken coal seam.

3. The press-injection synergistic permeability extraction and carbon sequestration method for soft coal seam roof according to claim 1, characterized in that, The step S2 of injecting CO2 into the sealing section to displace the coal seam gas comprises the following steps: S21: observing the change of the injection pressure in the first borehole branch in real time, after the fracturing operation is completed, developing the gas extraction operation of the first borehole branch, and the gas in the coal body around the fracture migrates to the first borehole branch; S22: after the gas extraction concentration in the first borehole branch is reduced and gradually tends to be stable, extending the 2# pressure-injection pipe into the second borehole branch, and sealing the borehole by using a sealing device; S23: developing roof hydraulic fracturing in the second borehole branch, after the fracturing operation is completed, developing the gas extraction operation of the second borehole branch, and the gas in the coal body around the fracture migrates to the second borehole branch; S24: injecting CO2 into the first borehole branch through the 1# pressure-injection pipe to displace the coal seam gas, and the CO2 migrates from the borehole branch to the coal body around the fracture, and continuously replaces and displaces the gas on the surface of the coal pore-fracture.

4. The press-injection synergistic permeability extraction and carbon sequestration method for soft coal seam roof according to claim 1, characterized in that, The step S3 of injecting liquid modification and grouting sealing comprises the following steps: S31: while extracting the gas in the second borehole branch by using the 2# pressure-injection pipe, observing the change of the CO2 concentration in the extraction pipeline, when the CO2 concentration rises and tends to be stable, stopping injecting CO2 into the first borehole branch; S32: injecting the plugging liquid containing hydrophobic surfactant into the first borehole branch by using the 1# pressure-injection pipe, the wettability of the coal pore-fracture surface is changed from hydrophilic to weak hydrophilic, and the water locking effect of high-concentration CO2 caused by capillary is plugged in the coal pore-fracture; S33: adopting the way of retreating fracturing, extending the 1# pressure-injection pipe into the third borehole branch adjacent to the second borehole branch, and sequentially developing the hydraulic fracturing and gas extraction operation; wherein, when developing the gas extraction in the third borehole branch, injecting CO2 into the second borehole branch through the 2# pressure-injection pipe to displace the coal seam gas; S34: when the CO2 concentration in the extraction pipeline in the third borehole branch rises and tends to be stable, stopping injecting CO2 into the second borehole branch, and injecting the plugging liquid containing hydrophobic surfactant into the second borehole branch by using the 2# pressure-injection pipe; S35: in this way, the fracturing-extraction-gas injection-plugging operation is carried out at different positions in the horizontal long borehole in an alternating manner, when all the regions develop the gas extraction and CO2 sequestration, the slurry is injected into the whole long borehole to seal the borehole.

5. The press-injection synergistic permeability extraction and carbon sequestration method for soft coal seam roof according to claim 2, characterized in that: In step S13, the fracturing fluid used in the roof hydraulic fracturing process contains a hydrophilic water lock breaker. After the fracturing fluid enters the coal body, the hydrophilic and low-surface-energy surfactant is adsorbed on the pore-fracture surface, reducing the adsorption capacity of the narrow pore surface and avoiding the adsorption, aggregation and blockage of the gas / CO2 migration channel in the pore.

6. The press-injection synergistic permeability extraction and carbon sequestration method for soft coal seam roof according to claim 1, characterized in that: In step S3, the plugging fluid used in the liquid injection modification process contains a hydrophobic surfactant. The surfactant is attached to the coal pore-fracture surface by physical adsorption, so that the surface wettability is changed from hydrophilic to weakly hydrophilic. Under the capillary force, water is aggregated and fills the pore throat with a smaller pore diameter, plugging the CO2 migration channel and forming a water lock effect. In turn, high-concentration CO2 is blocked in the coal pore-fracture.

7. The method according to claim 1, wherein the method is characterized in that: In step S3, the grouting plugging is achieved by injecting grout into the entire long borehole. The grout is cemented and bonded with the wall surface around the borehole, forming a stable and complete cover. In turn, CO2 leakage and dispersion are effectively prevented, and the purpose of carbon sequestration is achieved.

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