Method for fracturing low-permeability coal seam through linkage of CO2 and high-pressure water

Abstract

The invention discloses a method for fracturing a low-permeability coal seam by linkage of CO2 and high-pressure water, which comprises the following steps of: performing hydraulic fracturing to form a part of fractures, forming a weak acid environment by utilizing the characteristic that a CO2 medium is easily dissolved in high-pressure water in a high-pressure environment, and promoting chemical reaction between acid water and mineral components on the surfaces of the fractures under the driving of concentration gradient and high-pressure injection, the fractures are pressed by high-pressure water at the same time, so that cooperative work of high-pressure fracturing and CO2 acidification anti-reflection is achieved, and a complex fracture network is formed in the coal seam; after permeability increasing, the pressure is relieved, the normal pressure environment is recovered, at the moment, CO2 media dissolved in the high-pressure water in the layer penetrating drill holes are partially recovered to be in the gas state, CO2 gas and gas can form competitive adsorption in the coal seam, a large amount of gas is displaced and desorbed from the interior of the coal seam, in the whole process, the permeability increasing effect on the coal seam can be effectively achieved, and the coal seam permeability increasing effect is achieved. And no extra damage is caused to the coal seam, finally, on the premise that more energy is not consumed, effective transformation of the pore structure volume of the coal body is achieved to the maximum extent, and the gas extraction efficiency is improved.

Description

Technical Field

[0001] The invention relates to a method for fracturing a low-permeability coal seam by combining CO2 and high-pressure water, and belongs to the technical field of gas extraction in high-gas and low-permeability coal seams. Background Art

[0002] With the gradual depletion of shallow coal resources, coal resource development is advancing into the deep. The main characteristics of deep coal seams are high ground stress, high gas pressure, and low porosity and permeability. Before coal resources are mined, it is usually necessary to pre-extract the gas in the coal seam in advance. At present, the gas pre-extraction methods include intensive drilling extraction, directional long drilling extraction, etc., but due to the synergistic effect of ground stress and coal seam structure, the desorption of coal body gas near the borehole leads to a decrease in pore pressure, the increased effective stress causes the coal body to deform, and the pore fracture structure is compressed, resulting in poor porosity and permeability, making it impossible to achieve continuous and efficient extraction of coal seam gas.

[0003] In order to improve the efficiency of coal seam gas extraction, the main technical means used is to transform the volume of the coal seam structure, that is, to destroy the coal structure with the help of external energy, such as hydraulic fracturing technology, deep hole blasting technology, acid fracturing, etc. Hydraulic fracturing technology mainly uses high-pressure water as a working fluid to destroy the overall structure of the coal body, and the derived pore and fracture structure may be connected with the original pore and fracture structure of the coal body. Affected by ground stress, hydraulic fractures mainly expand in a direction perpendicular to the minimum principal stress, and the fracture morphology is distributed in a single manner. The existing stress shadows may produce corresponding stress concentration areas and gas extraction blank zones. At the same time, the reduction of water pressure in the fracturing borehole may cause the hydraulic fractures to close, and the residual water in the fractures will inhibit gas desorption, which will greatly affect the subsequent gas extraction efficiency.

[0004] The acid fracturing technology mainly involves injecting selected acidic fluids into the coal seam, and utilizing the physical and chemical reaction and dissolution process between the acidic fluids and the coal rock mass to expand the distribution of the coal body pore and fracture structure, and increase the advantageous migration channels of gas. However, in most cases, the selected acidic fluids have negative effects on the coal seam reservoir, and some even affect the working face environment. Therefore, in view of the technical limitations of hydraulic fracturing, such as limited cracks produced by hydraulic fracturing and strong acid fracturing damaging the reservoir, how to provide a new method for fracturing low-permeability coal seams, which can not only effectively achieve the effect of increasing the permeability of the coal seam, but also will not cause damage to the coal seam working face environment, and ultimately maximize the effective transformation of the coal body pore structure volume without increasing more energy consumption, and improve the gas extraction efficiency, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for fracturing low-permeability coal seams by combining CO2 and high-pressure water. The method adopts the coordinated approach of hydraulic fracturing, CO2 acidification dissolution and CO2 competitive adsorption to displace gas. It can not only effectively achieve the effect of increasing the permeability of the coal seam, but also will not cause damage to the environment of the coal seam working face. Ultimately, without consuming too much energy, it can maximize the effective transformation of the pore structure volume of the coal body and improve the gas extraction efficiency.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for fracturing low-permeability coal seams by combining CO2 and high-pressure water, the specific steps of which are:

[0007] Step 1: Construct multiple through-layer boreholes in the bottom rock tunnel toward the target coal seam to within 0.5m of the coal-rock boundary.

[0008] Step 2: Select a through-layer borehole from step 1, insert one end of the fracturing pipe into the through-layer borehole, and seal the through-layer borehole with a high-pressure resistant sealer. A high-pressure baffle is provided in the fracturing pipe to separate the fracturing pipe into an inner section and an outer section. The inner section is connected to the through-layer borehole. Then, one end of the high-pressure water branch pipe and one end of the high-pressure gas injection branch pipe are extended into the fracturing pipe and pass through the high-pressure baffle to enter the inner section.

[0009] Step three, connect the high-pressure water injection pump to the other end of the high-pressure water branch, and at the same time connect the CO2 gas injection pump to the other end of the high-pressure gas injection branch. First start the high-pressure water injection pump to inject high-pressure water into the perforated borehole through the high-pressure water branch until the injection water pressure in the high-pressure water branch reaches the set fracturing pressure P1 value and maintain the current injection water pressure value; observe the injection water pressure value of the high-pressure water branch. When the injection water pressure value is lower than the current pressure value, it means that hydraulic fracturing has caused cracks in the surrounding coal seams, and part of the high-pressure water has entered the cracks, resulting in a decrease in the injection water pressure value. At this time, continuous water injection is performed through the high-pressure water injection pump to ensure that the injection water pressure value reaches the P1 value.

[0010] Step 4: Start the CO2 gas injection pump and inject a large amount of CO2 medium into the perforated borehole through the high-pressure gas injection pipe at the set pressure P2 value. The gas injection time lasts for 30 minutes and then stops, and the injection water pressure is maintained at the P1 value for 24 hours. At this time, the injected CO2 medium will dissolve in the high-pressure water in the perforated borehole, and the CO2 dissolved concentration in the high-pressure water around the other end of the high-pressure gas injection pipe is the highest. Driven by the concentration gradient, the dissolved CO2 ions diffuse rapidly to the low-concentration area until they reach the cracks generated by hydraulic fracturing, and react chemically with the mineral components on the surface of the cracks to acidify their surface. At the same time, the cracks are pressurized by high-pressure water, thereby achieving the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement.

[0011] Step 5. When water flows in any other coal seam through-layer borehole, the gas-water mixed pressure in the current through-layer borehole is unloaded. At this time, the CO2 dissolved in the high-pressure water returns to gaseous state and diffuses into the coal seam through the cracks created by fracturing. It is adsorbed by the coal seam and drives the coal seam to quickly desorb gas. At this time, the through-layer borehole is connected to the gas extraction pipeline network for gas extraction.

[0012] Step 6: Repeat steps 2 to 5 for the remaining through-layer drill holes in step 1, so as to achieve the overall permeability enhancement and gas extraction work for the coal seam.

[0013] Furthermore, the diameter of the through-layer drill hole is 90 mm, the spacing between adjacent through-layer drill holes is 3 m to 3.5 m, and the length of the drill hole is 50 m to 60 m.

[0014] Furthermore, the materials used for the fracturing pipe, high-pressure water branch pipe and high-pressure gas injection branch pipe are all stainless steel, and the maximum tolerable tensile stress is not less than 20Mpa; a plurality of spherical cavities are arranged inside the fracturing pipe, and a plurality of through holes are arranged on the spherical cavities for mixing CO2 gas with high-pressure water and discharging them into the through-layer drilled hole.

[0015] Furthermore, in the step 2, the sealing length is 10 m from the opening of the through-layer drilling hole, and one or more high-pressure sealers are used for sealing. The maximum expansion pressure that a single high-pressure sealer can withstand is 30 MPa.

[0016] Furthermore, the P1 value is determined according to the ground stress and coal seam strength of the target coal seam, and the P2 value is 0.5 to 0.8 times the P1 value.

[0017] Furthermore, the high pressure water injection flow rate is controlled at 5-6 m 3 / min. Maintaining this flow rate can achieve a stable injection and hydraulic fracturing process.

[0018] Furthermore, the CO2 medium is in a gaseous state or a supercritical state; if it is in a gaseous state, the injection pressure is controlled at 4 to 7 MPa, and if it is in a supercritical state, the injection pressure is controlled at 8 to 10 MPa.

[0019] Furthermore, a one-way valve is installed at the other end of the high-pressure gas injection branch pipe to allow CO2 medium to be injected into the perforated borehole through the high-pressure gas injection branch pipe in one direction, and to prevent high-pressure water in the perforated borehole from flowing into the high-pressure gas injection branch pipe when no gas is injected.

[0020] Furthermore, if the gas concentration extracted from any through-layer borehole during the gas extraction process is lower than the set value, steps two to five are repeated for the through-layer borehole, and gas extraction is continued. This process is repeated until the extracted gas is still lower than the set value after a certain repetition. At this time, gas extraction from the through-layer borehole is stopped and the borehole is sealed.

[0021] Compared with the prior art, the present invention adopts the synergistic mode of hydraulic fracturing, CO2 acidification dissolution and CO2 competitive adsorption to displace gas, which has the following advantages:

[0022] 1. The present invention first forms partial cracks by hydraulic fracturing with high-pressure water, and further injects CO2 medium to compensate for the stress attenuation caused by the expansion of the cracks due to the hydraulic fracturing pressure, and maintains the pore pressure by means of a water-gas mixture to reduce the closure of the hydraulic cracks.

[0023] 2. The present invention utilizes high-pressure CO2 medium that is easily soluble in high-pressure water in a high-pressure environment to form a weakly acidic environment, and promotes chemical reactions between acidic water and mineral components on the surface of the fracture under the drive of concentration gradient (i.e., CO2 ions dissolved in water will diffuse from high-concentration positions to low-concentration positions) and high-pressure injection. The detached mineral particles accumulate in the fractures and act as proppants to further prevent the fractures from closing. At the same time, the fractures are pressurized by high-pressure water, thereby achieving the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement, forming a complex fracture network in the coal seam.

[0024] 3. After permeability enhancement, the present invention relieves the high-pressure environment and restores it to a normal atmospheric pressure environment. At this time, the CO2 medium dissolved in the high-pressure water in the through-layer borehole partially recovers to a gaseous state and diffuses into the coal seam through the cracks generated by fracturing. The CO2 gas and the gas gas will form a competitive adsorption, and the adsorption of the coal seam to CO2 gas is greater than that of the gas gas, which causes the gas to be displaced and desorbed from the coal seam in large quantities, and finally effectively improves the gas extraction efficiency after permeability enhancement.

[0025] 4. The present invention integrates multiple effects such as water intrusion weakening, CO2 acid etching, CO2 adsorption displacement, etc., which can not only effectively achieve the effect of increasing the permeability of coal seams and gas desorption, but also will not cause damage to the environment of the coal seam working face. Ultimately, without increasing more energy consumption, it maximizes the effective transformation of the pore structure volume of the coal body and improves the gas extraction efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is an overall construction schematic diagram of the present invention.

[0027] Figure 2 yes Figure 1 Cross-sectional view of the interior of a mid-through-layer borehole.

[0028] In the figure: 1-target coal seam; 2-bottom rock tunnel; 3-through-layer drilling; 4-high-pressure water branch pipe; 5-high-pressure gas injection branch pipe; 6-high-pressure resistant sealer; 7-fracturing pipe. DETAILED DESCRIPTION

[0029] The present invention will be further described below.

[0030] like Figure 1As shown, the specific steps of the present invention are:

[0031] Step 1: construct multiple through-layer drill holes 3 in the bottom rock tunnel 2 toward the target coal seam 1 to within 0.5m of the coal-rock boundary; the diameter of the through-layer drill holes 3 is 90mm, the spacing between adjacent through-layer drill holes 3 is 3m to 3.5m, and the drilling length is 50 to 60m.

[0032] Step 2: Select a through-layer borehole 3 from step 1, insert one end of the fracturing pipe 7 into the through-layer borehole 3, and seal the through-layer borehole 3 with a high-pressure sealer 6, such as Figure 2 As shown, a high-pressure baffle is provided inside the fracturing pipe 7, which is used to separate the fracturing pipe 7 into an inner section and an outer section, and the inner section is connected to the through-layer borehole 3, and then one end of the high-pressure water branch pipe 4 and one end of the high-pressure gas injection branch pipe 5 are extended into the fracturing pipe 7 and pass through the high-pressure baffle to enter the inner section; the materials used for the fracturing pipe 7, the high-pressure water branch pipe 4 and the high-pressure gas injection branch pipe 5 are all stainless steel, and the maximum tolerable tensile stress is not less than 20Mpa; a plurality of spherical cavities are arranged inside the fracturing pipe 7, and a plurality of through holes are arranged on the spherical cavities, which are used for mixing CO2 gas with high-pressure water and then discharging it into the through-layer borehole 3; a one-way valve is installed at the other end of the high-pressure gas injection branch pipe 5, which is used to allow the CO2 medium to be unidirectionally injected into the through-layer borehole 3 through the high-pressure gas injection branch pipe, and to prevent the high-pressure water in the through-layer borehole 3 from flowing into the high-pressure gas injection branch pipe 5 when no gas is injected.

[0033] Step 3, connect the high-pressure water injection pump to the other end of the high-pressure water branch 4, and connect the CO2 gas injection pump to the other end of the high-pressure gas injection branch 5. First start the high-pressure water injection pump to inject high-pressure water into the through-layer borehole 3 through the high-pressure water branch 4, and maintain the current water injection pressure until the injection water pressure value in the high-pressure water branch 4 reaches the set fracturing pressure P1 value; observe the injection water pressure value of the high-pressure water branch 4. When the injection water pressure value is lower than the current pressure value, it means that hydraulic fracturing causes cracks in the surrounding coal seams, and part of the high-pressure water enters the cracks, causing the injection water pressure value to decrease. At this time, continuous water injection is performed through the high-pressure water injection pump to ensure that the injection water pressure value reaches the P1 value; the high-pressure water injection flow rate is controlled at 5-6m 3 / min. Maintaining this flow rate can achieve a stable injection and hydraulic fracturing process.

[0034] Step 4: Start the CO2 gas injection pump to inject a large amount of CO2 medium into the through-layer borehole 3 through the high-pressure gas injection branch pipe 5 at the set pressure P2 value. The gas injection time lasts for 30 minutes and then stops, and the injection water pressure value is maintained at the P1 value for 24 hours; at this time, the injected CO2 medium will dissolve in the high-pressure water in the through-layer borehole 2, and the CO2 dissolved concentration in the high-pressure water around the other end of the high-pressure gas injection branch pipe 5 is the highest. Driven by the concentration gradient, the dissolved CO2 ions quickly diffuse to the low-concentration area until they reach the cracks generated by hydraulic fracturing, and chemically react with the mineral components on the surface of the cracks to acidify their surfaces, and the cracks are pressurized by high-pressure water at the same time, thereby realizing the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement; the CO2 medium is in a gaseous state or a supercritical state; if it is in a gaseous state, the injection pressure is controlled at 4 to 7 MPa, and if it is in a supercritical state, the injection pressure is controlled at 8 to 10 MPa; the P1 value is determined according to the ground stress and coal seam strength of the target coal seam, and the P2 value is 0.5 to 0.8 times the P1 value.

[0035] Step 5. When water flow occurs in any other through-layer borehole 3 of the coal seam, the gas-water mixed pressure in the current through-layer borehole 3 is unloaded. At this time, the CO2 dissolved in the high-pressure water returns to a gaseous state and diffuses into the interior of the coal seam through the cracks generated by fracturing. It is adsorbed by the coal seam and drives the coal seam to quickly desorb gas. At this time, the through-layer borehole 3 is connected to the gas extraction pipeline network for gas extraction.

[0036] Step 6: For the remaining through-layer boreholes 3 of step 1, repeat steps 2 to 5 respectively, so as to achieve the permeability enhancement and gas extraction work for the entire coal seam.

[0037] As an improvement of the present invention, the sealing length in step 2 is 10m from the opening of the through-layer drilling hole, and one or more high-pressure sealers 6 are used for sealing. The maximum expansion pressure of a single high-pressure sealer 6 is 30MPa.

[0038] As another improvement of the present invention, if the gas concentration extracted by any through-layer borehole 3 during the gas extraction process is lower than the set value, steps two to five are repeated for the through-layer borehole 3, and gas extraction is continued. This process is repeated until the extracted gas is still lower than the set value after a certain repetition. At this time, gas extraction of the through-layer borehole 3 is stopped and the through-layer borehole 3 is sealed.

[0039] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for fracturing low-permeability coal seams by combining CO2 with high-pressure water, characterized in that: The specific steps are: Step 1: construct multiple through-layer drill holes in the bottom rock tunnel towards the target coal seam to within 0.5m of the coal-rock boundary; Step 2: Select a through-layer borehole from step 1, insert one end of the fracturing pipe into the through-layer borehole, and seal the through-layer borehole with a high-pressure resistant sealer. A high-pressure baffle is provided in the fracturing pipe to separate the fracturing pipe into an inner section and an outer section. The inner section is connected to the through-layer borehole. Then, one end of the high-pressure water branch pipe and one end of the high-pressure gas injection branch pipe are extended into the fracturing pipe and pass through the high-pressure baffle to enter the inner section. Step 3, connect the high-pressure water injection pump to the other end of the high-pressure water branch, and connect the CO2 gas injection pump to the other end of the high-pressure gas injection branch. Start the high-pressure water injection pump to inject high-pressure water into the perforated borehole through the high-pressure water branch until the injection water pressure value in the high-pressure water branch reaches the set fracturing pressure P1 value and maintain the current injection water pressure value; observe the injection water pressure value of the high-pressure water branch. When the injection water pressure value is lower than the current pressure value, it means that hydraulic fracturing causes cracks in the surrounding coal seams, and part of the high-pressure water enters the cracks, resulting in a decrease in the injection water pressure value. At this time, the high-pressure water injection pump is used to continuously inject water to ensure that the injection water pressure value reaches the P1 value; Step 4: Start the CO2 gas injection pump to inject a large amount of CO2 medium into the perforated borehole through the high-pressure gas injection pipe at the set pressure P2 value. The gas injection time lasts for 30 minutes and then stops, and the injection water pressure value is maintained at the P1 value for 24 hours. At this time, the injected CO2 medium will dissolve in the high-pressure water in the perforated borehole, and the CO2 dissolved concentration in the high-pressure water around the other end of the high-pressure gas injection pipe is the highest. Driven by the concentration gradient, the dissolved CO2 ions diffuse rapidly to the low-concentration area until they reach the cracks generated by hydraulic fracturing, and chemically react with the mineral components on the surface of the cracks to acidify their surface. At the same time, the cracks are pressurized by high-pressure water, thereby realizing the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement. Step 5: When water flows in any other through-layer borehole of the coal seam, the gas-water mixed pressure in the current through-layer borehole is unloaded. At this time, the CO2 dissolved in the high-pressure water is restored to gaseous state and diffuses into the coal seam through the cracks generated by fracturing. It is adsorbed by the coal seam and displaces the coal seam to quickly desorb gas. At this time, the through-layer borehole is connected to the gas extraction pipeline network for gas extraction; Step 6: Repeat steps 2 to 5 for the remaining through-layer drill holes in step 1, so as to achieve the overall permeability enhancement and gas extraction work for the coal seam.

2. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: The diameter of the through-layer drill hole is 90 mm, the spacing between adjacent through-layer drill holes is 3 m to 3.5 m, and the length of the drill hole is 50 m to 60 m.

3. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: The materials used for the fracturing pipe, high-pressure water branch pipe and high-pressure gas injection branch pipe are all stainless steel, and the maximum tolerable tensile stress is not less than 20Mpa; a plurality of spherical cavities are arranged inside the fracturing pipe, and a plurality of through holes are arranged on the spherical cavities for mixing CO2 gas with high-pressure water and discharging them into the through-layer drilled hole.

4. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: In the step 2, the sealing length is 10 m from the opening of the through-layer drilling hole, and one or more high-pressure resistant sealers are used for sealing. The maximum expansion pressure that a single high-pressure resistant sealer can withstand is 30 MPa.

5. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: The P1 value is determined according to the ground stress and coal seam strength of the target coal seam, and the P2 value is 0.5 to 0.8 times the P1 value.

6. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: The high-pressure water injection flow rate is controlled at 5-6m 3 / min.

7. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: The CO2 medium is in a gaseous state or a supercritical state; if it is in a gaseous state, the injection pressure is controlled at 4-7 MPa, and if it is in a supercritical state, the injection pressure is controlled at 8-10 MPa.

8. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: A one-way valve is installed at the other end of the high-pressure gas injection branch pipe, which is used to allow the CO2 medium to be injected into the through-layer borehole through the high-pressure gas injection branch pipe in one direction, and to prevent the high-pressure water in the through-layer borehole from flowing into the high-pressure gas injection branch pipe when no gas is injected.

9. The method for fracturing low-permeability coal seams by combining CO2 and high-pressure water according to claim 1, characterized in that: If the gas concentration extracted from any through-layer borehole during the gas extraction process is lower than the set value, repeat steps 2 to 5 for the through-layer borehole and continue gas extraction. Repeat this process until the extracted gas is still lower than the set value after a certain repetition. At this time, stop gas extraction from the through-layer borehole and seal it.

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