A method for fracturing low-permeability coal seams by combining CO2 and high-pressure water

Abstract

The present invention discloses a method for fracturing low-permeability coal seams by combining CO2 with high-pressure water. The method comprises the following steps: first, hydraulic fracturing is performed to form partial cracks, and the CO2 medium is easily soluble in high-pressure water in a high-pressure environment to form a weakly acidic environment. Driven by the concentration gradient and high-pressure injection, the acidic water is promoted to chemically react with the mineral components on the surface of the cracks, and the cracks are pressurized by the high-pressure water at the same time, thereby realizing the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement, and forming a complex crack network in the coal seam; after permeability enhancement, the pressure is relieved to restore the normal pressure environment, at this time, the CO2 medium dissolved in the high-pressure water in the through-layer borehole is partially restored to a gaseous state, and the CO2 gas and gas gas inside the coal seam will form competitive adsorption, resulting in the gas being displaced and desorbed from the coal seam in large quantities. The whole process can not only effectively achieve the permeability enhancement effect on the coal seam, but also will not cause additional damage to the coal seam. Ultimately, without consuming too much energy, the effective transformation of the pore structure volume of the coal body is maximized, thereby improving the gas extraction efficiency.

Description

Technical Field

[0001] The invention relates to a method for fracturing a low-permeability coal seam by combining CO2 with 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 moving deeper. 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. Currently, gas pre-extraction methods include intensive drilling extraction and directional long drilling extraction. However, due to the synergistic effect of ground stress and coal seam structure, the desorption of coal 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] 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 use external energy to destroy the coal body structure, 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. The derived pore and fracture structure may penetrate the original pore and fracture structure of the coal body. Affected by ground stress, hydraulic fractures mainly expand in the direction perpendicular to the minimum principal stress, and the fracture morphology is uniformly distributed. 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, greatly affecting the subsequent gas extraction efficiency.

[0004] Acid fracturing technology mainly involves injecting a screened acidic fluid into the coal seam, and utilizing the physical and chemical reaction and dissolution process between the acidic fluid and the coal rock to expand the distribution of the coal body's pore and fracture structure, thereby increasing the gas's advantageous migration channels. However, in most cases, the screened acidic fluid has a negative effect 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 crack generation and strong acid fracturing damaging the reservoir, how to provide a new method for fracturing low-permeability coal seams that 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's pore structure volume without increasing energy consumption, thereby improving gas extraction efficiency, is the research direction required by the present invention. Summary of the Invention

[0005] In response to 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. It adopts a coordinated approach of hydraulic fracturing, CO2 acidification and 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 coal seam working face environment. 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 as follows:

[0007] 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.

[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 pipe and one end of the high-pressure gas injection pipe are extended into the fracturing pipe and pass through the high-pressure baffle into the inner section.

[0009] Step 3. 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, and maintain the current water injection pressure until the injection water pressure in the high-pressure water branch reaches the set fracturing pressure P1 value; observe the injection water pressure of the high-pressure water branch. When the injection water pressure is lower than the current pressure value, it means that hydraulic fracturing has caused cracks in the surrounding coal seams, and some high-pressure water has entered the cracks, resulting in a decrease in the injection water pressure. At this time, continue to inject water through the high-pressure water injection pump to ensure that the injection water pressure 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 is stopped after 30 minutes, 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 realizing the coordinated work of high-pressure fracturing and CO2 acidification and permeability enhancement.

[0011] Step 5: When water flow occurs in any other through-layer borehole in 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 returns 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.

[0012] Step 6: Repeat steps 2 to 5 for the remaining through-layer drill holes in step 1, thereby achieving permeability enhancement and gas extraction for the entire coal seam.

[0013] Furthermore, the diameter of the through-layer drill holes is 90 mm, the spacing between adjacent through-layer drill holes is 3 m to 3.5 m, and the drill hole length 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; multiple spherical cavities are arranged inside the fracturing pipe, and a number 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 step 2, the sealing length is 10 m from the through-hole opening, 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 5-6m 3 Maintaining this flow rate enables 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 through-layer drill hole through the high-pressure gas injection branch pipe in one direction, and to prevent high-pressure water in the through-layer drill hole 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 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 sealed.

[0021] Compared with the existing technology, the present invention adopts a synergistic approach 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, which is easily soluble in high-pressure water in a high-pressure environment to form a weakly acidic environment. Driven by the concentration gradient (i.e., CO2 ions dissolved in water will diffuse from high-concentration positions to low-concentration positions) and high-pressure injection, the acidic water is promoted to chemically react with the mineral components on the surface of the fracture. The detached mineral particles accumulate in the fracture and act as proppants to further prevent the fracture from closing. At the same time, the fracture is pressurized by high-pressure water, thereby realizing 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 normal atmospheric pressure. At this time, the CO2 medium dissolved in the high-pressure water in the through-layer drill hole partially returns to gaseous state and diffuses into the coal seam through the cracks generated by fracturing. The CO2 gas and gas gas will form a competitive adsorption, and the coal seam's adsorption capacity for CO2 gas is greater than that for gas gas, resulting in a large amount of gas being displaced and desorbed from the coal seam, ultimately effectively improving the gas extraction efficiency after permeability enhancement.

[0025] 4. The present invention integrates multiple effects such as water intrusion weakening, CO2 acid etching, and CO2 adsorption displacement. It can not only effectively achieve the effect of increasing the permeability of the coal seam and desorbing gas, but also will not cause damage to the coal seam working face environment. Ultimately, without increasing 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 the through-layer borehole.

[0028] In the figure: 1-target coal seam; 2-bottom rock roadway; 3-through-layer drilling hole; 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 a range of 0.5m within 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. Figure 2 As shown, a high-pressure baffle is provided in the fracturing pipe 7, which is used to separate the fracturing pipe 7 into an inner section and an outer section. The inner section is connected to the through-layer borehole 3, and then one end of the high-pressure water branch 4 and one end of the high-pressure gas injection branch 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 4 and the high-pressure gas injection branch 5 are all stainless steel, and the maximum tolerance tensile stress is not less than 20Mpa; a plurality of spherical cavities are provided 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; the other end of the high-pressure gas injection branch 5 is equipped with a one-way valve, which is used to allow the CO2 medium to be injected into the through-layer borehole 3 through the high-pressure gas injection branch, and to prevent the high-pressure water in the through-layer borehole 3 from flowing into the high-pressure gas injection branch 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 at the same time 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 value 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 has caused cracks in the surrounding coal seams, and some high-pressure water has entered the cracks, resulting in a decrease in the injection water pressure value. At this time, water is continuously injected 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 to 6 m 3 Maintaining this flow rate enables a stable injection and hydraulic fracturing process.

[0034] Step 4: Start the CO2 gas injection pump and inject a large amount of CO2 medium into the through-layer borehole 3 through the high-pressure gas injection pipe 5 at a 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 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 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. The CO2 medium is in a gaseous or 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 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: Repeat steps 2 to 5 for the remaining through-layer drill holes 3 of step 1, thereby achieving 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 inward from the opening of the through-layer drill hole, and one or more high-pressure sealers 6 are used for sealing. The maximum expansion pressure that a single high-pressure sealer 6 can withstand is 30MPa.

[0038] As another improvement of the present invention, if the gas concentration extracted from 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 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 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 principles 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: Drill multiple through-layer holes in the bottom rock roadway toward 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 inside 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 into the inner section. Step 3: 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, and maintain the current water injection pressure until the injection water pressure in the high-pressure water branch reaches the set fracturing pressure P1 value; observe the injection water pressure of the high-pressure water branch. When the injection water pressure is lower than the current pressure value, it means that hydraulic fracturing has caused cracks in the surrounding coal seams, and some high-pressure water has entered the cracks, resulting in a decrease in the injection water pressure. At this time, continue to inject water through the high-pressure water injection pump to ensure that the injection water pressure reaches the P1 value; 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 is stopped after 30 minutes, 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 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 flow occurs in any other through-layer borehole in 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 returns to a gaseous state and diffuses into the coal seam through the cracks created 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, thereby achieving permeability enhancement and gas extraction for the entire 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 holes is 90 mm, the spacing between adjacent through-layer drill holes is 3 m to 3.5 m, and the drill hole length 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; multiple spherical cavities are arranged inside the fracturing pipe, and several 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: The sealing length in step 2 is 10 m from the opening of the through-layer drill 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.

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 to 7 MPa; if it is in a supercritical state, the injection pressure is controlled at 8 to 10 MPa.

8. The method for fracturing low-permeability coal seams by combining CO2 with 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 to allow CO2 medium to be injected into the through-layer drill hole in one direction through the high-pressure gas injection branch pipe, and to prevent high-pressure water in the through-layer drill hole 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 with high-pressure water according to claim 1, characterized in that: During the gas extraction process, if the gas concentration extracted from any through-layer borehole 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 hole is sealed.

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