Hydraulic and ultrasonic composite fracturing coal seam pressure relief and permeability increasing method based on water pressure regulation and control

Abstract

According to the hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability increasing method based on water pressure regulation and control, in the initial stage, a low-pressure pressure-maintaining ultrasonic excitation softening pressure relief technology is adopted, and a coal reservoir is subjected to humidifying, strong moistening, softening and pressure relief under the action of low water pressure and ultrasonic excitation; in the middle stage, based on pump pressure changes, a high-pressure pressurization hydraulic ultrasonic composite fracturing process and a low-pressure pressure maintaining ultrasonic excitation softening pressure relief process are alternately carried out in a self-adaptive mode, physical transformation is carried out on the coal reservoir structure, a pore fracture network is enriched, and fracturing and permeation increasing are carried out; and in the later stage, a low-pressure pressure-maintaining hydraulic ultrasonic circulating excitation blockage removal and permeation enhancement process is adopted, hole fracture channels in the fractured coal reservoir are dredged, bridge blockage is removed, desorption is promoted, and permeation is increased. According to the method, in different stages of ultrasonic composite hydraulic fracturing, self-adaptive water pressure regulation and control are matched, respective unique advantages of an ultrasonic excitation technology and a hydraulic fracturing technology are fully played, the hydraulic and ultrasonic composite fracturing process can be remarkably optimized, and the pressure relief and permeability increasing effects of the coal reservoir are improved.

Description

Technical Field

[0001] The invention belongs to the field of coal seam permeability enhancement, and in particular relates to a hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability enhancement method based on water pressure regulation. Background Art

[0002] Commonly used methods for increasing the permeability of coal seams include hydraulic fracturing, ultrasonic excitation and other technologies, but these methods have certain limitations. Conventional hydraulic fracturing technology often has uncertainty in the cracking position and expansion direction of coal body cracks. At the same time, the number of cracks formed in the coal seam is often small and the fracture network structure is relatively simple. In the process of ultrasonic excitation of the original coal body, the sound intensity gradually decays with the increase of distance, and the impact radius is limited.

[0003] Although ultrasonic synergistic hydraulic fracturing is currently used to enhance the extraction of coalbed methane, the unique advantages of hydraulic fracturing and ultrasonic excitation have not yet been fully utilized. For example, CN202111125683.X discloses a method for enhancing coal seam fracturing and extraction effects by ultrasonic field. After the hydraulic fracturing construction is completed, the ultrasonic field is used to excite the target coal seam to further increase the permeability of the coal seam and enhance the fracturing effect; for example, CN202010560544.9 discloses a method for ultrasonic excitation-assisted hydraulic fracturing pressure relief and permeability enhancement in coal mines. Before implementing hydraulic fracturing, ultrasonic excitation is used to change the pore structure of the coal reservoir to assist in achieving relatively uniform fracturing of hydraulic fracturing.

[0004] In the ultrasonic synergistic hydraulic fracturing enhanced coalbed methane extraction method disclosed in the aforementioned patent, the ultrasonic excitation technology and the hydraulic fracturing technology are simply combined and sequenced or used in a superimposed manner, and the unique advantages of the two technologies of hydraulic fracturing and ultrasonic excitation and the role of environmental water pressure in each stage are not fully considered. This has to some extent limited the scope of influence and effect of the ultrasonic synergistic hydraulic fracturing enhanced coalbed methane extraction method, and therefore it is necessary to make improvements. Summary of the invention

[0005] The present invention aims to solve the technical problems existing in the prior art, considers the effect of environmental water pressure at different stages of ultrasonic composite hydraulic fracturing process, and provides a hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability enhancement method based on water pressure regulation.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a method for unloading and increasing permeability of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation. In the process of hydraulic ultrasonic composite fracturing of coal seams, different ranges of water pressure are applied to the permeability-enhancing boreholes at different stages of coal seam fracturing. Specifically: in the early stage of coal seam fracturing, a "low-pressure pressure-maintaining ultrasonic excitation softening and pressure-unloading" process is adopted. The water pressure of the permeability-enhancing boreholes of the coal seam is within a first set low-pressure range. Under the coupling effect of low water pressure and ultrasonic excitation, the coal reservoir is moistened and strengthened, and softened and pressure-unloaded. In the middle stage of coal seam fracturing, a "high-pressure boosting hydraulic ultrasonic composite fracturing" process is first adopted. The water pressure for water injection in the borehole is within the set high pressure range, which is greater than the first set low pressure range. The coal seam structure is physically transformed under the coupling effect of high water pressure and ultrasonic excitation. During the fracturing process, based on the change in the pump pressure of the injection pump, the "high-pressure boosting hydraulic ultrasonic composite fracturing" process and the "low-pressure pressure-maintaining ultrasonic excitation softening and unloading" process are adaptively and alternately performed to enrich the pore and fracture network and cause fracturing and increase permeability. In the later stage of coal seam fracturing, the "low-pressure pressure-maintaining ultrasonic cyclic excitation unblocking and permeability increase" process is adopted. The water pressure for water injection in the permeability-enhancing borehole of the coal seam is within the second set low pressure range to dredge the pore and fracture channels in the coal seam after fracturing, unblock bridges, promote desorption, and increase seepage.

[0007] The above technical scheme adopts the "low-pressure pressure-maintaining ultrasonic excitation softening pressure relief" process in the early stage, and moisturizes and strengthens the coal reservoir under the coupling effect of low water pressure and ultrasonic excitation, and softens and relieves pressure; in the middle stage, the pump pressure change based on the injection pump is adopted, and the "high-pressure boosting hydraulic ultrasonic composite fracturing" process and the "low-pressure pressure-maintaining ultrasonic excitation softening pressure relief" process are adaptively alternately carried out to physically transform the coal reservoir structure, enrich the pore and fracture network, and cause fracturing and permeability increase; in the later stage, the "low-pressure pressure-maintaining hydraulic ultrasonic circulation excitation unblocking and permeability increase" process is adopted to dredge the pore and fracture channels in the coal reservoir after fracturing, unblock bridges, promote desorption, and increase seepage. The present invention takes into account the effect of environmental water pressure at different stages of the ultrasonic composite hydraulic fracturing process, cooperates with adaptive water pressure regulation at different stages of the ultrasonic composite hydraulic fracturing, gives full play to the unique advantages of ultrasonic excitation technology and hydraulic fracturing technology, can significantly optimize the hydraulic ultrasonic composite fracturing process, and improve the coal reservoir pressure relief and permeability increase effect.

[0008] In a preferred embodiment of the present invention, the hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability enhancement method based on water pressure regulation comprises the following steps:

[0009] S1. Drilling: Select a target coal seam, construct an anti-permeability drill hole and a monitoring drill hole in the target coal seam, and an anti-permeability drill hole and a monitoring drill hole together constitute an anti-permeability fracturing unit;

[0010] S2. Arrange the anti-permeability system: arrange the ultrasonic fracturing module and the hydraulic fracturing module in the anti-permeability borehole;

[0011] S3, sealing and injecting liquid: using the sealing device to seal the permeability-enhancing borehole, starting the hydraulic fracturing module to inject water into the permeability-enhancing borehole, and filling the permeability-enhancing borehole with water;

[0012] S4, low-pressure pressure-maintaining ultrasonic excitation softening and pressure relief process: adjust the water injection pressure of the hydraulic fracturing module, continuously inject water into the permeability-enhancing borehole, stabilize the water pressure within the first set low-pressure range, start the ultrasonic fracturing module to continuously emit ultrasonic waves to the coal body around the permeability-enhancing borehole, complete low-pressure pressure-maintaining ultrasonic excitation for a period of time in the permeability-enhancing borehole, realize coal reservoir moistening and softening and pressure relief, and when it is judged that the low-pressure pressure-maintaining ultrasonic excitation is completed, execute step S5;

[0013] S5, high-pressure boosted hydraulic ultrasonic composite fracturing process: keep the ultrasonic fracturing module turned on, adjust the water injection pressure of the hydraulic fracturing module to the set high-pressure range, inject water into the permeability-enhancing borehole, and implement high-pressure boosted hydraulic ultrasonic composite fracturing on the target coal seam to increase fracturing and permeability. During this period, the maximum pump pressure of the hydraulic fracturing module is recorded as Pmax;

[0014] S6, during the high-pressure boosted hydraulic ultrasonic composite fracturing, if water comes out of the monitoring borehole, stop the fracturing and execute step S9; if no water comes out of the monitoring borehole, execute step S7;

[0015] S7, during the high-pressure boosted hydraulic ultrasonic composite fracturing, when the pump pressure of the hydraulic fracturing module begins to decrease, the pump pressure is recorded as P. During the process of pump pressure decrease, if the pump pressure P is less than the first pressure threshold, step S8 is executed; if the pump pressure P is greater than the first pressure threshold, the process returns to step S5;

[0016] S8, during the process of pump pressure decreasing, the pump pressure P is judged twice. If the pump pressure P is greater than the second pressure threshold, and the second pressure threshold is less than the first pressure threshold, it is judged that a large fracture network structure is generated during the high-pressure boosting hydraulic ultrasonic composite fracturing, and the process returns to step S4 to perform low-pressure pressure-maintaining ultrasonic excitation softening pressure relief; if the pump pressure P is less than the second pressure threshold, it is judged that the fracturing is completed, and step S9 is executed;

[0017] S9, low-pressure pressure-maintaining ultrasonic cycle excitation unblocking and permeability enhancement process: adjusting the injection pressure of the hydraulic fracturing module to the second set low-pressure range, keeping the ultrasonic fracturing module turned on, and continuously applying low water pressure and ultrasonic coupling excitation for a period of time in the permeability enhancement borehole to dredge the coal reservoir pore fissure channels generated by fracturing, unblocking and increasing permeability;

[0018] S10, end of operation: after the low-pressure ultrasonic cycle excitation unblocking and permeability enhancement process is completed, close the ultrasonic fracturing module and the hydraulic fracturing module, discharge the fracturing fluid in the permeability enhancement borehole, withdraw the ultrasonic fracturing module and the hydraulic fracturing module, seal the permeability enhancement borehole and the monitoring borehole, connect them to the coalbed methane extraction pipeline respectively, and extract coalbed methane.

[0019] The above technical scheme, in the middle stage of coal seam fracturing, adaptively and alternately carries out the "high-pressure boosting hydraulic ultrasonic composite fracturing" process and the "low-pressure pressure-maintaining ultrasonic excitation softening pressure unloading" process based on the change of pump pressure. As long as a new pore and fracture structure is formed in the process of coal seam fracturing, low-pressure pressure-maintaining ultrasonic excitation softening pressure unloading will be carried out again, which will help to achieve more uniform coal seam permeability, control high-pressure hydraulic fracturing to form a more complex fracture network structure, and further improve the pressure unloading and permeability-increasing effect of coal reservoirs.

[0020] In a preferred embodiment of the present invention, the first set low pressure range, the set high pressure range and the second set low pressure range are obtained in the following manner: the coal seam fracture initiation pressure P is obtained. q , the first set low pressure range is (1±20%)φ1P q ; Set the high pressure range to (1±20%)ε2P q ; The second low pressure setting range is (1±20%)φ1P q ; where φ i is the water pressure control coefficient, and the value of i is 1 or 2.

[0021] In the above technical scheme, the first set low pressure range, the set high pressure range and the second set low pressure range are associated with the fracturing pressure and water pressure control coefficient of the target coal seam. Different target coal seams are fractured using water pressures suitable for them to improve the pressure relief and permeability enhancement effect of the coal reservoir.

[0022] In a preferred embodiment of the present invention, the first set low pressure range, the set high pressure range, and the second set low pressure range are all calculated according to the following formula:

[0023] P=φ i ·P q

[0024] Among them, φ i is the water pressure control coefficient, i takes values ​​of 1 and 2; affects the water pressure control coefficient ε i The main parameters include coal seam permeability coefficient, coal seam solidity coefficient, coal seam porosity coefficient and fracturing margin coefficient, ε i The calculation formula is as follows:

[0025] ε i =α·β·γ·δ

[0026] Among them, α is the permeability coefficient of the coal seam; β is the solidity coefficient of the coal seam; γ is the porosity coefficient of the coal seam; δ is the fracturing margin coefficient,

[0027] For ε1, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ = 0.8;

[0028] For ε2, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ=1.2.

[0029] The above technical scheme takes into account the coal seam permeability coefficient, coal seam solidity coefficient, coal seam porosity coefficient and fracturing margin coefficient when calculating the first set low pressure range, the set high pressure range and the second set low pressure range, so as to adjust the water pressure in each period according to the different physical properties of different target coal seams and improve the permeability enhancement effect.

[0030] In a preferred embodiment of the present invention, the calculation formulas of α, β, and γ are as follows:

[0031]

[0032] In a preferred embodiment of the present invention, the coal seam fracture initiation pressure P is obtained according to the following formula: q :P q =3σ1-σ3+σ t , where P q is the coal seam fracture initiation pressure, σ1 is the minimum horizontal principal stress of the coal seam, σ3 is the maximum horizontal principal stress of the coal seam, σ t It is the tensile strength of coal.

[0033] The above technical solution obtains the coal seam fracture initiation pressure based on the minimum horizontal principal stress, the maximum horizontal principal stress and the tensile strength of the coal body.

[0034] In a preferred embodiment of the present invention, the first pressure threshold is 80% of the maximum pump pressure Pmax, and the second pressure threshold is 20% of the maximum pump pressure Pmax.

[0035] In another preferred embodiment of the present invention, in step S4, low pressure and pressure maintaining ultrasonic coupling excitation are performed in the anti-permeability drilling hole for 2 hours; and / or in step S9, low water pressure and ultrasonic coupling excitation are continued in the anti-permeability drilling hole for 2 hours.

[0036] In another preferred embodiment of the present invention, the ultrasonic fracturing module includes an ultrasonic transducer installed at the front end of an energized drill rod and located in the anti-reflection borehole. The ultrasonic transducer is electrically connected to the energized drill rod, and the end of the energized drill rod is connected to an ultrasonic generator located outside the anti-reflection borehole through an ultrasonic cable.

[0037] In another preferred embodiment of the present invention, the hydraulic fracturing module includes a fracturing pipe extending into the permeability-enhancing borehole, an injection pump connected to the liquid inlet of the fracturing pipe, and a liquid storage tank connected to the liquid inlet of the injection pump, and the injection pump and the liquid storage tank are arranged outside the permeability-enhancing borehole.

[0038] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0040] Figure 1 The invention is a flowchart of a method for decompression and permeability enhancement of a coal seam by hydraulic ultrasonic composite fracturing based on water pressure regulation in an embodiment.

[0041] Figure 2 It is a schematic diagram of using hydraulic ultrasonic composite fracturing to relieve pressure and increase permeability of coal seams in an embodiment.

[0042] The figure marks in the drawings of the specification include: target coal seam 1, permeability enhancement borehole 2, monitoring borehole 3, ultrasonic transducer 4, energized drill rod 5, ultrasonic cable 6, ultrasonic generator 7, sealer 8, fracturing pipe 9, injection pump 10, liquid storage tank 11, control valve 12, and extraction pipeline 13. DETAILED DESCRIPTION

[0043] Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0044] In the description of the present invention, it is to be understood that the terms “longitudinal”, “lateral”, “vertical”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside” and “outside” etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0045] In the description of the present invention, unless otherwise specified and limited, it should be noted that the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal connection between two components. It can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances.

[0046] The present invention provides a hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability enhancement method based on water pressure regulation, such as Figure 1As shown, in a preferred embodiment, during the hydraulic ultrasonic composite fracturing of coal seams, the water pressure ranges applied to the permeability enhancement boreholes are different at different stages of coal seam fracturing, specifically:

[0047] In the early stage of coal seam fracturing, the "low-pressure maintenance ultrasonic excitation softening and pressure relief" process is adopted. The water pressure of the coal seam's permeability enhancement drilling water injection is in the first set low pressure range. Under the coupling effect of low water pressure and ultrasonic excitation, the coal reservoir is moistened and lubricated, and softened and pressure relieved.

[0048] In the middle stage of coal seam fracturing, the "high-pressure boosting hydraulic ultrasonic composite fracturing" process is first used. The water pressure of the coal seam's permeability-enhancing drilling water injection is in the set high-pressure range, and the set high-pressure range is greater than the first set low-pressure range. The coal seam structure is physically transformed under the coupling effect of high water pressure and ultrasonic excitation. During the fracturing process, based on the change of the injection pump pressure, the "high-pressure boosting hydraulic ultrasonic composite fracturing" process and the "low-pressure pressure-maintaining ultrasonic excitation softening and unloading" process are adaptively alternated to enrich the pore and fracture network and increase fracturing and permeability.

[0049] In the later stage of coal seam fracturing, the "low-pressure maintained pressure ultrasonic circulation excitation unblocking and permeability enhancement" process is adopted. The water pressure of the coal seam's permeability enhancement drilling injection is within the second set low pressure range to dredge the pores and cracks in the coal seam after fracturing, unblock bridges, promote desorption, and increase seepage.

[0050] like Figure 1 and Figure 2 As shown, the hydraulic ultrasonic composite fracturing coal seam pressure relief and permeability enhancement method based on water pressure regulation of the present invention comprises the following steps:

[0051] S1. Drilling: Select the target coal seam 1. According to the construction aperture requirements, use a drilling rig to construct an anti-permeability borehole 2 and a monitoring borehole 3 in the target coal seam 1. The distance between the anti-permeability borehole 2 and the monitoring borehole 3 is 30 to 40 meters. An anti-permeability borehole 2 and a monitoring borehole 3 together constitute an anti-permeability fracturing unit. After the anti-permeability borehole 2 and the monitoring borehole 3 are drilled, the drill cuttings in the hole are flushed through a water injection pipeline.

[0052] S2. Arrange the permeability enhancement system: arrange the ultrasonic fracturing module and the hydraulic fracturing module in the permeability enhancement borehole 2. The ultrasonic fracturing module and the hydraulic fracturing module can adopt the existing technology, for example, the ultrasonic fracturing module includes an ultrasonic transducer 4 installed at the front end of the powered drill rod 5 and located in the permeability enhancement borehole 2, the ultrasonic transducer 4 is electrically connected to the powered drill rod 5 (for example, plug-in cable), the powered drill rod 5 has a powered core, and the end of the powered drill rod 5 is connected to the ultrasonic generator 7 located outside the permeability enhancement borehole 2 through the ultrasonic cable 6; the hydraulic fracturing module includes a fracturing pipe 9 extending into the permeability enhancement borehole 2, a liquid injection pump 10 connected to the liquid inlet of the fracturing pipe 9, and a liquid storage tank 11 connected to the liquid inlet of the liquid injection pump 10, and the liquid injection pump 10 and the liquid storage tank 11 are arranged outside the permeability enhancement borehole 2. During the specific arrangement, the ultrasonic transducer 4 and the fracturing pipe 9 are sequentially sent into the anti-permeability borehole 2, the ultrasonic transducer 4 is plugged and connected to the energized drill rod 5, the ultrasonic transducer 4 is located at the bottom of the anti-permeability borehole 2, the end of the energized drill rod 5 is connected to the ultrasonic generator 7 outside the anti-permeability borehole 2 through the ultrasonic cable 6, one end of the fracturing pipe 9 is located in the anti-permeability borehole 2, and the other end is connected to the injection pump 10 outside the anti-permeability borehole 2.

[0053] S3, sealing and injecting liquid: Use the sealer 8 to seal the permeability-enhancing borehole 2, improve the water and power supply of the ultrasonic fracturing module and the hydraulic fracturing module, start the injection pump 10 of the hydraulic fracturing module, and inject water into the permeability-enhancing borehole 2 through the fracturing pipe 9. When the injection pressure dial of the injection pump 10 shows an indication, the injection of liquid into the permeability-enhancing borehole 2 is completed and the permeability-enhancing borehole 2 is filled with water.

[0054] S4, low-pressure pressure-maintaining ultrasonic excitation softening and pressure relief process: start the injection pump 10 to inject liquid, adjust the injection pressure of the injection pump 10, and continuously inject water into the anti-permeability borehole 2 of the target coal seam 1 to stabilize the water pressure in the first set low pressure range (for example, 3.0-6.0 MPa) to control the water pressure in the anti-permeability borehole 2 to be stable at 3.0-6.0 MPa, set the ultrasonic excitation parameters and start the ultrasonic generator 7, the ultrasonic generator 7 continuously emits ultrasonic waves to the coal body around the anti-permeability borehole 2, complete the low-pressure pressure-maintaining ultrasonic excitation in the anti-permeability borehole 2 for a period of time (for example, 2 hours), realize the moisturizing and softening and pressure relief effect of the coal reservoir, and when it is judged that the low-pressure pressure-maintaining ultrasonic excitation is completed (that is, after the low-pressure pressure-maintaining ultrasonic excitation coupling operation time exceeds 2 hours), execute step S5.

[0055] S5, high-pressure boosted hydraulic ultrasonic composite fracturing process: keep the ultrasonic fracturing module turned on, continue ultrasonically exciting the coal seam, turn on the injection pump 10 again to inject liquid, adjust the injection pressure of the injection pump 10 to the set high pressure range (for example, 24.0-36.0MPa), inject water into the permeability-enhancing borehole 2, and perform high-pressure boosted hydraulic ultrasonic composite fracturing on the target coal seam 1 to increase fracturing and permeability, during which the maximum pump pressure of the injection pump 10 is recorded as Pmax. Among them, 24.0-36.0MPa is the set pump pressure, and Pmax is the maximum pump pressure during the actual change of the pump pressure curve of the injection pump 10.

[0056] S6, during the high-pressure boosted hydraulic ultrasonic composite fracturing, if water comes out of the monitoring borehole 3, then the fracturing is stopped and step S9 is executed; if water does not come out of the monitoring borehole 3, step S7 is executed;

[0057] S7. During the high-pressure boosting hydraulic ultrasonic composite fracturing, when the pump pressure of the injection pump 10 begins to decrease, the pump pressure is recorded as P. During the process of pump pressure decrease, if the pump pressure P is less than the first pressure threshold, preferably the first pressure threshold is 80% of the maximum pump pressure Pmax, then P<80%Pmax, and step S8 is executed; if the pump pressure P is greater than the first pressure threshold, specifically P>80%Pmax, then return to step S5 and continue the high-pressure boosting hydraulic ultrasonic composite fracturing process.

[0058] S8. During the process of pump pressure reduction, the pump pressure P is judged twice. If the pump pressure P is greater than the second pressure threshold, and the second pressure threshold is less than the first pressure threshold, preferably the second pressure threshold is 20% of the maximum pump pressure Pmax, and P>20%Pmax, it is determined that a large fracture network structure is generated during the high-pressure boosting hydraulic ultrasonic composite fracturing, and the process returns to step S4 to perform low-pressure pressure-maintaining ultrasonic excitation softening pressure relief; if the pump pressure P is less than the second pressure threshold, specifically P<20%Pmax, it is determined that the fracturing is completed and step S9 is executed.

[0059] S9, low-pressure maintained ultrasonic cycle excitation unblocking and permeability enhancement process: keep the injection pump 10 open to inject liquid, adjust the injection pressure of the hydraulic fracturing module to the second set low pressure range (for example, 3.0-6.0 MPa) to control the water pressure in the permeability enhancement borehole 2 to be stable at 3.0-6.0 MPa, keep the ultrasonic fracturing module open, continue low water pressure and ultrasonic coupling excitation in the permeability enhancement borehole 2 for a period of time (for example, 2 hours), and clear the coal reservoir pore fissure channels generated by fracturing to unblock and increase permeability.

[0060] S10, end the operation: After the low-pressure maintained ultrasonic cycle excitation unblocking and permeability enhancement process is completed, turn off the ultrasonic generator 7 of the ultrasonic fracturing module and the injection pump 10 of the hydraulic fracturing module, discharge the fracturing fluid in the permeability enhancement borehole 2, withdraw the ultrasonic fracturing module and the hydraulic fracturing module, seal the permeability enhancement borehole 2 and the monitoring borehole 3 and connect them to the grid, respectively connect them to the coalbed methane extraction pipeline 13, the extraction pipeline 13 is provided with a control valve 12, and the control valve 12 is opened to extract coalbed methane.

[0061] In the present invention, the first set low pressure range, the set high pressure range and the second set low pressure range are preferably obtained in the following manner:

[0062] The coal seam fracture pressure P is obtained according to the following formula q ,

[0063] P q =3σ1-σ3+σ t

[0064] Among them, P q is the coal seam fracture initiation pressure, σ1 is the minimum horizontal principal stress of the coal seam (obtained from formation data), σ3 is the maximum horizontal principal stress of the coal seam (obtained from formation data), σ t It is the tensile strength of coal (measured in the laboratory).

[0065] In the present invention, the first set low pressure range of the "low pressure holding ultrasonic excitation softening pressure relief" process is (1±20%)ε1P q ;The set high pressure range of the "high pressure boost hydraulic ultrasonic composite fracturing" process is (1±20%)ε2P q The second low pressure range of the "low pressure holding ultrasonic cycle excitation plugging and permeation enhancement" process is (1±20%)ε1P q ; Among them, ε i is the water pressure control coefficient, and the value of i is 1 or 2.

[0066] Specifically, the first set low pressure range, the set high pressure range, and the second set low pressure range are all calculated according to the following formula:

[0067] P=ε i ·P q

[0068] Among them, ε i is the water pressure control coefficient, i takes values ​​of 1 and 2; affects the water pressure control coefficient ε i The main parameters include coal seam permeability coefficient α, coal seam solidity coefficient β, coal seam porosity coefficient γ and fracturing margin coefficient δ, ε i The calculation formula is as follows:

[0069] ε i =α·β·γ·δ

[0070] For ε1, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ = 0.8;

[0071] For ε2, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ=1.2.

[0072] α, β, and γ are obtained using the following formula:

[0073]

[0074] Where k is the permeability of the coal seam, unit: m 2 , obtained by laboratory testing of standard coal pillar specimens in accordance with NB / T 11327-2023 "Test method for coal seam gas permeability";

[0075]

[0076] Wherein, f is the solidity coefficient of coal, which is obtained through laboratory testing in accordance with the method for determining the solidity coefficient of coal in Part 12 of GB / T 23561.12-2024 “Methods for determination of physical and mechanical properties of coal and rock”;

[0077]

[0078] Wherein, φ is the porosity of coal, in %, obtained through basic laboratory tests.

[0079] In the early stage of coal seam fracturing, the present invention adopts the "low-pressure pressure-maintaining ultrasonic excitation softening and pressure relief" process. The injection water pressure setting value in the permeability enhancement borehole of the target coal seam is in the first set low pressure range (for example, 3.0-6.0 MPa). After the water pressure is stable, continuous ultrasonic excitation operation is applied to the permeability enhancement borehole, which has the following three effects:

[0080] First, ultrasonic excitation can be used to transform the pore and fracture structure in the coal reservoir, forming a large number of evenly distributed fractures in the early stage of fracturing, providing guided fracture channels for the extended fracturing of high-pressure water splitting coal bodies during hydraulic ultrasonic composite fracturing, helping to achieve uniform fracturing, eliminate or reduce fracturing blank areas, and form a complex and rich fracture network structure.

[0081] Secondly, ultrasonic excitation can promote fluid to fill the pores and cracks in the coal body and wet the coal surface, increase the water content of the coal reservoir, enhance the energy conduction and penetration of ultrasonic waves, and enhance the mechanical vibration effect, cavitation effect and thermal effect in the coal reservoir during ultrasonic excitation.

[0082] Thirdly, the coal rock structure can be infiltrated, softened and weakened through low-pressure ultrasonic excitation, so as to reduce the strength and plasticity of the coal rock mass, thereby increasing the fracturing influence radius and fracturing effect of hydraulic ultrasonic composite fracturing in the coal reservoir.

[0083] In the middle stage of coal seam fracturing, the present invention adopts the "high-pressure boosted hydraulic ultrasonic composite fracturing" process to apply continuous ultrasonic excitation operation in the permeability-enhancing borehole. At the same time, liquid is continuously injected into the permeability-enhancing borehole to increase the water pressure. The injection pressure is set to a set high-pressure range (for example, 24.0-36.0 MPa). When water flows out of the monitored borehole or the pump pressure is lower than 20% Pmax (Pmax is the maximum pump pressure in the hydraulic ultrasonic composite fracturing process), the fracturing is stopped. It has the following three effects:

[0084] First, continuous ultrasonic excitation can continuously cause cavitation in the water environment, bringing local high temperature of 5000K and high pressure of 2000psi, accompanied by strong shock waves and microjets with speeds of up to 400km / h, which promote the development, expansion and extension of primary cracks in the coal seam and generate a large number of new cracks.

[0085] Secondly, under the high-pressure water environment, due to the incompressibility of the water medium, the contact between the ultrasonic transducer and the coal body is closer, which effectively enhances the energy conduction and fracturing effect of the ultrasonic mechanical vibration effect. The continuous ultrasonic excitation continuously damages the surface of the fracturing coal body.

[0086] Third, under the impact of high-pressure hydraulic pressure, on the one hand, the original microcracks in the coal body expand rapidly, and on the other hand, the weak surface structure in the coal body is expanded by high-pressure water; as the water pressure continues to increase, natural cracks and new cracks form a large fracture network, and the high-pressure hydraulic impact effectively changes the pore and fracture network structure of the coal body and improves the recoverability of coalbed methane.

[0087] In the later stage of coal seam fracturing, the present invention adopts the "low-pressure pressure-maintaining ultrasonic cycle excitation to remove blockage and increase permeability" process. The water pressure setting value of the permeability-enhancing drilling water injection of the target coal seam is the second set low pressure range (for example, 3.0-6.0 MPa). After the water pressure is stable, continuous ultrasonic excitation operation is applied to the permeability-enhancing drilling, which has the following three effects:

[0088] First, a large number of new macro-scale pore and fracture structures are formed after hydraulic ultrasonic composite fracturing. By filling the fracture space with water while maintaining pressure, the conduction range of ultrasonic waves can be enhanced with water as the propagation medium. The implementation of ultrasonic excitation again can effectively improve the porosity and permeability of the coal seam.

[0089] Secondly, after hydraulic ultrasonic composite fracturing, the cracks produced by fracturing will gradually close due to the influence of ground stress, mining disturbance, etc., especially for coal seams with complex fracture networks and broken and soft coal. The implementation of ultrasonic excitation again can effectively clean the coal dust in the fracture channel and open the gas migration channel.

[0090] Thirdly, after hydraulic ultrasonic composite fracturing, the remaining moisture in the coal seam pores and fissures will block the flow channel of gas due to the water locking effect. By implementing ultrasonic excitation again and utilizing the ultrasonic cavitation effect to decompose the moisture in the cracks and vaporize it from the coal seam cracks, the water locking effect in the coal seam pores and fissures can be effectively removed and the permeability of the coal seam can be improved.

[0091] In the description of this specification, the description with reference to the terms "preferred embodiment", "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0092] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation, characterized in that: In the process of hydraulic ultrasonic composite fracturing of coal seams, the water pressure range applied to the permeability enhancement borehole is different at different stages of coal seam fracturing, specifically: In the early stage of coal seam fracturing, the "low pressure maintenance ultrasonic excitation softening and pressure relief" process is adopted. The water pressure of the coal seam's permeability enhancement drilling water injection is within the first set low pressure range. Under the coupling effect of low water pressure and ultrasonic excitation, the coal reservoir is moistened and strengthened, and the pressure is softened and relieved. In the middle stage of coal seam fracturing, the "high-pressure boosting hydraulic ultrasonic composite fracturing" process is first used. The water pressure of the coal seam's permeability-enhancing borehole injection is within the set high-pressure range, and the set high-pressure range is greater than the first set low-pressure range. The coal seam structure is physically transformed under the coupling effect of high water pressure and ultrasonic excitation. During the fracturing process, based on the pump pressure change of the injection pump, the "high-pressure boosting hydraulic ultrasonic composite fracturing" process and the "low-pressure pressure-maintaining ultrasonic excitation softening and unloading" process are adaptively alternated to enrich the pore and fracture network and increase permeability; In the later stage of coal seam fracturing, the "low-pressure maintenance ultrasonic circulation excitation unblocking and permeability enhancement" process is adopted. The water pressure of the coal seam's permeability enhancement drilling water injection is within the second set low pressure range to dredge the pore and fracture channels in the coal seam after fracturing, unblock bridges, promote desorption, and increase seepage.

2. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 1, characterized in that: The steps include: S1. Drilling: Select a target coal seam, construct an anti-permeability drill hole and a monitoring drill hole in the target coal seam, and an anti-permeability drill hole and a monitoring drill hole together constitute an anti-permeability fracturing unit; S2. Arrange the anti-permeability system: arrange the ultrasonic fracturing module and the hydraulic fracturing module in the anti-permeability borehole; S3, sealing and injecting liquid: using a sealing device to seal the permeability-enhancing borehole, starting the hydraulic fracturing module to inject water into the permeability-enhancing borehole, and filling the permeability-enhancing borehole with water; S4, low-pressure pressure-maintaining ultrasonic excitation softening and pressure relief process: adjust the water injection pressure of the hydraulic fracturing module, continuously inject water into the permeability-enhancing borehole, stabilize the water pressure in the first set low-pressure range, start the ultrasonic fracturing module to continuously emit ultrasonic waves to the coal body around the permeability-enhancing borehole, complete low-pressure pressure-maintaining ultrasonic excitation in the permeability-enhancing borehole for a period of time, realize coal reservoir humidification and softening and pressure relief, and when it is judged that the low-pressure pressure-maintaining ultrasonic excitation is completed, execute step S5; S5, high-pressure boosted hydraulic ultrasonic composite fracturing process: keep the ultrasonic fracturing module turned on, adjust the water injection pressure of the hydraulic fracturing module to the set high-pressure range, inject water into the permeability-enhanced borehole, and perform high-pressure boosted hydraulic ultrasonic composite fracturing on the target coal seam to increase fracturing and permeability, during which the maximum pump pressure of the hydraulic fracturing module is recorded as Pmax; S6, during the high-pressure boosted hydraulic ultrasonic composite fracturing, if water comes out of the monitoring borehole, stop the fracturing and execute step S9; if no water comes out of the monitoring borehole, execute step S7; S7, during the high-pressure boosted hydraulic ultrasonic composite fracturing, when the pump pressure of the hydraulic fracturing module begins to decrease, the pump pressure is recorded as P. During the process of pump pressure decrease, if the pump pressure P is less than the first pressure threshold, step S8 is executed; if the pump pressure P is greater than the first pressure threshold, the process returns to step S5; S8, during the process of pump pressure decreasing, the pump pressure P is judged twice. If the pump pressure P is greater than the second pressure threshold, and the second pressure threshold is less than the first pressure threshold, it is judged that a large fracture network structure is generated during the high-pressure boosting hydraulic ultrasonic composite fracturing, and the process returns to step S4 to perform low-pressure pressure-maintaining ultrasonic excitation softening pressure relief; if the pump pressure P is less than the second pressure threshold, it is judged that the fracturing is completed, and step S9 is executed; S9, low-pressure pressure-maintaining ultrasonic cycle excitation unblocking and permeability enhancement process: adjusting the injection pressure of the hydraulic fracturing module to the second set low-pressure range, keeping the ultrasonic fracturing module turned on, and continuously applying low water pressure and ultrasonic coupling excitation for a period of time in the permeability enhancement borehole to dredge the coal reservoir pore fissure channels generated by fracturing, unblocking and increasing permeability; S10, end of operation: after the low-pressure ultrasonic cycle excitation unblocking and permeability enhancement process is completed, close the ultrasonic fracturing module and the hydraulic fracturing module, discharge the fracturing fluid in the permeability enhancement borehole, withdraw the ultrasonic fracturing module and the hydraulic fracturing module, seal the permeability enhancement borehole and the monitoring borehole, connect them to the coalbed methane extraction pipeline respectively, and extract coalbed methane.

3. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 1, characterized in that: The first set low pressure range, the set high pressure range and the second set low pressure range are obtained in the following manner: Get the coal seam fracture pressure P q , The first set low pressure range is (1±20%)ε1P q ; The set high pressure range is (1±20%)ε2P q ; The second set low pressure range is (1±20%)ε1P q ; Among them, φ i is the water pressure control coefficient, and the value of i is 1 or 2.

4. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 3 is characterized in that: The first set low pressure range, the set high pressure range, and the second set low pressure range are all calculated according to the following formula: P=φ i ·P q Among them, φ i is the water pressure control coefficient, i takes values ​​of 1 and 2; affects the water pressure control coefficient φ i The main parameters include coal seam permeability coefficient, coal seam solidity coefficient, coal seam porosity coefficient and fracturing margin coefficient, ε i The calculation formula is as follows: e i =a·b·c·d Among them, α is the permeability coefficient of the coal seam; β is the solidity coefficient of the coal seam; γ is the porosity coefficient of the coal seam; δ is the fracturing margin coefficient; For ε1, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ = 0.8; For φ2, the value range of α is [0, 1], the value range of β is [0, 1], the value range of γ is [0, 1], and δ=1.

2.

5. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 4, characterized in that: The calculation formulas of α, β, and γ are as follows: k is the permeability of the coal seam; f is the solidity coefficient of coal; φ is the porosity of coal.

6. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 3 is characterized in that: The coal seam fracture pressure P is obtained according to the following formula q : P q =3σ1-σ3+σ t Among them, P q is the coal seam fracture initiation pressure, σ1 is the minimum horizontal principal stress of the coal seam, σ3 is the maximum horizontal principal stress of the coal seam, σ t It is the tensile strength of coal.

7. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 2, characterized in that: The first pressure threshold is 80% of the maximum pump pressure Pmax, and the second pressure threshold is 20% of the maximum pump pressure Pmax.

8. The method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to claim 2, characterized in that: In step S4, low pressure and pressure-maintaining ultrasonic excitation is performed in the anti-permeability drilling hole for 2 hours; and / or in step S9, low water pressure and ultrasonic coupling excitation is continued in the anti-permeability drilling hole for 2 hours.

9. A method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to any one of claims 2 to 8, characterized in that: The ultrasonic fracturing module includes an ultrasonic transducer installed at the front end of an energized drill rod and located in the anti-permeability drill hole. The ultrasonic transducer is electrically connected to the energized drill rod, and the end of the energized drill rod is connected to an ultrasonic generator located outside the anti-permeability drill hole through an ultrasonic cable.

10. A method for decompression and permeability enhancement of coal seams by hydraulic ultrasonic composite fracturing based on water pressure regulation according to any one of claims 2 to 8, characterized in that: The hydraulic fracturing module comprises a fracturing pipe extending into the permeability-enhancing borehole, an injection pump connected to the liquid inlet of the fracturing pipe, and a liquid storage tank connected to the liquid inlet of the injection pump. The injection pump and the liquid storage tank are arranged outside the permeability-enhancing borehole.

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