Carbon dioxide assisted phase change fracturing method for coal seam anti-reflection

Abstract

The invention discloses a carbon dioxide assisted phase change fracturing method for coal seam anti-reflection. A vertical well and a horizontal well connected with the vertical well are drilled according to well site parameters; high-pressure sand-carrying fluid is pumped into the spraying unit close to the previous stage through the first oil pipe, the corresponding spraying gun is driven to rotate at a high speed, meanwhile, the high-pressure sand-carrying fluid is injected into the horizontal well through the corresponding spraying gun to conduct hole-forming and permeation-increasing operation, and operation is stopped after pulverized coal does not return out of the ground; the plugging ball body is put in, the first plunger pump is restarted, the next stage of hole forming and permeation increasing operation is carried out, and the hole forming and permeation increasing operation is repeatedly carried out till the hole forming and permeation increasing operation of all the hole forming sections is completed; the horizontal well and the cave set are filled with liquid CO2 and then blocked; clear water makes contact with quick lime and fully reacts with the quick lime to release heat, the released heat is used for rapidly increasing the temperature of liquid CO2, the liquid CO2 is rapidly subjected to phase change into gaseous CO2, the underground pressure intensity is sharply increased in the phase change process, the fracturing effect is achieved, and finally a fracture net is formed. According to the method, the fracturing and permeability increasing effects on the reservoir can be remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas production enhancement and transformation, and in particular relates to a carbon dioxide-assisted phase change fracturing method for increasing the permeability of coal seams. Background Art

[0002] Coalbed methane is an efficient and clean fossil energy. It occupies an important position in my country's energy structure, with a total resource volume of about 43 trillion m 3 The geological resources of coalbed methane in the shallower area of ​​2000m are as high as 30.05 trillion m 3 , of which recoverable resources are approximately 12.50 trillion m 3 As a new type of energy, the development of coalbed methane can make up for the shortage of conventional energy development and utilization to a great extent. However, my country's coal seams have the characteristics of low saturation, low permeability and low pressure, which leads to low production of coalbed methane wells. The most commonly used technology is hydraulic fracturing. However, traditional hydraulic fracturing development technology will lead to a large amount of water resources being lost, the invasion and retention of water phase will cause potential damage to the reservoir and serious water lock effect, groundwater pollution and difficulty in handling return fluid, etc., which is not conducive to the sustained high-speed growth of the economy and the transformation of the energy structure to green and low-carbon. Therefore, finding more efficient and environmentally friendly fracturing technology has become one of the research hotspots in the field of oil and gas extraction.

[0003] CO2 fracturing technology is a highly promising new technology for increasing the production of unconventional oil and gas reservoirs. While effectively avoiding the problems of water resource waste, formation damage, and environmental pollution caused by traditional hydraulic fracturing, this technology has the advantages of low fracturing pressure and complex fracture formation. In addition, it also has the dual gas enhanced extraction effects of fracturing permeability enhancement and displacement replacement, and can also achieve CO2 geological storage. It is a research hotspot in the current low-permeability coal seam fracturing transformation method. Its fracturing mechanism is to use the characteristics of CO2 liquid phase conversion to gas phase for blasting. CO2 exists in liquid state below 31°C and 7.2MPa pressure, and when it exceeds 31°C, no matter how high its pressure is, liquid CO2 instantly expands into gas, and the volume rapidly expands to blast the surrounding objects, making full use of the physical and chemical properties of CO2 itself to improve the transformation effect of the reservoir. This technology is a new waterless fracturing technology with broad application prospects, which will greatly promote the green and efficient development of unconventional oil and gas in my country. To this end, it is urgent to provide a carbon dioxide-assisted phase change fracturing method for coal seam permeability enhancement. Summary of the invention

[0004] In view of the above-mentioned problems of the prior art, the present invention provides a carbon dioxide-assisted phase change fracturing method for coal seam permeability enhancement. This method can overcome the technical problems of traditional fracturing methods such as single fracture formation, large water resource demand, difficulty in maintaining pressure, and easy pollution of the working face. It can significantly improve the fracturing and permeability enhancement effects on the reservoir.

[0005] In order to achieve the above object, the present invention provides a carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement, comprising the following steps:

[0006] Step 1: Coal seam resource positioning and well site design;

[0007] S11: Determine the geological conditions of the development area based on the geological parameters of the underground rock formations and exploration data, and accurately locate the resource location and well depth;

[0008] S12: designing well site parameters according to the determined resource location and well depth, and then drilling vertical wells and horizontal wells connected to the vertical wells according to the well site parameters;

[0009] Step 2: First hole-making and infiltration enhancement operation;

[0010] S21: prepare an abrasive bearing box, a liquid nitrogen storage tank, a stirring device, a plunger pump 1 and an oil pipe 1 on the ground; support the abrasive bearing box above the stirring device, and align the feed port of the abrasive bearing box with the main feed port at the upper end of the stirring device, connect the outlet end of the liquid nitrogen storage tank with the auxiliary feed port on one side of the upper end of the stirring device by using a connecting pipe 1, and connect the inlet end of the plunger pump 1 with the discharge port of the stirring device by using a connecting channel 2; connect the inlet end of the oil pipe 1 with the outlet end of the plunger pump 1, connect the multi-stage jet cavitation string to the outlet end of the oil pipe 1, and use the oil pipe 1 to extend the multi-stage jet cavitation string below the target operating section of the horizontal well;

[0011] The multi-stage jet cavitation column includes a filling pipeline and multi-stage jet units serially connected to the filling pipeline at intervals. One end of the filling pipeline is an open end and is connected to the outlet end of the oil pipe. The other end is a closed structure, and a plurality of installation notches are sequentially opened at intervals in the length direction. The multi-stage jet units are correspondingly distributed in the plurality of installation notches. Each stage of the jet unit includes two rotary sealing nipples, a spray gun and an eccentric director. The two rotary sealing nipples are relatively distributed at the two ends of the installation notch, and the ends away from each other are respectively connected to the filling pipelines on both sides. The ends are sealed and connected, and the connecting shafts at both ends of the spray gun are rotatably connected to the ends of the two rotating sealing short sections that are close to each other; the eccentric director is sleeved on the outside of the connecting shaft on one side of the spray gun, and is used to play a centering role during the rotation of the spray gun; the inner diameter of the multi-stage injection unit decreases from the end close to the oil pipe one to the end away from the oil pipe one, and a ball accommodating cavity is provided in the rotating sealing short section close to the end of the oil pipe one in each stage of the injection unit; after the sealing ball of the corresponding size is loaded into the ball accommodating cavity, the rotating sealing short section is transformed into a closed state;

[0012] S22: adding the abrasive in the abrasive carrying box and the liquid nitrogen in the liquid nitrogen storage tank into the stirring device, and using the stirring device to fully stir the abrasive and the liquid nitrogen to form a sand-carrying fluid;

[0013] S23: Start the plunger pump 1, and at the same time, open the injection unit of the first stage in the multi-stage injection cavitation column, use the plunger pump 1 to extract the sand-carrying fluid and pressurize it, so that the high-pressure sand-carrying fluid is pumped into the injection unit of the first stage through the oil pipe 1, and the corresponding spray gun is driven to rotate at high speed. At the same time, it is injected into the horizontal well through the corresponding spray gun to perform cavitation and permeability enhancement operations. During the cavitation and permeability enhancement operation, the sand-carrying fluid is subjected to high-pressure impact and low-temperature fracturing operations on the inner wall coal seam of the horizontal well. At the same time, a high-speed rotating spray gun is used to perform a high-speed rotating jet operation, and the radial turbulent pulsation is synchronously used to increase the jet diffusion angle and impact range, and a large-aperture cave is formed. After the coal powder is no longer returned from the ground, the plunger pump 1 is controlled to stop, and the cavitation and permeability enhancement operation is stopped;

[0014] Step 3: Carry out subsequent cavitation and infiltration enhancement operations;

[0015] S31: a plugging ball matching the size of the injection unit at the first stage is put into the oil pipe 1, and the plugging ball enters the injection unit at the first stage, and the rotary sealing nipple in the injection unit at the first stage is plugged by the plugging ball, so that the corresponding spray gun is set;

[0016] S32: opening the injection unit of the second previous stage in the multi-stage injection cavitation string, restarting the plunger pump 1, pumping the high-pressure sand-carrying fluid into the injection unit of the second previous stage through the oil pipe 1, and injecting it into the horizontal well through the corresponding spray gun, and continuing the cavitation and permeability enhancement operation of the next stage, until the coal powder stops returning from the ground, controlling the plunger pump 1 to stop, and stopping the cavitation and permeability enhancement operation;

[0017] S33: executing S31 to S32 repeatedly until the cavitation and permeability enhancement operations of all subsequent cavitation sections are completed, and finally a plurality of large-diameter caves are formed along the length direction of the horizontal well and are distributed in sequence. The plurality of large-diameter caves constitute a cave group.

[0018] Step 4: Run the double oil pipes and perform fracturing operations;

[0019] S41: Prepare a clean water storage tank, a CO2 storage tank, a second plunger pump, a third plunger pump, a second oil pipe, a third oil pipe, a bridge plug, a quicklime bearing tank and a water injection valve on the ground; the clean water storage tank, the CO2 storage tank and the quicklime bearing tank store clean water, liquid CO2 and quicklime respectively, and the quicklime bearing tank is provided with a heat dissipation hole connected with its inner cavity, and is provided with a water inlet connected with its inner cavity;

[0020] Use connecting pipe 3 to connect the outlet end of the clean water storage tank and the inlet end of plunger pump 2, connect the inlet end of oil pipe 2 to the outlet end of plunger pump 2, and connect the quicklime carrying box to the outlet end of oil pipe 2; use connecting pipe 4 to connect the outlet end of the CO2 storage tank and the inlet end of plunger pump 3, connect the inlet end of oil pipe 3 to the outlet end of plunger pump 3, and put the bridge plug on the outside of the outlet section of oil pipe 3;

[0021] S42: synchronously lowering the bridge plug and the quicklime bearing box to the wellhead of the horizontal well by using the oil pipe 2 and the oil pipe 3, and making the bridge plug located at the upstream side of the quicklime bearing box;

[0022] S43: Start the plunger pump 3, use the plunger pump 3 to extract liquid CO2 in the CO2 storage tank, and pump it into the horizontal well through the outlet end of the oil pipe 3, until the horizontal well and the cave group are filled, then stop the pumping operation of liquid CO2, use the control switch to set the bridge plug, and seal the wellhead position of the horizontal well;

[0023] S44: Start plunger pump 2, use plunger pump 2 to extract clean water from the clean water storage tank, and inject it into the quicklime carrying tank through oil pipe 2, so that the clean water contacts the quicklime and fully reacts to release heat. The released heat is used to rapidly increase the temperature of liquid CO2 in the horizontal well and cave, and rapidly change into gaseous CO2. The phase change process is used to increase the pressure underground to achieve the fracturing effect, and finally form a fracture network.

[0024] As a preference, in S11 in step one, the resource location is the location of the coal seam.

[0025] Preferably, in S12 of step 1, the well site parameters include the drilling depth of the vertical well, the drilling depth and the drilling distance of the horizontal well.

[0026] As a preference, in step 21, the multi-stage injection units are respectively a first-stage injection unit, a second-stage injection unit, a third-stage injection unit and a fourth-stage injection unit from an end away from the oil pipe 1 to an end close to the oil pipe 1.

[0027] Furthermore, in order to ensure the subsequent cavitation and fracturing effects, in step 2 S23, before the cavitation and permeability enhancement operation, liquid nitrogen or nitrogen gas is used to perform 2 to 3 well washing operations to displace the air and moisture in the pipelines and equipment. During the well washing operation, nitrogen recovery operations are carried out simultaneously.

[0028] Furthermore, in order to ensure the cavitation effect, in step 2 S23, the displacement of the plunger pump 1 is 0.3-0.5m 3 / min, the jet velocity of the rotating jet is 100-150m / s.

[0029] As a preference, in step 2 S21, the oil pipe 1 is made of a heat-insulating material, and in step 4 S41, the oil pipe 2 and the oil pipe 3 are both made of a heat-insulating material.

[0030] Furthermore, in order to ensure the fracturing effect, in step 4 S41, the quicklime stored in the quicklime carrying box accounts for 1 / 3 to 1 / 2 of the capacity of the quicklime carrying box.

[0031] As a preferred embodiment, in step 4 S43, after all operations are completed, a long-term well shut-in treatment is first carried out, with the shut-in time being not less than 5 hours, and then high-pressure nitrogen is injected through oil pipe 3 to displace the residual CO2 into the reservoir to achieve the storage operation of the residual CO2.

[0032] Furthermore, in order to effectively save energy, in step 2 S23, during the cavitation and infiltration process, the nitrogen backflow collection operation is carried out simultaneously; in step 3 S31 to S33, during the cavitation and infiltration process, the nitrogen backflow collection operation is carried out simultaneously.

[0033] The present invention aims to propose a deep coalbed methane resource permeability enhancement method based on a combination of abrasive jet technology and waterless fracturing technology, aiming to overcome the problems of low permeability enhancement efficiency and small fracturing range of low-saturation, low-permeability, and low-pressure coal seams in the prior art, and to provide effective technical support for the effective exploitation of deep coal seam resources. Specifically, the sand-carrying fluid formed by the mixture of liquid nitrogen and abrasive is pressurized, and a rotating jet operation is performed through a spray gun. Due to the ultra-low temperature of liquid nitrogen, the reservoir rock will produce tensile damage. At the same time, the high-pressure gas generated by the liquid nitrogen during the gasification and expansion process will have a strong impact on the reservoir rock, which will cause cracks and microcracks in the reservoir rock, significantly improving the permeability of the reservoir. At the same time, the use of a rotating jet can form turbulent pulsation in the radial direction, thereby increasing the jet diffusion angle and impact range, thereby forming a cave with a larger aperture. After the cave is formed, the liquid CO2 is filled into the horizontal well and cave group. The temperature of the liquid CO2 in the well will gradually increase, and then a phase change will occur and a large pressure will be generated on the cave and the horizontal well. This pressure can create a fracture network, thereby achieving the purpose of assisting in improving the fracturing effect. On this basis, the large amount of heat released during the reaction of clean water and quicklime is used to act on the liquid CO2, which can rapidly increase the temperature of the liquid CO2 in the well in a short period of time, thereby accelerating the phase change rate and effect of the liquid CO2, and rapidly increasing the surrounding pressure. Therefore, the fracturing effect is significantly improved by assisting in accelerating the phase change of CO2.

[0034] This method combines abrasive jet and waterless fracturing technology, in which the jet process also uses liquid nitrogen abrasive rotating jet to make holes, and the rotating jet plays a role in expanding the aperture. This method can greatly increase the contact area between the coal seam and the liquid CO2 fracturing fluid, providing a reliable guarantee for the subsequent improvement of the liquid CO2 fracturing effect. At the same time, it can also make full use of the ultra-low temperature characteristics and gasification expansion characteristics of liquid nitrogen to further achieve the effect of increasing permeability. Liquid nitrogen carrying abrasive jets will undergo phase change after entering the well, which will also complement the phase change of liquid CO2. In addition, abrasives can be used to support cracks, making it difficult for micro-cracks in the well to close, and can effectively improve the conductivity of micro-cracks, further increasing the contact area of ​​the fracturing fluid, significantly improving the fracturing effect, and forming a combination of multiple fracturing methods. The entire fracturing process does not adopt the operation method of water contacting the coal seam, which can avoid secondary damage to the reservoir by foreign fluids. Liquid nitrogen is a green and environmentally friendly material for increasing oil and gas production. The nitrogen returned to the ground can be recovered during operation, filtered, stored and reused later, and can also be used as a raw material for preparing liquid nitrogen. Even if the returned nitrogen cannot be fully recovered, it will not cause any pollution to the environment, underground and surface water resources. This method overcomes the technical problems of traditional fracturing methods such as single fracture formation, large water resource demand, difficulty in maintaining pressure, and easy contamination of the working surface. It uses multiple fracturing methods and auxiliary phase change methods to perform fracturing and permeability enhancement operations, which significantly improves the fracturing and permeability enhancement effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of drilling of a vertical well and a horizontal well in the present invention;

[0036] Figure 2 It is a schematic diagram of the state of the first cavitation and permeability enhancement operation using the sand-carrying fluid in the present invention;

[0037] Figure 3 It is a schematic diagram of the state of the secondary cavitation and infiltration enhancement operation using the sand-carrying fluid in the present invention;

[0038] Figure 4 It is a schematic diagram of the distribution of cave groups in the present invention;

[0039] Figure 5 It is a schematic diagram of the state of preparation work for liquid CO2 and clean water to enter the well in the present invention;

[0040] Figure 6 This is a schematic diagram of the state of liquid CO2 filling in the present invention;

[0041] Figure 7 This is a schematic diagram of the state in which the present invention utilizes the exothermic reaction of clean water and quicklime to cause the CO2 in the underground cave to undergo phase change;

[0042] Figure 8It is a schematic diagram of the state of CO2 phase change fracturing in the present invention.

[0043] In the figure: 1, rock formation, 2, vertical well, 3, horizontal well, 4, abrasive bearing box, 5, liquid nitrogen storage tank, 6, stirring device, 7, sand-carrying fluid, 8, multi-stage jet cavitation string, 9, first-stage jet unit, 10, second-stage jet unit, 11, third-stage jet unit, 12, fourth-stage jet unit, 13, eccentric director, 14, rotary seal short section, 15, oil pipe one, 16, plunger pump one, 17, cave, 18, cave group, 19, bridge plug, 20, quicklime bearing box, 21, water injection valve, 22, liquid CO2, 23, CO2 storage tank, 24, clean water, 25, clean water storage tank, 26, gaseous CO2, 27, fracture network, 28, filling pipeline, 29, spray gun, 30, oil pipe two, 31, oil pipe three, 32, plunger pump two, 33, plunger pump three. DETAILED DESCRIPTION

[0044] The present invention will be further described below in conjunction with the accompanying drawings.

[0045] like Figures 1 to 8 As shown, the present invention provides a carbon dioxide-assisted phase change fracturing method for coal seam permeability enhancement, that is, a multiple fracturing method combining liquid nitrogen jet technology with assisted CO2 phase change. First, a vertical well and a horizontal well connected thereto are drilled in the coal seam, and a liquid nitrogen abrasive rotary jet operation is performed in the horizontal well to create a plurality of jet caves. Compared with the operation mode of the straight jet, the turbulent pulsation in the radial direction of the rotary jet mode increases the jet diffusion angle and the impact range, and can form a cave with a larger aperture. Then, the well is filled with liquid CO2 and a quicklime bearing box filled with quicklime is lowered, and then the wellhead of the horizontal well is blocked with a bridge plug. Finally, clean water is injected into the quicklime bearing box, and the liquid CO2 phase change is assisted by utilizing the characteristic that a large amount of heat is released when the contacting clean water and quicklime react, so that the ambient temperature in the well is rapidly increased in a short time, the rate of CO2 changing from liquid phase to gas phase and the effect of thermal expansion are aggravated, and the pressure in the well and the cave surges instantly, thereby achieving the purpose of fracturing and permeability enhancement.

[0046] The specific steps include:

[0047] Step 1: Coal seam resource location and well site design, such as Figure 1 As shown;

[0048] S11: Determine the geological conditions of the development area based on the geological parameters and exploration data of the underground rock layer 1, and accurately locate the resource location and well depth;

[0049] S12: designing well site parameters according to the determined resource location and well depth, and then drilling a vertical well 2 and a horizontal well 3 connected to the vertical well 2 according to the well site parameters;

[0050] Step 2: First hole-making and infiltration enhancement operation;

[0051] S21: Figure 2 As shown, an abrasive bearing box 4, a liquid nitrogen storage tank 5, a stirring device 6, a plunger pump 16 and an oil pipe 15 are prepared on the ground; the abrasive bearing box 4 is supported above the stirring device 6, and the feed port of the abrasive bearing box 4 is aligned with the main feed port at the upper end of the stirring device 6, and the outlet end of the liquid nitrogen storage tank 5 is connected to the auxiliary feed port on one side of the upper end of the stirring device 6 by a connecting pipe 1, and the inlet end of the plunger pump 16 is connected to the discharge port of the stirring device 6 by a connecting channel 2; the inlet end of the oil pipe 15 is connected to the outlet end of the plunger pump 16, and the multi-stage jet cavitation string 8 is connected to the outlet end of the oil pipe 15, and the oil pipe 15 is used to extend the multi-stage jet cavitation string 8 to the target operation section of the horizontal well 3;

[0052] The multi-stage jet cavitation column 8 includes a filling pipeline 28 and multi-stage jet units sequentially connected to the filling pipeline 28 at intervals. One end of the filling pipeline is an open end and is connected to the outlet end of the oil pipe 15. The other end is a closed structure, and a plurality of installation notches are sequentially opened at intervals in the length direction. The multi-stage jet units are correspondingly distributed in the plurality of installation notches. Each stage of the jet unit includes two rotary sealing nipples 14, a spray gun 29 and an eccentric director 13. The two rotary sealing nipples 14 are relatively distributed at the two ends of the installation notch, and the ends away from each other are respectively sealed and connected to the ends of the filling pipeline 28 on both sides. The spray gun 2 The connecting shafts at both ends of 9 are rotatably connected to the ends of the two rotating seal short sections 14 that are close to each other, and are sealed to the two rotating seal short sections 14; the eccentric direction device 13 is sleeved on the outside of the connecting shaft on one side of the spray gun 19, and is used to play a centering role during the rotation process of the spray gun 29; the inner diameter of the multi-stage injection unit decreases from the end close to the oil pipe 15 to the end away from the oil pipe 15, and a ball accommodating cavity is provided in the rotating seal short section 14 close to the end of the oil pipe 15 in each stage of the injection unit; after the sealing ball of the corresponding size is loaded into the ball accommodating cavity, the rotating seal short section 14 is transformed into a closed state;

[0053] S22: Add the abrasive in the abrasive carrying box 4 and the liquid nitrogen in the liquid nitrogen storage tank 5 into the stirring device 6, and use the stirring device 6 to fully stir the abrasive and the liquid nitrogen to form a sand-carrying fluid 7;

[0054] S23: Start the plunger pump 16, and at the same time, open the injection unit of the first stage in the multi-stage injection cavitation column 8, use the plunger pump 16 to extract the sand-carrying fluid 7 and pressurize it, so that the high-pressure sand-carrying fluid 7 is pumped into the injection unit of the first stage through the oil pipe 15, and the corresponding spray gun 29 is driven to rotate at a high speed. At the same time, it is injected into the horizontal well 3 through the corresponding spray gun 29 to perform cavitation and permeability enhancement operations. During the cavitation and permeability enhancement operation, the sand-carrying fluid 7 is used to perform high-pressure impact and low-temperature fracturing operations on the inner wall coal seam 1 of the horizontal well 3. At the same time, the high-speed rotating spray gun 29 is used to perform high-speed rotating jet operations, and the radial turbulent pulsation is synchronously used to increase the jet diffusion angle and impact range, and a large-aperture cave 17 is formed. After the coal powder is no longer returned from the ground, the plunger pump 16 is controlled to stop, and the cavitation and permeability enhancement operation is stopped;

[0055] As a preferred embodiment, the large-aperture caves 17 are formed to be equally and symmetrically distributed on both sides of the horizontal well 3;

[0056] Step 3: Carry out subsequent cavitation and infiltration enhancement operations;

[0057] S31: Figure 3 and Figure 4 As shown, a plugging ball matching the size of the injection unit at the previous stage is put into the oil pipe 15, and the plugging ball enters the injection unit at the previous stage, and the rotating sealing nipple 14 in the injection unit at the previous stage is plugged by the plugging ball, so that the corresponding spray gun 29 is set;

[0058] S32: Open the injection unit of the second previous stage in the multi-stage injection cavitation string 8, restart the plunger pump 16, pump the high-pressure sand-carrying fluid 7 into the injection unit of the second previous stage through the oil pipe 15, and inject it into the horizontal well 3 through the corresponding spray gun 19, and continue the cavitation and infiltration operation of the next stage until the coal powder stops returning from the ground, control the plunger pump 6 to stop, and stop the cavitation and infiltration operation;

[0059] S33: S31 to S32 are executed repeatedly until the cavitation and permeability enhancement operations of all subsequent cavitation sections are completed, and finally a plurality of large-diameter caves 17 are formed along the length direction of the horizontal well 3 and are distributed in sequence. The plurality of large-diameter caves 17 form a cave group 18;

[0060] Step 4: Run the double oil pipes and perform fracturing operations;

[0061] S41: Figure 5As shown, a clean water storage tank 25, a CO2 storage tank 23, a plunger pump 2 32, a plunger pump 3 33, an oil pipe 2 30, an oil pipe 3 31, a bridge plug 19, a quicklime carrying tank 20 and a water injection valve 21 are prepared on the ground; the clean water storage tank 25, the CO2 storage tank 23 and the quicklime carrying tank 20 store clean water, liquid CO2 and quicklime respectively, and the quicklime carrying tank 20 is provided with a heat dissipation hole connected with its inner cavity, and is provided with a water inlet connected with its inner cavity; wherein, the volume expansion pressure of liquid CO2 at a temperature of 0°C and a pressure of 101kPa is about 600 times that of the liquid state, and the jet pressure of gaseous CO2 can reach 200MPa.

[0062] The outlet end of the clean water storage tank 25 and the inlet end of the plunger pump 32 are connected by connecting pipeline 3, the inlet end of the oil pipe 30 is connected to the outlet end of the plunger pump 32, and the quicklime carrying box 20 is connected to the outlet end of the oil pipe 32. As a preferred embodiment, the outlet end of the oil pipe 32 is also connected in series with a water injection valve 21. During the process of injecting clean water, the water injection valve 21 remains in an open state; the outlet end of the CO2 storage tank 23 and the inlet end of the plunger pump 33 are connected by connecting pipeline 4, the inlet end of the oil pipe 31 is connected to the outlet end of the plunger pump 33, and the bridge plug 19 is sleeved on the outside of the outlet section of the oil pipe 31;

[0063] S42: synchronously lowering the bridge plug 19 and the quicklime carrying box 20 to the wellhead position of the horizontal well 3 by using the oil pipe 2 30 and the oil pipe 3 31, and making the bridge plug 19 located at the upstream side of the quicklime carrying box 20;

[0064] S43: Figure 6 As shown, the plunger pump 33 is started, and the liquid CO2 in the CO2 storage tank 23 is extracted by the plunger pump 33, and pumped into the horizontal well 3 through the outlet end of the oil pipe 31, until the horizontal well 3 and the cave group 18 are filled, and then the pumping operation of the liquid CO2 is stopped, and the bridge plug 19 is set by the control switch to block the wellhead position of the horizontal well 3;

[0065] S44: Figure 7 and Figure 8 As shown, the plunger pump 2 32 is started, and the plunger pump 2 32 is used to extract the clean water in the clean water storage tank 25, and the clean water is pumped into the quicklime carrying tank 20 through the oil pipe 2 30, so that the clean water contacts the quicklime and fully reacts to release heat, and the released heat is used to rapidly increase the temperature of the liquid CO2 in the horizontal well 3 and the cave 17, and rapidly changes into gaseous CO2, and the pressure in the well is increased sharply by the phase change process, so as to achieve the effect of fracturing, and finally form a fracture network 27. As a preferred embodiment, during the water injection process, the ratio of quicklime to clean water is maintained between 1:1 and 1:5.

[0066] Among them, the phase change fracturing of liquid CO2 is actually physical work. In this method, a large amount of heat energy is generated by the reaction of water and quicklime, which helps to improve the effect and rate of the phase change of liquid CO2, so that the temperature and pressure of the liquid CO2 rise instantly until it exceeds the critical point and enters a critical state. The pressure in the well and the cave rises sharply, reaching or exceeding the fracture strength of the coal seam. The gaseous CO2 breaks through the coal seam constraints and acts on the coal seam in the form of high-pressure and high-speed jet impact, generating stress waves in the coal seam and realizing phase change rock breaking. Coal seams form cracks under the action of high pressure. The low-temperature liquid nitrogen and liquid nitrogen phase change in the cave formation stage of this method have already created cracks in the coal seams in caves and wells, allowing the liquid CO2 to contact a large area of ​​coal seams in the phase change fracturing stage. After the phase change, high-pressure gaseous CO2 enters the cracks. The splitting characteristics of the gas make the original cracks, liquid nitrogen and liquid CO2 cracks continue to develop and penetrate. At the same time, the pressure will continue to decrease with the consumption of energy until the stress at the crack tip is less than the fracture strength of the rock mass, and the fracturing ends.

[0067] As a preference, in S11 in step one, the resource location is the location of the coal seam 1 .

[0068] As a preference, in S12 of step one, the well site parameters include the drilling depth of the vertical well 2 , the drilling depth and the drilling distance of the horizontal well 3 .

[0069] As a preference, in step 21, the multi-stage injection units are respectively a first-stage injection unit 9, a second-stage injection unit 10, a third-stage injection unit 11 and a fourth-stage injection unit 12 from one end away from the oil pipe 15 to the end close to the oil pipe 15.

[0070] In order to ensure the subsequent cavitation and fracturing effects, in step 2 S23, before the cavitation and permeability enhancement operation, liquid nitrogen or nitrogen is used to perform 2 to 3 well washing operations to displace the air and moisture in the pipeline and equipment. During the well washing operation, nitrogen recovery operations are carried out simultaneously.

[0071] In order to ensure the cavitation effect, in step 2 S23, the displacement of the plunger pump 16 is 0.3-0.5m 3 / min, the jet velocity of the rotating jet is 100-150m / s.

[0072] As a preferred embodiment, in step 2 S21, the oil pipe 15 is made of insulating material, and in step 4 S41, the oil pipe 2 30 and the oil pipe 3 31 are both made of insulating material. In this way, it can be effectively ensured that the fluid will not undergo phase change during transportation.

[0073] In order to ensure the fracturing effect, in step 4 S41 , the quicklime stored in the quicklime carrying box 20 accounts for 1 / 3 to 1 / 2 of the capacity of the quicklime carrying box 20 .

[0074] As a preferred embodiment, in step 4 S43, after all operations are completed, a long-term well shut-in treatment is first carried out, with the shut-in time being not less than 5 hours, and then high-pressure nitrogen is injected through oil pipe 31 to displace the residual CO2 into the reservoir to achieve the storage operation of the residual CO2. This is not only conducive to the storage of CO2, but also can effectively prevent CO2 from being discharged back to the ground along with methane in the later production process, thereby achieving the effect of increasing production.

[0075] In order to effectively save energy, in step 2 S23, during the cavitation and infiltration process, the nitrogen backflow collection operation is carried out simultaneously; in step 3 S31 to S33, during the cavitation and infiltration process, the nitrogen backflow collection operation is carried out simultaneously.

[0076] The present invention aims to propose a deep coalbed methane resource permeability enhancement method based on a combination of abrasive jet technology and waterless fracturing technology, aiming to overcome the problems of low permeability enhancement efficiency and small fracturing range of low-saturation, low-permeability, and low-pressure coal seams in the prior art, and to provide effective technical support for the effective exploitation of deep coal seam resources. Specifically, the sand-carrying fluid formed by the mixture of liquid nitrogen and abrasive is pressurized, and a rotating jet operation is performed through a spray gun. Due to the ultra-low temperature of liquid nitrogen, the reservoir rock will produce tensile damage. At the same time, the high-pressure gas generated by the liquid nitrogen during the gasification and expansion process will have a strong impact on the reservoir rock, which will cause cracks and microcracks in the reservoir rock, significantly improving the permeability of the reservoir. At the same time, the use of a rotating jet can form turbulent pulsation in the radial direction, thereby increasing the jet diffusion angle and impact range, thereby forming a cave with a larger aperture. After the cave is formed, the liquid CO2 is filled into the horizontal well and cave group. The temperature of the liquid CO2 in the well will gradually increase, and then a phase change will occur and a large pressure will be generated on the cave and the horizontal well. This pressure can create a fracture network, thereby achieving the purpose of assisting in improving the fracturing effect. On this basis, the large amount of heat released during the reaction of clean water and quicklime is used to act on the liquid CO2, which can rapidly increase the temperature of the liquid CO2 in the well in a short period of time, thereby accelerating the phase change rate and effect of the liquid CO2, and rapidly increasing the surrounding pressure. Therefore, the fracturing effect is significantly improved by assisting in accelerating the phase change of CO2.

[0077] This method combines abrasive jet and waterless fracturing technology, in which the jet process also uses liquid nitrogen abrasive rotating jet to make holes, and the rotating jet plays a role in expanding the aperture. This method can greatly increase the contact area between the coal seam and the liquid CO2 fracturing fluid, providing a reliable guarantee for the subsequent improvement of the liquid CO2 fracturing effect. At the same time, it can also make full use of the ultra-low temperature characteristics and gasification expansion characteristics of liquid nitrogen to further achieve the effect of increasing permeability. Liquid nitrogen carrying abrasive jets will undergo phase change after entering the well, which will also complement the phase change of liquid CO2. In addition, abrasives can be used to support cracks, making it difficult for micro-cracks in the well to close, and can effectively improve the conductivity of micro-cracks, further increasing the contact area of ​​the fracturing fluid, significantly improving the fracturing effect, and forming a combination of multiple fracturing methods. The entire fracturing process does not adopt the operation method of water contacting the coal seam, which can avoid secondary damage to the reservoir by foreign fluids. Liquid nitrogen is a green and environmentally friendly material for increasing oil and gas production. The nitrogen returned to the ground can be recovered during operation, filtered, stored and reused later, and can also be used as a raw material for preparing liquid nitrogen. Even if the returned nitrogen cannot be fully recovered, it will not cause any pollution to the environment, underground and surface water resources. This method overcomes the technical problems of traditional fracturing methods such as single fracture formation, large water resource demand, difficulty in maintaining pressure, and easy contamination of the working surface. It uses multiple fracturing methods and auxiliary phase change methods to perform fracturing and permeability enhancement operations, which significantly improves the fracturing and permeability enhancement effects.

Claims

1. A carbon dioxide-assisted phase change fracturing method for increasing coal seam permeability, characterized in that: The following steps are involved: Step 1: Coal seam resource positioning and well site design; S11: Determine the geological conditions of the development area based on the geological parameters and exploration data of the underground rock layer (1), and accurately locate the resource location and well depth; S12: designing well site parameters according to the determined resource location and well depth, and then drilling a vertical well (2) and a horizontal well (3) connected to the vertical well (2) according to the well site parameters; Step 2: First hole-making and infiltration enhancement operation; S21: prepare an abrasive bearing box (4), a liquid nitrogen storage tank (5), a stirring device (6), a plunger pump (16) and an oil pipe (15) on the ground; support the abrasive bearing box (4) above the stirring device (6), and align the feed port of the abrasive bearing box (4) with the main feed port at the upper end of the stirring device (6); connect the outlet end of the liquid nitrogen storage tank (5) with the auxiliary feed port on one side of the upper end of the stirring device (6) by means of a connecting pipe (1); connect the inlet end of the plunger pump (16) with the discharge port of the stirring device (6) by means of a connecting channel (2); connect the inlet end of the oil pipe (15) with the outlet end of the plunger pump (16), connect the multi-stage jet cavitation column (8) to the outlet end of the oil pipe (15), and use the oil pipe (15) to extend the multi-stage jet cavitation column (8) downward to the target operation section of the horizontal well (3); The multi-stage jet cavitation column (8) comprises a filling pipeline (28) and multi-stage jet units which are sequentially connected to the filling pipeline (28) at intervals. One end of the filling pipeline is an open end and is connected to the outlet end of the oil pipe (15). The other end is a closed structure, and a plurality of installation notches are sequentially opened at intervals in the length direction. The multi-stage jet units are correspondingly distributed in the plurality of installation notches. Each stage of the jet unit comprises two rotary sealing short sections (14), a spray gun (29) and an eccentric director (13). The two rotary sealing short sections (14) are relatively distributed at the two ends of the installation notch, and the ends which are far away are respectively sealed with the ends of the filling pipeline (28) on both sides. The spray gun (29) is connected to the spray gun (29) by a sealing connection, wherein the connecting shafts at both ends of the spray gun (29) are rotatably connected to the adjacent ends of the two rotating sealing short sections (14); the eccentric direction device (13) is mounted on the outside of the connecting shaft on one side of the spray gun (19) to play a centering role during the rotation of the spray gun (29); the inner diameter of the multi-stage spray unit decreases from the end close to the oil pipe (15) to the end away from the oil pipe (15), and a ball accommodating cavity is provided in the rotating sealing short section (14) close to the end of the oil pipe (15) in each stage of the spray unit; after a sealing ball of corresponding size is loaded into the ball accommodating cavity, the rotating sealing short section (14) is transformed into a closed state; S22: Adding the abrasive in the abrasive carrying box (4) and the liquid nitrogen in the liquid nitrogen storage tank (5) into the stirring device (6), and using the stirring device (6) to fully stir the abrasive and the liquid nitrogen to form a sand-carrying fluid (7); S23: Start the plunger pump (16), and at the same time, open the injection unit of the first stage in the multi-stage injection cavitation column (8), use the plunger pump (16) to extract the sand-carrying fluid (7) and pressurize it, so that the high-pressure sand-carrying fluid (7) is pumped into the injection unit of the first stage through the oil pipe (15), and the corresponding spray gun (29) is driven to rotate at a high speed. At the same time, it is injected into the horizontal well (3) through the corresponding spray gun (29) to perform cavitation and permeability enhancement operations. During the cavitation and permeability enhancement operation, the sand-carrying fluid (7) is subjected to high-pressure impact and low-temperature fracturing operations on the inner wall coal layer (1) of the horizontal well (3). At the same time, the high-speed rotating spray gun (29) performs high-speed rotating jet operations, and synchronously utilizes radial turbulent pulsation to increase the jet diffusion angle and impact range, and form a large-aperture cave (17). After the coal powder is no longer returned from the ground, the plunger pump (16) is controlled to stop, and the cavitation and permeability enhancement operation is stopped; Step 3: Carry out subsequent cavitation and infiltration enhancement operations; S31: a plugging ball having a size matching that of the injection unit of the first stage is put into the oil pipe 1 (15), and the plugging ball is inserted into the injection unit of the first stage, and the rotary sealing nipple (14) in the injection unit of the first stage is plugged by the plugging ball, so that the corresponding spray gun (29) is set; S32: Open the injection unit of the second previous stage in the multi-stage injection cavitation column (8), restart the plunger pump one (16), pump the high-pressure sand-carrying fluid (7) into the injection unit of the second previous stage through the oil pipe one (15), and inject it into the horizontal well (3) through the corresponding spray gun (19), and continue the cavitation and infiltration operation of the next stage until the coal powder stops returning from the ground, then control the plunger pump one (6) to stop, and stop the cavitation and infiltration operation; S33: Execute S31 to S32 repeatedly until the cavitation and permeability enhancement operations of all subsequent cavitation sections are completed, and finally a plurality of large-diameter caves (17) are formed along the length direction of the horizontal well (3) and are distributed in sequence. The plurality of large-diameter caves (17) form a cave group (18); Step 4: Run the double oil pipes and perform fracturing operations; S41: Prepare a clean water storage tank (25), a CO2 storage tank (23), a second plunger pump (32), a third plunger pump (33), a second oil pipe (30), a third oil pipe (31), a bridge plug (19), a quicklime carrying tank (20) and a water injection valve (21) on the ground; the clean water storage tank (25), the CO2 storage tank (23) and the quicklime carrying tank (20) store clean water, liquid CO2 and quicklime respectively, and the quicklime carrying tank (20) is provided with a heat dissipation hole connected to its inner cavity, and is provided with a water inlet connected to its inner cavity; The outlet end of the clean water storage tank (25) and the inlet end of the second plunger pump (32) are connected by connecting pipe No. 3, the inlet end of the second oil pipe (30) is connected to the outlet end of the second plunger pump (32), and the quicklime carrying box (20) is connected to the outlet end of the second oil pipe (32); the outlet end of the CO2 storage tank (23) and the inlet end of the third plunger pump (33) are connected by connecting pipe No. 4, the inlet end of the third oil pipe (31) is connected to the outlet end of the third plunger pump (33), and the bridge plug (19) is sleeved on the outer side of the outlet section of the third oil pipe (31); S42: synchronously using the oil pipe 2 (30) and the oil pipe 3 (31) to lower the bridge plug (19) and the quicklime carrying box (20) to the wellhead position of the horizontal well (3), and making the bridge plug (19) located at the upstream side of the quicklime carrying box (20); S43: Start the plunger pump three (33), use the plunger pump three (33) to extract liquid CO2 in the CO2 storage tank (23), and pump it into the horizontal well (3) through the outlet end of the oil pipe three (31), until the horizontal well (3) and the cave group (18) are filled, then stop the pumping operation of liquid CO2, use the control switch to set the bridge plug (19), and seal the wellhead position of the horizontal well (3); S44: Start the second plunger pump (32), use the second plunger pump (32) to extract clean water from the clean water storage tank (25), and pump it into the quicklime carrying tank (20) through the second oil pipe (30), so that the clean water and quicklime contact and fully react to release heat. The released heat is used to rapidly increase the temperature of the liquid CO2 in the horizontal well (3) and the cave (17), and rapidly change into gaseous CO2. The phase change process is used to increase the pressure downhole, so as to achieve the fracturing effect, and finally form a fracture network (27).

2. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 1, characterized in that: In step S11 of step 1, the resource location is the location of the coal seam (1).

3. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 1 or 2, characterized in that: In step S12 of step 1, the well site parameters include the drilling depth of the vertical well (2), the drilling depth and drilling distance of the horizontal well (3).

4. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 3, characterized in that: In step 21, the multi-stage injection units are respectively a first-stage injection unit (9), a second-stage injection unit (10), a third-stage injection unit (11) and a fourth-stage injection unit (12) from an end away from the oil pipe 1 (15) to an end close to the oil pipe 1 (15).

5. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 4, characterized in that: In step 2 S23, before the cavitation and permeability enhancement operation, liquid nitrogen or nitrogen gas is used to perform 2 to 3 well flushing operations to displace the air and moisture in the pipeline and equipment. During the well flushing operation, nitrogen recovery operations are performed simultaneously.

6. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 5, characterized in that: In step 2 S23, the displacement of the plunger pump 1 (16) is 0.3-0.5 m 3 / min, the jet velocity of the rotating jet is 100-150m / s.

7. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 6, characterized in that: In step 2 S21, the oil pipe 1 (15) is made of a heat-insulating material. In step 4 S41, the oil pipe 2 (30) and the oil pipe 3 (31) are both made of a heat-insulating material.

8. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 7, characterized in that: In step 4 S41, the quicklime stored in the quicklime carrying box (20) accounts for 1 / 3 to 1 / 2 of the capacity of the quicklime carrying box (20).

9. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 8, characterized in that: In step 4 S43, after all operations are completed, a long-term well shut-in treatment is first performed, the well shut-in time is not less than 5 hours, and then high-pressure nitrogen is injected through oil pipe 3 (31) to displace the residual CO2 into the reservoir to achieve the storage operation of the residual CO2.

10. A carbon dioxide assisted phase change fracturing method for coal seam permeability enhancement according to claim 9, characterized in that: In step 2 S23, during the cavitation and permeation enhancement operation, the nitrogen backflow collection operation is carried out simultaneously; in step 3 S31 to S33, during the cavitation and permeation enhancement operation, the nitrogen backflow collection operation is carried out simultaneously.

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