CN112922577B - A methane explosion fracturing method for multi-layer radial horizontal wells in shale reservoirs - Google Patents

Abstract

The invention discloses a shale reservoir multi-level radial horizontal well methane explosion fracturing method, which is characterized in that existing methane of a shale reservoir is used as fuel, air injected from the ground is mixed with the shale reservoir methane in radial horizontal wells to form combustible mixed fluid and explode the combustible mixed fluid, and impact pressure generated by explosion is used for fracturing the reservoir, so that large-scale three-dimensional network fractures are formed in the shale reservoir. The invention combines the conventional explosive fracturing, radial horizontal well technology and supercritical CO₂The method has the advantages that the displacement technology is combined, the shale reservoir is divided into different transformation areas by reasonably arranging the layer number, the branch number and the azimuth angle of the radial horizontal well, and methane combustion and explosion fracturing in the radial horizontal well is implemented in the corresponding areas, so that the traditional combustion and explosion fracturing operation area is expanded from a shaft to the deep part of the stratum, the methane combustion and explosion position and direction can be controlled, the purpose of improving the combustion and explosion fracturing efficiency of the reservoir is achieved, and the efficient development of shale gas resources is realized.

Description

A methane explosion fracturing method for multi-layer radial horizontal wells in shale reservoirs

technical field

The invention relates to the technical field of unconventional natural gas development and shale gas reservoir fracturing and stimulation, in particular to a methane explosion fracturing method for multi-layer radial horizontal wells in shale reservoirs.

Background technique

At present, most of the main onshore oil and gas fields in my country have entered the middle and late stages of development and exploitation, showing remarkable characteristics of high recovery, high comprehensive water cut, and high decline rate of development productivity, which makes oilfield companies face a very tight trend of stable production. In 2019, my country's dependence on foreign oil and natural gas was 70.8% and 43.4% respectively, and the safe supply of oil and gas is facing huge challenges. Under such severe reality, it is particularly important to find alternative energy sources for conventional oil and gas. At present, unconventional natural gas represented by shale gas has become an important part of the world's natural gas supply. The shale gas resources in China are about 134.42×10 ¹² m ³ , which is 2.5 times that of conventional natural gas. The efficient development of gas resources is of great strategic significance for ensuring national energy security.

Shale reservoirs mainly use nano-pores as storage spaces, with a permeability of about (0.001-1)×10 ^-3 mD, making it extremely difficult to develop. More than 90% of shale gas reservoirs need to undergo hydraulic fracturing before they can survive A highly dense network of fractures is formed in the reservoir, so that artificial fractures and natural fractures are intertwined to form "man-made shale gas reservoirs", in order to obtain industrial production capacity. In 2017, the U.S. horizontal well fracturing technology contributed as much as 97% to shale gas production, becoming the “absolute main force” driving the growth of shale gas production. It can be seen that reservoir fracturing has become the core technology for the efficient development of shale gas. With the continuous expansion of fracturing scale, hydraulic fracturing technology also brings a series of problems, first of all, it is reflected in the gas reservoir damage caused by fracturing fluid retention. For most shale gas reservoirs, water is the wetting phase, and the invasion and retention of water phase will cause serious water lock and water sensitivity damage to the reservoir. will be more serious. It can be seen that how to eliminate or minimize the damage of water to the formation during the fracturing process is one of the keys to the stimulation of unconventional gas reservoirs such as shale gas. In addition, the shale gas reservoirs in my country are generally buried deep (2000-3500 m), the reservoir rocks have strong plasticity, and the horizontal stress difference of the formation is large. The conventional hydraulic fracturing method is not easy to form complex fractures, and it is difficult to achieve efficient development of shale gas. technical requirements. my country's shale gas blocks are characterized by mountainous and hilly landforms, narrow well sites and blocked traffic, which greatly affects the application of large-scale hydraulic fracturing technology. The terrain is relatively flat in the north and west, and most of them face the problems of water scarcity and serious water pollution. The hydraulic fracturing technology, which uses tens of thousands of cubic meters of water at every turn, will undoubtedly further aggravate the tension of water resources in these areas.

In order to improve the fracture degree of reservoir rocks, increase the complexity of fracturing fractures, and overcome the dependence of reservoir fracturing on water resources, some researchers fracturing the formation by means of wellbore blasting fracturing to form complex fractures near the wellbore. crack system. Research shows that the rapid combustion of combustibles in the wellbore can instantaneously generate high pressures above 60 MPa, and fracture expansion can break through the in-situ stress limit, forming multiple radial short fractures along the wellbore. In order to further increase the fracture extension distance, the researchers used the combination of different firing rates of different gunpowder to form multiple continuously loaded pulse pressures in the formation by controlling the firing rate of the fracturing gunpowder to increase the wellbore detonation time, and then achieve multi-stage The effect of explosive fracturing. However, in this method, the wellbore is still detonated, and it is difficult for the fracture to extend to the remote formation, and it is difficult to form a large-scale network fracture. Therefore, in the process of developing shale gas, it is necessary to break through the limitations of the existing technical framework and develop a new fracturing process to achieve green and efficient development of shale gas resources.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a methane explosion fracturing method for multi-layer radial horizontal wells in shale reservoirs, so as to solve the problems in the prior art, such as single fracture shape of shale reservoir fracturing and difficult formation of complex fracture network, etc. So as to realize the efficient development of shale gas resources.

In order to achieve the above purpose, the technical solution adopted in the present invention is as follows: a method for methane explosion and fracturing of multi-layer radial horizontal wells in shale reservoir, comprising the following steps:

1) Determining the fracturing interval: According to the logging data of the main shale gas wellbore and the geological characteristics of the reservoir encountered, the shale reservoir is divided into several fracturing intervals, and each interval acts as an independent work process;

2) Coiled tubing sidetracking radial horizontal wells: The coiled tubing sidetracking method is used to drill several radial horizontal wells perpendicular to the main wellbore inside the fracturing interval of the shale gas reservoir. The inner radial horizontal wells are arranged in upper and lower layers. The radial horizontal wells in the same fracturing interval are collectively called a construction well group, the radial horizontal well group in the lower layer is called an injection well group, and the radial horizontal well group in the upper layer is called a construction well group. For the fracturing well group;

3) Arranging downhole tools and surface equipment: After all construction well groups are drilled, the fracturing tools are lowered to the bottom of the well through coiled tubing, and surface equipment is installed and connected; the fracturing tools include a first-stage injector, a second-stage Ejector, first stage packer, second stage packer and third stage packer, said first stage packer, first stage ejector, second stage packer, second stage ejector The first-stage injector and the second-stage injector are all equipped with switchable sliding sleeves for opening and closing the fluid outlet on the injector, and the second-stage injector is An ignition electrode is also installed inside the device; the ground equipment includes a fluid injection system, coiled tubing equipment, a ground ignition device and a switchable sliding sleeve controller, the coiled tubing has a built-in cable, and the front end of the cable is respectively connected with the ignition electrode and the switchable sliding sleeve. The starting device is connected to the starting device, and the end of the cable is respectively connected to the ground ignition device and the switchable sliding sleeve controller;

In the initial stage, the fracturing tool is first lowered to the lowest fracturing operation interval. The specific arrangement plan of the fracturing tool is: the first stage packer is located below the injection well group in the fracturing operation interval, and the second stage packer is located below the injection well group in the fracturing operation interval. The first-stage packer is located between the injection well group and the fracturing well group, and the third-stage packer is located above the fracturing well group. The inlet of the fracturing well group is at the same depth; before the fracturing operation, the switchable sliding sleeves in the ejector are all closed, and the internal space of the fracturing tool is not connected to the wellbore at this time;

4) Supercritical _CO2 displacement operation: open the switchable sliding sleeve in the first-stage ejector, inject supercritical _CO2 fluid into the injection well group through the surface fluid injection system through the coiled tubing, and after the supercritical _CO2 enters the bottom hole The fluid outlet on the first-stage injector flows into the injection well group in the fracturing operation interval. A closed space is formed between the horizontal wellbore; under the action of fluid pressure, supercritical CO ₂ penetrates into the reservoir around the radial horizontal well of the injection well group; supercritical CO ₂ can be displaced and adsorbed on the shale when it flows in the reservoir and replace it into free state. With the continuous injection of supercritical CO ₂ , the free state methane gas in the reservoir is gradually displaced to the fracturing well group above the fracturing interval, and further flows into the reservoir. Fracturing well group; under the displacement of supercritical CO ₂ , a large amount of methane gas flows into the radial horizontal wellbore of the fracturing well group, and a methane accumulation zone is formed in the reservoir around the radial horizontal wellbore, resulting in this Increased methane concentrations in the region;

5) Pump air into the fracturing well group: close the switchable sliding sleeve in the first-stage injector, then open the switchable sliding sleeve in the second-stage injector, and inject the fluid into the fracturing well group through the coiled tubing through the surface fluid injection system The air is injected into the radial horizontal wellbore, and the air injected into the fracturing well group is mixed with the existing methane in the radial horizontal wellbore to form a methane-air mixed fluid;

6) Methane explosion fracturing operation: start the ground ignition device, and use the spark released by the ignition electrode in the second-stage injector to ignite the methane-air mixed fluid in the radial horizontal wellbore of the fracturing well group, so that the Explosion, using the instantaneous shock wave generated by methane explosion and the impact of high temperature and high pressure gas to fracture the reservoir rock, so as to form complex artificial fractures around the radial horizontal wellbore to realize methane explosion fracturing in shale reservoirs;

7) Lift up the fracturing tool: lift the fracturing tool in the main wellbore to the next fracturing section by recovering the coiled tubing, and the arrangement of the fracturing tool is the same as step 3);

8) Repeat steps 4) to 7) to continue supercritical CO ₂ flooding and methane explosion fracturing until the methane explosion fracturing of the shale reservoir in the entire main wellbore is completed.

Preferably, in step 2), the number and orientation of radial horizontal wells in each layer in each fracturing operation interval are the same.

Preferably, in step 4), the injection pressure of supercritical CO ₂ is higher than the formation pressure of the reservoir and lower than the fracture pressure or fracture extension pressure of the reservoir.

Further, in step 3), described fluid injection system comprises air bottle group, CO ₂ bottle group, air compressor, gas booster and heater, the air outlet of air bottle group and CO ₂ bottle group is respectively connected with gas The inlet of the supercharger is connected, the outlet of the air compressor is connected with another inlet of the gas supercharger, the outlet of the gas supercharger is connected with the inlet of the heater, and the outlet of the heater is connected with the injection end of the coiled tubing.

The invention combines conventional blasting and fracturing, radial horizontal well technology and supercritical CO ₂ displacement technology, and divides shale reservoirs into different stimulation areas by rationally arranging the number of layers, branches and azimuths of radial horizontal wells , and implement methane explosion fracturing in the radial horizontal wellbore in the corresponding area, which not only expands the traditional explosion fracturing operation area from the wellbore to the depth of the formation, but also helps to control the location of methane explosion and explosion. direction, and then achieve the purpose of improving the efficiency of reservoir blasting and fracturing. Due to the extremely low surface tension and strong diffusivity of supercritical _CO2 , it is easier to flow in tight shale reservoirs than other fluids. The use of supercritical _CO2 flooding is beneficial to improve the flow of methane into the radial wellbore. Desorption efficiency. Moreover, the adsorption capacity of supercritical CO ₂ to shale is greater than that of methane molecules, so supercritical CO ₂ can also replace part of the adsorbed methane into free state, and further displace it into the radial horizontal wellbore to be fractured.

The methane blasting fracturing method for shale reservoirs proposed by the present invention firstly utilizes radial horizontal well technology to provide shale reservoir methane with a larger blasting space and a farther blasting position, thereby effectively reducing conventional blasting and fracturing. The scope of action extends from around the main wellbore to the depth of the reservoir, and then complex artificial fractures are formed inside the reservoir, which effectively solves the limitation of insufficient fracture expansion range in blast fracturing. In addition, the instantaneous impact generated by methane explosion can overcome the limitation of in-situ stress on the direction of fracture propagation, which helps to prevent fractures from expanding and extending in a single direction, which in turn helps to form a complex fracture network in the reservoir.

Description of drawings

Fig. 1 is a schematic diagram of the distribution of fracturing operation intervals in a shale reservoir in the method of the present invention;

Fig. 2 is the schematic diagram of radial horizontal well distribution in the method of the present invention;

Fig. 3 is the schematic diagram of downhole fracturing tool and surface equipment in the method of the present invention;

Fig. 4 is the schematic diagram of closing the sliding sleeve in the downhole fracturing tool in the method of the present invention;

5 is a schematic diagram of the opening of the sliding sleeve in the downhole fracturing tool in the method of the present invention;

Fig. 6 is supercritical CO ₂ displacement process schematic diagram in the method of the present invention;

7 is a schematic diagram of the process of pumping air into the fracturing well group in the method of the present invention;

Fig. 8 is the schematic diagram of methane explosion fracturing in radial horizontal well in the method of the present invention;

9 is a schematic diagram of the lifting of the downhole fracturing tool in the method of the present invention;

Fig. 10 is the second supercritical CO ₂ displacement schematic diagram in the method of the present invention;

11 is a schematic diagram of the second methane explosion fracturing of radial horizontal wells in the method of the present invention;

12 is a schematic diagram of the overall fracturing effect of the reservoir in the method of the present invention;

In the figure: 1. Main wellbore, 2. Fracturing section 1, 3. Fracturing section 2, 4. Fracturing section 3, 5a-5f, radial horizontal well (injection well group), 6a ~6f, radial horizontal well (fractured well group), 7, coiled tubing, 8a, first stage packer, 8b, second stage packer, 8c, third stage packer, 9a, first stage packer Stage Injector, 9b, Second Stage Injector, 10, Injector Fluid Outlet, 11, Ignition Electrode, 12, Cable, 13, Switchable Sleeve, 14, Ignition Device, 15, Switchable Sleeve Controller, 16 , air bottle group, 17, CO ₂ bottle group, 18, gas booster, 19, air compressor, 20, heater, 21, coiled tubing vehicle, 22, coiled tubing roller, 23, coiled tubing injection head, 24, _CO2 fluid, 25, Supercritical _CO2 fluid, 26, Methane, 27, Supercritical _CO2 displacement zone, 28, Supercritical _CO2 -methane mixing zone, 29, Methane accumulation zone, 30, Air, 31 , high pressure air, 32, complex cracks.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

The invention provides a methane explosion fracturing method for multi-layer radial horizontal wells in shale reservoirs, which aims to improve the control volume of a single well of shale gas and form complex fractures in the reservoir. In the specific implementation, the existing methane in the shale reservoir is used as fuel, the air injected from the ground is mixed with the methane in the shale reservoir to form a combustible mixed fluid, and then the combustible fluid is ignited in the radial horizontal wellbore by means of downhole ignition. The explosion generates shock pressure to fracture the reservoir, and then forms a large-scale three-dimensional network fracture, which specifically includes the following steps:

1) Determine the fracturing interval of the shale reservoir. According to the logging data of the main shale gas wellbore and the geological characteristics of the reservoir encountered, the reservoir is divided into several fracturing intervals. In the embodiment shown in FIG. 1 , the reservoir drilled by the main shale gas wellbore 1 is divided into three fracturing operation intervals: namely fracturing operation interval 1 2, fracturing operation interval 2 3, Fracturing operation interval three 4, each operation interval as an independent working process;

2) Coiled tubing sidetracks radial horizontal wells. As shown in Figure 2, the coiled tubing sidetracking method is used to drill two layers of radial horizontal wells perpendicular to the main wellbore 1 in each fracturing interval. The number and orientation of radial horizontal wells in each layer are the same. The drilled radial horizontal wells are all located in the divided fracturing intervals. In the embodiment shown in FIG. 2 , the radial horizontal wells located at the same depth are arranged with symmetrical double wings, and the axis of the wellbore is parallel to the direction of the minimum horizontal in-situ stress. The radial horizontal wells in the same fracturing interval are collectively referred to as a construction well group, that is, each construction well group contains two upper and lower radial horizontal well groups, and each group contains several radial horizontal wells. The horizontal well group is called injection well group, and the radial horizontal well group in the upper layer is called fracturing well group. As shown in FIG. 2 , the construction well group in the fracturing operation interval 1 2 includes radial horizontal wells 5a, 5b, 6a, 6b, and the construction well group in the fracturing operation interval 2 3 includes radial horizontal wells 5c, 5d, 6c, 6d, the construction well group in the fracturing operation interval 34 includes radial horizontal wells 5e, 5f, 6e, 6f. Among them, radial horizontal wells 5a-5f are injection well groups in corresponding fracturing operation intervals, and radial horizontal wells 6e-6f are fracturing well groups in corresponding fracturing operation intervals.

3) Arrange downhole tools and surface equipment. After all the construction well groups are drilled, the fracturing tool is lowered to the bottom fracturing operation section 1 2 through the coiled tubing 7 as shown in Figure 3, and ground equipment is installed and connected. The fracturing tool used consists of a first-stage packer 8a, a first-stage injector 9a, a second-stage packer 8b, a second-stage injector 9b and a third-stage packer 8c from bottom to top , that is, the two-stage injectors are located between the two packers, respectively. The specific arrangement plan of the fracturing tool is as follows: the first-stage packer 8a is located below the injection well group (radial horizontal wells 5a-5b) of the fracturing operation interval 1 2, and the second-stage packer is located in the fracturing operation section 12. Between the injection well group (radial horizontal wells 5a-5b) and the fracturing well group (radial horizontal wells 6a-6b) of the fracturing interval 12, the third-stage packer is located in the fracturing well group (radial horizontal wells 6a-6b). Above the horizontal wells 6a-6b), the fluid outlet 10 of the first-stage injector 9a is aimed at the inlet of the injection well group (radial horizontal wells 5a-5b), and the fluid outlet 10 of the second-stage injector 9b is aimed at the fracturing The inlet of the well group (radial horizontal wells 6a-6b). The used second-stage injector 9 b has a built-in ignition electrode 11 , and the coiled tubing 7 has a built-in cable 12 . The front end of the cable 12 is connected to the ignition electrode 11 in the fracturing tool and the switchable sliding sleeve 13 on the injectors 9a-9b (Figs. 4 and 5), and the end of the cable is connected to the ground ignition device 14 and the switchable sliding sleeve controller 15. connect. As shown in FIG. 4 , before the fracturing operation, the sliding sleeves 13 of the injectors 9 a to 9 b are all closed, and at this time, the fluid cannot enter the wellbore 1 from the fluid outlet 10 on the injector.

In addition to the surface ignition device 14 and the switchable sliding sleeve controller 15, the surface equipment also includes a fluid injection system and coiled tubing equipment. The fluid injection system includes an air bottle group 16, a CO ₂ bottle group 17, a gas booster 18, an air compressor 19 and a heater 20. The outlets of the air bottle group 16 and the CO ₂ bottle group 17 are respectively connected with the outlet of the gas booster 18. The inlet is connected, the outlet of the air compressor 19 is connected with the other inlet of the gas booster 18, the outlet of the gas booster 18 is connected with the inlet of the heater 20, and the outlet of the heater 20 is connected with the injection end of the coiled tubing 7 . In addition to the coiled tubing 7, the coiled tubing equipment also includes a coiled tubing vehicle 21, a coiled tubing drum 22, and a coiled tubing injection head 23. The coiled tubing drum 22 is arranged on the frame of the coiled tubing operation vehicle 21. The coiled tubing One end of the coiled tubing 7 is wound on the coiled tubing drum 22, the other end of the coiled tubing 7 is connected to the fracturing tool, and the coiled tubing injection head 23 is arranged on the coiled tubing 7 exposed from the wellhead device.

4) Supercritical CO ₂ displacement operation. Activate the switchable sliding sleeve controller 15 on the surface, so that the sliding sleeve 13 in the first-stage injector 9a moves downward to open the sliding sleeve 13 (Fig. 5). fluid outlet 10 on the fracturing tool. As shown in FIG. 6 , the valve of the CO ₂ bottle group 17 is opened to allow the _{CO 2} fluid 24 to enter the gas booster 18 through the ground pipeline. 24 is pressurized to above the critical pressure (7.38MPa), and then heated to above the critical temperature (31.1° C.) through the heater 20 . At this time, the fluid injected through the coiled tubing 7 is the supercritical CO ₂ fluid 25 . After the supercritical CO ₂ fluid 25 enters the bottom hole, it flows out from the fluid outlet 10 on the fracturing tool body 9a. Under the sealing action of the first-stage packer 8a and the second-stage packer 8b, the packer and the injection well A closed space will be formed between the radial horizontal wells 5a-5b of the group, and the supercritical CO ₂ fluid 25 will penetrate into the surrounding reservoirs of the radial horizontal wells 5a-5b of the injection well group. Since the adsorption strength of supercritical CO ₂ and shale is much greater than that of methane and shale, supercritical CO ₂ can replace the methane adsorbed on shale when it flows in the reservoir, and replace it into a free state. With the continuous injection of supercritical _CO2 fluid 25, the free methane gas in the reservoir is gradually displaced around the fracturing well group, and further flows from the reservoir into the radial horizontal wells 6a-6b of the fracturing well group, Methane 26 is formed in the free state. Under the continuous displacement of supercritical _CO2 fluid 25, supercritical _CO2 displacement will form between the injection well group (radial horizontal wells 5a-5b) and the fracturing well group (radial horizontal wells 6a-6b) Zone 27, supercritical _CO2 -methane mixing zone 28 and methane accumulation zone 29. The methane accumulation zone 29 will cause the methane concentration to increase around the radial horizontal wells 6a-6b, and a large amount of methane 26 will enter the wellbore. During supercritical _CO2 flooding, the injection pressure of supercritical _CO2 fluid 25 is slightly higher than the formation pressure of the reservoir and lower than the reservoir fracture or fracture propagation pressure.

5) Pump air into the fracturing well group. By controlling the ground switchable sliding sleeve controller 15, the sliding sleeve 13 in the first-stage injector 9a is moved upward to close the sliding sleeve according to the state shown in FIG. 4, and the sliding sleeve 13 in the second-stage injector 9b is controlled downward. The sliding sleeve is opened by the movement as shown in FIG. 5 , at this time, the fluid in the coiled tubing 7 can enter the radial horizontal wells 6a-6b through the fluid outlet 10 on the second-stage injector 9b. As shown in FIG. 7 , open the valve of the air cylinder group 16, and use the gas booster 18 to pressurize the air 30 flowing out of the air cylinder group 16 to form high-pressure air 31 in the manner of step 4). When the high-pressure air 31 is pumped, the heater 20 is turned off, that is, the air injected into the bottom of the well is not heated. After the high-pressure air 31 enters the bottom of the well, it enters the fracturing well group (radial horizontal wells 6a-6b) in this interval through the fluid outlet 10 on the second-stage injector 9b, and mixes with the methane 26 in the wellbore A methane-air mixed fluid is formed.

6) Methane explosion fracturing operation. As shown in FIG. 8, the ground ignition device 14 is activated, and the methane-air mixed fluid in the radial horizontal wells 6a-6b is ignited by the electric spark released by the ignition electrode 11 in the second-stage injector 9b, so that it explodes . The shock wave generated by the instantaneous explosion of methane and the high temperature and high pressure gas are used to impact the reservoir rock, and complex fractures 32 are formed around the radial horizontal wells 6a-6b, so as to realize the explosion and fracturing of methane in the shale reservoir. Due to the short action time of methane detonation and the fast pressure boosting rate in the radial horizontal wellbore, the complex fractures 32 generated by methane detonation fracturing not only include conventional tensile fractures, but also undergo shear damage with the reservoir rock. It also causes complex mechanical behaviors such as slippage and dislocation of the crack surface. Since the dislocation and slip of the rocks on both sides of the shear fracture in the reservoir are irrecoverable, this greatly enhances the self-supporting ability of the fracture, and can effectively prevent the fracture from closing after compression, thereby improving the effect of reservoir fracturing.

7) As shown in Fig. 9, the fracturing tool in the main wellbore 1 is lifted up to the fracturing operation interval 2 3 by recovering the coiled tubing 7. The arrangement of the fracturing tool is the same as that in step 3): the fracturing tool The first-stage packer 8a is located below the injection well group (radial horizontal wells 5c-5d) in this interval, and the second-stage packer 8b is located at the injection well group (radial horizontal wells 5c-5d) and fracturing wells Between groups (radial horizontal wells 6c-6d), the third-stage packer 8c is located above the fracturing well group (radial horizontal wells 6c-6d), and the first-stage injector 9a and the second-stage injector 9b The fluid outlets 10 are aligned with the inlets of the injection well group and the fracturing well group in the interval, respectively. The switchable sliding sleeves 13 in both stage injectors are closed.

8) Repeat steps 4) to 6) to continue supercritical CO ₂ flooding and methane explosion fracturing operations. The supercritical CO ₂ flooding and methane explosion fracturing in section 23 of the fracturing operation is shown in the figure 10 and Figure 11.

9) As shown in FIG. 12 , steps 7) to 8) are repeated until the methane explosion fracturing of the shale reservoir of the entire main wellbore 1 is completed.

The methane blasting fracturing method for shale reservoirs proposed by the present invention firstly utilizes radial horizontal well technology to provide shale reservoir methane with a larger blasting space and a farther blasting position. Fuel or explosives can only be extended to deep reservoir detonation in the main wellbore detonation operation, and complex artificial fractures are formed inside the reservoir, which effectively solves the limitation of insufficient expansion range of fractures in detonation fracturing. In addition, the instantaneous impact produced by methane explosion can overcome the limitation of in-situ stress on the direction of fracture propagation, which helps to prevent fractures from expanding and extending in a single direction, thereby helping to form a complex fracture network in the reservoir. Due to the short action time of methane detonation and the fast pressure increase in radial horizontal wells, the fractures generated by methane detonation fracturing not only include conventional tensile fractures, but also shear damage along with the reservoir rocks and cause fractures on the fracture surface. Complex mechanical behaviors such as slippage and dislocation. Since the dislocation and slip of the rocks on both sides of the shear fracture in the reservoir are irrecoverable, this greatly enhances the self-supporting ability of the fracture, and can effectively prevent the fracture from closing after compression, thereby improving the effect of reservoir fracturing.

Claims (4)

1. a shale reservoir multi-level radial horizontal well methane explosion fracturing method, is characterized in that, comprises the following steps: 1) Determining the fracturing interval: According to the logging data of the main shale gas wellbore and the geological characteristics of the reservoir encountered, the shale reservoir is divided into several fracturing intervals, and each interval acts as an independent work process; 2) Coiled tubing sidetracking radial horizontal wells: The coiled tubing sidetracking method is used to drill several radial horizontal wells perpendicular to the main wellbore inside the fracturing interval of the shale gas reservoir. The inner radial horizontal wells are arranged in upper and lower layers. The radial horizontal wells in the same fracturing interval are collectively called a construction well group, the radial horizontal well group in the lower layer is called an injection well group, and the radial horizontal well group in the upper layer is called a construction well group. For the fracturing well group; 3) Arranging downhole tools and surface equipment: After all construction well groups are drilled, the fracturing tools are lowered to the bottom of the well through coiled tubing, and surface equipment is installed and connected; the fracturing tools include a first-stage injector, a second-stage Ejector, first stage packer, second stage packer and third stage packer, said first stage packer, first stage ejector, second stage packer, second stage ejector The first-stage injector and the second-stage injector are all equipped with switchable sliding sleeves for opening and closing the fluid outlet on the injector, and the second-stage injector is An ignition electrode is also installed inside the device; the ground equipment includes a fluid injection system, coiled tubing equipment, a ground ignition device and a switchable sliding sleeve controller, the coiled tubing has a built-in cable, and the front end of the cable is respectively connected with the ignition electrode and the switchable sliding sleeve. The starting device is connected to the starting device, and the end of the cable is respectively connected to the ground ignition device and the switchable sliding sleeve controller; In the initial stage, the fracturing tool is first lowered to the lowest fracturing operation interval. The specific arrangement plan of the fracturing tool is: the first stage packer is located below the injection well group in the fracturing operation interval, and the second stage packer is located below the injection well group in the fracturing operation interval. The first-stage packer is located between the injection well group and the fracturing well group, and the third-stage packer is located above the fracturing well group. The inlet of the fracturing well group is at the same depth; before the fracturing operation, the switchable sliding sleeves in the ejector are all closed, and the internal space of the fracturing tool is not connected to the wellbore at this time; 4) Supercritical _CO2 displacement operation: open the switchable sliding sleeve in the first-stage ejector, inject supercritical _CO2 fluid into the injection well group through the surface fluid injection system through the coiled tubing, and after the supercritical _CO2 enters the bottom hole The fluid outlet on the first-stage injector flows into the injection well group in the fracturing operation interval. A closed space is formed between the horizontal wells. Under the action of fluid pressure, supercritical CO ₂ will penetrate into the reservoir around the radial horizontal wells of the injection well group; when supercritical CO ₂ flows in the reservoir, it can displace and adsorb in the shale. The methane above the fracturing layer is replaced by the free state. With the continuous injection of supercritical _CO2 , the free methane gas in the reservoir is gradually displaced to the fracturing well group above the fracturing interval, and further from the reservoir. Flow into the fracturing well group; under the displacement of supercritical CO ₂ , a large amount of methane gas flows into the radial horizontal wellbore of the fracturing well group, and the reservoir forms around the radial horizontal wellbore of the fracturing well group Methane accumulation zone, resulting in increased methane concentration in the reservoir around the radial horizontal wells of the fracturing well group; 5) Pump air into the fracturing well group: close the switchable sliding sleeve in the first-stage injector, then open the switchable sliding sleeve in the second-stage injector, and inject the fluid into the fracturing well group through the coiled tubing through the surface fluid injection system The air is injected into the radial horizontal wellbore, and the air injected into the fracturing well group is mixed with the existing methane in the radial horizontal wellbore to form a methane-air mixed fluid; 6) Methane explosion fracturing operation: start the ground ignition device, and use the spark released by the ignition electrode in the second-stage injector to ignite the methane-air mixed fluid in the radial horizontal wellbore of the fracturing well group, so that the Explosion, using the instantaneous shock wave generated by methane explosion and the impact of high temperature and high pressure gas to fracture the reservoir rock, so as to form complex artificial fractures around the radial horizontal wellbore to realize methane explosion fracturing in shale reservoirs; 7) Lift up the fracturing tool: lift the fracturing tool in the main wellbore to the next fracturing section by recovering the coiled tubing, and the arrangement of the fracturing tool is the same as step 3); 8) Repeat steps 4) to 7) to continue supercritical CO ₂ flooding and methane explosion fracturing until the methane explosion fracturing of the shale reservoir in the entire main wellbore is completed. 2. a kind of shale reservoir multi-level radial horizontal well methane explosion fracturing method according to claim 1, is characterized in that, in step 2), in each fracturing operation interval, the radial direction of each layer The number and orientation of horizontal wells are the same. 3. a kind of shale reservoir multilayer radial horizontal well methane explosion fracturing method according to claim 1, is characterized in that, in step 4), the injection pressure of supercritical CO ₂ is higher than the formation of reservoir pressure, and is lower than the reservoir fracture pressure or fracture propagation pressure. 4. a kind of shale reservoir multi-layer radial horizontal well methane explosion fracturing method according to claim 1, is characterized in that, in step 3), described fluid injection system comprises air cylinder group, CO ₂ Bottle group, air compressor, gas booster and heater, the air outlet of the air bottle group and _CO2 bottle group are respectively connected with the inlet of the gas booster, and the outlet of the air compressor is connected with the other inlet of the gas booster , the outlet of the gas booster is connected with the inlet of the heater, and the outlet of the heater is connected with the injection end of the coiled tubing.

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