A method and system for improving the complexity of pulsating CO2 composite fracture network in tight oil and gas reservoirs

Abstract

The present invention relates to a method and system for improving the complexity of a pulsating CO2 composite fracturing network in a tight oil and gas reservoir, comprising: pre-acid pretreatment; using CO2 and water for pulsating alternating construction to perform a first CO2 composite fracturing construction; stopping the pump for a period of time; performing a second high-viscosity fracturing fluid fracturing construction; depressurizing and backflowing, and the fracturing construction is completed. The present invention uses acid pretreatment to solve the formation pollution and improve the physical properties of the rock. The present invention combines the high permeability and diffusivity of CO2 with the fatigue damage caused to the rock by pulsating fracturing to promote the formation of a complex fracture network. The pressure is held and the pump is stopped for a period of time, so that the CO2 can fully expand to the deep formation, and the local stress field is turned to reduce the horizontal stress difference of the reservoir. A secondary high-viscosity fracturing fluid fracturing construction is carried out to carry the proppant to the deep formation, support and expand the fracture network, and ultimately improve the complexity of the fracture network.

Description

Technical Field

[0001] The invention relates to the technical field of tight oil and gas reservoir fracturing transformation technology, and in particular to a method and system for improving the complexity of a pulsating CO2 composite fracturing network in a tight oil and gas reservoir. Background Art

[0002] With the development of the oil and gas industry, most of the conventional oil and gas reservoirs have been developed, and the amount of resources in conventional oil and gas reservoirs has gradually decreased. In recent years, unconventional oil and gas reservoirs have received extensive attention. Among them, the representative tight oil and gas reservoirs are rich in resources and have huge development potential, which is the focus of unconventional oil and gas reservoir development.

[0003] Tight oil and gas reservoirs have great development potential, but compared with conventional oil and gas reservoirs, tight oil and gas reservoirs have tiny pores, complex pore structures, most of them have microcracks, high rock stress, high fracture pressure, and great development difficulty. Therefore, most tight oil and gas reservoirs face problems such as low production, low utilization rate, high mining cost and low economic effect. Therefore, tight oil and gas reservoirs generally use long horizontal well section hydraulic fracturing technology to transform reservoirs in order to achieve commercial development. Field practice has proved that due to the high clay content of tight reservoirs and large horizontal stress differences, the effect of conventional hydraulic fracturing is not ideal. Using CO2 for reservoir fracturing transformation is a technology with important potential. CO2 fracturing has the advantages of high rock breaking efficiency, strong permeability, and complex fracture creation, but the crack width of CO2 fracturing is relatively limited, generally only 1 / 3 to 1 / 6 of the crack width of conventional hydraulic fracturing. At the same time, the viscosity of pure CO2 fracturing fluid is low and the sand carrying capacity is relatively weak.

[0004] Pulsating fracturing is one of the important technological methods to improve the complexity of the fracture network. Its advantage is that it can produce fatigue damage through hydraulic pulsation, and repeated alternating loads can enhance the fatigue of rocks. The CO2-water alternating injection mode has the advantages of synergizing the two fracturing processes. A large amount of liquid CO2 is instantly gasified into high-temperature and high-pressure gas to act on the water body and the inside of the rock. The phase change produced will instantly generate a huge force, and the reaction force formed will impact the CO2 gas again, and finally form an instantaneous contraction-expansion-contraction alternating force. During the whole change process, a larger alternating load will be generated, which will make the formed cracks more complex, and the pulsating CO2 injection mode will be beneficial to reduce friction and increase the injection range of CO2. Considering the characteristics of CO2 fracturing with low crack width, limited support pressure bearing capacity, and easy closure of cracks. After pulsating CO2 fracturing, secondary high-viscosity fracturing fluid fracturing construction is carried out to support the expansion of the crack network, improve the efficiency of composite fracturing, and ultimately improve the complexity of the crack network.

[0005] In summary, there is a need for a reservoir fracturing reconstruction technology that can combine the reservoir characteristics of tight oil and gas reservoirs and form a complex fracture network. Summary of the invention

[0006] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for increasing the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs. This method can increase the complexity of the fracture network of tight oil and gas reservoirs with an original ground stress difference of less than 25MPa, improve the ability of the reservoir matrix to provide oil and gas to the artificial fractures, and ultimately achieve a better production increase effect. Field tests have proved that this method has achieved a good production increase effect in the fracturing operation of tight oil and gas reservoirs.

[0007] In summary, in order to solve the above problems, the present invention is based on the following three theories: ① The smaller the horizontal stress difference in tight oil and gas reservoirs, the more conducive it is to the expansion of cracks to form a complex fracture network; ② Pulsating fracturing can produce fatigue damage through hydraulic pulsation, and repeated alternating loads increase the fatigue of rocks; ③ Properly increasing the viscosity of the fracturing fluid is conducive to the entry of proppants and gas migration. In order to solve the shortcomings of traditional hydraulic fracturing in tight oil and gas reservoirs, such as difficulty in forming a complex fracture network, reservoir damage, and low pressure bearing capacity of proppants and easy closure of fractures caused by small fracture width. By pre-acid pretreatment of the target reservoir before fracturing, two fracturing processes (pulsating CO2 fracturing and high-viscosity fracturing fluid fracturing) are used.

[0008] First, the target reservoir is pretreated with acid to solve the formation pollution; prevent the expansion, migration and precipitation of clay particles; improve the physical properties of the reservoir rock and increase the permeability and porosity of the rock around the well. Then, CO2 pulsation fracturing (low viscosity fracturing fluid system) is used, and low viscosity CO2 and water are alternately injected. CO2 has high permeability (generally under reservoir conditions, it is easy to reach the supercritical state, and the solubility and diffusion capacity are further enhanced), which is easy to communicate with natural fractures; in addition, the CO2-water alternating injection mode adopted can coordinate the advantages of the two fracturing processes. A large amount of liquid CO2 is instantly gasified to form high-temperature and high-pressure gas to act on the water body and the rock. The phase change produced will instantly generate a huge force, and the reaction force formed will impact the CO2 gas again, and can reduce the friction of CO2, forming an instantaneous contraction-expansion-contraction alternating force. During the whole change process, a larger alternating load will be generated. Combined with the high permeability of CO2-water fracturing fluid and the rock fatigue caused by the alternating load, it will eventually promote the formation of a complex fracture network. After that, the pump is stopped for a period of time to allow the CO2 to fully expand to the deep formation, causing the local stress field to shift and reducing the horizontal stress difference of the reservoir. Finally, a secondary high-viscosity fracturing fluid fracturing operation is carried out to carry the proppant to the deep formation, support and expand the fracture network, and ultimately increase the complexity of the fracture network.

[0009] Specifically, the present invention is implemented through the following technical solutions:

[0010] A method for increasing the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir, comprising:

[0011] Pre-acid pretreatment;

[0012] The first CO2 composite fracturing operation was carried out by using CO2 and water for alternating pulsation.

[0013] Stop the pump for a while;

[0014] Carry out the second fracturing operation with high viscosity fracturing fluid;

[0015] The pressure is released and the fracturing operation is completed.

[0016] According to the preferred embodiment of the present invention, the pre-acid includes a cleaning acid, and the cleaning acid includes a concentrated acid and an organic acid;

[0017] Pre-acid pretreatment includes: pumping pre-acid into the formation under a pressure higher than the fracture pressure of the reservoir rock, so that part of the pre-acid is at the forefront of the fracture, and part of the pre-acid is lost to both sides of the fracture wall. When the pre-acid flows along the fracture into the formation and contacts the reservoir rock matrix, it dissolves the mud and acid-soluble components in the reservoir rock, connects micro-cracks and solves the formation pollution problem.

[0018] More preferably, the volume of the preacid is 15 to 30 m 3 .

[0019] Preferably, according to the present invention, CO2 and water are used for pulsating alternating construction to carry out the first CO2 composite fracturing construction; comprising:

[0020] Place CO2 and water in different mixing tanks;

[0021] Use 1.0~3.0m 3 / min displacement, using CO2 and water for pulsating alternating construction, and carry out the first CO2 composite fracturing operation.

[0022] Preferably according to the present invention, under the same pressure condition, the volume ratio of CO2 to water ranges from (1.5 to 3):1.

[0023] Further preferably, the volume ratio of CO2 to water is 2:1.

[0024] Preferably according to the present invention, the pump stop time is as follows:

[0025] The displacement of the first CO2 composite fracturing operation is 1.0~1.5m 3 / min, the pump stop time is 20 to 40 minutes;

[0026] The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 80 to 100 minutes;

[0027] The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 180~220min;

[0028] The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 280-320min.

[0029] Preferably, the displacement of the first CO2 composite fracturing operation is 1.0 to 1.5 m 3 / min, the pump stop time is 30min;

[0030] The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 90min;

[0031] The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 210min;

[0032] The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 300min.

[0033] Preferably, the second high-viscosity fracturing fluid fracturing operation according to the present invention includes:

[0034] Use 5.0~18.0m 3 / min displacement, and carry out the second high viscosity fracturing fluid fracturing construction.

[0035] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, steps of a method for improving the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir are implemented.

[0036] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of a method for increasing the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir.

[0037] A system for improving the complexity of a pulsating CO2 composite fracture network in tight oil and gas reservoirs, comprising:

[0038] The pre-acid pretreatment module is configured to: perform pre-acid pretreatment;

[0039] The first CO2 composite fracturing construction module is configured to: use CO2 and water to perform pulsating alternating construction to perform the first CO2 composite fracturing construction;

[0040] The pump stop module is configured to: stop the pump for a period of time;

[0041] The second high-viscosity fracturing fluid fracturing construction module is configured to: perform the second high-viscosity fracturing fluid fracturing construction.

[0042] The pressure relief and flowback module is configured to: pressure relief and flowback, and the fracturing construction is completed.

[0043] Compared with the existing technology, the present invention has the following advantages and beneficial effects:

[0044] The present invention utilizes acid pretreatment to solve formation contamination; clay expansion, particle migration, and precipitation; and improves rock physical properties (increasing the permeability and porosity of the rock around the well). Then, pulsating CO2 composite fracturing is performed, which combines the high permeability and diffusivity of CO2 and the fatigue damage to the rock caused by pulsating fracturing to promote the formation of a complex fracture network. Afterwards, the pump is stopped for a period of time to allow the CO2 to fully expand to the deep formation, causing the local stress field to shift and reducing the horizontal stress difference of the reservoir. Finally, a secondary high-viscosity fracturing hydraulic fracturing construction is carried out to carry the proppant to the deep formation, support and expand the fracture network, and ultimately increase the complexity of the fracture network. The method was used to carry out fracturing production in a certain oil field, and the oil increase was 2 to 5 times that of conventional hydraulic fracturing. It can be seen that a better oil increase effect has been achieved by adopting this method. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0046] Figure 1(a) is a schematic diagram of the stress field change before pulsating CO2 fracturing;

[0047] Figure 1(b) is a schematic diagram of the stress field change after pulsating CO2 fracturing;

[0048] Figure 2 It is a schematic diagram of the fracturing construction curve of Well 42-81 (comparison well) in the W well area of ​​Changqing Oilfield during conventional fracturing.

[0049] Figure 3 It is a schematic diagram of the fracturing construction curve of Well 41-0172 (experimental well) in Well Block W of Changqing Oilfield in an embodiment of the present invention. DETAILED DESCRIPTION

[0050] The present invention is further described below in conjunction with the accompanying drawings and specific examples so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention.

[0051] Example 1

[0052] A method for increasing the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir, comprising:

[0053] Pre-acid pretreatment;

[0054] The first CO2 composite fracturing operation was carried out by using CO2 and water for alternating pulsation.

[0055] Stop the pump for a while;

[0056] Carry out the second fracturing operation with high viscosity fracturing fluid;

[0057] The pressure is released and the fracturing operation is completed.

[0058] Example 2

[0059] The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs described in Example 1 is different in that:

[0060] The pre-acid is an acid liquid system mainly composed of cleaning acid, which includes concentrated acid and organic acid;

[0061] Preacid system: 8% HCI + 4% HCOOH + 3% CH3COOH + 0.8% HF + 0.5% FC951 (drainage aid, Xinxiang Shengqing New Materials Co., Ltd.) + 1% NH4Cl (clay stabilizer) + 0.5% SGR-0409 (corrosion inhibitor, Shandong Xingerui Environmental Technology Co., Ltd.). Among them, the drainage aid, clay stabilizer and corrosion inhibitor are based on the on-site ratio and manufacturer's guidance. The mass concentrations of HCI, HCOOH, CH3COOH and HF are derived from indoor experimental data.

[0062] At present, soil acid (HCI+HF) is commonly used in tight reservoirs on site. It can dissolve the filter cake formed by oil well drilling fluid and dissolve some minerals in the reservoir, while expanding the porosity and permeability of the reservoir, and is used as the basic acid. Chlorite (clay mineral) in tight reservoirs is sensitive to hydrochloric acid, so low acid-sensitive acid should be used to keep the acid system at a low pH value; the reservoir minerals also contain iron-containing minerals, so it is necessary to avoid acid slag produced by acid to block the pore throat. Therefore, it is necessary to select some organic acids as additives to reduce the reaction between soil acid and iron-containing minerals to produce residues. Therefore, the basic system is: HCI+HCOOH+CH3COOH+HF, and the ratio is determined by indoor experiments. The role of drainage aid: reduce the surface / interfacial tension of the liquid, thereby reducing the capillary resistance of the fluid and improving the drainage efficiency of residual acid. Clay stabilizer: Clay is easily decomposed in hydrochloric acid, which harms the reservoir. Although acetic acid does not decompose clay, it will cause clay expansion. Adding it to the acid system can solve such problems. Corrosion inhibitor: Reduces the corrosion of acid on downhole equipment such as oil casing and acidizing construction equipment.

[0063] Pre-acid pretreatment includes: pumping pre-acid into the formation under a pressure higher than the fracture pressure of the reservoir rock, so that part of the pre-acid is at the forefront of the fracture, and part of the pre-acid is lost to both sides of the fracture wall. When the pre-acid flows along the fracture into the formation and contacts the reservoir rock matrix, it dissolves the mud and acid-soluble components in the reservoir rock, connects micro-cracks and solves the formation pollution problem.

[0064] The volume of preacid is 15~30m 3 .

[0065] The first CO2 composite fracturing operation was carried out by using CO2 and water for pulsating alternating operations; including:

[0066] Place CO2 and water in different mixing tanks;

[0067] Use 1.0~3.0m 3 / min displacement, using CO2 and water for pulsating alternating construction, and carry out the first CO2 composite fracturing operation.

[0068] The first CO2 composite fracturing operation used two liquid tanks: one for liquid CO2 and the other for water-based fracturing fluid. The specific fracturing operation included: adding a downhole low-frequency pulsation device to the original design pipe string, adapting the throttling nozzle, and ensuring that the output frequency was in the range of 15 to 30 Hz. On the premise of pre-acid to solve the formation contamination, CO2 pulse fracturing was carried out first, and hydraulic pulse fracturing was carried out after the CO2 fracturing was completed.

[0069] Under the same pressure conditions, the volume ratio of CO2 and water ranges from (1.5 to 3):1.

[0070] The stress field change before the first CO2 composite fracturing operation is shown in Figure 1(a), and the stress field change after the first CO2 composite fracturing operation is shown in Figure 1(b). The comparison shows that the CO2 fracturing with stronger permeability causes a drastic change in the stress field of the reservoir around the well.

[0071] The essence of fatigue damage caused by pulsating load is that the cumulative effect of plastic strain gradually reduces the mechanical strength of the specimen, so that the rock can be destroyed at a lower peak load.

[0072] In theory, the propagation of pulsating pressure is regarded as the propagation of pulsating waves in the cracks of stratum rocks. The wave equation during propagation is:

[0073]

[0074] Where: ε is the displacement of the particle; x is the equilibrium position of the particle; v is the wave velocity, which depends on the medium; t is the time.

[0075] The wave equation for a plane wave is:

[0076]

[0077] Where: A is the pulsation amplitude; ω is the angular velocity.

[0078] Equation (2) describes the change of the displacement of each particle in the medium with the equilibrium position and time of each particle.

[0079] The propagation of pulsating pressure is the propagation of wave energy, which includes potential energy and kinetic energy. The expression of particle kinetic energy is:

[0080]

[0081] Where: W k is the kinetic energy of the particle; △m is the mass of the particle; ρ is the density; △V is the volume of the particle.

[0082] The expression of particle potential energy is:

[0083]

[0084] Where W p is the potential energy of the particle; k is the elastic coefficient.

[0085] because,

[0086] so,

[0087] The total mechanical energy W of the particle is obtained as:

[0088]

[0089] From formula (7), it can be concluded that the total mechanical energy is a function of time and equilibrium position. At a certain moment, the energy of a particle moves periodically with the particle; the energy of a particle at a certain position changes periodically with the passage of time. In other words, during fluctuations, the latter particle periodically obtains energy from the previous particle and periodically transfers energy to the next particle. Pulsating fracturing technology uses this principle to make certain physical quantities of particles in the medium change repeatedly, so as to cause fatigue damage to the reservoir rock, facilitate the formation of microcracks, and improve the conductivity of the cracks.

[0090] After the first CO2 composite fracturing operation is completed, the wellhead is closed without dismantling the fracturing equipment and pipelines, and the pump is stopped for a period of time. The specific pump stop time is as follows:

[0091] The displacement of the first CO2 composite fracturing operation is 1.0~1.5m 3 / min, the pump stop time is 20 to 40 minutes;

[0092] The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 80 to 100 minutes;

[0093] The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 180~220min;

[0094] The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 280-320min.

[0095] The second high viscosity fracturing fluid fracturing operation includes:

[0096] Use 5.0~18.0m 3 / min displacement, and carry out the second high viscosity fracturing fluid fracturing construction.

[0097] The second high-viscosity fracturing fluid fracturing construction process is as follows: prepare a new fracturing tank filled with high-viscosity water-based fracturing fluid. After the pump is stopped, carry out ordinary hydraulic fracturing construction.

[0098] Example 3

[0099] The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs described in Example 2 is different in that:

[0100] The volume ratio of CO2 to water is 2:1.

[0101] The displacement of the first CO2 composite fracturing operation is 1.0~1.5m 3 / min, the pump stop time is 30min;

[0102] The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 90min;

[0103] The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 210min;

[0104] The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 300min.

[0105] Example 4

[0106] The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs described in Example 3 is different in that:

[0107] Example well: The fractured section of well 42-81 in a certain oil field is 3582.3-3595.4m. The average porosity of the fractured layer is 6.8%, and the average permeability is 2.4mD, which is a typical tight oil reservoir.

[0108] The horizontal stress difference of the fractured layer calculated based on the logging data is 11-13 MPa, indicating that this method can be applied.

[0109] According to the design of the present invention, first, 21m 3 The pretreatment was carried out with an acid system mainly based on clean acid; the displacement used in the first CO2 fracturing operation was 4.5m 3 / min, the pump stop time after the first fracturing is 90min, and the displacement of the second fracturing operation is 10.0m 3 / min, the fracturing operation curve shows that the second operation pressure is lower than the first operation pressure, indicating that the fracture initiation pressure is reduced and the crack expansion of the second fracturing operation is more complex and sufficient. The fracturing operation curve of Well 42-81 (comparison well) in W well area of ​​Changqing Oilfield in conventional fracturing is shown in the figure below. Figure 2 As shown in Figure 2, the fracturing operation curve of Well 41-0172 (experimental well) in Well Block W of Changqing Oilfield is as follows: Figure 3 As shown, the cumulative oil production of the comparison well 42-81 in the same block is 18.45 tons, and the cumulative oil production of the experimental well 41-0172 is 97.67 tons. After adopting the method of the present invention, the well 42-81 achieved an excellent fracturing effect.

[0110] Example 5

[0111] A computer device includes a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, the steps of the method for improving the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir described in any one of Examples 1-4 are implemented.

[0112] Example 6

[0113] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method for increasing the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir as described in any one of Examples 1-4.

[0114] Example 7

[0115] A system for improving the complexity of a pulsating CO2 composite fracture network in tight oil and gas reservoirs, comprising:

[0116] The pre-acid pretreatment module is configured to: perform pre-acid pretreatment;

[0117] The first CO2 composite fracturing construction module is configured to: use CO2 and water to perform pulsating alternating construction to perform the first CO2 composite fracturing construction;

[0118] The pump stop module is configured to: stop the pump for a period of time;

[0119] The second high-viscosity fracturing fluid fracturing construction module is configured to: perform the second high-viscosity fracturing fluid fracturing construction.

[0120] The pressure relief and flowback module is configured to: pressure relief and flowback, and the fracturing construction is completed.

Claims

1. A method for increasing the complexity of a pulsating CO2 composite fracture network in a tight oil and gas reservoir, characterized in that: include: Pre-acid pretreatment; The first CO2 composite fracturing operation was carried out by using CO2 and water for alternating pulsation. Stop the pump for a while; Carry out the second fracturing operation with high viscosity fracturing fluid; Pressure relief and flowback, fracturing operation is completed; Preacids include concentrated acids and organic acids; Pre-acid pretreatment includes: pumping pre-acid into the formation under a pressure higher than the fracture pressure of the reservoir rock, so that part of the pre-acid is at the forefront of the fracture, and part of the pre-acid is lost to both sides of the fracture wall. When the pre-acid flows along the fracture into the formation and contacts the reservoir rock matrix, it dissolves the mud and acid-soluble components in the reservoir rock, connects micro-fractures and solves the formation pollution problem; The first CO2 composite fracturing operation was carried out by using CO2 and water for pulsating alternating operations; including: Place CO2 and water in different liquid distribution tanks; install a downhole low-frequency pulsation device in the original design pipe string, adapt the throttling nozzle, and ensure that the output frequency is in the range of 15~30Hz; Use 1.0~3.0m 3 / min displacement, using CO2 and water for pulsating alternating construction, first CO2 pulse fracturing, and then hydraulic pulse fracturing after CO2 fracturing is completed; Carry out the first CO2 composite fracturing operation; The second high viscosity fracturing fluid fracturing operation includes: Use 5.0~18.0m 3 / min displacement, and carry out the second high viscosity fracturing fluid fracturing construction.

2. The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to claim 1, characterized in that: The volume of preacid is 15~30m 3 .

3. The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to claim 1, characterized in that: Under the same pressure conditions, the volume ratio of CO2 and water ranges from (1.5~3):

1.

4. A method for increasing the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to claim 3, characterized in that: Under the same pressure conditions, the volume ratio of CO2 and water ranges from 2:

1.

5. The method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to claim 1, characterized in that: The pump stop times are as follows: The displacement of the first CO2 composite fracturing operation is 1.0~1.5m 3 / min, the pump stop time is 20~40min; The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 80~100min; The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 180~220min; The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 280~320min.

6. A method for increasing the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to claim 5, characterized in that: The displacement of the first CO2 composite fracturing operation is 1.0~1.5m 3 / min, the pump stop time is 30min; The displacement of the first CO2 composite fracturing operation is 1.5~2.0m 3 / min, the pump stop time is 90min; The displacement of the first CO2 composite fracturing operation is 2.0~2.5m 3 / min, the pump stop time is 210min; The displacement of the first CO2 composite fracturing operation is 2.5~3.0m 3 / min, the pump stop time is 300min.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: When the processor executes the computer program, the steps of the method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to any one of claims 1-6 are implemented.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs according to any one of claims 1 to 6 are implemented.

9. A system for improving the complexity of the pulsating CO2 composite fracture network in tight oil and gas reservoirs, characterized in that: include: The pre-acid pretreatment module is configured to: perform pre-acid pretreatment; The first CO2 composite fracturing construction module is configured to: use CO2 and water to perform pulsating alternating construction to perform the first CO2 composite fracturing construction; The pump stop module is configured to: stop the pump for a period of time; The second high-viscosity fracturing fluid fracturing construction module is configured to: perform the second high-viscosity fracturing fluid fracturing construction; The pressure relief and flowback module is configured to: pressure relief and flowback, and the fracturing operation is completed; Preacids include concentrated acids and organic acids; Pre-acid pretreatment includes: pumping pre-acid into the formation under a pressure higher than the fracture pressure of the reservoir rock, so that part of the pre-acid is at the forefront of the fracture, and part of the pre-acid is lost to both sides of the fracture wall. When the pre-acid flows along the fracture into the formation and contacts the reservoir rock matrix, it dissolves the mud and acid-soluble components in the reservoir rock, connects micro-fractures and solves the formation pollution problem; The first CO2 composite fracturing operation was carried out by using CO2 and water for pulsating alternating operations; including: Place CO2 and water in different liquid distribution tanks; add a downhole low-frequency pulsation device to the original design pipe string, adapt the throttling nozzle, and ensure that the output frequency is in the range of 15~30Hz; Use 1.0~3.0m 3 / min displacement, using CO2 and water for pulsating alternating construction, first CO2 pulse fracturing, and then hydraulic pulse fracturing after CO2 fracturing is completed; Carry out the first CO2 composite fracturing operation; The second high viscosity fracturing fluid fracturing operation includes: Use 5.0~18.0m 3 / min displacement, and carry out the second high viscosity fracturing fluid fracturing construction.

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