Complex crack fracturing method for compact sandstone reservoir

Abstract

The invention relates to the technical field of oil and gas field downhole operation, in particular to a compact sandstone reservoir complex crack fracturing method, and aims to solve the problem that in an existing fracturing mode, damage to a stratum is large. According to the compact sandstone reservoir complex fracture fracturing method, by adopting a mode of combining supercritical carbon dioxide and conventional hydraulic fracturing, by utilizing the super-strong fluidity and diffusivity of the supercritical carbon dioxide, on the basis that long fractures and high guide fractures are made through conventional hydraulic fracturing, the fracture complexity is further increased, and the fracture yield is increased. Meanwhile, a weak acid environment brought by carbon dioxide is utilized, the effects of inhibiting clay expansion and reducing reservoir damage are achieved, and finally the transformation effect of the compact sandstone is improved.

Description

Technical Field

[0001] The invention relates to the technical field of downhole operations in oil and gas fields, and in particular to a method for fracturing complex fractures in dense sandstone reservoirs. Background Art

[0002] my country's recoverable reserves of tight sandstone gas have reached 10 trillion cubic meters, accounting for 40% of the country's total proven natural gas reserves. It is an important area for increasing my country's natural gas reserves and production.

[0003] Tight sandstone gas in my country generally has the characteristics of undeveloped natural fractures, strong heterogeneity, high fracture pressure, and large horizontal stress difference, making it difficult to form complex network fractures like shale oil and gas reservoirs. At present, the main transformation ideas are medium displacement, small construction scale, high viscosity liquid, and large particle size ceramsite to create long fractures and high conductivity fractures. However, with the deepening of development, the current situation faced by major large gas fields in my country is that the reservoir quality continues to decline and the gas test effect decreases year by year. How to increase the degree of fractures on the basis of creating long fractures, further improve the transformation effect, and achieve stable production and increased production of gas wells is a problem that the oil industry is currently paying close attention to.

[0004] Patent CN104747158A provides a method for transforming high-argillaceous sandstone, which uses pre-acid solution to pretreat the formation, reduce construction pressure, and increase the success rate of high-argillaceous sandstone fracturing, thereby improving the transformation effect. This method can reduce the construction difficulty of high-argillaceous reservoirs and improve the construction success rate, but it cannot increase the complexity of the cracks. Patent CN114607341A provides a temporary plugging and diverting fracturing method, which adds a temperature-sensitive phase change temporary plugging agent, sets the target range to liquid and the target range to solid, and uses displacement fluid to send the liquid temporary plugging agent to a specified position. After reaching the target temperature, it forms a solid state, which plays a role of temporary plugging and diverting. Compared with conventional solid temporary plugging agents, this temporary plugging method reaches a deeper depth and can achieve the purpose of fixed-point plugging and fixed-point fracturing. It is more conducive to forming complex cracks and improving the transformation effect. Although the temporary plugging and diverting process can achieve the purpose of increasing the complexity of the cracks, the temporary plugging material injected into the formation will have the problem of being unable to be completely degraded, which is easy to cause greater damage to the formation. Summary of the invention

[0005] In order to improve the complexity of tight sandstone reservoir fractures and reduce reservoir damage, the present invention proposes a method for complex fracture fracturing of tight sandstone reservoirs, which can better improve the transformation effect of tight sandstone reservoirs.

[0006] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0007] The present invention provides a method for fracturing complex fractures in a tight sandstone reservoir, comprising:

[0008] (1) Pumping pre-pad fluid into the target layer;

[0009] (2) Pumping liquid carbon dioxide into the target layer;

[0010] (3) Pumping bio-gel fracturing fluid systems of different viscosities into the target layer in sequence. This step is divided into three stages, including:

[0011] The first stage: pump low-viscosity bio-gel fracturing fluid mixed with 70 / 140 mesh ceramsite into the target layer; wherein the viscosity of the low-viscosity bio-gel fracturing fluid is 8-15 mPa·s, the 70 / 140 mesh ceramsite is added in a slug manner, and the sand ratio is 3%-7%;

[0012] The second stage: pumping medium-high viscosity bio-glue fracturing fluid mixed with 40 / 70 mesh ceramsite into the target layer; wherein the viscosity of the medium-high viscosity bio-glue fracturing fluid is 60-80 mPa·s, 40 / 70 mesh ceramsite is added continuously, and the sand ratio is 7%-18%;

[0013] The third stage: pumping high-viscosity bio-gel fracturing fluid mixed with 20 / 40 mesh ceramsite into the target layer; wherein the viscosity of the high-viscosity bio-gel fracturing fluid is 90-110 mPa·s, 20 / 40 mesh ceramsite is continuously added, and the sand ratio is 19-33%;

[0014] (4) Inject medium-viscosity bio-gel fracturing fluid with a viscosity of 30 to 50 mPa·s and clean water into the target layer to perform displacement construction in sequence.

[0015] Furthermore,

[0016] In step (1), the pre-fluid is a high-viscosity bio-gel fracturing fluid, which is obtained by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 90-110 mPa·s, 170s -1 The viscosity under high-speed shear is >80mPa·s.

[0017] Furthermore,

[0018] In step (1), the pre-liquid volume is 10%-20% of the total construction liquid volume, and the pumping displacement is 6-10m 3 / min.

[0019] Furthermore,

[0020] In step (2), the injection volume of liquid carbon dioxide is 50 to 100 m 3 , injection displacement is 3-4m 3 / min.

[0021] Furthermore,

[0022] In step (3), the low viscosity biogel fracturing fluid is obtained by mixing 0.3-0.5% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 8-15 mPa·s.

[0023] Furthermore,

[0024] In step (3), the medium-high viscosity biogel fracturing fluid is prepared by mixing 1.3-1.6% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 60-80 mPa·s, 170s -1 The viscosity under high-speed shear is >50mPa·s.

[0025] Furthermore,

[0026] In step (3), the high viscosity bio-gel fracturing fluid is obtained by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 90-110 mPa·s, 170s -1 The viscosity under high-speed shear is >80mPa·s.

[0027] Furthermore,

[0028] In step (3), the usage amount of 70 / 140 mesh ceramsite is 5%-10%, the usage amount of 40 / 70 mesh ceramsite is 10%-20%, and the usage amount of 20 / 40 mesh ceramsite is 70%-85%.

[0029] Furthermore,

[0030] The pumping rate of the three-stage fracturing fluid in step (3) is 6-10m 3 / min.

[0031] Furthermore,

[0032] In step (4), the medium viscosity biogel fracturing fluid is prepared by mixing 0.8-1.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 30-50 mPa·s, 170s -1 The viscosity under high-speed shearing is >20mPa·s;

[0033] In step (4), the amount of medium viscosity biogel displacement fluid is 0.6 times the wellbore, the amount of clean water displacement is 0.5 times the wellbore, and the pumping displacement is 6-10m 3 / min.

[0034] Compared with the existing technology, this complex fracture fracturing method for tight sandstone reservoirs adopts a combination of liquid carbon dioxide and conventional hydraulic fracturing. The liquid carbon dioxide reaches a supercritical state under the formation temperature and pressure. The super-strong fluidity and diffusivity of supercritical carbon dioxide are utilized to further increase the complexity of the fractures on the basis of conventional hydraulic fracturing to create long fractures and high-conductivity fractures. At the same time, the weakly acidic environment brought by carbon dioxide is utilized to inhibit clay expansion and reduce reservoir damage, thereby ultimately improving the transformation effect of tight sandstone. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 Flow chart for on-site CO2 pumping

[0037] Figure 2 This is a curve diagram of the complex fracture fracturing construction in a tight sandstone reservoir provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments.

[0039] Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0040] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the reagents, materials, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0041] The present invention provides a method for fracturing complex fractures in a tight sandstone reservoir, comprising the following steps:

[0042] (1) Pump high-viscosity biogel fracturing fluid into the target layer as a pre-fluid to create fractures.

[0043] Furthermore, the pre-fluid is prepared by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance of water, and the base fluid viscosity is 90-110 mPa·s, 170s -1 The viscosity under high-speed shearing is >80mPa·s. The amount of pre-liquid is 10%-20% of the total construction liquid volume, and the pumping displacement is 6-10m 3 / min.

[0044] In this step, the purpose of pumping the pre-fluid is to form the main fracture near the wellbore with high-viscosity liquid, reduce liquid loss and improve fracture creation efficiency, while creating conditions for supercritical carbon dioxide to create complex fractures at the far end.

[0045] (2) Pump liquid carbon dioxide into the target layer.

[0046] Furthermore, the amount of liquid carbon dioxide is preferably 50 to 100 m 3 The injection displacement is preferably 3-4m 3 / min.

[0047] In this step, the liquid carbon dioxide on the ground is pressurized and stored at -20℃. The pumping equipment injects the liquid carbon dioxide into the bottom of the well through a circulating cold pump. Under the action of the bottom hole temperature and pressure, it reaches a supercritical state (temperature>31.06℃, pressure>7.38MPa). At this time, the carbon dioxide is in a special mixed phase state with a density of 0.6-0.9g / cm3 and a viscosity of 0.02mPa·s, which can achieve deep penetration and increase the complexity of the fractures.

[0048] (3) Pumping bio-gel fracturing fluid systems of different viscosities into the target layer in sequence. This step is divided into three stages, specifically:

[0049] The first stage: pump low-viscosity bio-gel fracturing fluid mixed with 70 / 140 mesh ceramsite into the target layer; the viscosity of the low-viscosity bio-gel fracturing fluid is 8-15 mPa·s, the 70 / 140 mesh ceramsite is added in a plug-type manner, and the sand ratio is 3%-7%;

[0050] The second stage: pumping medium-high viscosity bio-glue fracturing fluid mixed with 40 / 70 mesh ceramsite into the target layer; wherein the viscosity of the medium-high viscosity bio-glue fracturing fluid is 60-80 mPa·s, 40 / 70 mesh ceramsite is added continuously, and the sand ratio is 7%-18%;

[0051] The third stage: pumping high-viscosity bio-gel fracturing fluid mixed with 20 / 40 mesh ceramsite into the target layer; wherein the viscosity of the high-viscosity bio-gel fracturing fluid is 90-110 mPa·s, 20 / 40 mesh ceramsite is continuously added, and the sand ratio is 19-33%;

[0052] Furthermore, in the first stage, the low viscosity biogel fracturing fluid is obtained by mixing 0.3-0.5% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 8-15 mPa·s.

[0053] Furthermore, in the second stage, the medium-high viscosity bio-gel fracturing fluid is prepared by mixing 1.3-1.6% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance of water, with a base fluid viscosity of 60-80 mPa·s and a flow rate of 170s -1 The viscosity under high-speed shear is >50mPa·s.

[0054] Furthermore, in the above third stage, the high viscosity biogel fracturing fluid is obtained by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the remainder of water. The base fluid viscosity is 90-110 mPa·s, and the viscosity is >80 mPa·s under high-speed shear of 170 s-1.

[0055] Furthermore, in the above three stages, the usage of 70 / 140 mesh ceramsite is 5%-10%, the usage of 40 / 70 mesh ceramsite is 10%-20%, and the usage of 20 / 40 mesh ceramsite is 70%-85%.

[0056] Furthermore, in the above three stages, the pumping displacement is 6-10m 3 / min.

[0057] In this step, the purposes of sequentially pumping bio-gel fracturing fluid systems of different viscosities include: low-viscosity bio-gel carries 70 / 140 mesh ceramsite to fill the tiny complex fracture system formed by supercritical carbon dioxide; medium-high viscosity bio-gel carries 40 / 70 mesh ceramsite to polish the fracture wall surface and improve the fracture creation efficiency, while filling the branch fractures below the main fracture; high-viscosity bio-gel carries 20 / 40 mesh ceramsite to fill the main fracture system and improve the flow conductivity.

[0058] (4) Inject medium-viscosity bio-gel fracturing fluid with a viscosity of 30 to 50 mPa·s and clean water into the target layer to perform displacement construction in sequence.

[0059] Furthermore, in this step, the medium viscosity biogel fracturing fluid is obtained by mixing 0.8-1.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance of water, and the base fluid viscosity is 30-50mPa·s, 170s -1 The viscosity under high-speed shear is >20mPa·s.

[0060] Furthermore, the displacement volume of medium-viscosity biogel is 0.6 times the wellbore, the displacement volume of clean water is 0.5 times the wellbore, and the pumping displacement is 6-10m 3 / min.

[0061] In this step, the purpose of using medium-viscosity bio-glue fracturing fluid and clean water in sequence for displacement construction is that the medium-viscosity bio-glue can better displace the sand in the wellbore into the formation, and excessive displacement of clean water can better clean the pumping equipment and pipelines to ensure the smoothness of subsequent operations.

[0062] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and examples.

[0063] Example 1

[0064] The method of the present application was used to carry out fracturing construction on a well in the Sulige area. The basic conditions of the well were as follows: reservoir depth 3300m, permeability 0.5mD, porosity 12%, reservoir thickness 10m, well temperature 100°C, formation pressure 30MPa, maximum principal stress 60-65MPa, minimum principal stress 52-58MPa, clay mineral content 15-20%, the main component being illite-montmorillonite mixed layer, Young's modulus 22GPa, Poisson's ratio 0.23, Ф139.7mm casing cementing completion, and casing injection fracturing construction.

[0065] The specific steps are as follows:

[0066] Step (1): Inject pre-fluid into the target layer and use high-viscosity liquid to create main fractures in the near-wellbore area, reduce near-wellbore liquid loss, and increase fracture length.

[0067] When pumping the pre-liquid, quickly increase the displacement to 6.5m 3 / min, which helps to reduce the formation of multiple fractures near the wellbore. The amount of pre-fluid injected in this step is 60m 3 The pre-drip liquid formula is 2.2% polymer thickener, 0.1% nano-displacement agent, 0.3% high-efficiency anti-swelling agent and the balance is water. The base liquid viscosity is 98mPa·s, 170s -1 The viscosity under high-speed shear is 85mPa·s.

[0068] Step (2): Inject liquid carbon dioxide into the target layer at a rate of 80 m 3 The injection displacement is preferably 3.5m 3 / min.

[0069] When pumping carbon dioxide, follow Figure 1 In the process shown, the carbon dioxide pumping equipment is first circulated and cooled through a circulating pressure-maintaining pipeline, and then the carbon dioxide in the storage tank is pumped to the target layer through a high-pressure pipeline.

[0070] In multi-stage pumping fracturing construction, after the current stage of carbon dioxide pumping is completed, the circulating pressure-maintaining pipeline is continued to circulate to ensure that the carbon dioxide is always in a flowing state in the pipeline. There is no need to cold pump before pumping in subsequent layers, thus achieving continuous construction.

[0071] Step (3): Pumping bio-gel fracturing fluid of different viscosities into the formation with a total volume of 380 m 3 , including three stages:

[0072] Phase 1: Use low-viscosity bio-glue fracturing fluid to carry 70 / 140 mesh ceramsite and pump it to the target layer. The amount of low-viscosity bio-glue liquid is 120m 3 , 70 / 140 mesh ceramsite 3.4m 3 , sand-mixing ratio 5-7%, construction displacement 8m 3 / min. In this stage, the small-size ceramsite carried by the low-viscosity liquid can help fill the supercritical carbon dioxide in the early stage to form complex fractures, and can also help reduce near-wellbore filtration and increase the length of the main fracture.

[0073] The second stage: medium-high viscosity bio-glue fracturing fluid is used to carry 40 / 70 mesh ceramsite and continuously pumped to the target layer. The medium-high viscosity bio-glue liquid dosage is 100m 3 , 40 / 70 mesh ceramsite 8m 3 , sand-mixing ratio 7-16%, construction displacement 8m 3 / min. In this stage, the medium- and high-viscosity liquid carrying small and medium-sized ceramsite helps to increase the proppant migration distance and improve the paving effect at the front end of the fracture.

[0074] The third stage: high viscosity bio-gel fracturing fluid is used to carry 20 / 40 mesh ceramsite and continuously pumped to the target layer. The amount of high viscosity bio-gel liquid is 150m 3 , 20 / 40 mesh ceramsite 38.5m 3 , sand-mixing ratio 19-32%, construction displacement 8m 3 / min, in this stage, the high viscosity liquid carrying large-size ceramsite helps to improve the conductivity of the main fracture.

[0075] Step (4): Pump medium-viscosity bio-glue and clean water into the wellbore for displacement construction. 20m of medium-viscosity bio-glue 3 , clean water 15m 3 , construction displacement 8m 3 / min.

[0076] In this embodiment, the low-viscosity biogel fracturing fluid is obtained by mixing 0.35% polymer thickener, 0.1% nano-displacement agent, 0.3% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 10 mPa·s.

[0077] The medium viscosity biogel fracturing fluid is obtained by mixing 1.0% polymer thickener, 0.1% nano-displacement agent, 0.3% high-efficiency anti-swelling agent and the rest of water. The base fluid viscosity is 42mPa·s, 170s -1 The viscosity under high-speed shear is 25mPa·s.

[0078] The medium-high viscosity biogel fracturing fluid is obtained by mixing 1.6% polymer thickener, 0.1% nano-displacement agent, 0.3% high-efficiency anti-swelling agent and the rest of water. The base fluid viscosity is 75mPa·s, 170s -1 The viscosity under high-speed shear is 55mPa·s.

[0079] The high viscosity bio-gel fracturing fluid is obtained by mixing 2.2% polymer thickener, 0.1% nano-displacement agent, 0.3% high-efficiency anti-swelling agent and the rest of water. The base fluid viscosity is 98mPa·s, 170s -1 The viscosity under high-speed shear is 85mPa·s.

[0080] By adopting this method, after fracturing construction, the unobstructed flow rate of gas production test was tested to be 322,817 cubic meters per day, which was increased by more than 30% compared with the conventional fracturing test unobstructed flow rate of adjacent wells on the same platform.

[0081] Figure 2 This is the pumping curve for the case well construction. The construction liquid is 545m 3 , liquid carbon dioxide 80m 3 , add sand 50m 3 It is divided into 4 stages. Stage 1 is the pre-liquid pumping stage with a pumping volume of 6.5m 3 / min, construction pressure 43-46MPa, liquid volume 60m 3 The base liquid viscosity is 98mPa·s. The construction pressure drops significantly in the later stage of pumping, which reflects that the cracks extend well and the high viscosity liquid has a good crack-making effect.

[0082] Stage 2 is the CO2 pumping stage, with an injection volume of 3.5m 3 / min, injection volume is 80m 3 The construction pressure continued to decline, from 24.5MPa to 23.5MPa, reflecting that carbon dioxide reached a supercritical state under the bottom hole temperature and pressure, exhibiting good fluidity and increasing the complexity of the fractures.

[0083] Stage 3 is the sand mixing stage, which includes three parts: the first part uses low-viscosity liquid to carry 70 / 140 mesh small-size ceramsite to fill complex cracks, with a construction displacement of 8m 3 / min, liquid volume 120m 3 The base fluid viscosity is 10mPa·s, and the sand is added by plugging. The amount of sand added is 3.4m 3, sand ratio 5-7%, due to the change of wellbore friction, the construction pressure will be reduced from the initial 48.15MPa to 35.5MPa,

[0084] The second part is a medium-high viscosity liquid carrying 40 / 70 mesh small-sized ceramsite to increase the proppant migration distance and improve the paving effect at the front end of the fracture. The construction displacement is 8m 3 / min, liquid volume 100m 3 The base fluid viscosity is 75mPa·s, and the sand is added continuously with a slug and a low sand ratio. The amount of sand added is 8m 3 , sand ratio is 7-16%. In this stage, after the proppant enters the formation, the fracturing increases slightly. After the segment plug is polished, the pressure drops from 37.8 to 32.7 MPa, reflecting that the crack expansion state and proppant laying effect are good.

[0085] The third part uses high viscosity liquid to carry 20 / 40 mesh large-size ceramsite to continuously add sand to improve the conductivity of the main fractures in the near-wellbore area. The construction displacement is 8m 3 / min, liquid volume 150m 3 , base fluid viscosity is 98mPa·s, sand addition amount is 38.5m 3 , sand ratio is 19-32%. The construction pressure at this stage is relatively stable, reflecting that the crack width is better and the sand addition is smoother.

[0086] Stage 4 is the displacement fluid stage, using medium viscosity bio-glue and clean water for displacement construction, medium viscosity bio-glue 20m 3 , clean water 15m 3 , construction displacement 8m 3 / min. Due to the changes in wellbore friction and liquid column pressure, the construction pressure increased from 33MPa to 43MPa.

[0087] Comparative Example

[0088] In this case, the adjacent wells in the same layer on the same platform were constructed using conventional hydraulic fracturing technology with a construction displacement of 8m 3 / min, the pre-flushing fluid uses medium-high viscosity bio-gel fracturing fluid, the base fluid viscosity is 55mPa·s, the sand-carrying fluid uses high viscosity bio-gel fracturing fluid, the base fluid viscosity is 100mPa·s, and the total fluid volume is 500m 3 , sand adding amount 55m 3 , particle size 20 / 40 mesh ceramsite, sand ratio 5-33%, after fracturing, the gas test flow rate is 245,128 cubic meters / day. This case uses a combination of supercritical carbon dioxide and conventional hydraulic fracturing technology. Under the condition of similar construction scale, the gas test flow rate is increased by 31.7%.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for fracturing complex fractures in a tight sandstone reservoir, characterized in that: include: (1) Pumping pre-pad fluid into the target layer; (2) Pumping liquid carbon dioxide into the target layer; (3) Pumping bio-gel fracturing fluid systems of different viscosities into the target layer in sequence. This step is divided into three stages, including: The first stage: pump low-viscosity bio-gel fracturing fluid mixed with 70 / 140 mesh ceramsite into the target layer; wherein the viscosity of the low-viscosity bio-gel fracturing fluid is 8-15 mPa·s, the 70 / 140 mesh ceramsite is added in a slug manner, and the sand ratio is 3%-7%; The second stage: pumping medium-high viscosity bio-glue fracturing fluid mixed with 40 / 70 mesh ceramsite into the target layer; wherein the viscosity of the medium-high viscosity bio-glue fracturing fluid is 60-80 mPa·s, 40 / 70 mesh ceramsite is added continuously, and the sand ratio is 7%-18%; The third stage: pumping high-viscosity bio-gel fracturing fluid mixed with 20 / 40 mesh ceramsite into the target layer; wherein the viscosity of the high-viscosity bio-gel fracturing fluid is 90-110 mPa·s, 20 / 40 mesh ceramsite is continuously added, and the sand ratio is 19-33%; (4) Inject medium-viscosity bio-gel fracturing fluid with a viscosity of 30 to 50 mPa·s and clean water into the target layer to perform displacement construction in sequence.

2. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (1), the pre-fluid is a high-viscosity bio-gel fracturing fluid, which is obtained by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 90-110 mPa·s, 170s -1 The viscosity under high-speed shear is >80mPa·s.

3. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (1), the pre-liquid volume is 10%-20% of the total construction liquid volume, and the pumping displacement is 6-10m 3 / min.

4. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (2), the injection volume of liquid carbon dioxide is 50 to 100 m 3 , injection displacement is 3-4m 3 / min.

5. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (3), the low viscosity biogel fracturing fluid is obtained by mixing 0.3-0.5% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 8-15 mPa·s.

6. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (3), the medium-high viscosity biogel fracturing fluid is prepared by mixing 1.3-1.6% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 60-80 mPa·s, 170s -1 The viscosity under high-speed shear is >50mPa·s.

7. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (3), the high viscosity bio-gel fracturing fluid is obtained by mixing 1.9-2.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 90-110 mPa·s, 170s -1 The viscosity under high-speed shear is >80mPa·s.

8. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (3), the usage amount of 70 / 140 mesh ceramsite is 5%-10%, the usage amount of 40 / 70 mesh ceramsite is 10%-20%, and the usage amount of 20 / 40 mesh ceramsite is 70%-85%.

9. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: The pumping rate of the three-stage fracturing fluid in step (3) is 6-10m 3 / min.

10. The method for complex fracture fracturing of tight sandstone reservoirs according to claim 1, characterized in that: In step (4), the medium viscosity biogel fracturing fluid is prepared by mixing 0.8-1.2% polymer thickener, 0.1-0.15% nano-displacement agent, 0.3-0.5% high-efficiency anti-swelling agent and the balance water, and the base fluid viscosity is 30-50 mPa·s, 170s -1 The viscosity under high-speed shearing is >20mPa·s; In step (4), the amount of medium viscosity biogel displacement fluid is 0.6 times the wellbore, the amount of clean water displacement is 0.5 times the wellbore, and the pumping displacement is 6-10m 3 / min.

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