A composite fracturing method for continental shale oil reservoirs

Abstract

The present invention provides a composite fracturing method for continental shale oil reservoirs, comprising: pre-treating the reservoir with acid; injecting liquid carbon dioxide into the pre-treated reservoir; creating main fractures with acid gel liquid; opening and expanding micro fractures and branch fracture systems with slick water; supporting the micro fracture system and branch fracture system with a first proppant; supporting the main fracture system with a second proppant and a third proppant; and displacing the proppant to the fracture opening. In the process of shale oil exploration and development, the present invention selects low-damage fracturing fluid according to the characteristics of its reservoir; and proposes a mixed fracturing fluid of slick water + acid gel, multi-scale small-size proppants and fiber plus sand to form a high-conductivity complex volume fracture network to increase the transformation volume of the reservoir; and adopts pre-liquid CO2 energy enhancement to increase the post-fracturing return rate.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field fracturing, and in particular relates to a composite fracturing method for continental shale oil reservoirs. Background Art

[0002] China's recoverable reserves of shale (tight) oil are 4.41 billion tons, ranking third in the world, accounting for about 10% of the oil resources, mainly distributed in Songliao Basin, Bohai Bay Basin, Ordos Basin, Nanxiang Sag, Subei Basin, Santanghu Basin, Junggar Basin, Sichuan Basin, etc. Domestic continental shale oil is mainly formed in the front delta of continental fault lake basins and semi-deep lake-deep lake phases, with a burial depth of 2200-3500m, an organic carbon content of 1.2%-2.6%, a low content of brittle minerals (40%-50%), a low degree of thermal evolution (R0 is 0.5%-1.1%), poor crude oil mobility, permeability of 0.01-0.3mD, porosity of 3.4-8.9%, and a formation pressure coefficient of 1.05-1.15, which is a normal pressure coefficient. The reservoir space mainly develops shale matrix pores, organic pores and a small amount of microcracks.

[0003] In recent years, the shale oil exploration tests conducted by Sinopec and PetroChina have not yielded satisfactory results. It has been found that domestic continental shale oil fracturing generally has the poor compressibility phenomenon of "cannot be opened, cannot be supported, low flowback, and difficult to maintain stable production". The specific difficulties in fracturing transformation are: ① The mud content is high, and the reservoir is easily damaged by fracturing fluid; ② The formation is plastic, making it difficult to add sand, with a high degree of proppant embedding and low fracture conductivity; ③ It is a low-porosity, low-permeability, and low-pressure formation, making it difficult to flow back the fracturing fluid after fracturing; ④ The horizontal stress difference is large, and natural fractures are underdeveloped, making it difficult to form a volume fracture network for conventional shale transformation.

[0004] Therefore, how to solve the problems of reservoir damage, easy embedding of proppants, low conductivity, and difficulty in fracturing fluid return, as well as how to form a complex fracture network and achieve volume fracturing have become urgent issues to be solved by technical personnel in this field. Summary of the invention

[0005] In view of this, the object of the present invention is to provide a composite fracturing method for continental shale oil reservoirs. The method provided by the present invention can not only solve the problems of reservoir vulnerability, easy embedding of proppants, low conductivity, and difficulty in fracturing fluid return, but also realize volume fracturing transformation of complex fracture networks.

[0006] The present invention provides a composite fracturing method for continental shale oil reservoirs, comprising:

[0007] Using acid to pretreat the reservoir;

[0008] injecting liquid carbon dioxide into the pretreated reservoir;

[0009] Use acid gel liquid to create the main crack;

[0010] Use slick water to open and expand micro-cracks and branching crack systems;

[0011] Using a first proppant to support a micro-fracture system and a branch fracture system;

[0012] Using secondary proppants and tertiary proppants to support the main fracture system;

[0013] Displace the proppant to the crack opening;

[0014] The particle size of the first proppant is 70 to 140 mesh;

[0015] The particle size of the second proppant is 40 to 70 mesh;

[0016] The particle size of the third proppant is 30 to 50 mesh;

[0017] The second proppant and / or the third proppant contain degradable fibers.

[0018] Preferably, the acid solution includes hydrochloric acid and / or earth acid.

[0019] Preferably, the displacement of the injected liquid carbon dioxide is 1.0 to 1.5 m 3 / min.

[0020] Preferably, the ingredients of the slippery water are:

[0021] 0.06-0.08wt% of drag reducing agent;

[0022] 0.08-0.12wt% of drainage aid;

[0023] 0.1-0.3wt% of an anti-swelling agent;

[0024] The balance is water.

[0025] Preferably, the drag reducing agent is a polyacrylamide substance;

[0026] The drainage aid is a compound of polyoxyethylene amine ethers and fluorocarbon surfactants;

[0027] The anti-swelling agent is a poly-N-hydroxymethyl acrylamide substance.

[0028] Preferably, the acidic jelly solution comprises:

[0029] 0.3-0.4wt% thickener;

[0030] 0.4-0.6wt% clay stabilizer;

[0031] 0.04-0.06wt% of demulsifier;

[0032] 0.08-0.12wt% of drainage aid;

[0033] 0.08-0.12 wt% of a fungicide;

[0034] 0.5-0.7 wt% of a cross-linking agent;

[0035] 0.5-0.7% pH adjuster;

[0036] 0.01-0.15wt% of a breaker;

[0037] The balance is water.

[0038] Preferably, the thickener is carboxymethyl hydroxypropyl guar gum;

[0039] The clay stabilizer is one or a combination of ammonium chloride and quaternary ammonium salt;

[0040] The demulsifier is one or both of alkyl phosphate and alkoxy carboxylate;

[0041] The drainage aid is one or both of a fatty alcohol polyether substance and a fatty alcohol polyether cationic compound;

[0042] The bactericide is one or more of formaldehyde, glutaraldehyde and quaternary ammonium salt;

[0043] The cross-linking agent is an organic zirconium salt substance;

[0044] The pH regulator is an acetic acid substance;

[0045] The debonding agent is an ammonium persulfate substance.

[0046] Preferably, the first proppant is a ceramsite proppant;

[0047] The second proppant is a resin-coated quartz sand proppant;

[0048] The third proppant is a resin-coated quartz sand proppant.

[0049] Preferably, the viscosity of the slippery water is 2 to 6 mPa.s;

[0050] The viscosity of the acidic jelly liquid is 100-150 mPa.s.

[0051] Preferably, the degradable fiber is polyester fiber.

[0052] The composite fracturing method provided by the present invention is a fracturing process of "pre-liquid carbon dioxide energization + slick water and acid gel liquid mixed fracturing fluid system + multi-scale small-particle proppant + degradable fiber plus sand". In the process of shale oil exploration and development, the present invention optimizes low-damage fracturing fluid according to its reservoir characteristics; and proposes slick water + acid gel mixed fracturing fluid, multi-scale small-particle proppant and fiber plus sand to form a high-conductivity complex volume fracture network to increase the transformed volume of the reservoir; pre-liquid CO2 energization is used to increase the post-fracturing return rate. The present invention solves the problems of domestic continental shale oil reservoir damage, easy embedding of proppants, low conductivity, and difficult return of fracturing fluid, which is of great significance for guiding the exploration and development of domestic continental shale oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 A distribution map of hydraulic fractures obtained by the method provided in the embodiment;

[0054] Figure 2 This is a distribution diagram of the fracturing transformation volume obtained by the method provided in the embodiment. DETAILED DESCRIPTION

[0055] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other examples improved or modified by ordinary technicians in this field belong to the scope of protection of the present invention. It should be understood that the embodiments of the present invention are only used to illustrate the technical effects of the present invention, rather than to limit the scope of protection of the present invention. In the embodiments, the methods used are conventional methods unless otherwise specified.

[0056] The present invention provides a composite fracturing method for continental shale oil reservoirs, comprising:

[0057] Using acid to pretreat the reservoir;

[0058] injecting liquid carbon dioxide into the pretreated reservoir;

[0059] Use acid gel liquid to create the main crack;

[0060] Use slick water to open and expand micro-cracks and branching crack systems;

[0061] Using a first proppant to support a micro-fracture system and a branch fracture system;

[0062] Using secondary proppants and tertiary proppants to support the main fracture system;

[0063] Displace the proppant to the crack opening;

[0064] The particle size of the first proppant is 70 to 140 mesh;

[0065] The particle size of the second proppant is 40 to 70 mesh;

[0066] The particle size of the third proppant is 30 to 50 mesh;

[0067] The second proppant and / or the third proppant contain degradable fibers.

[0068] In the present invention, the type of acid and injection parameters are preferably selected according to the conditions of the reservoir; the conditions of the reservoir preferably include the mineral components of the reservoir; the acid preferably includes hydrochloric acid and / or mud acid; the injection parameters preferably include the displacement and dosage of the acid, and the displacement of the acid is preferably 0.5-1.0 m 3 / min, more preferably 0.6 to 0.8m 3 / min; the amount of the acid solution is preferably 10 to 20m 3 , more preferably 12 to 18 m 3 , the most preferred is 14 to 16 m 3 .

[0069] In the present invention, the components of the acid solution include:

[0070] 12-18 wt% industrial hydrochloric acid;

[0071] 1-2 wt% corrosion inhibitor;

[0072] 1-2 wt% of an iron ion stabilizer;

[0073] 0.2-0.8 wt% of a clay stabilizer;

[0074] 0.08~0.12wt% of drainage aid.

[0075] In the present invention, the mass content of the industrial hydrochloric acid is preferably 13-17%, more preferably 14-16%, and most preferably 15%.

[0076] In the present invention, the mass content of the corrosion inhibitor is preferably 1.2-1.8%, more preferably 1.4-1.6%, and most preferably 1.5%; the corrosion inhibitor is preferably one or a combination of quaternary ammonium pyridinium salt, Mannich base and aromatic amine.

[0077] In the present invention, the mass content of the iron ion stabilizer is preferably 1.2-1.8%, more preferably 1.4-1.6%, and most preferably 1.5%; the iron ion stabilizer is preferably ethylenediaminetetraacetic acid, nitrilotriacetic acid complexing agent, thiourea or isoascorbic acid reducing agent.

[0078] In the present invention, the mass content of the clay stabilizer (clay stabilizer in acid solution) is preferably 0.3-0.7%, more preferably 0.4-0.6%, and most preferably 0.5%; the clay stabilizer (clay stabilizer in acid solution) is preferably one or a combination of ammonium chloride and quaternary ammonium salt.

[0079] In the present invention, the mass content of the drainage aid (drainage aid in acid solution) is preferably 0.09-0.11%, more preferably 0.1%; the drainage aid (drainage aid in acid solution) is preferably one or a combination of alcohols, ethers and fluorocarbon surfactants.

[0080] In the present invention, the injection displacement of the injected liquid carbon dioxide is preferably 1.0 to 1.5 m 3 / min, more preferably 1.1 to 1.4 m / min 3 / min, the most preferred is 1.2 to 1.3m 3 In the present invention, the liquid carbon dioxide is mainly used to increase energy and promote the return of fracturing fluid after fracturing. The amount of the liquid carbon dioxide is preferably 80 to 120 m 3 , more preferably 90 to 110 m 3 , preferably 100m 3 .

[0081] The present invention adopts pre-liquid carbon dioxide energy enhancement, and uses a small displacement (1.0-2.0m 3 / min) method to inject liquid carbon dioxide into the formation. The injection amount is based on the material balance principle. The relationship between the amount of liquid CO2 used and the rise in formation pressure is calculated. When the mixed phase pressure of liquid CO2 and crude oil is reached, this is the optimal amount of liquid CO2. After liquid CO2 is injected into the formation, it combines with the water in the oil layer and generates carbonic acid with a relatively stable form, which quickly dissolves in the crude oil to achieve the purpose of reducing the viscosity of the crude oil. The pH value of CO2 in water under saturated conditions is within the range of 3 to 3.7, and the acidic characteristics are obvious. This acidic characteristic has a certain dissolution ability for the cementing materials of rock particles, and the dissolved materials can be discharged with the return fluid, and the oil and gas seepage space is increased. In this acidic environment, the expansion of clay minerals in the formation will be effectively controlled. As the residence time of CO2 in the reservoir increases, its temperature gradually recovers and CO2 will be vaporized faster. 1m 3 Liquid CO2 can produce 546 standard cubic meters of gaseous CO2, generating great flowback power in the limited space of the reservoir; the method provided by the present invention is suitable for the fracturing transformation of low-pressure reservoirs of continental shale oil, reducing damage to the reservoir and improving the flowback rate of the fracturing fluid.

[0082] In the present invention, the method of using acidic gel solution to create main cracks preferably includes:

[0083] Inject 5 to 7 wellbore volumes of acidic gel solution to obtain the main fracture system; inject gel breaker at the same time.

[0084] In the present invention, the displacement of the acid gelling liquid in the main seam making process is preferably a step-by-step increase in displacement; the initial displacement of the step-by-step increase in displacement is preferably 2m 3 / min~3m 3 / min; It is particularly preferred that the displacement is increased in steps of 2 to 3 times, and the displacement increased in steps of 1.5 to 2.0 m 3 / min, until the displacement reaches 10-12m 3 In the present invention, the amount of the acidic gel solution is preferably 100 to 150 m 3 , more preferably 110 to 140 m 3 , the most preferred is 120 to 130 m 3 .

[0085] In the present invention, the viscosity of the acidic jelly solution is preferably 100-150 mPa.s, more preferably 110-140 mPa.s, and most preferably 120-130 mPa.s.

[0086] In the present invention, the components of the acidic jelly solution are preferably:

[0087] 0.3-0.4wt% thickener;

[0088] 0.4-0.6wt% clay stabilizer;

[0089] 0.04-0.06wt% of demulsifier;

[0090] 0.08-0.12wt% of drainage aid;

[0091] 0.08-0.12 wt% of a fungicide;

[0092] 0.5-0.7 wt% of a cross-linking agent;

[0093] 0.5-0.7 wt % of a pH regulator;

[0094] 0.01-0.15wt% of a breaker;

[0095] The balance is water.

[0096] In the present invention, the mass content of the thickener is preferably 0.35%; the mass content of the clay stabilizer (clay stabilizer in the acidic jelly solution) is preferably 0.5%; the mass content of the demulsifier is preferably 0.05%; the mass content of the drainage aid (drainage aid in the acidic jelly solution) is preferably 0.1%; the mass content of the bactericide is preferably 0.1%; the mass content of the cross-linking agent is preferably 0.6%; the mass content of the pH adjuster is preferably 0.6%; the mass content of the breaker (breaker in the acidic jelly solution) is preferably 0.05-0.1%, more preferably 0.06-0.08%.

[0097] In the present invention, the thickener is preferably carboxymethyl hydroxypropyl guar gum; the clay stabilizer (clay stabilizer in the acidic gel solution) is one or a combination of ammonium chloride and quaternary ammonium salt; the demulsifier is preferably one or both of alkyl phosphate and alkoxy carboxylate; the drainage aid (drainage aid in the acidic gel solution) is preferably one or both of fatty alcohol polyether substances and fatty alcohol polyether cationic complexes; the bactericide is preferably one or more of formaldehyde, glutaraldehyde and quaternary ammonium salt; the cross-linking agent is preferably an organic zirconium salt substance; the pH adjuster is preferably an acetic acid substance; the breaker (breaker in the acidic gel solution) is preferably an ammonium persulfate substance, and the breaker is preferably added from a sand mixing truck during fracturing construction; the water is preferably clean water.

[0098] In the present invention, the cross-linking time of the acidic jelly solution is preferably 30 to 180 s, more preferably 50 to 150 s, more preferably 80 to 120 s, and most preferably 100 s.

[0099] In the present invention, the acidic gelling liquid is cross-linked in an acidic environment (PH=3-5), has good heat and shear resistance, low residue, easy gel breakage and flowback, and little damage. The acidic gelling liquid provided by the present invention has a low damage effect, and the acidic fracturing fluid system-carboxymethyl hydroxypropyl guar fracturing fluid is used to effectively inhibit the expansion and migration of clay minerals caused by the negative charge of the clay surface, and play a role in stabilizing the clay; thereby effectively reducing the damage of the fracturing fluid to the reservoir.

[0100] In the present invention, the degumming agent is preferably an ammonium persulfate substance.

[0101] In the present invention, the discharge volume of the slippery water is preferably 12 to 14 m 3 / min, more preferably 13m 3 / min; the amount of slippery water is preferably 600 to 800m 3 , more preferably 650 to 750 m 3 , preferably 700m 3 .

[0102] In the present invention, the ingredients of the slippery water are preferably:

[0103] 0.06-0.08wt% of drag reducing agent;

[0104] 0.08-0.12wt% of drainage aid;

[0105] 0.1-0.3wt% of an anti-swelling agent;

[0106] The balance is water.

[0107] In the present invention, the mass content of the drag reducing agent is preferably 0.07%; the mass content of the drainage aid (drainage aid in slippery water) is preferably 0.1%; and the mass content of the anti-swelling agent is preferably 0.2%.

[0108] In the present invention, the drag reducing agent is preferably a polyacrylamide-based substance; the drainage aid (drainage aid in slippery water) is preferably a compound of polyoxyethylene amine ether and fluorocarbon surfactant; the anti-swelling agent is preferably a composite anti-swelling agent, more preferably a poly (N-hydroxymethyl) acrylamide-based substance.

[0109] In the present invention, the viscosity of the slippery water is preferably 2-6 mPa.s, more preferably 3-5 mPa.s, and most preferably 4 mPa.s; the density is preferably 1.0-1.05 g / cm 3 The anti-swelling rate is preferably 75-85%, more preferably 78-82%, and most preferably 80%; the surface tension is preferably 25-28 mN / m, more preferably 26-27 mN / m; the interfacial tension is preferably 2.5-3.0 mN / m, more preferably 2.6-2.8 mN / m; it has the advantages of effectively inhibiting clay expansion and reducing formation damage.

[0110] In the present invention, the method for filling microcracks and branch cracks preferably includes:

[0111] The first proppant with a sand ratio of 3% to 12% is carried by slick water and injected into micro fractures and branch fractures in a plug-type sand adding manner.

[0112] In the present invention, the method of filling microcracks and branch cracks more preferably includes:

[0113] Sand slugs (first sand slugs) and liquid slugs (first liquid slugs) are injected alternately.

[0114] In the present invention, the first sand-adding segment plug is preferably a sand-carrying fluid containing 2 to 3 wellbore volumes of slick water carrying the first proppant.

[0115] In the present invention, the first liquid slug is preferably 2 to 3 wellbore volumes of slick water.

[0116] In the present invention, in the process of alternately injecting the first sand plug and the first liquid plug, the first proppant is preferably added starting from the sand plug with a sand ratio of 3-5%; in the alternating process, the sand ratio of the latter sand plug is improved to a certain extent than that of the former sand plug, and the sand ratio improvement is preferably 1-2%; the number of the first sand plugs is preferably 5-7.

[0117] In the present invention, the first proppant is preferably a ceramsite proppant of 70 to 140 meshes, more preferably 80 to 120 meshes, and most preferably 100 meshes.

[0118] In the present invention, before the second proppant and the third proppant are used to support the main fracture system, the method preferably further includes:

[0119] Inject acid gel solution to create the main seam again.

[0120] In the present invention, the displacement of the acid gel solution in the process of re-making the main seam is preferably 10 to 12 m 3 / min, the amount of acid gel solution is preferably 600-700m 3 , more preferably 620 to 680 m 3 , the most preferred is 640~660m 3 .

[0121] In the present invention, the method for supporting the main fracture system preferably comprises:

[0122] The acid gel solution is used to carry the second proppant and the third proppant with a sand ratio of 6 to 20% and is injected in a plug-type sand adding manner.

[0123] In the present invention, the method for supporting the main fracture system more preferably includes:

[0124] Sand slugs (second sand slugs) and liquid slugs (second liquid slugs) are injected alternately.

[0125] In the present invention, the second sand plug is preferably 2 to 3 wellbore volumes of an acidic gel solution carrying the second proppant and the third proppant.

[0126] In the present invention, the second sand-adding segment plug preferably also contains degradable fibers; the present invention preferably adopts a fiber adding device to add a certain amount of degradable fibers to the second sand-adding segment plug. In the present invention, the degradable fibers are preferably added to the second sand-adding segment plug in the middle and late stages of fracturing, after the sand ratio of the second sand-adding segment plug is increased to 10%, and degradable fibers with a mass concentration of 0.08 to 0.12% are added during the sand-adding stage; the mass content of the degradable fibers in the sand-adding segment plug is preferably 0.08 to 0.12%, more preferably 0.09 to 0.11%, and most preferably 0.1%.

[0127] In the present invention, the degradable fiber is preferably a polyester fiber material, and the length of the degradable fiber is preferably 6 to 9 mm, more preferably 7 to 8 mm; the degradable fiber preferably automatically degrades after being soaked at a temperature of 75 to 85°C for 45 to 55 hours; the temperature is preferably 78 to 82°C, more preferably 80°C; the soaking time is preferably 48 to 50 hours.

[0128] The addition of degradable fibers in the present invention can reduce the damage to the fracture conductivity caused by the breakage and embedding of proppants due to the closing pressure, and has a good sand-carrying capacity, the proppant can be transported to a deep part, and the fracture conductivity is improved. The addition of degradable fibers in the present invention can reduce the sedimentation rate of the proppant, so that the fracture efficiency will not decrease due to the sedimentation of the proppant during the fracture closure process; has a good sand-carrying capacity, the proppant can be transported to a deep part, and the fracture conductivity is high; reduces the damage to the fracture conductivity caused by the breakage and embedding of proppants due to the closing pressure.

[0129] In the present invention, the second liquid segment plug is preferably an acidic gel liquid with 1 to 2 wellbore volumes.

[0130] In the present invention, in the process of alternately injecting the second sand-added plug and the second liquid plug, the second proppant and the third proppant are preferably added starting from the sand-added plug with a sand ratio of 6-8%; in the alternating process, the sand ratio of the latter sand-added plug is improved to a certain extent than that of the former sand-added plug, and the sand ratio improvement is preferably 1-2%; the number of sand-added plugs to which the second proppant is added is preferably 8-10; the number of sand-added plugs to which the third proppant is added is preferably 3-4.

[0131] In the present invention, the second proppant is preferably a resin-coated quartz sand proppant of 40 to 70 meshes, more preferably 50 to 60 meshes, and most preferably 55 meshes; the second proppant is preferably added with a sand-added plug with a sand ratio of 6 to 18%, more preferably 8 to 16%, more preferably 10 to 14%, and most preferably 12 to 13%.

[0132] In the present invention, the third proppant is preferably a resin-coated quartz sand proppant of 30-50 mesh, more preferably 35-45 mesh, and most preferably 40 mesh; the third proppant is preferably added with a sand plug having a sand ratio of 18-20%.

[0133] In the present invention, the total amount of the first proppant, the second proppant and the third proppant added in each layer fracturing is preferably 60 to 80 m 3 , more preferably 65 to 75 m 3 , the most preferred is 70m 3 ; The added mass ratio of the first proppant, the second proppant and the third proppant is preferably (2-4):(5-7):1, and more preferably 3:6:1.

[0134] In the present invention, the second proppant and the third proppant are resin-coated quartz sand. Compared with ordinary proppant, resin-coated quartz sand can effectively prevent embedding in the formation and has a higher pressure bearing capacity. The present invention adopts a multi-scale small-particle size combined segment plug sand adding mode of 70 / 140 mesh powdered pottery + 40 / 70 mesh resin-coated quartz sand + 30 / 50 mesh resin-coated quartz sand; in the early stage, 70 / 140 mesh powdered pottery is used to grind the hole friction resistance, reduce filtration loss, and support microcracks and secondary cracks; 40 / 70 mesh resin-coated quartz sand supports branch cracks and main cracks; in the later stage, 30 / 50 mesh resin-coated quartz sand is added with a high sand ratio to form a high-conductivity main crack at the crack mouth, realizing "branch crack network + high-conductivity main crack".

[0135] In the present invention, the method of displacing the proppant to the crack opening preferably includes:

[0136] Use slick water to displace the last section of proppant in the wellbore to the crack opening.

[0137] In the present invention, the timing of using slick water to displace the last section of proppant in the wellbore to the crack mouth is preferably when the sand addition amount designed for fracturing is reached; the last section of proppant is preferably the second proppant or the third proppant described in the above technical solution; the amount of slick water is preferably the sum of the wellbore volume and the surface pipeline volume.

[0138] The present invention adopts a mixed fracturing fluid system of slippery water and acidic jelly liquid. The fracturing fluid system used for fracturing is slippery water and acidic jelly liquid. The mass ratio of slippery water and acidic jelly liquid in the total amount of injected fracturing fluid is preferably 1: (0.8-1.2), more preferably 1: (0.9-1.1), and most preferably 1: 1.

[0139] In the present invention, the process of creating the main crack, initiating and spreading microcracks and branch cracks, and filling the main crack preferably includes:

[0140] After the formal fracturing begins, the acid gel solution is first injected 100 to 150 m 3 , injection displacement 10~12m 3 / min, promote the formation of effective main fractures; then inject slick water for 600-800m 3 , injection displacement 12~14m 3 / min, using the good communication ability of slippery water to form a complex seam network; then inject acid gel solution 600-700m 3 , injection displacement 10~12m 3 / min, using its good crack-making and sand-carrying capabilities to increase the sand-to-liquid ratio and fill the main cracks.

[0141] In the present invention, the composite fracturing method for continental shale oil reservoirs preferably comprises:

[0142] Pre-treating the reservoir using acid;

[0143] Injection of liquid carbon dioxide

[0144] Use acid gel liquid to create the main crack;

[0145] Slippery water carrying 70-140 mesh ceramsite proppants with a sand ratio of 3% to 12% is injected to support the micro-fracture system and the branch fracture system, and the concentration of each proppant is different;

[0146] Injecting acid gel solution carrying 40-70 mesh + 30-50 mesh resin coated quartz sand proppant with sand ratio of 6% to 20% to support the main fracture system. The proppant concentration in each section is different, and a certain concentration of degradable fiber is injected in the later high sand ratio stage;

[0147] The last section of proppant in the wellbore is completely displaced to the crack opening by using slick water.

[0148] In the present invention, the composite fracturing method for continental shale oil reservoirs more preferably comprises:

[0149] Pre-treating the reservoir using acid;

[0150] Inject liquid carbon dioxide with an injection volume of 1.0 to 1.5 m 3 / min;

[0151] Inject 5 to 7 wellbore volumes of pre-acid gel solution to create main fractures to obtain a main fracture system, and inject a gel breaker at the same time. The acid gel solution has a viscosity of 100 mPa.s to 150 mPa.s;

[0152] Injecting slick water to open and expand the microcracks and branch fracture system, and start adding the first proppant into the slick water to fill the microcracks and branch fractures, alternately injecting 2 to 3 wellbore volumes of sand-carrying liquid (slick water carrying the first proppant) as a sand-adding segment plug (first sand-adding segment plug) and 1 to 2 wellbore volumes of slick water as a liquid segment plug (first liquid segment plug), starting with a 3% to 5% sand ratio segment plug to add the first proppant, the sand ratio of the latter sand-adding segment plug is improved by 1 to 2% compared with the previous sand-adding segment plug, and the number of sand-adding segment plugs is 5 to 7; the viscosity of the slick water is 2 mPa.s to 6 mPa.s;

[0153] Acid gelling liquid is injected to form a main fracture and carry a second proppant and a third proppant to support the main fracture, 2 to 3 wellbore volumes of sand-carrying liquid (acid gelling liquid carrying the second proppant and the third proppant) are alternately injected as a sand-adding segment plug (second sand-adding segment plug) and 1 to 2 wellbore volumes of acid gelling liquid are injected as a liquid segment plug (second liquid segment plug), and a certain amount of degradable fiber is added to the sand-adding segment plug by a fiber adding device, and the second proppant and the third proppant are added in sequence starting with a 6% to 8% sand ratio segment plug, and the sand ratio of the latter sand-adding segment plug is improved by 1 to 2% compared with the previous sand-adding segment plug; the number of the second proppant sand-adding segment plugs is 8 to 10, and the number of the third proppant sand-adding segment plugs is 3 to 4, and a gel breaker is injected at the same time, and the viscosity of the acid gelling liquid is 100 mPa.s to 150 mPa.s;

[0154] When the designed fracturing sand addition amount is reached, the second proppant or the third proppant in the well casing is completely displaced to the crack opening by using slick water, and the amount of slick water used is the sum of the well casing volume and the surface pipeline volume.

[0155] In the process of shale oil exploration and development, the present invention adopts low-damage fracturing fluid according to the characteristics of its reservoir; and proposes a mixed fracturing fluid of slick water + acid gel, multi-scale small-size proppants and fiber plus sand to form a high-conductivity complex volume fracture network to increase the transformation volume of the reservoir; and adopts pre-liquid CO2 energy enhancement to improve the post-fracturing return rate. The present invention solves the problems of damage to domestic continental shale oil reservoirs, easy embedding of proppants, low conductivity, and difficulty in fracturing fluid return, which is of great significance for guiding the exploration and development of domestic continental shale oil.

[0156] The acid solution formula used in the following examples of the present invention includes: 15wt% industrial hydrochloric acid + 1.5wt% corrosion inhibitor + 1.5wt% iron ion stabilizer + 0.5wt% clay stabilizer + 0.1wt% drainage aid, which is provided by Beijing Hongyi Enze Energy Technology Co., Ltd. The acid gel solution formula is: 0.35wt% carboxymethyl hydroxypropyl guar gum + 0.5wt% clay stabilizer + 0.05wt% demulsifier + 0.1wt% drainage aid + 0.1wt% bactericide + 0.6wt% cross-linking agent + 0.6wt% pH conditioner + (0.01-0.15wt%) degelling agent + the rest is water, which is provided by Beijing Baofengchun Petroleum Technology Co., Ltd. The slippery water formula is: 0.07wt% drag reducer + 0.1wt% drainage aid + 0.2wt% composite anti-swelling agent + the rest is water, which is provided by Beijing Baofengchun Petroleum Technology Co., Ltd. The 70-140 mesh ceramsite is provided by Zhengzhou Shaolin Filter Material Co., Ltd., and the model is a ceramsite product with a particle size of 0.112-0.224mm. The 40-70 mesh resin-coated quartz sand is provided by Beijing Qixiangda New Materials Co., Ltd., and the model is a resin-coated quartz sand product with a particle size of 0.224-0.45mm. The 30-50 mesh resin-coated quartz sand is provided by Beijing Qixiangda New Materials Co., Ltd., and the model is a resin-coated quartz sand product with a particle size of 0.35-0.6mm. The degradable fiber is a polyester fiber, provided by Beijing Hongyi Enze Energy Technology Co., Ltd., and is a polyester fiber product with a length of 6-8mm.

[0157] Example

[0158] A shale oil horizontal well, located in the northern depression of the Songliao Basin, successfully completed 10 stages of fracturing construction using the method provided by the present invention, and obtained industrial oil flow after fracturing. The specific implementation steps of each stage of fracturing construction are shown in Table 1:

[0159] Table 1 Each stage of fracturing construction steps

[0160]

[0161]

[0162] The well was fractured in 10 stages and a total of 100m3 of acid was injected. 3 、Slippery Water 8551m 3 、Acid gel solution 8448m 3 、Pre-liquid carbon dioxide 600m 3 , add 70 / 140 mesh powder pottery 165m 3 , 40 / 70 mesh resin coated quartz sand 481m 3 , 30 / 50 mesh resin coated quartz sand 75m 3 , biodegradable fiber 2320m 3 .

[0163] The cumulative flowback volume of fracturing fluid after fracturing in this well is 3185.3m 3 , flowback rate 18.63%, self-flow production to obtain daily oil production of 14.37m 3 , and the stable production time is long, and the fracturing effect is significant; through ground microseismic monitoring, it is found that the fracturing cracks are distributed in a complex network, such as Figure 1 As shown in the figure, the total transformation volume (SRV) was measured to be 1253.61×10 4 m 3 ,like Figure 2 shown.

[0164] In the process of shale oil exploration and development, the present invention adopts low-damage fracturing fluid according to the characteristics of its reservoir; and proposes a mixed fracturing fluid of slick water + acid gel, multi-scale small-size proppants and fiber plus sand to form a high-conductivity complex volume fracture network to increase the transformation volume of the reservoir; and adopts pre-liquid CO2 energy enhancement to improve the post-fracturing return rate. The present invention solves the problems of damage to domestic continental shale oil reservoirs, easy embedding of proppants, low conductivity, and difficulty in fracturing fluid return, which is of great significance for guiding the exploration and development of domestic continental shale oil.

[0165] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A composite fracturing method for continental shale oil reservoirs, comprising: Using acid to pretreat the reservoir; The acid solution includes hydrochloric acid and / or mud acid; injecting liquid carbon dioxide into the pretreated reservoir; The displacement of the injected liquid carbon dioxide is 1.0 to 1.5 m 3 / min; Acid gelling liquid is used to form the main crack; the displacement of the acid gelling liquid during the main crack formation is a step-by-step increase; the acid gelling liquid comprises: 0.3-0.4wt% thickener; 0.4-0.6wt% clay stabilizer; 0.04-0.06wt% demulsifier; 0.08-0.12wt% drainage aid; 0.08-0.12wt% bactericide; 0.5-0.7wt% cross-linking agent; 0.5-0.7% pH regulator; 0.01-0.15wt% breaker; the balance is water; the thickener is carboxymethyl hydroxypropyl guar gum; Slippery water is used to open and expand microcracks and branch crack systems; the slippery water comprises: 0.06-0.08wt% of a drag reducing agent; 0.08-0.12wt% of a drainage aid; 0.1-0.3wt% of anti-swelling agent; the balance is water; the drag reducing agent is a polyacrylamide substance; the drainage agent is a compound of polyoxyethylene amine ether and fluorocarbon surfactant; the anti-swelling agent is a poly-N-hydroxymethyl acrylamide substance; the first proppant is used to support the micro-fracture system and the branch fracture system; Using secondary proppants and tertiary proppants to support the main fracture system; Displace the proppant to the crack opening; The particle size of the first proppant is 70 to 140 mesh; The particle size of the second proppant is 40 to 70 mesh; The particle size of the third proppant is 30 to 50 mesh; The second proppant and / or the third proppant contain degradable fibers.

2. The method according to claim 1, characterized in that The clay stabilizer is one or a combination of ammonium chloride and quaternary ammonium salt; The demulsifier is one or both of alkyl phosphate and alkoxy carboxylate; The drainage aid is one or both of a fatty alcohol polyether substance and a fatty alcohol polyether cationic compound; The bactericide is one or more of formaldehyde, glutaraldehyde and quaternary ammonium salt; The cross-linking agent is an organic zirconium salt substance; The pH regulator is an acetic acid substance; The debonding agent is an ammonium persulfate substance.

3. The method according to claim 1, characterized in that The first proppant is a ceramsite proppant; The second proppant is a resin-coated quartz sand proppant; The third proppant is a resin-coated quartz sand proppant.

4. The method according to claim 1, characterized in that The viscosity of the slippery water is 2 to 6 mPa.s; The viscosity of the acidic jelly liquid is 100-150 mPa.s.

5. The method according to claim 1, characterized in that The degradable fiber is polyester fiber.

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