Microspheres, a preparation method thereof, a liquid carbon dioxide phase change fracturing method and application thereof

Abstract

The application discloses a kind of microspheres and its preparation method, liquid carbon dioxide phase change fracturing method and application.The microspheres of the application include composite shell, the composite shell includes rigid shell, heat preservation layer, first rupture control layer and second rupture control layer from outside to inside;The composite shell is equipped with injection port and one-way valve flap;The one-way valve flap is used to allow injection port to inject material in the composite shell or prevent injection port to inject material in the composite shell.The microspheres of the application form liquid carbon dioxide microspheres after injecting liquid carbon dioxide, which can be applied in liquid carbon dioxide phase change fracturing.The liquid carbon dioxide phase change fracturing technology of the application is based on liquid carbon dioxide microspheres, by pumping liquid carbon dioxide microspheres, further improves net pressure in fracture by the additional pressure released by liquid carbon dioxide microspheres, so as to improve the volume of reconstruction, which can provide a new efficient fracturing technology for efficient development of complex oil and gas reservoirs.

Description

Technical Field

[0001] This invention relates to the technical field of oil and gas field development engineering, and more specifically, to a microsphere and its preparation method, a liquid carbon dioxide phase change fracturing method and its application. Background Technology

[0002] Currently, liquid carbon dioxide phase change fracturing technology is mainly used for coal seam fracturing to improve gas drainage efficiency. CN115749926A discloses a liquid carbon dioxide phase change fracturing technology. The main invention of this patent is to use a multi-section phase change fracturing pipe component, and automatically adjust the spacing between each section of the phase change fracturing pipe component by the spacing of the borehole and the coal seam cutting, thereby achieving the effect of expanding the borehole and increasing permeability.

[0003] Yingchun Fan (2020) also presented results on liquid carbon dioxide phase change fracturing technology and its application in coal mine gas extraction. Zhou Keping et al. (2017) and Xiao Chengxu (2018) conducted in-depth research on the rock-breaking mechanism of liquid carbon dioxide phase change fracturing, mainly involving the optimization of phase change parameters and fracturing effect, but did not develop corresponding liquid carbon dioxide phase change fracturing technology.

[0004] Overall, the working environment and technical content of the currently developed liquid carbon dioxide phase change fracturing technology cannot be effectively applied to the development of oil and gas reservoirs (deep reservoir fracturing). In other words, liquid carbon dioxide phase change fracturing technology has not yet been formed in the field of oil and gas development.

[0005] Therefore, it is necessary to carry out relevant technical research to provide technical support for the efficient development of complex oil and gas reservoirs (coal-bearing reservoirs, coal seam / rock gas, shale reservoirs, normal and low-pressure reservoirs, and highly water-sensitive formations, etc.). Summary of the Invention

[0006] To address the problems in existing technologies, this invention proposes a microsphere and its preparation method, a liquid carbon dioxide phase change fracturing method, and its application. This invention designs a liquid carbon dioxide microsphere and, based on this microsphere, develops a liquid carbon dioxide phase change fracturing technology applicable to oil and gas reservoir fracturing. This improves the reservoir stimulation technology system and promotes the efficient development of complex oil and gas reservoirs (coal-bearing reservoirs, coalbed / rock gas, shale reservoirs, atmospheric and low-pressure reservoirs, and highly water-sensitive formations, etc.).

[0007] One objective of this invention is to provide a microsphere comprising a composite shell, wherein the composite shell comprises, from the outside to the inside, a rigid outer shell, a thermal insulation layer, a first rupture control layer, and a second rupture control layer; the composite shell is provided with an injection port and a one-way valve baffle; the one-way valve baffle is used to allow the injection port to inject material into the composite shell or to prevent the injection port from injecting material into the composite shell.

[0008] In the microspheres described in this invention, preferably,

[0009] One side of the one-way valve baffle is fixed inside the second rupture control layer, and the other side can move according to the pressure difference inside and outside the composite shell; the injection port is located at a position where the one-way valve baffle can cover and seal the injection port when closed, and a position where the material flowing out of the injection port can flow into the composite shell when open; preferably, one side of the one-way valve baffle is fixed inside the second rupture control layer by a spring hinge; and / or, the area of ​​the injection port is less than or equal to the area of ​​the one-way valve baffle; and / or,

[0010] The injected material is liquid carbon dioxide; and / or,

[0011] The microspheres have a mesh size of 2–70 (diameter 0.212–8 mm, produced as needed); and / or,

[0012] The breaking strength of the rigid outer shell and the insulation layer must be less than that of the first breaking control layer and the second breaking layer; and / or,

[0013] The shear failure strength of the rigid outer shell and the insulation layer must be less than the shear failure strength of the first fracture control layer and the second fracture layer; and / or,

[0014] Preferably, the fracture strength or shear failure strength of the rigid outer shell, the first fracture control layer, and the second fracture control layer are all ≥11 MPa; and / or,

[0015] The microspheres are resistant to temperatures ranging from -15℃ to 200℃ (the pressure resistance time is adjustable, and the dissolution or degradation time of each component can be adjusted according to different material selections or combinations).

[0016] In the microspheres described in this invention, preferably,

[0017] The material of the rigid outer shell is selected from at least one of PGA (polyglycolic acid), PLA (polylactic acid), poly(3-hydroxyalkanoate) (PHA), poly(ε-caprolactone) (PCL), aliphatic aromatic copolyester, polyvinyl alcohol (PVA), or carbon dioxide / epoxy copolymer resin; and / or,

[0018] The thickness of the rigid outer shell is 0.01–0.8 mm; preferably 0.01–0.4 mm; and / or,

[0019] The material of the insulation layer is selected from at least one of aluminum silicate fiber mixture, expanded polypropylene (EPP) material, aerogel, or elastic foam; preferably, the aerogel is selected from at least one of silicon-based or carbon-based aerogel; the elastic foam is selected from at least one of flexible polyurethane foam; and / or,

[0020] The thickness of the insulation layer is 0.02–1.0 mm; preferably 0.02–0.8 mm.

[0021] In the microspheres described in this invention, preferably,

[0022] The material of the first crack control layer is a mixture including copolymer, starch, glycerol, liquid paraffin, fragrance, preservative, and weight gain agent; preferably, based on 100 wt% of the total weight of the mixture, the mixture includes (40-60) wt% of a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, (5-10) wt% of starch, (5-10) wt% of glycerol, (5-10) wt% of liquid paraffin, (5-10) wt% of fragrance, (5-10) wt% of preservative, and (10-20) wt% of weight gain agent; and / or,

[0023] The thickness of the first fracture control layer is 0.01–0.8 mm; preferably 0.01–0.5 mm; and / or,

[0024] The material of the second crack control layer is a mixture including copolymer, starch, glycerol, liquid paraffin, fragrance, preservative, and weight gain agent; preferably, the mixture includes (40-60) wt% of a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, (5-10) wt% of starch, (5-10) wt% of glycerol, (5-10) wt% of liquid paraffin, (5-10) wt% of fragrance, (5-10) wt% of preservative, and (10-20) wt% of weight gain agent; then the above substances are heated and mixed together, the heated mixture is poured into a mold to prepare the second crack control layer, and then it is dried for 24 hours or more until it is completely cured; and / or,

[0025] The thickness of the second fracture control layer is 0.01–0.8 mm; preferably 0.01–0.4 mm; and / or,

[0026] The material of the one-way valve baffle is selected from at least one of PGA (polyglycolic acid), PLA (polylactic acid), poly(3-hydroxyalkanoate) (PHA), poly(ε-caprolactone) (PCL), aliphatic aromatic copolyester, polyvinyl alcohol (PVA), or carbon dioxide / epoxy copolymer resin.

[0027] The thickness of the one-way valve baffle is 0.1–0.5 mm; preferably 0.1–0.2 mm.

[0028] In the microspheres of this invention, the first and second rupture control layers primarily control the pressure at which the liquid carbon dioxide in the microspheres releases energy. For example, if the design requires a pressure release of 20 MPa, these two control layers must ensure that the microspheres remain in a non-energy-releasing state until this pressure is reached. The insulation layer is mainly used for heat preservation, ensuring that under a certain ambient temperature, the liquid carbon dioxide in the microspheres reaches a rapid vaporization energy release state only after a predetermined time. It also ensures that the microspheres remain in a safe state during storage and transportation, i.e., they do not reach the rapid vaporization energy release state. The rigid outer shell is mainly used to protect the insulation layer from damage and enhance the transportability of the microspheres in fluids. The composite shell of the microspheres of this invention can be designed with specific material proportions according to strength requirements.

[0029] In the microspheres of this invention, a one-way valve baffle is provided at the liquid carbon dioxide injection port. This baffle can automatically adjust according to the pressure difference between the inside and outside of the injection port. During injection, when the external pressure is higher than the internal pressure of the sphere, the baffle moves towards the center of the sphere, opening the injection channel and allowing liquid carbon dioxide to enter the sphere. When the internal pressure reaches a preset design value, the internal pressure pushes the baffle away from the center of the sphere, closing the channel, thereby effectively preventing liquid carbon dioxide from flowing out and ensuring the system's sealing and safety.

[0030] The one-way valve baffle is fixed to the second rupture control layer by a spring hinge. When liquid carbon dioxide is injected, the one-way valve baffle opens inward. When the injection stops, the one-way valve baffle closes outward under the action of the spring hinge (when the injection stops, the external pressure of the ball is atmospheric pressure, and the internal pressure of the ball is higher than atmospheric pressure, so it can be closed) to prevent liquid carbon dioxide from overflowing.

[0031] A second objective of this invention is to provide a method for preparing microspheres as described in one objective of this invention, comprising the following steps:

[0032] S1: The semi-solidified second fracture control layer material is injected into a spherical mold one, with an opening reserved, to initially form a spherical shape with an opening; then a one-way valve baffle is installed inside the spherical shape with an opening to obtain intermediate sample one; then the intermediate sample one is placed into mold two, and then the semi-solidified second fracture control layer material is injected into mold two to obtain a second fracture control layer with a one-way valve baffle installed;

[0033] S2: Apply a layer of glue to the outside of the second crack control layer, then evenly cover the second crack control layer with the semi-solid first crack control layer material, and place it in mold three to cool and form a sphere. Complete the installation of the insulation layer and the rigid shell in this manner.

[0034] A third objective of this invention is to provide an application of microspheres, as described in one objective of this invention, in liquid carbon dioxide phase change fracturing.

[0035] The fourth objective of this invention is to provide a liquid carbon dioxide phase change fracturing method, which includes at least the steps of injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing after the pre-fracturing stage and injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing before the injection of displacement fluid; preferably, the liquid carbon dioxide microspheres are liquid carbon dioxide microspheres formed after injecting liquid carbon dioxide as described in one objective of this invention.

[0036] In the liquid carbon dioxide phase change fracturing method of the present invention, preferably, the following steps are included:

[0037] (1) After acid treatment, perform the pre-treatment liquid seam-making step;

[0038] (2) A step of phase change fracturing by injecting fracturing fluid containing liquid carbon dioxide microspheres in one injection;

[0039] (3) Sand fracturing step;

[0040] (4) The step of secondary injection of fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing;

[0041] (5) Optional repetition of step (3) and / or step (4);

[0042] (6) Inject the displacement solution.

[0043] This invention designs the fracturing fluid volume, fracturing sand volume, and pumping procedure for the target reservoir according to a general fracturing scheme design method. Then, after the pre-fluidization stage of the pumping procedure, a pumping stage is added for pumping liquid carbon dioxide microspheres for fracturing. The liquid carbon dioxide microspheres can be added according to the designed average sand ratio, and the mesh size of the liquid carbon dioxide microspheres can be consistent with the mesh size of the medium-sized proppant in the fracturing scheme. A pumping stage for liquid carbon dioxide microspheres for fracturing is added before each displacement stage.

[0044] To ensure safe operation, the operating pressure window must be at least 5 to 10 MPa larger than that of conventional fracturing schemes for similar reservoirs, and the wellhead, casing, and other fracturing materials, equipment, and tools should be selected accordingly.

[0045] Through scheme design, it is ensured that the pressure release of the liquid carbon dioxide microspheres must occur only after they have fully entered the formation, and that the wellhead pressure remains within the safe pressure limit after the microsphere pressure is released. In summary, the liquid carbon dioxide phase change fracturing process of this invention, after injecting liquid carbon dioxide microspheres, causes the liquid carbon dioxide to heat up, vaporize, and expand over a period of time, generating high pressure to assist in fracturing the reservoir, which is not available in conventional fracturing processes.

[0046] In the liquid carbon dioxide phase change fracturing method of the present invention, preferably, step (1),

[0047] The acid used for acid treatment is HCl or oxalic acid; and / or,

[0048] The mass concentration of the acid solution used for acid treatment is 15%–20%; and / or,

[0049] The discharge rate of the acid solution used for acid treatment is 1.5–2 m³. 3 / min; and / or,

[0050] The injection volume of acid solution used for acid treatment is 10-20 ml. 3 ;

[0051] Step (1),

[0052] The pre-fluid injection process includes the steps of first injecting low-viscosity slickwater and then injecting ultra-high-viscosity slickwater.

[0053] Preferably,

[0054] The viscosity of the low-viscosity slippery water is 1–11 mPa·s; preferably 3–6 mPa·s; and / or,

[0055] The discharge rate of the low-viscosity slippery water is 2-20 m³. 3 / min; preferably 2-14m 3 / min; and / or,

[0056] The injection volume of the low-viscosity slippery water is 40-70 m³. 3 Preferably 50-60m 3 ; and / or,

[0057] The viscosity of the ultra-high viscosity slippery liquid is 30–100 mPa·s; preferably 50–60 mPa·s; and / or,

[0058] The discharge capacity of the ultra-high viscosity slippery water is 8-20 m³. 3 / min; preferably 10-20m 3 / min; and / or,

[0059] The injection volume of the ultra-high viscosity slippery water is 70-100 m³. 3 The preferred depth is 80-90m. 3 .

[0060] In the liquid carbon dioxide phase change fracturing method of the present invention, preferably, step (2),

[0061] The fracturing fluid containing liquid carbon dioxide microspheres has a sand ratio of 3% to 16%; preferably 6% to 10%; and / or,

[0062] The fracturing fluid containing the liquid carbon dioxide microspheres is ultra-high viscosity slickwater; preferably with a viscosity of 30–100 mPa·s; and / or,

[0063] The fracturing fluid containing liquid carbon dioxide microspheres has a flow rate of 6–20 m³ / h. 3 / min; preferably 8-18m 3 / min; and / or,

[0064] The injection volume of the fracturing fluid containing liquid carbon dioxide microspheres is 0.5 to 1.0 times the volume of the pre-filled fluid.

[0065] In the liquid carbon dioxide phase change fracturing method of the present invention, preferably, step (3),

[0066] The displacement of the sand-fracturing is 10-20 m³. 3 / min; preferably 15-20m 3 / min; and / or,

[0067] The sand ratio for the sand-fracturing is 3%-5%; and / or,

[0068] The fracturing fluid used in the proppant fracturing is low-viscosity slickwater; preferably, the viscosity of the low-viscosity slickwater is 1–11 mPa·s; more preferably 3–6 mPa·s; and / or,

[0069] The injection volume for each fracturing operation with proppant is 60–100 m³. 3 The preferred depth is 80-90m. 3 .

[0070] In the liquid carbon dioxide phase change fracturing method of the present invention, preferably, step (4),

[0071] The fracturing fluid containing liquid carbon dioxide microspheres has a sand ratio of 70%–80% of the previous sand ratio (for example, it could be 70%–80% of the second sand ratio in a sand-fracturing operation); and / or,

[0072] The fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is low-viscosity slickwater; preferably, the viscosity of the low-viscosity slickwater is 1–11 mPa·s; and / or,

[0073] The fracturing fluid containing liquid carbon dioxide microspheres has a flow rate of 6–20 m³ / h. 3 / min; preferably 8-18m 3 / min; and / or,

[0074] The injection volume of the fracturing fluid containing liquid carbon dioxide microspheres is 1 to 1.5 times the wellbore volume.

[0075] In the liquid carbon dioxide phase change fracturing method described in this invention, preferably,

[0076] Step (6),

[0077] The displacement fluid is a low-viscosity slippery water; preferably, the viscosity of the low-viscosity slippery water is 3–100 mPa·s; more preferably 3–30 mPa·s; and / or,

[0078] The displacement fluid has a flow rate of 6–20 m³. 3 / min; preferably 16-20m 3 / min; and / or,

[0079] The injection volume of the displacement fluid is 1.0 to 2.0 times the wellbore volume; preferably 1.0 to 1.5 times the wellbore volume.

[0080] The fifth objective of this invention is to provide an application of the liquid carbon dioxide phase change fracturing method as described in the fourth objective of this invention in oil and gas extraction.

[0081] The substances and parameters not limited in this invention can be selected according to existing technology, which is a conventional technical means in this field.

[0082] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0083] Compared with the prior art, the present invention has at least the following advantages:

[0084] This invention proposes a liquid carbon dioxide phase change fracturing technology. Based on liquid carbon dioxide microspheres, it increases the net pressure within the fracture by adding the injection of liquid carbon dioxide microspheres to the conventional pumping procedure. This increases the volume of the fracturing process. This technology can provide a new and efficient fracturing technique for the efficient development of complex oil and gas reservoirs (coal-bearing reservoirs, coal seam / rock gas, shale reservoirs, atmospheric and low-pressure reservoirs, and highly water-sensitive formations, etc.).

[0085] This invention proposes a fracturing technology based on liquid carbon dioxide microspheres, which can further increase the volume of fracturing on the basis of conventional fracturing technology, and at the same time utilize the special properties of carbon dioxide fluid to achieve reservoir permeability enhancement and production improvement. Attached Figure Description

[0086] Figure 1 This is a schematic diagram of the structure of the microspheres of the present invention;

[0087] Figure 2 This is a schematic diagram of the molding process in mold one of the present invention;

[0088] Figure 3 This is a schematic diagram of the molding process in mold two of the present invention.

[0089] Explanation of reference numerals in the attached figures:

[0090] 1- Rigid outer shell; 2- Thermal insulation layer; 3- First rupture control layer; 4- Second rupture control layer; 5- Liquid carbon dioxide; 6- Injection pipe; 7- One-way valve baffle; 8- Mold one; 9- Gas injection pipe; 10- Second rupture control layer material in semi-solid state; 11- Mold two; 12- Injection pipe; 13- Detachable suspension arm. Detailed Implementation

[0091] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0092] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0093] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0094] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0095] Example 1

[0096] like Figure 1As shown, the liquid carbon dioxide microspheres of the present invention include a composite shell, which comprises, from the outside to the inside, a rigid outer shell 1, a thermal insulation layer 2, a first rupture control layer 3, and a second rupture control layer 4. The composite shell is provided with an injection port and a one-way valve baffle; an injection pipe 6 is installed on the injection port of the composite shell. The one-way valve baffle 7 is used to allow the injection pipe 6 to inject material into the composite shell or to prevent the injection pipe 6 from injecting material into the composite shell, wherein the injected material is liquid carbon dioxide 5.

[0097] One side of the one-way valve baffle 7 is fixed inside the second rupture control layer 4 by a spring hinge, while the other side can move according to the pressure difference inside and outside the composite shell. The injection pipe 6 is located at a position where the injection port of the injection pipe 6 can be covered and sealed when the one-way valve baffle 7 is closed, and at a position where material can flow out of the injection port of the injection pipe 6 into the composite shell when it is open. The area of ​​the liquid carbon dioxide 5 injection port is smaller than the area of ​​the one-way valve baffle 7.

[0098] The working principle of the one-way valve baffle 7 of the present invention is as follows: the one-way valve baffle 7 is fixed in the second rupture control layer 4 by a spring hinge (not shown in the figure). When liquid carbon dioxide 5 is injected, the external pressure is higher than the internal pressure of the microsphere. The one-way valve baffle 7 moves towards the center of the microsphere to form an injection channel for liquid carbon dioxide 5. When the contents of the microsphere reach the design pressure, the injection stops. The internal pressure of the microsphere and the elasticity of the spring hinge will play a role in pushing the one-way valve baffle 7 away from the center of the microsphere, thus preventing the liquid carbon dioxide 5 from flowing out of the microsphere.

[0099] The method for preparing the microspheres includes the following steps:

[0100] S1: The shape of mold 18 is as follows Figure 2 As shown, mold one consists of two detachable and combinable parts. A semi-solidified second rupture control layer material 10 is injected into mold one. Then, air is injected into the semi-solidified second rupture control layer material 10 using the air injection pipe 9. Using the air pressure and the shape of mold one, it is initially formed into an open sphere. A one-way valve baffle is then installed inside the open sphere via a spring hinge, ensuring it automatically springs back to form a seal, resulting in intermediate sample one. Since the initially formed sphere is semi-solid and relatively soft, one installation method involves directly inserting the spring hinge with the one-way valve baffle into the initially formed sphere. After the initially formed sphere solidifies, the spring hinge and the initially formed sphere are fixed inside the sphere. The shape of mold two 11 is as follows... Figure 3As shown, mold two is also a detachable and combinable two-part structure. The intermediate sample one is positioned in mold two using a detachable suspension arm 13 (such as a hook, one end of which is hooked to the opening of the intermediate sample one, and the other end is hooked to mold two). Then, the semi-solid state of the second rupture control layer material is injected into mold two using the injection pipe 12 to obtain the second rupture control layer with a one-way valve baffle installed. (At this time, the second rupture control layer can be a fully enclosed sphere. After all parts of the composite shell are formed, the injection pipe 6 is installed on the opening of the composite shell. Alternatively, the injection port can be reserved during the forming process, and the remaining parts can be a closed sphere. After all parts of the composite shell are formed, the injection pipe 6 is installed. As long as the injection pipe is located at a position that can cover and seal the injection port of the injection pipe when the one-way valve baffle is closed, and allows the material to flow out of the injection port of the injection pipe when it is open, it is acceptable.)

[0101] S2: Apply a layer of glue to the outside of the second crack control layer, then evenly cover the second crack control layer with the semi-solidified first crack control layer material, and place it in mold three (which has a structure that is basically the same as mold two, but is different in size) to cool and form a sphere. Complete the installation of the insulation layer and the rigid shell in this way.

[0102] The rigid outer shell is made of PGA (polyglycolic acid); the thickness of the rigid outer shell is 0.05 mm; the insulation layer is made of carbon aerogel; the thickness of the insulation layer is 0.08 mm.

[0103] The material of the first crack control layer is a mixture including a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, starch, glycerol, liquid paraffin, fragrance, preservative and weight gain agent; wherein, the mixture includes 50 wt% of the mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer (the mass ratio of polyvinyl alcohol copolymer and vinyl acetate-ethylene copolymer is 1:1), 10 wt% of starch, 10 wt% of glycerol, 5 wt% of liquid paraffin, 5 wt% of fragrance (thymol), 5 wt% of preservative (sodium benzoate) and 15 wt% of weight gain agent (sodium stannate), and the components are mixed evenly according to the above proportions; the thickness of the first crack control layer is 0.1 mm.

[0104] The material of the second crack control layer is a mixture including a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, starch, glycerol, liquid paraffin, fragrance, preservative and weighting agent; wherein, the mixture includes 55 wt% of the mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer (the mass ratio of polyvinyl alcohol copolymer and vinyl acetate-ethylene copolymer is 1:1), 10 wt% of starch, 5 wt% of glycerol, 5 wt% of liquid paraffin, 5 wt% of fragrance (thymol), 5 wt% of preservative (sodium benzoate) and 15 wt% of weighting agent (sodium stannate), and the components are mixed evenly according to the above proportions; the thickness of the second crack control layer is 0.08 mm.

[0105] The material of the one-way valve baffle is selected from PGA (polyglycolic acid); the thickness of the one-way valve baffle is 0.1mm, and the shape of the one-way valve baffle is a circular piece.

[0106] The mesh size of the microspheres prepared by the above method can be 10 to 16 mesh, depending on the requirements; the microspheres are resistant to temperatures from -15℃ to 150℃. The rupture strength or shear failure strength of the rigid outer shell and the insulation layer is less than that of the first rupture control layer and the second rupture control layer; the rupture strength or shear failure strength of the rigid outer shell, the first rupture control layer, and the second rupture control layer is ≥11MPa.

[0107] Example 2

[0108] The same microspheres and preparation method as in Example 1 were used, the difference being that the materials used for the microspheres and the thickness of each layer of the composite shell were different, as detailed below:

[0109] The rigid outer shell is made of poly(3-hydroxyalkanoate) (PHA); the thickness of the rigid outer shell is 0.08 mm; the insulation layer is made of aluminum silicate fiber mixture; the thickness of the insulation layer is 0.1 mm.

[0110] The material of the first crack control layer is a mixture including a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, starch, glycerol, liquid paraffin, fragrance, preservative and weight gain agent; wherein, the mixture includes 60 wt% of the mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer (the mass ratio of polyvinyl alcohol copolymer and vinyl acetate-ethylene copolymer is 1:1), 5 wt% of starch, 10 wt% of glycerol, 5 wt% of liquid paraffin, 5 wt% of fragrance (thymol), 5 wt% of preservative (sodium benzoate) and 10 wt% of weight gain agent (sodium stannate), and the components are mixed evenly according to the above proportions; the thickness of the first crack control layer is 0.11 mm.

[0111] The material of the second crack control layer is a mixture including a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, starch, glycerol, liquid paraffin, fragrance, preservative and weight gain agent; wherein, the mixture includes 55 wt% of the mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer (the mass ratio of polyvinyl alcohol copolymer and vinyl acetate-ethylene copolymer is 1:1), 10 wt% of starch, 5 wt% of glycerol, 5 wt% of liquid paraffin, 5 wt% of fragrance (thymol), 5 wt% of preservative (sodium benzoate) and 15 wt% of weight gain agent (sodium stannate), and the components are mixed evenly according to the above proportions; the thickness of the second crack control layer is 0.08 mm.

[0112] The material of the one-way valve baffle is selected from poly(3-hydroxyalkanoate) (PHA); the thickness of the one-way valve baffle is 0.1 mm.

[0113] The mesh size of the microspheres prepared by the above method can be set to 5-10 mesh as needed; the microspheres are resistant to temperatures from -15℃ to 180℃. The rupture strength or shear failure strength of the rigid outer shell and the insulation layer is less than that of the first rupture control layer and the second rupture control layer; the rupture strength or shear failure strength of the rigid outer shell, the first rupture control layer, and the second rupture control layer is ≥11MPa.

[0114] Example 3

[0115] A liquid carbon dioxide phase change fracturing method, comprising at least the steps of injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing after the pre-fracturing fluid fracturing stage and injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing before the injection of displacement fluid; the liquid carbon dioxide microspheres are the liquid carbon dioxide microspheres (10 mesh) prepared in Example 1. Specifically, the liquid carbon dioxide phase change fracturing method includes the following steps:

[0116] (1) First, acid treatment is performed; the acid used for acid treatment is HCl; the mass concentration of the acid solution used for acid treatment is 15%; the discharge rate of the acid solution for acid treatment is 2m. 3 / min; the acid injection volume for acid treatment is 10m³. 3 After acid treatment, a pre-fluidized joint formation step is performed. This step includes injecting low-viscosity slurry followed by ultra-high-viscosity slurry. The low-viscosity slurry used has a viscosity of 3 mPa·s and a flow rate of 14 m³ / s. 3 / min; the injection volume of low-viscosity slickwater is 50m³. 3 The viscosity of the ultra-high viscosity slick is 50 mPa·s; the displacement of the ultra-high viscosity slick is 16 m³ / s. 3 / min; the injection volume of ultra-high viscosity slippery water is 80m³. 3 .

[0117] (2) A phase change fracturing step involving the injection of fracturing fluid containing liquid carbon dioxide microspheres; wherein the fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is ultra-high viscosity slickwater; the viscosity of ultra-high viscosity slickwater is 50 mPa·s; the sand ratio of the fracturing fluid containing liquid carbon dioxide microspheres is 7%; and the flow rate of the fracturing fluid containing liquid carbon dioxide microspheres is 18 m³ / s. 3 / min; the injection volume of fracturing fluid containing liquid carbon dioxide microspheres is 0.8 times the volume of the pre-fracturing fluid;

[0118] (3) Sand fracturing step; wherein, the displacement of sand fracturing is 18m³. 3 / min; the sand ratio for sand fracturing is 5%; the fracturing fluid for sand fracturing is low-viscosity slickwater with a viscosity of 3 mPa·s; the injection volume for sand fracturing is 80 m³ / min. 3 .

[0119] (4) A second injection of fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing; in step (4), the fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is low-viscosity slickwater with a viscosity of 6 mPa·s; the sand ratio of the fracturing fluid containing liquid carbon dioxide microspheres is 70% of the previous sand ratio; the discharge rate of the fracturing fluid containing liquid carbon dioxide microspheres is 16 m³ / s. 3 / min; the injection volume of the fracturing fluid containing liquid carbon dioxide microspheres is 1.0 times the wellbore volume.

[0120] (5) Inject the displacement fluid; the displacement fluid is low-viscosity slickwater with a viscosity of 10 mPa·s; the displacement fluid flow rate is 18 m³ / s. 3 / min; the injection volume of the displacement fluid is 1.0 times the wellbore volume.

[0121] The fracturing effect of the present invention after fracturing using the above method is shown in Table 1.

[0122] Table 1

[0123] Fracturing methods Crack half length / m Seam height / m <![CDATA[SRV×104 / m 3 ]]> Conventional fracturing process 108 30 3070 Liquid carbon dioxide phase change fracturing process 124 28 3540 Improvement of crack parameters Increase by 15% Basically equivalent Increase by 15%

[0124] Example 4

[0125] A liquid carbon dioxide phase change fracturing method, comprising at least the steps of injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing after the pre-fracturing fluid fracturing stage and injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing before the injection of displacement fluid; the liquid carbon dioxide microspheres are the liquid carbon dioxide microspheres (5 mesh) prepared in Example 2. Specifically, the liquid carbon dioxide phase change fracturing method includes the following steps:

[0126] (1) First, acid treatment is performed; the acid used for acid treatment is HCl; the mass concentration of the acid solution used for acid treatment is 20%; the discharge rate of the acid solution used for acid treatment is 1.5m. 3 / min; the acid injection volume for acid treatment is 15m³. 3 After acid treatment, a pre-fluidized joint formation step is performed. This step includes injecting low-viscosity slurry followed by ultra-high-viscosity slurry. The low-viscosity slurry used has a viscosity of 6 mPa·s and a flow rate of 14 m³ / s. 3 / min; the injection volume of low-viscosity slickwater is 50m³. 3 The viscosity of the ultra-high viscosity slick is 60 mPa·s; the displacement of the ultra-high viscosity slick is 14 m³ / s. 3 / min; the injection volume of ultra-high viscosity slippery water is 90m³. 3 .

[0127] (2) A step of phase change fracturing by injecting fracturing fluid containing liquid carbon dioxide microspheres in a single injection; wherein the fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is ultra-high viscosity slickwater; the viscosity of ultra-high viscosity slickwater is 60 mPa·s; the sand ratio of the fracturing fluid containing liquid carbon dioxide microspheres is 10%; and the displacement of the fracturing fluid containing liquid carbon dioxide microspheres is 16 m³ / s. 3 / min; the injection volume of fracturing fluid containing liquid carbon dioxide microspheres is 0.5 times the volume of the pre-fracturing fluid;

[0128] (3) Sand fracturing step; wherein, the displacement of sand fracturing is 16m³. 3 / min; the sand ratio for sand fracturing is 5%; the fracturing fluid for sand fracturing is low-viscosity slickwater with a viscosity of 6 mPa·s; the injection volume for sand fracturing is 90 m³ / min. 3 .

[0129] (4) A second injection of fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing; in step (4), the fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is low-viscosity slickwater, 10 mPa·s; the sand ratio of the fracturing fluid containing liquid carbon dioxide microspheres is 70% of the previous sand ratio; the discharge rate of the fracturing fluid containing liquid carbon dioxide microspheres is 18 m³ / s. 3 / min; the injection volume of fracturing fluid containing liquid carbon dioxide microspheres is 1.5 times the wellbore volume.

[0130] (5) If the fracturing parameters do not meet the design requirements, such as fracture height and fracture length, steps (3) and (4) can be repeated with the same process parameters.

[0131] (6) Inject the displacement fluid; the displacement fluid is low-viscosity slickwater with a viscosity of 6 mPa·s; the displacement fluid flow rate is 20 m³ / s. 3 / min; the injection volume of the displacement fluid is 1.5 times the wellbore volume.

[0132] The fracturing effect of the present invention after fracturing using the above method is shown in Table 2.

[0133] Table 2

[0134] Fracturing methods Crack half length / m Seam height / m <![CDATA[SRV×10 4 / m 3 ]]> Conventional fracturing process 156 20 3370 Liquid carbon dioxide phase change fracturing process 172 28 3940 Improvement of crack parameters Increase by 10% Increase by 40% Increase by 17%

[0135] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

[0136] All publications, patent applications, patents, and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings commonly understood by those skilled in the art. In case of conflict, the definitions in this specification shall prevail.

[0137] When this specification uses the prefixes “known to those skilled in the art,” “prior art,” or similar terms to derive materials, substances, methods, steps, apparatus, or components, the objects derived from such prefixes cover those commonly used in the art at the time of this application’s filing, but also include those that are not currently commonly used but will become generally recognized in the art as suitable for similar purposes.

[0138] In the context of this specification, except where expressly stated otherwise, any matters or issues not mentioned shall apply directly to those known in the art without any modification.

Claims

1. A microsphere, characterized in that: The microspheres include a composite shell, which comprises, from the outside to the inside, a rigid outer shell, a thermal insulation layer, a first rupture control layer, and a second rupture control layer; the composite shell is provided with an injection port and a one-way valve baffle; the one-way valve baffle is used to allow the injection port to inject material into the composite shell or to prevent the injection port from injecting material into the composite shell.

2. The microspheres according to claim 1, characterized in that, One side of the one-way valve baffle is fixed inside the second rupture control layer, and the other side can move according to the pressure difference inside and outside the composite shell; the injection port is located at a position where the one-way valve baffle can cover and seal the injection port when closed, and a position where the material flowing out of the injection port can flow into the composite shell when open; preferably, one side of the one-way valve baffle is fixed inside the second rupture control layer by a spring hinge; and / or, the area of ​​the injection port is less than or equal to the area of ​​the one-way valve baffle; and / or, The injected material is liquid carbon dioxide; and / or, The microspheres have a mesh size of 2 to 70; and / or, The breaking strength of the rigid outer shell and the insulation layer must be less than that of the first breaking control layer and the second breaking layer; and / or, The shear failure strength of the rigid outer shell and the insulation layer must be less than the shear failure strength of the first fracture control layer and the second fracture layer; and / or, Preferably, the fracture strength or shear failure strength of the rigid outer shell, the first fracture control layer, and the second fracture control layer are all ≥11 MPa; and / or, The microspheres are resistant to temperatures ranging from -15℃ to 200℃.

3. The microspheres according to claim 1, characterized in that, The material of the rigid outer shell is selected from at least one of polyglycolic acid, polylactic acid, poly(3-hydroxyalkanoate), poly(ε-caprolactone), aliphatic aromatic copolyester, polyvinyl alcohol, or carbon dioxide / epoxy copolymer resin; and / or, The thickness of the rigid outer shell is 0.01–0.8 mm; preferably 0.01–0.4 mm; and / or, The material of the insulation layer is selected from at least one of aluminum silicate fiber mixture, foamed polypropylene material, aerogel, or elastic foam; preferably, the aerogel is selected from at least one of silicon-based or carbon-based aerogel; the elastic foam is selected from at least one of flexible polyurethane foam; and / or, The thickness of the insulation layer is 0.02–1.0 mm; preferably 0.02–0.8 mm.

4. The microspheres according to claim 1, characterized in that, The material of the first crack control layer is a mixture including copolymer, starch, glycerol, liquid paraffin, fragrance, preservative, and weight gain agent; preferably, based on 100 wt% of the total weight of the mixture, the mixture includes (40-60) wt% of a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, (5-10) wt% of starch, (5-10) wt% of glycerol, (5-10) wt% of liquid paraffin, (5-10) wt% of fragrance, (5-10) wt% of preservative, and (10-20) wt% of weight gain agent; and / or, The thickness of the first fracture control layer is 0.01–0.8 mm; preferably 0.01–0.5 mm; and / or, The material of the second crack control layer is a mixture including copolymer, starch, glycerol, liquid paraffin, fragrance, preservative, and weight gain agent; preferably, based on 100 wt% of the total weight of the mixture, the mixture includes (40-60) wt% of a mixture of polyvinyl alcohol and vinyl acetate-ethylene copolymer, (5-10) wt% of starch, (5-10) wt% of glycerol, (5-10) wt% of liquid paraffin, (5-10) wt% of fragrance, (5-10) wt% of preservative, and (10-20) wt% of weight gain agent; and / or, The thickness of the second fracture control layer is 0.01–0.8 mm; preferably 0.01–0.4 mm; and / or, The material of the one-way valve baffle is selected from at least one of polyglycolic acid, polylactic acid, poly(3-hydroxyalkanoate), poly(ε-caprolactone), aliphatic aromatic copolyester, polyvinyl alcohol, or carbon dioxide / epoxy copolymer resin; and / or, The thickness of the one-way valve baffle is 0.1–0.5 mm; preferably 0.1–0.2 mm.

5. A method for preparing microspheres as described in any one of claims 1-4, characterized in that, Includes the following steps: S1: The semi-solidified second fracture control layer material is injected into a spherical mold one, with an opening reserved, to initially form a spherical shape with an opening; then a one-way valve baffle is installed inside the spherical shape with an opening to obtain intermediate sample one; then the intermediate sample one is placed into mold two, and then the semi-solidified second fracture control layer material is injected into mold two to obtain a second fracture control layer with a one-way valve baffle installed; S2: Apply a layer of glue to the outside of the second crack control layer, then evenly cover the second crack control layer with the semi-solid first crack control layer material, and place it in mold three to cool and form a sphere. Complete the installation of the insulation layer and the rigid shell in this manner.

6. The application of the microspheres as described in any one of claims 1-4 in liquid carbon dioxide phase change fracturing.

7. A liquid carbon dioxide phase change fracturing method, characterized in that, The liquid carbon dioxide phase change fracturing method includes at least the steps of injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing after the pre-fracturing stage and injecting fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing before the injection of displacement fluid; preferably, the liquid carbon dioxide microspheres are the liquid carbon dioxide microspheres formed after injecting liquid carbon dioxide into the microspheres described in any one of claims 1-4.

8. The liquid carbon dioxide phase change fracturing method according to claim 7, characterized in that, Includes the following steps: (1) After acid treatment, perform the pre-treatment liquid seam-making step; (2) A step of phase change fracturing by injecting fracturing fluid containing liquid carbon dioxide microspheres in one injection; (3) Sand fracturing step; (4) The step of secondary injection of fracturing fluid containing liquid carbon dioxide microspheres for phase change fracturing; (5) Optional repetition of step (3) and / or step (4); (6) Inject the displacement solution.

9. The liquid carbon dioxide phase change fracturing method according to claim 8, characterized in that: Step (1), The acid used for acid treatment is HCl or oxalic acid; and / or, The mass concentration of the acid solution used for acid treatment is 15%–20%; and / or, The discharge rate of the acid solution used for acid treatment is 1.5–2 m³. 3 / min; and / or, The injection volume of acid solution used for acid treatment is 10-20 ml. 3 ; Step (1), The pre-fluid injection process includes the steps of first injecting low-viscosity slickwater and then injecting ultra-high-viscosity slickwater. Preferably, The viscosity of the low-viscosity slippery water is 1–11 mPa·s; preferably 3–6 mPa·s; and / or, The discharge rate of the low-viscosity slippery water is 2-20 m³. 3 / min; preferably 2-14m 3 / min; and / or, The injection volume of the low-viscosity slippery water is 40-70 m³. 3 Preferably 50-60m 3 ; and / or, The viscosity of the ultra-high viscosity slippery liquid is 30–100 mPa·s; preferably 50–60 mPa·s; and / or, The discharge capacity of the ultra-high viscosity slippery water is 8-20 m³. 3 / min; preferably 10-20m 3 / min; and / or, The injection volume of the ultra-high viscosity slippery water is 70-100 m³. 3 The preferred depth is 80-90m. 3 .

10. The liquid carbon dioxide phase change fracturing method according to claim 8, characterized in that: Step (2), The fracturing fluid containing liquid carbon dioxide microspheres has a sand ratio of 3% to 16%; preferably 6% to 10%; and / or, The fracturing fluid containing the liquid carbon dioxide microspheres is ultra-high viscosity slickwater; preferably with a viscosity of 30–100 mPa·s; and / or, The fracturing fluid containing liquid carbon dioxide microspheres has a flow rate of 6–20 m³ / h. 3 / min; preferably 8-18m 3 / min; and / or, The injection volume of the fracturing fluid containing liquid carbon dioxide microspheres is 0.5 to 1.0 times the volume of the pre-filled fluid.

11. The liquid carbon dioxide phase change fracturing method according to claim 8, characterized in that: Step (3), The displacement of the sand-fracturing is 10-20 m³. 3 / min; preferably 15-20m 3 / min; and / or, The sand ratio for the sand-fracturing is 3%-5%; and / or, The fracturing fluid used in the proppant fracturing is low-viscosity slickwater; preferably, the viscosity of the low-viscosity slickwater is 1–11 mPa·s; more preferably, it is 3–6 mPa·s; and / or, The injection volume for each fracturing operation with proppant is 60–100 m³. 3 The preferred depth is 80-90m. 3 .

12. The liquid carbon dioxide phase change fracturing method according to claim 8, characterized in that: Step (4), The fracturing fluid containing liquid carbon dioxide microspheres has a sand ratio of 70% to 80% of the previous sand ratio; and / or, The fracturing fluid in the fracturing fluid containing liquid carbon dioxide microspheres is low-viscosity slickwater; preferably, the viscosity of the low-viscosity slickwater is 1–11 mPa·s; and / or, The fracturing fluid containing liquid carbon dioxide microspheres has a flow rate of 6–20 m³ / h. 3 / min; preferably 8-18m 3 / min; and / or, The injection volume of the fracturing fluid containing liquid carbon dioxide microspheres is 1 to 1.5 times the wellbore volume.

13. The liquid carbon dioxide phase change fracturing method according to claim 8, characterized in that: Step (6), The displacement fluid is a low-viscosity slippery water; preferably, the viscosity of the low-viscosity slippery water is 3–100 mPa·s; more preferably, it is 3–30 mPa·s; and / or, The displacement fluid has a flow rate of 6–20 m³. 3 / min; preferably 16-20m 3 / min; and / or, The injection volume of the displacement fluid is 1.0 to 2.0 times the wellbore volume; preferably 1.0 to 1.5 times the wellbore volume.

14. The application of the liquid carbon dioxide phase change fracturing method as described in any one of claims 7-13 in oil and gas extraction.

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