Carbon-water mixed phase fracturing method, used fracturing fluid, preparation method and application

Abstract

The invention discloses a carbon-water mixed phase fracturing method, used fracturing fluid, a preparation method and application. The preparation method of the carbon-water mixed-phase fracturing fluid comprises the following steps: keeping the water temperature at 0-40 DEG C, and adding a water-phase thickening agent, a liquid carbon dioxide phase thickening agent and a gel breaker into water according to a designed proportion to prepare a water-phase fracturing fluid; a proppant with the designed sand ratio is added into the water-phase fracturing fluid, and the water-phase fracturing fluid is pressurized to the designed pressure after sand mixing is uniform; and pressurizing liquid carbon dioxide accounting for 95-5Wt% of the finally prepared carbon-water mixed-phase fracturing fluid to design pressure, mixing the liquid carbon dioxide with the water-phase fracturing fluid, and carrying out synergistic dual-thickening on the carbon-water mixed phase by using the two-phase thickening agent to obtain the carbon-water mixed-phase fracturing fluid. According to the invention, only the two-phase thickening agent, the gel breaker and the propping agent need to be added into the water phase, so that the carbon-water mixed-phase fracturing fluid with the functions of thickening, carrying sand, forming cracks and reducing resistance can be prepared; the anti-swelling and discharge-aiding functions are achieved, and reservoir protection is facilitated; a waterproof lock and a waterproof sensitive function are achieved, and damage is greatly reduced; the energy of the stratum is greatly increased, flowback is promoted, and oil and gas are replaced; the yield-increasing effect is excellent and the cost performance is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of fracturing stimulation, and particularly relates to a fracturing method of a carbon - water mixed - phase fracturing by organically combining water - based fracturing, carbon dioxide pre - energizing fracturing, carbon dioxide foam fracturing, and carbon dioxide dry fracturing, and forming a carbon - water mixed - phase fracturing fluid by two - phase synergistic thickening. Specifically, it relates to a carbon - water mixed - phase fracturing fluid, a preparation method of the carbon - water mixed - phase fracturing fluid, a carbon - water mixed - phase fracturing method using the carbon - water mixed - phase fracturing fluid, and the application of the carbon - water mixed - phase fracturing fluid in oil and gas fields. Background Art

[0002] The purpose of fracturing is to form fractures with conductivity in the reservoir, and the fracturing fluid used largely determines the fracturing effect. At the same time, there are certain requirements for the viscosity of the fracturing fluid, so that it can become a low - viscosity fluid and be easily flow - back after fracturing to avoid damage to the oil and gas layers in the reservoir.

[0003] Currently, the conventional fracturing methods mainly include oil - based fracturing, water - based fracturing, carbon dioxide dry fracturing, carbon dioxide pre - energizing fracturing, and carbon dioxide foam fracturing, etc. Among them: (1) Oil - based fracturing has high costs, poor safety, and prominent environmental protection problems, and accounts for a relatively low proportion in fracturing methods; (2) Water - based fracturing has the advantages of low cost, high safety, and convenient construction, and is currently the most widely used. However, due to the influence of water sensitivity, water lock effect, etc., the formation damage is generally high, and the stimulation effect is not ideal enough; (3) Carbon dioxide dry fracturing has the advantages of no aqueous - phase pollution, no residue, low damage or even no damage, and energy - increasing effect, etc. However, due to the need for special high - pressure sand - mixing equipment, it cannot effectively thicken and carry sand, and has high costs; (4) Carbon dioxide pre - energizing fracturing can effectively supplement formation energy, improve flow - back ability, and reduce sensitivity damage. However, it has high friction and large filtration loss, weak fracture - forming ability, and the stimulation effect is still unstable; (5) Carbon dioxide foam fracturing, where the foam formed by water is the continuous phase, is essentially water - based fracturing, and also has problems of large filtration loss, low sand ratio, and low sand - carrying capacity.

[0004] At present, the preparation methods and processes of water-based fracturing fluids at home and abroad are relatively mature. For large-scale volume transformation of unconventional oil and gas reservoirs such as staged large-scale fracturing construction of shale gas horizontal wells, the development and application of fracturing equipment and supporting technologies have been realized. However, in unconventional oil and gas reservoirs such as shale oil and gas, tight oil and gas, and coalbed methane, they all show the "three lows" characteristics of low porosity, low pressure, and low permeability, and the reservoir sensitivity is strong, such as water sensitivity, water blockage, stress sensitivity, and temperature sensitivity. Inevitably, the water-based fracturing fluid will cause varying degrees of damage to the reservoir after entering the reservoir, resulting in a significant decline in oil and gas production, and ultimately the stimulation effect is not satisfactory. On the other hand, for dry fracturing, pre-increasing energy fracturing, foam fracturing, etc. related to carbon dioxide fracturing, although the various advantages of carbon dioxide can be utilized, if liquid carbon dioxide cannot be effectively thickened, problems such as low viscosity, low sand ratio, small sand volume, and large friction will occur, making it difficult to form effective fractures in the reservoir, and thus it is difficult to achieve continuous and effective stimulation.

[0005] Therefore, how to effectively solve the damage problem of water-based fracturing fluid and the drag reduction and sand carrying problem of carbon dioxide fracturing at the same time, and how to reduce costs and increase efficiency have become the key to achieving effective stimulation in fracturing at the present stage. Summary of the Invention

[0006] In view of this, the main object of the present invention is to provide a carbon-water mixed-phase fracturing method, the fracturing fluid used, a preparation method and an application, in order to at least partially solve the technical problems existing in the above-mentioned existing related technologies.

[0007] To achieve the above object, according to one aspect of the present invention, a preparation method of a carbon-water mixed-phase fracturing fluid is provided, including the following steps:

[0008] Step A: Keep the water temperature at 0-40°C, add an aqueous-phase thickener, a liquid carbon dioxide-phase thickener, and a breaker to water according to a designed ratio to prepare an aqueous-phase fracturing fluid, and the aqueous-phase fracturing fluid accounts for 5-95 wt% of the finally prepared carbon-water mixed-phase fracturing fluid;

[0009] Step B: Add a proppant with a designed sand ratio to the aqueous-phase fracturing fluid prepared in Step A, and after mixing the sand evenly, pressurize it to a designed pressure;

[0010] Step C: Pressurize liquid carbon dioxide accounting for 95-5 wt% of the finally prepared carbon-water mixed-phase fracturing fluid to a designed pressure, and then merge it with the aqueous-phase fracturing fluid obtained in Step B. The two-phase thickeners synergistically double-thicken the carbon-water mixed phase to obtain a carbon-water mixed-phase fracturing fluid.

[0011] In the above solution, by mass percentage, the ratio of the aqueous phase to the liquid carbon dioxide phase in the carbohydrate mixed-phase fracturing fluid is (5-95%):(95-5%); wherein, the mass fraction of the aqueous phase thickener in the aqueous phase is 0.1-1.0%, and the mass fraction of the dry carbon dioxide phase thickener in the liquid carbon dioxide phase is 0.1-1.0%.

[0012] In the above solution, the water in step A is conventional source water, including one or more of tap water, surface water, groundwater, fracturing fluid flowback fluid or crude oil separated water; the liquid carbon dioxide in step A is the captured gaseous carbon dioxide pressurized to 1.5-2.5 MPa, cooled to -25 to -15 °C by a pressure pump truck and stored in a special tanker or storage tank; the aqueous phase thickener in step A is one of plant gum and modified plant gum thickeners, viscoelastic surfactant thickeners, synthetic polymer thickeners; the liquid carbon dioxide phase thickener in step A includes one of fluorocarbon polymer-based carbon dioxide thickeners, carbon-silicon polymer-based carbon dioxide thickeners, and hydrocarbon polymer-based carbon dioxide thickeners.

[0013] In the above solution, the designed ratio of the aqueous phase thickener in step A, when used as a drag reducer, accounts for 0.1-0.3% of the mass of the aqueous phase, and when used as a sand-carrying fluid, it accounts for 0.3-1.0% of the mass of the aqueous phase; the designed ratio of the liquid carbon dioxide phase thickener in step A, when used as a drag reducer, accounts for 0.1-0.3% of the mass of the liquid carbon dioxide phase, and when used as a sand-carrying fluid, it accounts for 0.3-1.0% of the mass of the liquid carbon dioxide phase; the breaker in step A includes at least one of ammonium persulfate and capsule breaker; the designed ratio of the breaker in step A accounts for 0.005-0.2% of the mass of the finally prepared carbohydrate mixed-phase fracturing fluid; the proppant in step B includes at least one of quartz sand, ceramsite, and resin-coated sand; the designed sand ratio of the proppant in step B, when used as a drag reducer, the sand ratio is 0-10%, and when used as a sand-carrying fluid, the sand ratio is 5-50%; the designed pressure of the aqueous phase fracturing fluid after pressurization in step B is 10-40 MPa, preferably 20-30 MPa; the designed pressure of the liquid carbon dioxide after pressurization in step C is 10-40 MPa, preferably 20-30 MPa; wherein, the designed pressures of the liquid carbon dioxide phase and the aqueous phase fracturing fluid after pressurization are kept consistent.

[0014] According to another aspect of the present invention, there is also provided a carbohydrate mixed-phase fracturing fluid prepared by using the preparation method of the carbohydrate mixed-phase fracturing fluid described above.

[0015] In the above solution, the temperature of the carbohydrate mixed-phase fracturing fluid is -10 to 40 °C, the density is 0.8 to 1.1 g / cm 3 3, and the pH value is 3.0 to 6.0.

[0016] In the above solution, when used as a drag reduction fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid is 1 to 20 mPa·s, the drag reduction rate ≥ 60%, and the sand ratio is 0 to 10%; when used as a sand-carrying fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid ≥ 20 mPa·s, the sand ratio is 5 to 50%, the core damage rate ≤ 10%, the anti-swelling rate ≥ 80%, and the flowback rate ≥ 50%.

[0017] According to another aspect of the present invention, there is also provided a method for fracturing a carbohydrate mixed phase, comprising the following steps: after pressurizing the carbohydrate mixed-phase fracturing fluid to the designed pressure, it is transported to the reservoir, then shut in for a certain period of time, and finally broken gel and flowed back to complete the whole process of fracturing construction.

[0018] In the above solution, before the carbohydrate mixed-phase fracturing fluid is transported to the formation, it is finally pressurized to 40 to 100 MPa, preferably 40 to 70 MPa; the shut-in time is 1 h to 14 d, preferably 2 to 24 h.

[0019] According to another aspect of the present invention, there is provided an application of the carbohydrate mixed-phase fracturing fluid as a drag reduction fluid or a sand-carrying fluid in an oil and gas field.

[0020] It can be seen from the above technical solutions that the carbohydrate mixed-phase fracturing method, the fracturing fluid used, the preparation method and the application provided by the present invention can achieve the following positive progress effects compared with the prior art:

[0021] (1) Only water phase needs to add a water phase thickener, a liquid carbon dioxide phase thickener, a breaker and a proppant, and continuously prepare the mixed-phase fracturing fluid online, which is convenient for preparation;

[0022] (2) The mixed-phase fracturing fluid has the functions of thickening and sand-carrying and fracture-making and drag reduction, realizing integrated fracturing;

[0023] (3) The mixed-phase fracturing fluid has the functions of anti-swelling and flowback assistance, which is beneficial to reservoir protection;

[0024] (4) The mixed-phase fracturing fluid has the functions of preventing water lock and water sensitivity, and can greatly reduce damage;

[0025] (5) The mixed-phase fracturing fluid has the functions of greatly increasing the energy of the formation and maintaining a high permeability, and at the same time displacing oil and gas;

[0026] (6) The production increase effect is excellent and the cost performance is high. Description of the Drawings

[0027] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings. When considered in conjunction with the accompanying drawings, the present invention can be more completely and better understood and many of the attendant advantages thereof can be easily known by referring to the following detailed description. However, the accompanying drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their description are used to explain the present invention and do not constitute an improper limitation on the present invention.

[0028] Figure 1 The present invention is a flow chart of a method for preparing a carbon-water mixed phase fracturing fluid according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0030] In view of the technical problems existing in the prior art, the researchers of the present invention gradually realized during the research and development process that:

[0031] (1) Water-based fracturing has the advantages of low cost, convenient preparation, perfect fracturing equipment and technical processes, etc. However, in unconventional oil and gas reservoirs such as shale oil and gas, tight oil and gas, conglomerate oil and gas and coalbed methane, they all show the "three lows" characteristics of low porosity, low pressure and low permeability. The reservoirs are highly sensitive, such as water sensitivity, water lock, stress sensitivity and temperature sensitivity. After the water-based fracturing fluid enters the reservoir, it will inevitably cause different degrees of damage to the reservoir. At the same time, it is necessary to add additional anti-swelling agents to improve clay stability and add drainage aids to improve the flowback effect after fracturing.

[0032] (2) Dry fracturing fluid has many advantages, such as no water phase, no water-sensitive water lock pollution, no residue, low damage or even no damage, energy enhancement and easy flowback, strong fluidity to connect reservoirs, carbon dioxide dissolving in crude oil to reduce viscosity and displacement adsorption of oil and gas, etc. However, it is difficult to carry sand due to its low viscosity and difficult to create fractures due to its high friction resistance. It also requires special closed sand mixing equipment. At the same time, the construction process is time-consuming and labor-intensive, the operation is inconvenient and the safety requirements are strict. The cost is high and the overall cost performance is low.

[0033] (3) Carbon dioxide pre-energized fracturing can effectively replenish formation energy, improve flowback capacity, and reduce sensitivity damage, but it has high friction and large filtration loss, weak fracture creation ability, and the production increase effect is still unstable;

[0034] (4) Carbon dioxide foam fracturing: the foam formed by the water phase is a continuous phase. It is essentially a water-based fracturing. However, it also has the problems of large filtration loss, low sand ratio and low sand carrying capacity.

[0035] Furthermore, the inherent properties of water-based fracturing, carbon dioxide dry fracturing, carbon dioxide pre-energized fracturing and carbon dioxide foam fracturing limit their further promotion and application.

[0036] The applicant of the present invention has long been focused on the research and development of oilfield fracturing fluid thickener systems, and has also been dedicated to the applied research of fracturing field technologies and processes. Through years of technological innovation, rich experience has been accumulated. In terms of fracturing methods and fracturing fluids, the present invention fully considers the advantages and disadvantages of the above-mentioned fracturing fluids, gives full play to the advantages of liquid carbon dioxide while organically combining with the advantages of water-based fracturing fluids, and creatively proposes a new fracturing method of "carbohydrate mixed-phase fracturing", and forms a supporting carbohydrate mixed-phase fracturing fluid, its preparation and application. This carbohydrate mixed-phase fracturing method organically combines the water-based fracturing method and the fracturing method involving carbon dioxide, and simultaneously has the technical advantages of the water phase and the liquid carbon dioxide phase, and combines the two into one to form a more uniquely advantageous stable carbohydrate mixed-phase. The carbohydrate mixed-phase fracturing fluid more fully demonstrates the unique effects of good fracture formation, sufficient sand volume, low damage, and high energy, with excellent performance, and a significant increase in production will be inevitable.

[0037] Furthermore, the carbohydrate mixed-phase fracturing method implemented by using the carbohydrate mixed-phase fracturing fluid provided by the present invention is to organically combine the water-based fracturing method and the fracturing method involving carbon dioxide, and simultaneously has the technical advantages of the water phase and the liquid carbon dioxide phase, and combines the two into one to form an innovative fracturing method with more unique advantages. The present invention names it "carbohydrate mixed-phase fracturing method". In this carbohydrate mixed-phase fracturing method, after adding water-phase thickener and liquid carbon dioxide-phase thickener in the designed proportions to the water phase, it is mixed with the liquid carbon dioxide phase in the designed proportion. The two-phase thickeners synergistically double-thicken the mixed phase to form a carbohydrate mixed-phase fracturing fluid with a certain viscosity, and then carry out the carbohydrate mixed-phase fracturing construction.

[0038] As Figure 1 shown, Figure 1 FIG. is a flowchart of a method for preparing a carbohydrate mixed-phase fracturing fluid according to an embodiment of the present invention, and this method includes the following steps:

[0039] Step A: Keep the water temperature at 0 - 40 °C, add a water-phase thickener, a liquid carbon dioxide-phase thickener, and a breaker to the water in the designed proportions to prepare a water-phase fracturing fluid, and the water-phase fracturing fluid accounts for 5 - 95 wt% of the finally prepared carbohydrate mixed-phase fracturing fluid;

[0040] Step B: Add a proppant with a designed sand ratio to the water-phase fracturing fluid prepared in Step A, and after mixing the sand evenly, pressurize it to the designed pressure;

[0041] Step C: Pressurize the liquid carbon dioxide accounting for 95 - 5 wt% of the finally prepared carbohydrate mixed-phase fracturing fluid to the designed pressure, and then converge it with the water-phase fracturing fluid obtained in Step B. The two-phase thickeners synergistically double-thicken the carbohydrate mixed-phase to obtain a carbohydrate mixed-phase fracturing fluid.

[0042] According to an embodiment of the present invention, the water in step A is conventional source water, including one or more of tap water, surface water, groundwater, fracturing fluid flowback fluid, or crude oil separated water; the liquid carbon dioxide is the captured gaseous carbon dioxide pressurized to 1.5 - 2.5 MPa and cooled to -25 - -15 °C by a pressure pump truck and stored in a special tanker or storage tank; the aqueous phase thickener includes one of plant gum and modified plant gum thickener, viscoelastic surfactant thickener, and synthetic polymer thickener; the liquid carbon dioxide phase thickener includes one of fluorocarbon polymer-based carbon dioxide thickener, carbon-silicon polymer-based carbon dioxide thickener, and hydrocarbon polymer-based carbon dioxide thickener.

[0043] According to an embodiment of the present invention, the designed proportion of the aqueous phase thickener in step A, when used as a drag-reducing fluid, accounts for 0.1 - 0.3% of the mass fraction of the aqueous phase, and when used as a sand-carrying fluid, accounts for 0.3 - 1.0% of the mass fraction of the aqueous phase; the designed proportion of the liquid carbon dioxide phase thickener, when used as a drag-reducing fluid, accounts for 0.1 - 0.3% of the mass fraction of the liquid carbon dioxide phase, and when used as a sand-carrying fluid, accounts for 0.3 - 1.0% of the mass fraction of the liquid carbon dioxide phase; the breaker includes at least one of ammonium persulfate and capsule breaker; the designed proportion of the breaker accounts for 0.005 - 0.2% of the mass fraction of the finally prepared carbohydrate mixed-phase fracturing fluid;

[0044] According to an embodiment of the present invention, the proppant in step B includes at least one of quartz sand, ceramic proppant, and resin-coated sand; the designed sand ratio of the proppant, when used as a drag-reducing fluid, is 0 - 10%, and when used as a sand-carrying fluid, is 5 - 50%; the designed pressure after pressurizing the aqueous phase fracturing fluid is 10 - 40 MPa, preferably 20 - 30 MPa;

[0045] According to an embodiment of the present invention, the designed pressure after pressurizing the liquid carbon dioxide in step C is 10 - 40 MPa, preferably 20 - 30 MPa; wherein, the designed pressures after pressurizing the liquid carbon dioxide phase and the aqueous phase fracturing fluid are kept consistent.

[0046] The carbohydrate mixed-phase fracturing fluid prepared by the preparation method of the carbohydrate mixed-phase fracturing fluid according to an embodiment of the present invention includes the following components: by mass percentage, the ratio of the aqueous phase to the liquid carbon dioxide phase in the carbohydrate mixed-phase fracturing fluid is (5 - 95%):(95 - 5%), the mass fraction of the aqueous phase thickener in the aqueous phase is 0.1 - 1.0%, and the mass fraction of the dry carbon dioxide phase thickener in the liquid carbon dioxide phase is 0.1 - 1.0%.

[0047] Among them, when used as a drag-reducing fluid, the water-phase thickener accounts for 0.1-0.3% of the mass of the water phase, and the liquid carbon dioxide-phase thickener accounts for 0.1-0.3% of the mass of the liquid carbon dioxide phase; when used as a proppant-carrying fluid, the water-phase thickener accounts for 0.3-1.0% of the mass of the water phase, and the liquid carbon dioxide-phase thickener accounts for 0.3-1.0% of the mass of the liquid carbon dioxide phase.

[0048] The carbohydrate mixed-phase fracturing fluid prepared by the preparation method of the carbohydrate mixed-phase fracturing fluid according to the embodiment of the present invention has a temperature of -10 to 40 °C, a density of 0.8 to 1.1 g / cm 3 and a pH value of 3.0 to 6.0.

[0049] When the carbohydrate mixed-phase fracturing fluid prepared by the preparation method of the carbohydrate mixed-phase fracturing fluid according to the embodiment of the present invention is used as a drag-reducing fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid is 1 to 20 mPa·s, the drag reduction rate is ≥60%, and the sand ratio is 0 to 10%; when used as a proppant-carrying fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid is ≥20 mPa·s, the sand ratio is 5 to 50%, the core damage rate is ≤10%, the anti-swelling rate is ≥80%, and the flowback rate is ≥50%.

[0050] The present invention also provides an application of the carbohydrate mixed-phase fracturing fluid described above, that is, a carbohydrate mixed-phase fracturing method. In this method, the carbohydrate mixed-phase fracturing fluid is pressurized to the designed pressure and then transported to the formation, then shut in for a certain period of time, and finally broken and flowed back to complete the fracturing construction process.

[0051] Among them, before the carbohydrate mixed-phase fracturing fluid is transported to the formation, it is finally pressurized to 40-100 MPa, preferably 40-70 MPa; the shut-in time is 1 h to 14 d, preferably 2-24 h.

[0052] Furthermore, the embodiment of the present invention also provides a preparation process of the carbohydrate mixed-phase fracturing fluid and the application of the carbohydrate mixed-phase fracturing fluid as a drag-reducing fluid or a proppant-carrying fluid in oil and gas fields. The specific steps are as follows:

[0053] Step 1: Addition of two-phase thickeners and additives:

[0054] Keep the water temperature at 0-40 °C. Using a liquid mixing truck, add the water-phase thickener and the liquid carbon dioxide-phase thickener respectively through the parallel thickener feeding equipment according to the designed ratio, and add a breaker at the feeding port of the solid additive equipment to prepare a water-phase fracturing fluid. The water-phase fracturing fluid accounts for 5-95 wt% of the finally prepared carbohydrate mixed-phase fracturing fluid.

[0055] In this step, the water is conventional source water, including one or more of tap water, surface water, groundwater, fracturing fluid flowback fluid, or crude oil separation water. The liquid carbon dioxide is the gas carbon dioxide captured and pressurized to 1.5 - 2.5 MPa and cooled to -25 - -15 °C by a pressure pump truck and stored in a special tanker or storage tank. The aqueous phase thickener is one of plant gum and modified plant gum thickeners, viscoelastic surfactant thickeners, and synthetic polymer thickeners. The liquid carbon dioxide phase thickener includes one of fluorocarbon polymer-based carbon dioxide thickeners, carbon-silicon polymer-based carbon dioxide thickeners, and hydrocarbon polymer-based carbon dioxide thickeners. The designed proportion of the aqueous phase thickener, when used as a drag reduction fluid, accounts for 0.1 - 0.3% of the mass fraction of the aqueous phase, and when used as a sand-carrying fluid, it accounts for 0.3 - 1.0% of the mass fraction of the aqueous phase. The designed proportion of the liquid carbon dioxide phase thickener, when used as a drag reduction fluid, accounts for 0.1 - 0.3% of the mass fraction of the liquid carbon dioxide phase, and when used as a sand-carrying fluid, it accounts for 0.3 - 1.0% of the mass fraction of the liquid carbon dioxide phase. The breaker includes, but is not limited to, one or more of ammonium persulfate and capsule breaker. The designed proportion of the breaker accounts for 0.005 - 0.2% of the mass fraction of the finally prepared water-carbon mixed phase fracturing fluid.

[0056] Step 2: Addition of proppant and sand mixing:

[0057] Add the proppant with the designed sand ratio to the aqueous phase fracturing fluid prepared in Step 1. After the sand is evenly mixed, pressurize it to the designed pressure.

[0058] In this step, the proppant includes, but is not limited to, one or more of quartz sand, ceramic proppant, and resin-coated sand. The designed sand ratio of the proppant, when used as a drag reduction fluid, the sand ratio is 0 - 10%, and when used as a sand-carrying fluid, the sand ratio is 5 - 50%. The designed pressure after pressurizing the aqueous phase fracturing fluid is designed according to the actual situation of the inlet and outlet of the booster pump, including but not limited to 10 - 40 MPa, preferably 20 - 30 MPa.

[0059] Step 3: Double thickening of water-carbon mixed phase:

[0060] The liquid carbon dioxide accounting for 95 - 5% wt of the water-carbon mixed phase fracturing fluid in another parallel manifold is pressurized to the designed pressure by a booster pump, then merges with the aqueous phase fracturing fluid at the two high-pressure manifold interfaces and is further pressurized to the designed pressure, quickly mixed and heat-exchanged. The liquid carbon dioxide phase thickener enters the liquid carbon dioxide and quickly dissolves and thickens, and the two phases cooperate for double thickening to become the water-carbon mixed phase fracturing fluid.

[0061] In this step, the designed pressure after boosting the liquid carbon dioxide is designed in combination with the actual conditions at the inlet and outlet of the booster pump, including but not limited to 10 - 40 MPa, preferably 20 - 30 MPa, and is kept consistent with the designed pressure after boosting the aqueous fracturing fluid. The designed pressure of the carbon - water mixed - phase fracturing fluid is designed in combination with actual conditions such as the formation fracture pressure, including but not limited to 40 - 100 MPa, preferably 40 - 70 MPa.

[0062] Step 4: The fracturing fluid enters the reservoir to complete processes such as fracturing and flowback:

[0063] The carbon - water mixed - phase fracturing fluid successively passes through the surface high - pressure pipeline, the tubing or casing in the well, and the perforations to enter the reservoir, carrying the proppant into the main fractures and branch fractures of the reservoir for fracture creation and sand filling, while enhancing the energy of the reservoir. Then, the well is shut in and pressured for a certain period of time, and finally, it is broken gel and flowed back to complete the entire process of fracturing construction.

[0064] In this step, the shut - in and pressure - holding time can be designed in combination with actual conditions such as on - site construction conditions, formation conditions, and production requirements after fracturing, including but not limited to 1 h - 14 d, preferably 2 - 24 h.

[0065] The following are several specific examples and comparative examples to illustrate the technical solutions of the present invention in more detail. It should be noted that the specific values and dosages in the following examples are only for illustration, and can be scaled according to the mixing ratio relationship in specific applications.

[0066] Example 1

[0067] Indoor application and performance test of the carbon - water mixed - phase fracturing method:

[0068] Keep the water temperature at 30°C. Add 50 g of tap water to the liquid - mixing equipment, keep stirring, and successively add 0.05 g of aqueous thickener, 2.85 g of liquid - carbon - dioxide - phase thickener, and 0.5 g of capsule breaker. Stir and mix evenly; then add 100 cm 3 of ceramsite. After mixing the sand evenly, it is pressurized to 20 MPa by a booster pump and enters the high - pressure manifold. In another parallel manifold, keep the temperature of the liquid carbon dioxide at - 20°C. Pressurize 950 g of liquid carbon dioxide to 20 MPa by a booster pump, and it converges with the aqueous fracturing fluid at the interface of the two high - pressure manifolds and is further pressurized to 40 MPa, and stirred and mixed evenly to form a carbon - water mixed - phase fracturing fluid. Heat it up to 90°C, shut in and pressure - hold for 8 h, and finally break gel and flow back to complete the indoor evaluation process.

[0069] Among them, existing aqueous phase thickeners can be used as the aqueous phase thickener, for example, the oil-based liquid fracturing fluid thickener disclosed in Example 2 of Patent CN103911136B; existing liquid carbon dioxide phase thickeners can be used as the liquid carbon dioxide phase thickener, for example, the dry fracturing fluid drag reduction thickener disclosed in Example 1 of Patent CN108264604B.

[0070] The above-prepared carbohydrate mixed-phase fracturing fluid is used as a drag reduction fluid. After testing, at a temperature of -5°C, a density of 0.92 g / cm 3 , a pH value of 3.5, a viscosity of 10 mPa·s, a drag reduction rate of 72%, and a sand ratio of 10%.

[0071] Example 2

[0072] Indoor application and performance testing of the carbohydrate mixed-phase fracturing method:

[0073] Keep the water temperature at 5°C. Add 2850 g of surface water to the liquid preparation equipment, keep stirring, and sequentially add 28.5 g of aqueous phase thickener, 1.5 g of liquid carbon dioxide phase thickener, and 3 g of ammonium persulfate, and stir and mix evenly; then add 1200 cm 3 quartz sand. After mixing the sand evenly, pressurize it to 40 MPa through a booster pump and enter the high-pressure manifold. Keep the temperature of liquid carbon dioxide at -15°C in another parallel manifold. Pressurize 150 g of liquid carbon dioxide to 40 MPa through a booster pump, and converge with the aqueous phase fracturing fluid at the interfaces of the two high-pressure manifolds, and further pressurize it to 70 MPa, stir and mix evenly to form a carbohydrate mixed-phase fracturing fluid. Heat it up to 70°C, close the pressure for 16 h, and finally break the gel and flow back to complete the indoor evaluation process.

[0074] Among them, existing aqueous phase thickeners can be used as the aqueous phase thickener, for example, the integrated self-crosslinking emulsion type fracturing fluid thickener disclosed in Example 1 of Patent CN111205390B; existing liquid carbon dioxide phase thickeners can be used as the liquid carbon dioxide phase thickener, for example, the dry fracturing fluid drag reduction thickener disclosed in Example 2 of Patent CN108264604B.

[0075] The above-prepared carbohydrate mixed-phase fracturing fluid is used as a sand-carrying fluid. After testing, at a temperature of 0°C, a density of 1.05 g / cm 3 , a pH value of 5.0, a viscosity of 120 mPa·s, a sand ratio of 40%, a core damage rate of 8%, an anti-swelling rate of 85%, and a flow-back rate of 58%.

[0076] Example 3

[0077] Indoor application and performance testing of the carbohydrate mixed-phase fracturing method:

[0078] Maintain the water temperature at 20°C, add 1400 g of groundwater to the liquid preparation equipment, keep stirring, and successively add 11.2 g of aqueous phase thickener, 3 g of liquid carbon dioxide phase thickener, and 3 g of ammonium persulfate, and stir and mix evenly; then add 600 cm 3 Resin-coated sand. After mixing the sand evenly, it is pressurized to 30 MPa by a booster pump and enters the high-pressure manifold. In another parallel manifold, the temperature of liquid carbon dioxide is maintained at -18°C. 600 g of liquid carbon dioxide is pressurized to 30 MPa by a booster pump and converges with the aqueous phase fracturing fluid at the interfaces of the two high-pressure manifolds, and is further pressurized to 60 MPa, and stirred and mixed evenly to become a carbon-water mixed phase fracturing fluid. Heat it up to 60°C, close the pressure for 24 h, and finally break the gel and flow back to complete the indoor evaluation process.

[0079] Among them, the aqueous phase thickener can adopt the existing aqueous phase thickener, for example, the oil-based liquid fracturing fluid thickener disclosed in Example 3 of Patent CN103911136B; the liquid carbon dioxide phase thickener can adopt the existing liquid carbon dioxide phase thickener, for example, the dry fracturing fluid drag reduction and thickening agent disclosed in Example 3 of Patent CN108264604B.

[0080] The above-prepared carbon-water mixed phase fracturing fluid is used as a sand-carrying fluid. After testing, at a temperature of 5°C, a density of 1.00 g / cm 3 , a pH value of 4.5, a viscosity of 63 mPa·s, a sand ratio of 30%, a core damage rate of 5%, an anti-swelling rate of 91%, and a flow-back rate of 65%.

[0081] Example 4

[0082] Field application and performance test of the carbon-water mixed phase fracturing method:

[0083] Field application in a shale gas well in a certain oilfield in northern Shaanxi:

[0084] Prepare 200 t of surface water in a water tank, maintain the water temperature at 15°C, continuously add 1.2 t of aqueous phase thickener through one liquid addition port of the continuous liquid preparation vehicle, continuously add 1.8 t of liquid carbon dioxide phase thickener through another parallel liquid addition port, and at the same time continuously add 0.15 t of capsule gel breaker at the feeding port of the solid additive equipment, and mix evenly in the aqueous phase fracturing fluid; then the aqueous phase fracturing fluid enters the atmospheric sand mixer, and 150 m 3 Ceramsite, and after mixing the sand evenly, it is pressurized to 25 MPa by a booster pump.

[0085] The on-site equipped liquid carbon dioxide storage tank and tank truck are filled with sufficient liquid carbon dioxide. 300 t of liquid carbon dioxide at -20°C is continuously pumped in through another parallel manifold, pressurized to 25 MPa by a booster pump, converges with the aqueous phase fracturing fluid at the interfaces of the two high-pressure manifolds, and is further pressurized to 50 MPa, and quickly mixed evenly to become a carbon-water mixed phase fracturing fluid.

[0086] The carbohydrate mixed-phase fracturing fluid sequentially passes through the surface high-pressure pipeline, the wellbore casing, and the perforations and enters the reservoir with a well temperature of 120 °C, carrying the ceramsite into the main fractures and branch fractures of the reservoir for fracture creation and sand filling. At the same time, it enhances the energy of the reservoir. Then, the well is shut in and pressured for 4 h, and finally, it is broken and flowed back to complete the entire process of fracturing construction.

[0087] Among them, the aqueous phase thickener can adopt existing aqueous phase thickeners. For example, the integrated self-crosslinking emulsion-type fracturing fluid thickening disclosed in Example 2 of Patent CN111205390B can be used; the liquid carbon dioxide phase thickener can adopt existing liquid carbon dioxide phase thickeners. For example, the dry fracturing fluid drag reduction and thickening agent disclosed in Example 1 of Patent CN108264604B can be used.

[0088] The above-prepared carbohydrate mixed-phase fracturing fluid is used as a sand-carrying fluid. After testing, at a temperature of 3 °C, a density of 0.98 g / cm 3 , a pH value of 4.0, a viscosity of 54 mPa·s, a sand ratio of 30%, a core damage rate of 3%, an anti-swelling rate of 94%, and a flowback rate of 67%.

[0089] Comparative Example 1

[0090] Indoor application and performance testing of the fracturing fluid of Comparative Example 1:

[0091] Keep the water temperature at 20 °C. Add 200 g of tap water to the liquid preparation equipment, keep stirring, and sequentially add 0.1 g of aqueous phase thickener, 0.9 g of liquid carbon dioxide phase thickener, and 3 g of ammonium persulfate, and stir and mix evenly; then add 20 cm 3 of ceramsite. After mixing the sand evenly, it is pressurized to 10 MPa by a booster pump and enters the high-pressure manifold. Keep the temperature of the liquid carbon dioxide at -15 °C in another parallel manifold. Pressurize 1800 g of liquid carbon dioxide to 10 MPa by a booster pump and converge with the aqueous phase fracturing fluid at the interfaces of the two high-pressure manifolds, and further pressurize to 50 MPa, stir and mix evenly to form the fracturing fluid of Comparative Example 1. Heat up to 60 °C, shut in and pressure for 18 h, and finally break and flow back to complete the indoor evaluation process.

[0092] Among them, the aqueous phase thickener can adopt existing aqueous phase thickeners. For example, the oil-based liquidized fracturing fluid thickener disclosed in Example 2 of Patent CN103911136B can be used; the liquid carbon dioxide phase thickener can adopt existing liquid carbon dioxide phase thickeners. For example, the dry fracturing fluid drag reduction and thickening agent disclosed in Example 1 of Patent CN108264604B can be used.

[0093] The above-prepared fracturing fluid of Comparative Example 1, after testing, at a temperature of -8 °C, a density of 0.95 g / cm 3 , a pH value of 4.0, a viscosity of 0.5 mPa·s, a drag reduction rate of 21%, and a sand ratio of 1%.

[0094] Comparative Example 2

[0095] Laboratory application and performance test of the fracturing fluid in Comparative Example 2:

[0096] Keep the water temperature at 20°C. Add 2700 g of surface water to the liquid preparation equipment, keep stirring, and sequentially add 48.6 g of aqueous phase thickener, 5.4 g of liquid carbon dioxide phase thickener, and 3 g of capsule breaker, and stir and mix evenly; then add 1050 cm 3 quartz sand. After mixing the sand evenly, pressurize it to 20 MPa through a booster pump and enter the high-pressure manifold. Keep the temperature of liquid carbon dioxide at -18°C in another parallel manifold. Pressurize 300 g of liquid carbon dioxide to 20 MPa through a booster pump, and merge with the aqueous phase fracturing fluid at the interfaces of the two high-pressure manifolds, and further pressurize it to 60 MPa, stir and mix evenly to form the fracturing fluid of Comparative Example 2. Heat up to 70°C, keep the pressure closed for 12 h, and finally break the gel and flow back to complete the laboratory evaluation process.

[0097] Among them, the aqueous phase thickener can adopt the existing aqueous phase thickener, for example, the integrated self-crosslinking emulsion type fracturing fluid thickener disclosed in Example 3 of Patent CN111205390B; the liquid carbon dioxide phase thickener can adopt the existing liquid carbon dioxide phase thickener, for example, the dry fracturing fluid drag reduction and thickening agent disclosed in Example 2 of Patent CN108264604B.

[0098] The above-prepared fracturing fluid of Comparative Example 2 is used as a sand-carrying fluid. After testing, at a temperature of 6°C, a density of 1.02 g / cm 3 , a pH value of 5.0, a viscosity of 195 mPa·s, a sand ratio of 35%, a core damage rate of 18%, an anti-swelling rate of 78%, and a flow-back rate of 45%.

[0099] The main materials, ratios, two-phase ratios, and performance test result data of the above examples and comparative examples are listed in Table 1 below.

[0100] Table 1 Statistical table of the main materials, ratios, two-phase ratios, and performance test results of examples and comparative examples

[0101]

[0102] The carbon-water mixed phase fracturing method provided by the above examples has been shown through practical applications to have the following significant advantages:

[0103] 1. Only need to add an aqueous phase thickener, a liquid carbon dioxide phase thickener, a breaker, and a proppant to the aqueous phase, and continuously prepare the mixed phase fracturing fluid online; at the same time, no special closed sand mixing equipment is required, and the liquid preparation can be completed through an atmospheric pressure sand mixing truck, which is convenient for preparation.

[0104] 2. In the carbohydrate mixed-phase fracturing fluid, by rapidly and real-time adjusting the concentrations of the two-phase thickeners, the switching between formulating a drag-reducing fluid with low concentrations of the two-phase thickeners and formulating a proppant-carrying fluid with relatively high concentrations of the two-phase thickeners is achieved. It has the functions of thickening proppant carrying and creating fractures to reduce drag, realizing integrated fracturing.

[0105] 3. In the carbohydrate mixed-phase fracturing fluid, part of the liquid carbon dioxide reacts and combines with water to form a saturated carbonic acid solution, greatly inhibiting the hydration swelling of clay. At the same time, liquid carbon dioxide has an ultra-low surface tension, and the mixed-phase fracturing fluid has excellent anti-swelling and flowback-assisting functions, which is beneficial to reservoir protection and greatly reduces the damage of the fracturing fluid to the reservoir.

[0106] 4. In the carbohydrate mixed-phase fracturing fluid, after the two-phase thickeners synergistically thicken, a carbohydrate mixed-phase with viscosity is formed, and the water activity is greatly reduced, and the water lock and water sensitivity phenomena are greatly inhibited, thereby greatly reducing the damage.

[0107] 5. In the carbohydrate mixed-phase fracturing fluid, liquid carbon dioxide can greatly increase the energy of the formation throughout the whole process, thoroughly communicate the large and small fractures in the formation and deeply communicate the micro-fractures, realizing high permeability of the formation, and at the same time combining with oil and gas and displacing oil and gas.

[0108] 6. With excellent production increase effect and high cost performance, the prominent advantages of the carbohydrate mixed-phase fracturing method have been verified through field applications and tests. Compared with the water-based fracturing of adjacent wells and the carbon dioxide pre-increase energy fracturing, the production increase effect reaches several times or even more than ten times.

[0109] So far, the embodiments of the present invention have been described in detail with reference to the accompanying drawings. It should be noted that, in the accompanying drawings or the main text of the specification, the implementation manners that are not illustrated or described are all forms known to those of ordinary skill in the art and have not been described in detail. In addition, the definitions of the above-mentioned various elements and methods are not limited to the specific structures, shapes or manners mentioned in the embodiments. Those of ordinary skill in the art can make simple changes or substitutions to them.

[0110] Similarly, it should be understood that, in order to streamline the present invention and help understand one or more of the various disclosed aspects, in the above description of the exemplary embodiments of the present invention, the various features of the present invention are sometimes grouped together into a single embodiment, figure, or description thereof. However, the disclosed method should not be construed as reflecting the intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as reflected in the following claims, the disclosed aspects lie in less than all the features of the preceding disclosed single embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into the detailed description, where each claim stands on its own as a separate embodiment of the present invention.

[0111] The specific embodiments described above further elaborate on the objective, technical solution and beneficial effects of the present invention. It should be understood that the above description is only for the specific embodiments of the present invention and is not used to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a carbohydrate mixed-phase fracturing fluid, characterized in that It includes the following steps: Step A: Keep the water temperature at 0 - 40°C, add an aqueous phase thickener, a liquid carbon dioxide phase thickener, and a gel breaker to the water according to the designed ratio to prepare an aqueous phase fracturing fluid, and the aqueous phase fracturing fluid accounts for 5 - 95 wt% of the finally prepared carbohydrate - mixed - phase fracturing fluid; Step B: Add a proppant with a designed sand ratio to the aqueous phase fracturing fluid prepared in Step A, and after mixing the sand evenly, pressurize it to the designed pressure; Step C: Pressurize the liquid carbon dioxide accounting for 95 - 5 wt% of the finally prepared carbohydrate - mixed - phase fracturing fluid to the designed pressure, and then converge it with the aqueous phase fracturing fluid obtained in Step B. The two thickeners synergistically double - thicken the carbohydrate - mixed phase to obtain a carbohydrate - mixed - phase fracturing fluid.

2. The preparation method of the carbohydrate - mixed - phase fracturing fluid according to claim 1, wherein: By mass percentage, the ratio of the aqueous phase to the liquid carbon dioxide phase in the carbohydrate - mixed - phase fracturing fluid is (5 - 95%)∶(95 - 5%); Among them, the mass fraction of the aqueous phase thickener in the aqueous phase is 0.1 - 1.0%, and the mass fraction of the dry carbon dioxide phase thickener in the liquid carbon dioxide phase is 0.1 - 1.0%.

3. The preparation method of the carbohydrate - mixed - phase fracturing fluid according to claim 1, wherein: The water in Step A is conventional source water, including one or more of tap water, surface water, groundwater, fracturing fluid flowback fluid, or crude oil separated water; The liquid carbon dioxide in Step A is the liquid carbon dioxide obtained by pressurizing the captured gaseous carbon dioxide to 1.5 - 2.5 MPa, cooling it to - 25 - 15°C with a pressure pump truck and storing it in a special tank truck or storage tank; The aqueous phase thickener in Step A is one of plant gum and modified plant gum thickeners, viscoelastic surfactant thickeners, and synthetic polymer thickeners; The liquid carbon dioxide phase thickener in Step A includes one of fluorocarbon polymer - type carbon dioxide thickeners, carbon - silicon polymer - type carbon dioxide thickeners, and hydrocarbon polymer - type carbon dioxide thickeners.

4. The preparation method of the carbohydrate - mixed - phase fracturing fluid according to any one of claims 1 - 3, wherein: The designed ratio of the aqueous phase thickener in Step A, when used as a drag - reducing fluid, accounts for 0.1 - 0.3% of the mass fraction of the aqueous phase, and when used as a sand - carrying fluid, accounts for 0.3 - 1.0% of the mass fraction of the aqueous phase; The designed ratio of the liquid carbon dioxide phase thickener in Step A, when used as a drag - reducing fluid, accounts for 0.1 - 0.3% of the mass fraction of the liquid carbon dioxide phase, and when used as a sand - carrying fluid, accounts for 0.3 - 1.0% of the mass fraction of the liquid carbon dioxide phase; The gel breaker in Step A includes at least one of ammonium persulfate and capsule gel breaker; The designed ratio of the gel breaker in Step A accounts for 0.005 - 0.2% of the mass fraction of the finally prepared carbohydrate - mixed - phase fracturing fluid; The proppant in Step B includes at least one of quartz sand, ceramsite, and resin - coated sand; The designed sand ratio of the proppant in Step B, when used as a drag - reducing fluid, the sand ratio is 0 - 10%, and when used as a sand - carrying fluid, the sand ratio is 5 - 50%. In step B, the designed pressure after boosting the aqueous fracturing fluid is 10 - 40 MPa, preferably 20 - 30 MPa; In step C, the designed pressure after boosting the liquid carbon dioxide is 10 - 40 MPa, preferably 20 - 30 MPa; wherein, the designed pressure after boosting the liquid carbon dioxide phase is the same as that of the aqueous fracturing fluid.

5. A carbohydrate mixed-phase fracturing fluid prepared by the preparation method of the carbohydrate mixed-phase fracturing fluid according to any one of claims 1 - 4.

6. The carbohydrate mixed-phase fracturing fluid according to claim 5, characterized in that, The temperature of the carbohydrate mixed-phase fracturing fluid is -10 to 40 °C, the density is 0.8 to 1.1 g / cm 3 , and the pH value is 3.0 to 6.

0.

7. The carbohydrate mixed-phase fracturing fluid according to claim 5, characterized in that When used as a drag-reducing fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid is 1 - 20 mPa·s, the drag reduction rate ≥ 60%, and the sand ratio is 0 - 10%; When used as a sand-carrying fluid, the viscosity of the carbohydrate mixed-phase fracturing fluid ≥ 20 mPa·s, the sand ratio is 5 - 50%, the core damage rate ≤ 10%, the anti-swelling rate ≥ 80%, and the flowback rate ≥ 50%.

8. A fracturing method for a carbohydrate mixed phase, characterized in that, It includes the following steps: After boosting the carbohydrate mixed-phase fracturing fluid according to any one of claims 5 - 7 to the designed pressure, it is transported to the formation, then shut-in and pressured for a certain time, and finally broken and flowed back to complete the fracturing construction process.

9. The carbohydrate mixed-phase fracturing method according to claim 8, characterized in that Before the carbohydrate mixed-phase fracturing fluid is transported to the formation, it is finally boosted to 40 - 100 MPa, preferably 40 - 70 MPa; The shut-in and pressured time is 1 h - 14 d, preferably 2 - 24 h.

10. An application of the carbohydrate mixed-phase fracturing fluid according to any one of claims 5 - 7 as a drag-reducing fluid or a sand-carrying fluid in oil and gas fields.

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