Fracturing method and fracturing device for carbonate reservoir

Abstract

The invention provides a fracturing method and a fracturing device for a carbonate reservoir, and relates to the technical field of petroleum drilling. The fracturing method for the carbonate reservoir comprises the steps that transformation fluid is injected into the carbonate reservoir to form a fracture network, and the transformation fluid is used for preventing fractures from being closed; injecting acid liquor into the crack to form a flow guide channel, wherein the acid liquor can be used for dissolving calcite in the crack; and alternately injecting acid liquids with different concentrations into the crack to form acid etching waves for recovering the communication between the natural pores and the crack, and injecting an acid fracturing fluid into the crack for etching the middle part and the root part of the crack. According to the technical scheme, all-directional transformation of the ultra-deep carbonate reservoir can be achieved, and the fracturing transformation effect of the carbonate reservoir is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil drilling, in particular to a fracturing method and device for carbonate reservoirs. BACKGROUND

[0002] With the increasing exploration and development of oil and gas resources at home and abroad, the amount of high-quality resources is decreasing, and complex lithologic oil and gas reservoirs have gradually become the focus. Unlike conventional carbonate reservoirs (large thickness, mainly limestone or dolomite), complex lithologic carbonate reservoirs have the characteristics of strong heterogeneity and thin single-layer thickness, and various lithology types, including argillaceous dolomite, gypsiferous dolomite, and calcareous dolomite.

[0003] For low-permeability carbonate reservoirs or carbonate reservoirs with underdeveloped near-well natural fractures, the current main reconstruction technologies include matrix acidizing, acid fracturing, hydraulic sand fracturing, and various stimulation methods derived for different purposes. However, in complex lithologic carbonate reservoirs, the matrix acidizing method can only reconstruct the reservoir near the wellbore due to the limited acidizing distance, and cannot achieve effective reconstruction on a large scale. Although the acid fracturing method can form long artificial fractures, the fracturing fluid does not react with carbonate rocks, and the ability to communicate natural fractures is poor. In addition, in low-permeability carbonate reservoirs, the poor reservoir properties, high argillaceous content, and developed microfractures result in fast reaction rate and large filtration loss of acid fluid and rock, which further leads to short acid-etched fractures. At the same time, acid-insoluble substances easily block the fracture channels, resulting in low conductivity of acid-etched fractures, and ultimately poor stimulation effect of acid fracturing reconstruction. SUMMARY

[0004] The purpose of the embodiments of the present application is to provide a fracturing method for carbonate reservoirs to solve the problem of poor ability of existing fracturing reconstruction technologies to communicate natural fractures in complex lithologic carbonate reservoirs.

[0005] To solve the above technical problems, the embodiments of the present application provide the following technical solutions:

[0006] The present application provides a fracturing method for carbonate reservoirs, comprising:

[0007] S1, injecting a reconstruction fluid into a carbonate reservoir to form a fracture network, the reconstruction fluid being used to prevent the fracture from closing;

[0008] S2, injecting an acid fluid into the fracture to form a flow channel, and the acid fluid being used to dissolve calcite in the fracture;

[0009] S3, alternately injecting acid fluids of different concentrations into the fracture to form an acid-etched wave, used to restore the communication between natural pores and the fracture;

[0010] S4, injecting acid fracturing fluid into the fracture for etching the middle and root of the fracture.

[0011] In some embodiments of the present application, the reconstruction fluid is composed of water, proppant and cross-linking agent, the proppant is common ceramic particles with particle size of 30-50 mesh or high-strength ceramic particles with particle size of 20-40 mesh.

[0012] In some embodiments of the present application, the proppant accounts for 10%-20% of the mass percentage of the reconstruction fluid.

[0013] In some embodiments of the present application, the acid fracturing fluid is a self-generating acid fracturing fluid, which is composed of reactants, additives and base fluid, the PH value of the reactants is not less than 7 at normal temperature and is less than 7 at preset formation temperature.

[0014] In some embodiments of the present application, the acid fluid is composed of water, acid, retarder and cross-linking agent.

[0015] In some embodiments of the present application, the concentration of the acid fluid is 10%-20%, and the viscosity of the acid fluid is 35 mPa·s-40 mPa·s.

[0016] In some embodiments of the present application, the step S3 of alternately injecting acid fluid with different concentrations is to alternately inject high-concentration acid fluid and low-concentration acid fluid, the flow ratio of the high-concentration acid fluid to the low-concentration acid fluid is 1:2 each time, the concentration of the acid fluid for the last time is 15%, and the flow rate is 30 m 3 .

[0017] In some embodiments of the present application, the step S1 of injecting reconstruction fluid into the carbonate reservoir adopts high-pressure injection and stepwise displacement injection, first, a preset displacement is injected to form a main fracture, and then the displacement is gradually increased to form a branch fracture network.

[0018] In some embodiments of the present application, the step S4 of injecting acid fracturing fluid into the fracture includes using variable displacement fracture control technology to optimize the morphology of the fracture.

[0019] The present application also provides a fracturing device for carbonate reservoirs for implementing the above method. It comprises:

[0020] A first injection unit is configured to inject reconstruction fluid into the carbonate reservoir to form a fracture network, and the reconstruction fluid is used to prevent the fracture from closing;

[0021] A second injection unit is configured to inject acid fluid into the fracture to form a flow channel, and the acid fluid can be used to dissolve calcite in the fracture.

[0022] a third injection unit configured to inject acid fluids of different concentrations into the fracture alternately to form acid etching waves for restoring the natural pores to be connected with the fracture;

[0023] a fourth injection unit configured to inject acid fracturing fluid into the fracture for etching the middle and root of the fracture.

[0024] Compared with the prior art, the fracturing method for carbonate reservoirs provided by the present application can solve the problem that the fracture morphology is controlled by the mechanical properties of the rock in the prior art by first establishing a stable fracture network to prevent closure, and then establishing a high-conductivity channel, thereby realizing accurate regulation of the fracture morphology. Then, the acid etching waves are formed by alternating acid injection to communicate the pores and expand the etching width, which can etch and expand the fracture and remove the pollution. Since the reconstruction fluid can prop open the fracture and resist the closure stress of the formation, the acid fluid can penetrate deep into the fracture, thereby avoiding the problems of short acid etching distance and acid-insoluble substances blocking the fracture channel in the prior art, and restoring the natural pores to be connected with the artificial fracture. Finally, the acid fracturing fluid is used to etch the middle and root of the fracture in depth to significantly extend the effective acid etching distance, thereby realizing all-around reconstruction of the ultra-deep carbonate reservoir and significantly improving the fracturing reconstruction effect of the carbonate reservoir. BRIEF DESCRIPTION OF DRAWINGS

[0025] The above and other objects, features and advantages of the example embodiments of the present application will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which several embodiments of the present application are shown by way of example, and wherein like or corresponding elements refer to like or corresponding parts throughout the several figures. In the drawings:

[0026] Figure 1 A flowchart schematically showing a fracturing method for carbonate reservoirs according to an embodiment of the present application is shown;

[0027] Figure 2 A structural diagram schematically showing a fracturing device for carbonate reservoirs according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0028] The example embodiments of the present application will be described in greater detail below, with reference to the accompanying drawings. Although example embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present application can be more thoroughly understood, and the scope of the present application can be accurately conveyed to those skilled in the art.

[0029] It should be noted that, unless otherwise specified, the technical terms or scientific terms used in the present application should be understood as the usual meanings understood by those skilled in the art to which the present application belongs.

[0030] In the existing acid fracturing technology, a four-stage sand fracturing method is usually adopted. This method has the problems of fast acid-rock reaction rate, large acid fluid filtration loss, short acid-etched fracture distance, and easy plugging of fracture channels by acid-insoluble substances in the dense carbonate reservoir, resulting in low acid-etched fracture conductivity and poor acid fracturing stimulation effect. Meanwhile, in the existing sand fracturing technology, the fracture morphology is controlled by the mechanical properties of the rock, and the fracture morphology cannot be accurately controlled, so it is difficult to establish a high-conductivity channel, resulting in limited acid treatment range and etching width, and it is difficult to realize the all-round reconstruction of the ultra-deep carbonate reservoir.

[0031] Therefore, the embodiments of the present application provide a fracturing method for a carbonate reservoir, which forms a fracture network by injecting a reconstruction fluid capable of avoiding fracture closure, and establishes a conductive channel by acid to realize etching and fracture expansion and remove pollution, thereby improving the fracture conductivity, restoring the connection between natural pores and artificial fractures, and improving the stimulation effect.

[0032] Embodiment 1

[0033] The embodiments of the present application provide a fracturing method for a carbonate reservoir, as shown in the following figure, which includes the following steps: Figure 1

[0034] S1, injecting a reconstruction fluid into the carbonate reservoir to form a fracture network, the reconstruction fluid being used to prevent fracture closure;

[0035] S2, injecting acid into the fracture to form a conductive channel, and the acid being capable of dissolving calcite in the fracture;

[0036] S3, alternately injecting acid of different concentrations into the fracture to form an acid-etched wave, for restoring the connection between natural pores and fractures;

[0037] S4, injecting an acid fracturing fluid into the fracture, for etching the middle and root parts of the fracture.

[0038] First, a reconstruction fluid (such as a viscous fracturing fluid, a linear gel, a crosslinked gel, or a fracturing fluid carrying proppants) can be injected into the carbonate reservoir, which can have sufficient viscosity or carrying capacity to prop open the fracture and resist the closure stress of the formation. By using fluid pressure, multiple branch fractures are generated in the reservoir to form a fracture network, and the reconstruction fluid can prevent the fracture from closing after the pressure is released, providing a stable space for the subsequent action of the acid.

[0039] Then, acid (such as hydrochloric acid or retarded acid) is injected into the formed fracture, the acid reacts with and dissolves the calcite on the fracture wall, and the calcite is the main component of the carbonate rock. Through the etching effect, an uneven etching morphology (such as a groove or a hole) is formed on the fracture wall, thereby forming a preliminary conductive channel in the fracture and improving the permeability of the fracture. ​

[0040] Then, high-concentration and low-concentration acid solutions are alternately injected into the fractures. The concentration difference drives the acid solution to penetrate into the natural pores, forming an acid etching wave. This wave effect can disrupt the rapid reaction equilibrium between the acid solution and the rock, expand the effective range of the acid solution, promote the acid solution to enter the reservoir's original pores, and restore the connectivity between natural pores and artificial fractures.

[0041] Finally, acid fracturing fluid, such as acid or a fluid containing acid precursors, is injected into the fracture. Targeted acidizing can be used, pumping the acid into the fracture at specific points to avoid acid loss. Alternatively, a fluid containing acid precursors can be used to slowly react and generate acid deep within the fracture. The acid fracturing fluid is then slowly released into the middle and root of the fracture, etching areas far from the wellbore to extend the effective acid etching distance and prevent premature acid consumption in the near-wellbore zone.

[0042] The fracturing method for carbonate reservoirs provided in this application first establishes a stable fracture network to prevent closure, and then creates high-conductivity channels. This solves the problem in existing technologies where fracture morphology is controlled by rock mechanical properties, enabling precise control of fracture morphology. Then, alternating acid injection is used to create acid etching waves to connect pores and expand the dissolution width, both etching and widening fractures and removing contaminants. Because the fracturing fluid can open fractures and resist formation closure stress, the acid can penetrate deep into the fractures, avoiding the problems of short acid etching distances and acid-insoluble substances clogging fracture channels in existing technologies, thus restoring the connection between natural pores and artificial fractures. Finally, the acid fracturing fluid is used to deeply etch the middle and root of the fractures to significantly extend the effective acid etching distance, achieving comprehensive fracturing of ultra-deep carbonate reservoirs and significantly improving the fracturing effect of carbonate reservoirs.

[0043] In some embodiments, the modified fluid consists of water, a proppant, and a crosslinking agent. The proppant is ordinary ceramsite with a particle size of 30-50 mesh or high-strength ceramsite with a particle size of 20-40 mesh.

[0044] The stimulation fluid is a mixture of water, proppant, and crosslinking agent in a specific ratio. This ratio can be an existing one, set according to actual working conditions. The water can be treated freshwater, formation water, or backflow water. The proppant can be selected based on reservoir geological conditions, using either ordinary ceramsite with a particle size of 30-50 mesh or high-strength ceramsite with a particle size of 20-40 mesh. For example, when the reservoir is shallow and the rock compressive strength is low, ordinary ceramsite with a particle size of 30-50 mesh can be used. When the reservoir is ultra-deep, the rock is dense, and the pressure is high, high-strength ceramsite with a particle size of 20-40 mesh can be used. The crosslinking agent can increase the viscosity of the stimulation fluid, enhancing its sand-carrying capacity and fracture-forming effect.

[0045] In step S1, the modified fluid is injected into the carbonate reservoir at a predetermined flow rate (calculated based on the reservoir fracture pressure). High pressure induces multi-directional branching fractures in the reservoir rock, forming an interconnected fracture network. Propionate enters and fills the fractures with the fluid. Through the action of a cross-linking agent, the proppant embeds itself into the fracture walls, forming a three-dimensional network of molecular chains. This effectively prevents the fractures from closing after pressure relief, providing a stable spatial carrier for subsequent acid injection and action.

[0046] In some embodiments, the proppant accounts for 10%-20% of the mass percentage of the modified fluid.

[0047] The proportion of proppant in the modified fluid is 10%-20%, and by combining particle size, a proppant system adapted to different reservoir conditions can be formed. For example, when the reservoir is shallow to medium-depth, with narrow fracture width and low in-situ stress, ordinary ceramsite with a particle size of 30 mesh or high-strength ceramsite with a particle size of 40 mesh can be used as proppant, with the proportion controlled at 10%. By filling narrow fracture branches with a lower proportion of proppant, the fracture is prevented from closing while avoiding excessive proppant accumulation that could hinder subsequent acid flow. Or, when the reservoir is ultra-deep, with wide fracture width and high in-situ stress, ordinary ceramsite with a particle size of 50 mesh or high-strength ceramsite with a particle size of 20 mesh can be used as proppant, with the proportion controlled at 20%. A higher proportion of high-strength proppant is used to resist high stress and ensure the stable opening of the main fracture and wide branch fractures.

[0048] In some embodiments, the acid fracturing fluid is a self-generating acid fracturing fluid, which is composed of reactants, additives and base fluid. The reactants have a pH value of not less than 7 at room temperature and a pH value of less than 7 at a preset formation temperature.

[0049] Acid fracturing fluid can be autogenous acid fracturing fluid, which is composed of reactants, additives, and base fluid mixed in a specific ratio. The specific ratio can be any existing ratio. The reactants can be organic acid esters, amides, fluorides, etc., the additives can be thickeners, breaker agents, corrosion inhibitors, etc., and the base fluid can be water or brine.

[0050] The reactants maintain a stable pH of 7-9 at room temperature (around 25°C), exhibiting chemical stability and preventing premature acid formation. When fracturing fluid is injected into the middle and root of the fracture, the high formation temperature (typically tens to hundreds of degrees Celsius) activates the reactants, causing hydrolysis or decomposition reactions that slowly release acids (such as hydrochloric acid, hydrofluoric acid, and organic acids). For example, esters hydrolyze to form carboxylic acids at high temperatures, and fluorides decompose to form hydrofluoric acid.

[0051] The generated acid dissolves and erodes the rock walls, especially in the middle and root of the fracture—areas where traditional acids struggle to reach effectively. This creates irregular grooves or cavities, widening the fracture channel and increasing its conductivity. Because the acid is generated gradually, it avoids the problems of rapid reaction and premature consumption near the wellbore associated with traditional acids, ensuring the fracturing fluid can reach the middle and root of the fracture smoothly and thus act more deeply at the distal end.

[0052] In some embodiments, the acid solution consists of water, acid, a retarder, and a crosslinking agent.

[0053] The acid solution is composed of water, acid, a retarder, and a crosslinking agent mixed in a specific ratio, which can be any existing ratio. The acid can be hydrochloric acid or a retarding acid, used to dissolve the calcite on the crack walls. The retarder can be an organic amine compound, such as hexadecyltrimethylammonium chloride, used to slow down the reaction rate between the acid and the rock. The crosslinking agent can be borax or an organozirconium crosslinking agent; the amount added is adjusted according to the required acid viscosity, which can increase the acid viscosity to reduce filtration loss. Water serves as the base solution, used to dissolve the components and adjust the overall concentration of the acid solution.

[0054] By adding a retarder, an adsorption film can be formed between the acid and the rock, reducing the diffusion rate of hydrogen ions to the rock surface and extending the acid reaction time by several times. Simultaneously, a crosslinking agent increases the acid viscosity, extending its residence time within the cracks, reducing filtration loss, and allowing the acid to penetrate deeper into branch cracks and micro-cracks. This solves the problems of rapid reaction, high filtration loss, and severe clogging associated with traditional acid solutions.

[0055] In some embodiments, the concentration of the acid solution is 10%-20%, and the viscosity of the acid solution is 35 mPa·s-40 mPa·s.

[0056] Before injecting acid in step S2, concentration and viscosity parameters can be determined using reservoir logging data (such as porosity and permeability). For example, when the reservoir is a tight carbonate rock with medium to low permeability, a high-concentration acid of 20% can be used, combined with a high viscosity of 40 mPa·s. The strong dissolving power of the high-concentration acid can break through the tight rock structure, while the high viscosity reduces filtration loss, ensuring that the acid concentrates its effect on the target fractures. When the reservoir has medium to high permeability and well-developed natural fractures, a low-concentration acid of 10% can be used, with the viscosity controlled at 35 mPa·s. This avoids excessive dissolution of natural fractures leading to excessive formation of acid-insoluble substances, while the appropriate viscosity guides the acid to permeate into the natural pores. By precisely matching the acid concentration and viscosity within specific ranges, the targeting, fluidity, and stability of the acid action are improved, further enhancing the overall effect of fracturing in carbonate reservoirs.

[0057] In some embodiments, the alternating injection of acid solutions of different concentrations in step S3 involves alternating injection of high-concentration acid solutions and low-concentration acid solutions. The flow rate ratio of the high-concentration acid solution to the low-concentration acid solution during each alternating injection is 1:2. The final injection has an acid concentration of 15% and a flow rate of 30 m³ / s. 3 .

[0058] The alternating injection operation in step S3 can be a regular alternation between high-concentration acid and low-concentration acid. The high-concentration acid can be a 20% hydrochloric acid solution, and the low-concentration acid can be a 10% hydrochloric acid solution. The two are injected alternately in the order of high concentration, low concentration, high concentration, low concentration, with the flow rate ratio of high-concentration acid to low-concentration acid controlled at 1:2 during each alternation. For example, the initial injection of high-concentration acid is at a flow rate of 10 m³ / h, followed by the injection of low-concentration acid at a flow rate of 20 m³ / h; the second alternation uses the same high-concentration acid flow rate of 10 m³ / h. 3 / h, low-concentration acid solution maintained for 20m 3 / h, and so on.

[0059] The number of alternating injection rounds can be determined based on the degree of natural porosity development in the reservoir. For example, for reservoirs with relatively dense natural porosity, three alternating injection rounds can be used, for a total of six injections; for tight reservoirs with sparse porosity, five alternating injection rounds can be used, for a total of six injections. The acid concentration of the last injection is 15%, and the flow rate is 30 m3, which can balance the concentration gradient formed by the previous alternating injections and stabilize the diffusion effect of the acid etching wave.

[0060] This embodiment uses a 1:2 flow rate ratio to alternately inject high- and low-concentration acid solutions, creating a periodic concentration gradient driving force within the fracture. The high-concentration acid diffuses into the low-concentration area, while the high flow rate of the low-concentration acid propels the high-concentration acid to penetrate deeper into the pores, forming a continuously advancing acid etching wave. The low-concentration acid has a slower reaction rate, and the high flow rate occupies more fracture space, delaying the reaction between the high-concentration acid and the rock. Meanwhile, the high-concentration acid, protected by the low-concentration acid, can penetrate deeper into the fracture and contact natural pores. This alternation creates a slow, highly soluble cycle, extending the effective action time of the acid and solving the problem of limited modification range caused by premature acid consumption. The final injection of 15% concentration acid at a flow rate of 30 m³ neutralizes the extreme concentration gradient formed by the previous alternations, avoiding uneven acid etching due to concentration fluctuations, and the large flow rate of 30 m³ pushes the acid etching wave to even more distant natural pores.

[0061] In some embodiments, the injection of the alteration fluid into the carbonate reservoir in step S1 adopts a high-pressure injection and a stepped displacement injection method. First, the main fracture is formed by injecting at a preset displacement, and then the displacement is gradually increased to form a branch fracture network.

[0062] Specifically, the stepped displacement injection method can begin with a 3m³ injection rate. 3Injecting 30m³ / min at a displacement of / min 3 Fluid, then at 5m 3 Injecting 30m³ / min at a displacement of / min 3 Fluid, then at 7m 3 Injecting 60m³ / min at a displacement of / min 3 The fluid allows the fracture half-length to reach 50m. Through the action of a cross-linking agent, the proppant is embedded in the fracture wall, forming a three-dimensional network of molecular chains, preventing fracture closure and establishing a highly conductive channel.

[0063] Or we can start with 2m 3 Injecting 40m³ / min at a displacement of / min 3 Fluid, then at 4m 3 Injecting 40m³ / min at a displacement of / min 3 Fluid, then at 8m 3 Injecting 80m³ / min at a displacement of / min 3 The fluid allows the fracture half-length to reach 60m. Through the action of a cross-linking agent, the proppant is embedded in the fracture wall, forming a three-dimensional network of molecular chains, preventing fracture closure and establishing a highly conductive channel.

[0064] In this embodiment, the modification fluid injection method in step S1 is a combination of high-pressure injection and stepped displacement control. By precisely controlling the pressure and displacement, a network of main fractures and branch fractures is formed, and the fracture half-length can be greater than 50m. This achieves comprehensive modification of ultra-deep carbonate reservoirs and improves the modification range and dissolution width of acid fluid.

[0065] In some embodiments, the injection of acid fracturing fluid into the fracture in step S4 includes employing variable displacement fracture control technology to optimize the fracture morphology.

[0066] Variable displacement fracture control technology dynamically adjusts the injection rate or flow rate of fracturing fluid, utilizing periodic or stepwise changes in fluid pressure and velocity to achieve controllable optimization of fracture length, width, branch density, and extension direction, thereby improving reservoir stimulation. When the displacement increases, the injected fluid kinetic energy is enhanced, and the bottom hole pressure rises, which can drive fracture widening or extension to more distant locations. When the displacement decreases, the pressure weakens, and the fracture extension rate slows down, but the fluid is more likely to penetrate into branch fractures, promoting microfracture development.

[0067] In existing technologies, fixed-displacement injection can easily lead to fracture morphology being limited by rock mechanical properties, resulting in mostly single main fractures and limited modification range. This embodiment uses a stepped displacement design, with an initial low displacement focusing on forming stable main fractures to avoid fluid energy dispersion, and then gradually increasing the displacement to utilize kinetic energy to actively impact and create branch fractures, thus solving the problems of single fracture morphology and narrow modification range in traditional technologies.

[0068] Example 2

[0069] This application also provides a fracturing device for carbonate reservoirs, such as... Figure 2 The diagram illustrates the implementation of the aforementioned method. It includes: a first injection unit 21, used to inject a moderating fluid into the carbonate reservoir to form a fracture network, the moderating fluid being used to prevent fracture closure; a second injection unit 22, used to inject acid into the fractures to form flow channels, the acid being able to dissolve calcite within the fractures; a third injection unit 23, used to alternately inject acid of different concentrations into the fractures to form acid etching waves, used to restore the connection between natural pores and fractures; and a fourth injection unit 24, used to inject acid fracturing fluid into the fractures, used to etch the middle and root of the fractures.

[0070] The fracturing unit can employ coiled tubing and a tool string, comprising a first injection unit 21, a second injection unit 22, a third injection unit 23, and a fourth injection unit 24, capable of being deployed to the target reservoir section for staged fracturing operations. After the coiled tubing and tool string reaches the predetermined target section, the first injection unit 21 first performs targeted high-speed injection to modify the reservoir fluid, forming a multi-branched fracture network. The proppant in the modified fluid prevents fracture closure. After fracture formation, the second injection unit 22 first injects acid at a low flow rate for test flushing, etching initial wormholes within the fractures as basic flow channels. Subsequently, the third injection unit 23 injects acid at high flow rates of varying concentrations, utilizing the concentration difference and high pressure to create localized dissolution cavities within the channels, promoting channel extension to deeper layers and expanding branched wormholes along the channel walls, thus increasing the acid etching range. Finally, the fourth injection unit 24 performs drag acidizing, precisely etching the middle and root of the fractures. After the first stage is completed, the coiled tubing and tool string are pulled up to the next target layer, and the above process is repeated for the second stage of targeted acidizing operation, and so on, until the segmented fracturing of the entire carbonate reservoir is completed.

[0071] This device embodiment corresponds to the aforementioned method embodiment. For ease of reading, this device embodiment will not repeat the details of the aforementioned method embodiment one by one. However, it should be clear that the device in this embodiment can implement all the contents of the aforementioned method embodiment.

[0072] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A fracturing method for carbonate reservoirs, characterized in that, include: S1. Injecting a modification fluid into a carbonate reservoir to form a fracture network, the modification fluid being used to prevent the fractures from closing. S2. Inject acid into the crack to form a flow channel, and the acid can be used to dissolve the calcite in the crack; S3. Alternately inject acid solutions of different concentrations into the crack to form an acid etching wave, which is used to restore the connection between the natural pores and the crack; S4. Inject acid fracturing fluid into the crack to etch the middle and root of the crack.

2. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, The modified fluid consists of water, a proppant, and a crosslinking agent. The proppant is made of ordinary ceramsite with a particle size of 30-50 mesh or high-strength ceramsite with a particle size of 20-40 mesh.

3. The fracturing method for carbonate reservoirs according to claim 2, characterized in that, The proppant accounts for 10%-20% of the mass percentage of the modified fluid.

4. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, The acid fracturing fluid is a self-generating acid fracturing fluid, which is composed of reactants, additives and base fluid. The reactants have a pH value of not less than 7 at room temperature and a pH value of less than 7 at a preset formation temperature.

5. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, The acid solution consists of water, acid, a retarder, and a crosslinking agent.

6. The fracturing method for carbonate reservoirs according to claim 5, characterized in that, The concentration of the acid solution is 10%-20%, and the viscosity of the acid solution is 35 mPa·s-40 mPa·s.

7. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, In step S3, the alternating injection of acid solutions of different concentrations involves sequentially injecting high-concentration acid solutions and low-concentration acid solutions alternately. The flow rate ratio of the high-concentration acid solution to the low-concentration acid solution during each alternating injection is 1:

2. The final injection has an acid concentration of 15% and a flow rate of 30 m³ / h. 3 .

8. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, In step S1, the injection of alteration fluid into the carbonate reservoir is carried out using high-pressure injection and stepped displacement injection. First, the main fracture is formed by injecting at a preset displacement, and then the displacement is gradually increased to form the branch fracture network.

9. The fracturing method for carbonate reservoirs according to claim 1, characterized in that, The injection of acid fracturing fluid into the fracture in step S4 includes the use of variable displacement fracture control technology to optimize the fracture morphology.

10. A fracturing device for carbonate reservoirs, characterized in that, include: The first injection unit is used to inject a modification fluid into the carbonate reservoir to form a fracture network, the modification fluid being used to prevent the fractures from closing. The second injection unit is used to inject acid into the crack to form a flow channel, and the acid can be used to dissolve the calcite in the crack. The third injection unit is used to alternately inject acid of different concentrations into the crack to form an acid etching wave, which is used to restore the connection between the natural pores and the crack. The fourth injection unit is used to inject acid fracturing fluid into the crack to etch the middle and root of the crack.

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