Acid fracturing transformation method for complex lithologic carbonate reservoir

Abstract

The invention belongs to the technical field of reservoir transformation of oil and gas field development engineering, and particularly relates to an acid fracturing transformation method for a complex lithologic carbonate reservoir. According to the method, during acid fracturing construction, the fracturing fluid and the acid fluid are alternately injected for multiple times, on one hand, the reservoir temperature is reduced, the acid rock reaction rate is slowed down, and the effective action distance of acid etching is increased, and on the other hand, the fingering phenomenon of the acid fluid in the crack is formed, non-uniform etching is formed, and the flow conductivity of the crack is improved. The low-viscosity gelled acid has low viscosity and high acid rock reaction rate, the low-viscosity gelled acid is injected firstly, and the corrosion crack width can be further enlarged on the basis that early-stage fracturing fluid forms a long-distance fracturing crack; according to the method, the high-viscosity acid liquid is injected, then the high-viscosity gelled acid is injected, the characteristics that the high-viscosity acid liquid is high in viscosity, low in acid rock reaction rate and long in acid etching effective action distance can be utilized, on the basis of an existing crack channel, the acid liquid is communicated to a far-end grey matter / cloudy matter filling crack, and a corrosion crack net is formed in a long distance.

Description

Technical Field

[0001] The invention belongs to the technical field of reservoir transformation in oil and gas field development engineering, and in particular relates to an acid fracturing transformation method for a carbonate reservoir with complex lithology. Background Art

[0002] With the increasing efforts in oil and gas resource exploration and development at home and abroad, the amount of high-quality resources with good quality is decreasing, and complex lithology oil and gas reservoirs are gradually coming into people's view. Among them, carbonate oil and gas reservoirs have developed into an important type of oil and gas resources in China, and occupy a very important position in oil and gas production. Different from conventional carbonate reservoirs with large thickness and mainly limestone or dolomite, complex lithology carbonate reservoirs have strong heterogeneity and thin single layer thickness, including mud dolomite, gypsum dolomite, gray dolomite and other types of lithology. This type of oil and gas reservoir not only has poor physical properties such as porosity and permeability, but also has a high content of mud and gypsum, and reservoir transformation is required to achieve efficient development.

[0003] For carbonate rocks with complex lithology, acid fracturing is a commonly used reservoir transformation method. That is, acid is injected into the formation under conditions higher than the formation fracture pressure, and oil and gas flow channels are formed by the dissolution of the acid, thereby increasing the production of a single well. However, the main disadvantages of this method are large acid loss, fast acid-rock reaction speed, and short acid-etched crack length. In particular, materials such as mud and paste do not react with acid, and are prone to block acid-etched crack channels after acid fracturing, resulting in a rapid decrease in conductivity and a rapid decline in production.

[0004] The literature "Application of Cross-linked Acid and Sand Acid Fracturing Technology in Complex Lithology Reservoirs" (Acta Petrolei Sinica, 2008, Vol. 29) and "Multi-stage Injection and Sand Acid Fracturing Technology in Daniudi Gas Field" (Inner Mongolia Petrochemical, 2011, Vol. 37) organically combines sand fracturing and acid fracturing technology to achieve simultaneous acid etching and sand carrying, forming multiple acid etching-support composite fractures in the reservoir section.

[0005] The Chinese invention patent application with publication number CN103089228A discloses a method for ground cross-linked acid-carrying sand acid fracturing of mud dolomite. By combining acid fracturing with sand-adding acid fracturing technology, a propped fracture with deep penetration and longer-lasting conductivity is formed to the maximum extent, giving play to the dual role of acid-etched fractures and propped fractures, thereby improving the conductivity of artificial fractures and oil well production capacity.

[0006] In the above existing technologies of sand-adding acid fracturing, the pre-fluid is injected with a single cross-linked acid to create fractures, which has the problems of fast acid-rock reaction speed and short acid-etched fracture length. At the same time, the influence of different lithologies on the acid-rock reaction rate is not considered, making it difficult to achieve efficient transformation of carbonate reservoirs with complex lithologies. Summary of the invention

[0007] The purpose of the present invention is to provide an acid fracturing reconstruction method for complex lithology carbonate reservoirs, so as to solve the problem that the existing acid fracturing reconstruction method has poor reconstruction effect on complex lithology carbonate reservoirs.

[0008] In order to achieve the purpose, the technical solution adopted by the present invention is:

[0009] A method for acid fracturing transformation of a carbonate reservoir with complex lithology comprises the following steps: first injecting fracturing fluid to form a fracture, then multi-stage injecting acid to increase the length of the acid-etched fracture, and then injecting a sand-carrying fluid to form a support fracture; the multi-stage acid injection comprises injecting at least two levels of acid solution, and the two adjacent levels of acid solution are separated by fracturing fluid; the at least two levels of acid solution comprise a first level of acid solution and other levels of acid solution, the first level of acid solution is a low-viscosity gelling acid, and at least one of the other levels of acid solution is a high-viscosity gelling acid.

[0010] The acid fracturing transformation method of the complex lithology carbonate reservoir provided by the present invention is that the fracturing fluid and the acid liquid are injected alternately for many times during the acid fracturing construction, which is beneficial to reduce the reservoir temperature, slow down the acid-rock reaction rate, and increase the effective action distance of acid etching on the one hand, and on the other hand, the acid liquid forms a fingering phenomenon in the crack, forms non-uniform etching, and improves the conductivity of the crack. At the same time, low-viscosity gelling acid has a lower viscosity and a faster acid-rock reaction rate and has a large difference in reaction rate with calcite and dolomite. The low-viscosity gelling acid is injected first, and the width of the dissolution crack can be further expanded on the basis of the long-distance fracturing crack formed by the early fracturing fluid; and then the high-viscosity gelling acid is injected, and the characteristics of the high-viscosity acid liquid with higher viscosity, slower acid-rock reaction rate, and longer effective action distance of acid etching can be utilized. On the basis of the existing fracture channel, the acid liquid is connected to the gray / cloudy filling crack at the far end, and a dissolution crack network is formed at a long distance.

[0011] Preferably, in the multi-stage acid injection stage, the limiting acid displacement is first determined based on the pipe friction and the wellhead pressure limit; then the preferred acid dosage is determined based on the relationship between the effective reservoir action distance and the acid dosage under the limiting acid displacement; finally, the acid injection stage and the preferred acid displacement are determined based on the relationship chart between the acid displacement and the fingering efficiency. This method can effectively improve the pertinence of the acid displacement, acid injection stage, and acid dosage during acid fracturing construction, thereby effectively increasing the length of the acid-etched seam and further improving the effectiveness of complex lithology carbonate reservoir reconstruction.

[0012] Preferably, in the multi-stage acid injection stage, the acid injection stages are 3, the first stage acid is low-viscosity gelling acid, and the second and third stage acid are both high-viscosity gelling acid. Through the above-mentioned coordinated design of one-stage low-viscosity gelling acid and two-stage high-viscosity gelling acid, on the basis of long-distance fracture compression, the width of the dissolution fracture is first expanded, and then the distal filling fracture is connected, so that a complex fracture network system of "main fracture + dissolution fracture network" can be formed, which effectively improves the flow conductivity.

[0013] The fracturing fluid has high viscosity and good fracture-forming performance, so that long-distance fractures can be formed first during fracturing. Preferably, during the fracture-forming stage of injecting fracturing fluid, the preferred fracturing fluid displacement is determined based on the relationship chart between the fracturing fluid displacement and the fracture length; and then the limit dosage of fracturing fluid under the preferred fracturing fluid displacement is determined based on the relationship chart between the dynamic fracture length and the limit dosage of fracturing fluid. Determining the fracturing fluid displacement and the limit dosage of fracturing fluid by the above method can further improve the accuracy and effectiveness of fracturing construction and provide a basis for obtaining a longer acid-etched fracture length.

[0014] Preferably, the fracturing fluid injection includes injecting non-sand-carrying fracturing fluid and sand-carrying fracturing fluid, wherein the sand-carrying fracturing fluid accounts for 30-40% of the sum of the volumes of the non-sand-carrying fracturing fluid and the sand-carrying fracturing fluid, and the total sand ratio of the sand-carrying fracturing fluid is 5-7%, wherein the particle size of the proppant is 70 / 140 mesh. During the injection of the fracturing fluid, adding 70 / 140 mesh ceramsite at a low sand ratio can support microcracks and prevent the formed microcracks from closing.

[0015] Further preferably, the sand-carrying fracturing fluid is injected in multiple stages, each stage of the sand-carrying fracturing fluid is separated by a non-sand-carrying fracturing fluid, and the sand ratio of each stage of the sand-carrying fracturing fluid is gradually increased. In this way, the fracture-making effect can be further optimized, providing conditions for the subsequent formation of a complex fracture network.

[0016] Fracturing fluid sand-carrying propping is to use the sand-carrying performance of fracturing fluid to carry proppant into the cracks on the basis of the long-distance dissolution cracks formed by acid, so as to form proppant cracks under high closure pressure and further improve the flow conductivity. Preferably, during the sand-carrying fluid injection stage, the maximum width of dynamic cracks under different fracturing displacements is obtained according to the dynamic extension model of the cracks and the fracture filtration model, and then the amount of proppant is determined in combination with the effective action distance and thickness of the reservoir. In this way, the pertinence and effectiveness of the amount of proppant can be improved, and the flow conductivity can be guaranteed.

[0017] Further preferably, during the injection of the sand-carrying fluid, the total sand ratio of the sand-carrying fluid is 15-20%, wherein the particle size of the proppant is 40 / 70 mesh. In this way, the supporting capacity of the fracture can be further optimized and the flow conductivity can be improved. More preferably, the sand-carrying fluid is injected in multiple stages, and the sand ratio of each stage of the sand-carrying fluid is gradually increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a flow chart of the acid fracturing transformation method of complex lithology carbonate reservoir of the present invention;

[0019] Figure 2 : is a graph showing the relationship between the displacement of the Mawu 1+2 reservoir fracturing fluid and the fracture length at different viscosities in an embodiment of the present invention;

[0020] Figure 3 is a graph showing the relationship between the amount of fracturing fluid used and the dynamic crack length at different fracturing fluid displacements in an embodiment of the present invention;

[0021] Figure 4 is a graph showing the relationship between displacement and wellhead predicted pressure at different extension pressures in an embodiment of the present invention;

[0022] Figure 5 is a graph showing the relationship between displacement and wellhead predicted pressure under different casing / tubing sizes in an embodiment of the present invention;

[0023] Figure 6 is the relationship between the effective action distance of the Mawu 1+2 reservoir and the amount of acid solution used under different displacements in the embodiment of the present invention;

[0024] Figure 7 is a graph showing the relationship between the acid liquid displacement and the fingering efficiency in an embodiment of the present invention;

[0025] Figure 8 is a graph showing the relationship between the acid fluid displacement and the maximum width of dynamic fractures under different natural fracture densities in an embodiment of the present invention;

[0026] Fig. 9 1 is a graph showing the relationship between the acid fluid displacement and the optimal proppant dosage under different natural fracture densities in the embodiment of the present invention. DETAILED DESCRIPTION

[0027] Complex lithology carbonate reservoirs have many types of lithology, poor physical properties, and strong heterogeneity. The single layer thickness of its reservoir is thin, and it contains various types of lithology such as mudstone, gypsum dolomite, and gray dolomite. The physical properties such as porosity and permeability are poor, and the content of mud and gypsum is high. Reservoir transformation is required to achieve efficient development.

[0028] The existing sand-added acid fracturing technology has problems such as the length of the acid-etched seam, the influence of different lithologies on the acid-rock reaction rate is not considered, and the optimization of key parameters (acid solution dosage, construction displacement, proppant dosage, etc.) lacks basis. The present invention is aimed at the acid fracturing transformation of carbonate rocks with complex lithologies. When considering the influence of different lithologies on the acid-rock reaction rate, and determining the relationship between the effective action distance of the reservoir and the acid solution dosage, the core mineral component test and the acid-rock reaction experimental test are first carried out to establish the relationship between the content of different mineral components and the acid-rock reaction rate, and then the relationship between the acid solution dosage and the effective action distance of the reservoir is established through the acid-rock reaction rate. The dynamic seam width is obtained through the fracture dynamic extension model and the fracture filtration model, and then the relationship between the dynamic seam length, the effective action distance of acid etching, and the fracturing fluid and the acid solution dosage is established. Then, the optimal fracturing fluid and acid solution displacement are obtained through simulation, and finally, the limit acid etching effective action distance and the reservoir thickness (seam height) are combined to calculate the proppant dosage.

[0029] The implementation process of the present invention is described in detail below in conjunction with specific embodiments.

[0030] The specific embodiment of the acid fracturing transformation method of the complex lithology carbonate reservoir of the present invention is as follows: Figure 1 The specific instructions are as follows:

[0031] Well A is a gas reservoir evaluation well deployed on the residual hillock of the ancient slope in a certain well area. The well mainly evaluates the Mawu 1 The development and preservation of the karst reservoirs in the eroded residual hills near the fault zone and the gas production capacity. 1 The lithology of the layer is gray dolomite, with marlstone bands developed in the lower part. The reservoir is filled with mud. Considering reservoir protection, waterproof lock agent and clay stabilizer are added to the fracturing fluid to control the scale of acid addition. 1 There are many calcite-filled cracks on the top. 1 2 , Ma Wu 1 3 0.15cm needle-shaped residual dissolved pores were found in each layer. This layer was transformed by combining pre-fluid fracturing + large-volume acid fracturing + sand fracturing.

[0032] The design idea is as follows: In the first stage, the pre-fracturing fluid uses cross-linking fluid with variable displacement to control the fracture height and create long fractures, connect as many natural fractures as possible, and add 70 / 140 mesh expanded clay for distal support; in the second stage, gelled acid and cross-linking fluid are injected alternately, and acid etching is used to form a diversion channel, while dissolving the calcite in the fracture to further expand the fracture communication range; in the third stage, the cross-linking fluid carries 40 / 70 mesh expanded clay into the formation to ensure that the acid-etched fractures have a high diversion capacity.

[0033] The design and basis of fracturing construction parameters are as follows:

[0034] (1) According to the DST test data, the average geothermal gradient of the Mawu 1 layer is 3.1°C / 100m, the vertical depth of the middle part of the perforation section is 2961m, and the predicted formation temperature is 103.3°C. According to the formation temperature, the preferred thickening acid system with 0.45% thickener (i.e., high viscosity gelling acid), low viscosity acid system (i.e., low viscosity gelling acid), and 0.45% HPG cross-linking fluid system (i.e., fracturing fluid) are selected.

[0035] (2) Simulate the five-horse race using finite element analysis software 1 The fracture extension under different fracturing fluid viscosities and different fracturing fluid displacements at different layers. Through calculation and analysis, the relationship between fracturing fluid displacement and fracture length under different layers and viscosities can be obtained ( Figure 2 ).

[0036] Among them, the viscosity of the fracturing fluid is determined according to the specific working conditions of the on-site construction. Figure 2 It shows how to give the optimal fracturing fluid injection rate in combination with the tubing structure under actual working conditions under the fracturing fluid viscosity.

[0037] Specifically, combined with the pipe string structure, it can be determined that the optimal displacement of the fracturing fluid injection is 6m 3 / min, then use 4-6m 3 / min injection rate to control the fracture height and create long fractures. During this period, 4.6m 3 70 / 140 mesh expanded clay is used to support micro cracks.

[0038] In this step, the crack extension under different displacements is simulated by finite element analysis software, and the crack extension length (i.e., crack length) at the same time point is read to form a chart of different displacements and crack lengths.

[0039] Then, according to the fracture dynamic extension model and fracture filtration model, the dynamic fracture length (i.e. fracture length) and the limit dosage of fracturing fluid under different fracturing displacements are calculated ( Figure 3 ), determine 6m 3 The maximum fracturing fluid dosage at the displacement rate of 500m / min is 3 .

[0040] Among them, the dynamic extension model of the fracture is determined by the PKN model (refer to Chapter 6, Section 4 (page 227) of Li Yingchuan's Oil Production Engineering (Second Edition)), and the fracture loss model is obtained by the working fluid loss experiment. The limit dosage refers to the dosage under the limit injection capacity of the string structure. During on-site construction, the string pressure limit and construction safety must be considered, and the actual dosage will be less than the limit dosage.

[0041] (3) According to the wellhead pressure limit and pipe string structure, the design adopts the maximum acid fluid displacement of 8m 3 / min( Figure 4 , Figure 5 ), and then disassemble the Mazda5 according to different displacements 1+2 Relationship between effective reservoir distance and acid dosage ( Figure 6 ), the optimal acid dosage is designed to be 300m 3 Finally, the Ma Wu was simulated using the fluid mechanics finite element analysis software. 1+2 The distribution of acid flow and fingering effect in acid fracturing fractures under different alternating levels in the layer, and the relationship between acid displacement and fingering efficiency are obtained ( Figure 7 ), thus determining the optimal acid injection rate to be 8m 3 / min, the alternating level is 3.

[0042] Figure 4In the figure, 0.021MPa / m, 0.022MPa / m, and 0.023MPa / m show different extension pressure gradients. The wellhead predicted pressure can be obtained by adding the fracture extension pressure to the string friction and the perforation friction, minus the liquid column pressure. Therefore, the wellhead predicted pressure under different displacements and different extension pressure gradients can be calculated, and the wellbore string structure pressure limit and the maximum fracture pressure gradient ( Figure 5 ), the optimal acid pumping rate can be determined (the predicted wellhead pressure must be less than the wellhead limit pressure).

[0043] Among them, when determining the relationship between the effective action distance of the reservoir and the amount of acid used, the target layer core is first selected, and core mineral component tests and acid-rock reaction experimental tests are carried out to establish a relationship between the content of different mineral components and the acid-rock reaction rate. The relationship between the mineral content and the effective action distance is established through the acid-rock reaction rate, and the optimal acid dosage is then determined.

[0044] The principle of determining the amount of acid is as follows: the amount of acid is determined by the controlled volume of the acid fracturing fracture (i.e., the fracture width, fracture height, and effective fracture length, where the effective fracture length is the effective action distance of acid erosion), and the effective action distance of acid erosion is affected by the acid-rock reaction rate, which is affected by the mineral content. Since carbonate reservoirs are highly heterogeneous, the distribution of mineral components varies greatly, and the acid-rock reaction rates vary greatly, it is necessary to establish the relationship between mineral content and effective action distance in order to clarify the effective action distance under the formation conditions of the construction well, thereby calculating the optimal amount of acid to ensure a balance between construction effect and construction economy. (The mineral content is determined by X-ray diffraction experiments, and the acid-rock reaction rate is determined by acid-rock reaction experiments. Through these two experiments, the relationship between mineral content and acid-rock reaction rate can be established, and then the effective action distance is deduced from the acid-rock reaction rate, and then the controlled volume of the acid fracturing fracture is calculated from the effective action distance, which is the optimal amount of acid.

[0045] During construction, due to the structural strength limitation of the wellbore tubing, in order to ensure the safety of construction, the actual amount of acid used should be less than the recommended limit amount of acid.

[0046] Furthermore, the dosage of low-viscosity gelling acid and high-viscosity gelling acid is determined as follows:

[0047] The amount of low-viscosity gelling acid used is determined by the desired dissolution seam width. For example, if the effective distance of low-viscosity gelling acid is 100m and the seam height is 50m, to obtain a seam width of 2-4mm, 10-20m needs to be dissolved. 3 Carbonate rock will consume 100-150m 3The concentration of low-viscosity gelling acid is 20%. The amount of high-viscosity gelling acid is determined by the dynamic fracture length of the pre-hydraulic fracturing. In this embodiment, the dynamic fracture width under the optimal fracturing fluid displacement is 150m. In order to increase the effective action distance of the acid, high-viscosity gelling acid is selected. At the same time, in order to expand the average fracture width to 4-6mm, it is necessary to dissolve 10m in the first 100m fracture length section. 3 Carbonate rock, dissolution 10m in 100-150m long fracture section 3 Carbonate rock, so it needs to consume 150-200m 3 The concentration is 20% high viscosity gelling acid.

[0048] When obtaining the relationship chart between acid displacement and fingering efficiency, the flow state of alternating injection of fracturing fluid and acid at different displacements is simulated by fluid mechanics finite element analysis software to obtain the fingering pattern of acid in the fracture, thereby calculating the fingering efficiency. The fingering efficiency is the numerical product of the fingering length of the acid in the fracture divided by the fracture length and the fingering width of the acid in the fracture divided by the fracture width.

[0049] Figure 7 In the above, primary injection refers to the injection of fracturing fluid (cross-linking fluid in Table 1) first and then the injection of acid; secondary injection refers to the injection of fracturing fluid and acid alternately twice; tertiary injection refers to the injection of fracturing fluid and acid alternately three times.

[0050] (4) Since the lithology of the target transformation layer is mainly gray dolomite and the calcite-filled fractures are relatively developed, a three-stage alternating injection process of "fracturing fluid + low-viscosity gelling acid + fracturing fluid + high-viscosity gelling acid + fracturing fluid + high-viscosity gelling acid" is adopted. First, the fracturing fluid is injected to reduce the temperature in the fracture, and then the low-viscosity gelling acid is injected. On the one hand, a wider dissolution fracture width is obtained, and on the other hand, the calcite-filled fractures are dissolved as much as possible to form a dissolution fracture network. Then, the fracturing fluid and high-viscosity gelling acid are injected alternately. On the one hand, the effective action distance of acid etching is increased, and on the other hand, a fingering effect is formed, which increases the degree of non-uniform etching and improves the conductivity.

[0051] In Table 1, the type of fracturing fluid injected in the multi-stage injection stage is the same as that in the pre-fluid injection stage, both of which are cross-linked fluids. In the actual construction process, the characteristics of "first injecting fracturing fluid and then injecting acid" are still reflected. Table 1 is divided into multi-stage injection stages starting from acid injection. The overall construction essence is still the three-stage alternating injection process of "fracturing fluid + low-viscosity gelling acid + fracturing fluid + high-viscosity gelling acid + fracturing fluid + high-viscosity gelling acid".

[0052] (5) First, by combining the fracture dynamic extension model and the fracture filtration model, the maximum width of the dynamic fracture under different fracturing rates can be calculated ( Figure 8 ), and then combined with the effective action distance of the limit acid etching and the reservoir thickness (fracture height), the proppant dosage can be calculated to be 20-30m 3 ( Fig. 9 ). Fig. 9 In the figure, 1 / m, 1.5 / m, and 2 / m represent the density of natural fractures.

[0053] In the sand-carrying fluid stage, in order to ensure that the proppant can be carried to the far end of the fracture to improve the conductivity of the far end of the fracture, the sand-carrying fluid is injected at the maximum displacement to provide maximum energy for the migration of the proppant and allow it to migrate farther.

[0054] (6) According to the construction data of adjacent wells, the formation closure pressure gradient is about 0.02 MPa / m, the formation closure pressure is 59.2 MPa, and a combination of 70 / 140 mesh and 40 / 70 mesh 69 MPa pressure-resistant ceramsite is used as the proppant.

[0055] (7) According to steps (1) to (6), the pumping procedure for fracturing operation of Well A is finally determined (Table 1). 3 / min injection of fracturing fluid 262m 3 , use high viscosity fracturing fluid to control the fracture height and create long fractures; then increase the displacement to 8m 3 / min injection of low viscosity acid 120m 3 , using the strong penetrability of low-viscous acid to communicate calcite to fill the cracks, forming a dissolution crack network, and then continue to 8m 3 / min displacement alternately twice in sequence 100m 3 Fracturing fluid and 80m 3 High viscosity acid is used to obtain dissolution cracks with a longer action distance and non-uniform etching; finally, 185m3 of sand-carrying liquid with a sand ratio of 15.5% is injected, 40 / 70 mesh ceramsite is used, and the amount of sand added is 28.7m 3 , forming a highly conductive complex fracture network system of "main fractures + dissolution fracture network".

[0056] Table 1A Well Fracturing Construction Pumping Procedure

[0057]

[0058] In Table 1, the composition of each working liquid is as follows (the ratios are all in mass fraction):

[0059] Original glue solution: 0.45% HPG (first grade) + 2% KCl + 0.05% bactericide + 0.3% drainage agent + 0.2% Na 2 CO 3 The technical indicators of the original glue solution (i.e. base solution) are shown in Table 2 below.

[0060] Low viscosity acid: 20% HCl + 2.5% corrosion inhibitor + 0.3% iron ion stabilizer + 0.3% drainage agent + 0.15% thickener. Low viscosity acid apparent viscosity (25℃, 170s -1):6-18mPa·s.

[0061] Cross-linking liquid: base liquid + 0.4%-0.6% organic boron cross-linking agent.

[0062] Thickening acid: 20% HCl + 2.5% corrosion inhibitor + 0.3% iron ion stabilizer + 0.3% drainage agent + 0.45% thickener. Thickening acid apparent viscosity (25℃, 170s -1 ): 24-45mPa·s.

[0063] Active water: clean water + 1% KCl.

[0064] Table 2 Technical indicators of original glue solution

[0065]

[0066] Among them, the pretreatment injection of low-viscosity acid plays a role in reducing the formation fracture pressure. The role of the spacer fluid injected into the original glue liquid is to form a filter cake and reduce the subsequent working fluid loss.

[0067] (8) After multi-stage injection and acid fracturing, the test gas production of Well A was 14405m 3 / d, which is 20.99% higher than the production of adjacent wells. In comparison, Well B in the same well area adopted the acid fracturing process of "high viscosity acid + low viscosity acid + conventional acid + fracturing fluid" (conventional acid refers to acid without thickener, and its viscosity is generally 1-5mPa·s; the conventional acid formula in this example is: 20% HCl + 2% corrosion inhibitor + 0.2% iron ion stabilizer + 0.2% drainage agent), and the final test gas production was 1860m 3 / d, which is only 12.91% of the test gas production of Well A.

Claims

1. A method for acid fracturing of complex lithology carbonate reservoirs, characterized in that: The method comprises the following steps: first injecting fracturing fluid to form seams, then injecting acid in multiple stages to increase the length of the acid-etched seams, and then injecting sand-carrying fluid to form support seams; the multiple stages of acid injection include injecting at least two stages of acid liquid, and the two adjacent stages of acid liquid are separated by fracturing fluid; the at least two stages of acid liquid include a first stage of acid liquid and other stages of acid liquid, the first stage of acid liquid is low-viscosity gelling acid, and at least one stage of acid liquid in the other stages of acid liquid is high-viscosity gelling acid.

2. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 1, characterized in that: In the multi-stage acid injection stage, the limiting acid displacement is first determined based on the string friction and the wellhead pressure limit; then the optimal acid dosage is determined based on the relationship between the effective reservoir action distance and the acid dosage under the limiting acid displacement; finally, the acid injection stages and the optimal acid displacement are determined based on the relationship chart between the acid displacement and the fingering efficiency.

3. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 1 or 2, characterized in that: In the multi-stage acid injection stage, the acid injection stages are 3, the first-stage acid is low-viscosity gelling acid, and the second-stage acid and the third-stage acid are both high-viscosity gelling acid.

4. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 1, characterized in that: During the fracture creation stage of injecting fracturing fluid, the preferred fracturing fluid displacement is determined based on the relationship chart between the fracturing fluid displacement and the fracture length; and the maximum fracturing fluid dosage under the preferred fracturing fluid displacement is determined based on the relationship chart between the dynamic fracture length and the maximum fracturing fluid dosage.

5. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 1, characterized in that: The fracturing fluid injection method includes injecting non-sand-carrying fracturing fluid and sand-carrying fracturing fluid, wherein the sand-carrying fracturing fluid accounts for 30-40% of the total volume of the non-sand-carrying fracturing fluid and the sand-carrying fracturing fluid, and the total sand ratio of the sand-carrying fracturing fluid is 5-7%, wherein the particle size of the proppant is 70 / 140 mesh.

6. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 5, characterized in that: The sand-carrying fracturing fluid is injected in multiple stages, each stage of the sand-carrying fracturing fluid is separated by non-sand-carrying fracturing fluid, and the sand ratio of each stage of the sand-carrying fracturing fluid is gradually increased.

7. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 1, 5 or 6, characterized in that: During the sand-carrying fluid injection stage, the maximum width of dynamic fractures under different fracturing displacements is obtained based on the fracture dynamic extension model and fracture filtration model, and then the proppant dosage is determined in combination with the effective action distance and reservoir thickness of the reservoir.

8. The acid fracturing reconstruction method for complex lithology carbonate reservoirs according to claim 7, characterized in that: During the sand-carrying fluid injection stage, the total sand ratio of the sand-carrying fluid is 15-20%, wherein the particle size of the proppant is 40 / 70 mesh.

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