A model and method for evaluating the damage of fracturing fluid to tight gas reservoirs

Abstract

The present invention relates to the field of unconventional oil and gas field development, and specifically to a model and method for evaluating the damage caused by fracturing fluid to tight gas reservoirs. The model comprises a microscopic etching matrix model and a microscopic etching fracture model. Nitrogen and fracturing fluid filtrate are injected, and the injection pressure and model image are recorded to calculate the extent of damage caused by the fracturing fluid to the matrix of the tight gas reservoir. The microscopic etching matrix model is replaced with a microscopic etching fracture model. Nitrogen and fracturing fluid breaker are injected, and the injection pressure and model image are recorded to calculate the extent of damage caused by fracturing fluid residue to the cracks of the tight gas reservoir. By establishing a reusable microscopic visual etching model to simulate the tight gas reservoir, and based on this model, a corresponding evaluation test program is established to evaluate the damage caused by fracturing fluid to the reservoir, the technical difficulties encountered in existing technologies, such as the lack of representativeness of test fluids, the lack of damage mechanism evaluation, large errors, and poor repeatability, are addressed.

Description

Technical Field

[0001] The present invention relates to the field of unconventional oil and gas field development, and in particular to a model and method for evaluating damage of fracturing fluid to tight gas reservoirs. Background Art

[0002] Tight gas reservoirs are characterized by a rich abundance of nano- and micro-pore throats, poor pore throat connectivity, and low matrix permeability. Low single-well production, short production plateaus, rapid production decline, and low recovery rates are common challenges during development. Therefore, tight gas reservoirs typically require fracturing to achieve economic development. However, fracturing fluids can cause a degree of reservoir damage during fracturing, reducing the effectiveness of fracturing. Particularly in newly developed areas, reservoir damage from fracturing fluids has become a bottleneck restricting the efficient exploration and development of tight gas reservoirs, severely hindering their timely discovery, accurate evaluation, and efficient development. Therefore, a scientific, systematic, and accurate evaluation of the extent of fracturing fluid damage to tight gas reservoirs and a clear understanding of the underlying mechanisms are crucial.

[0003] Current evaluations of fracturing fluid damage to tight gas reservoirs primarily draw on petroleum industry standards for conventional oil and gas reservoirs. However, these conventional evaluation methods and standards are not fully applicable to tight gas reservoir damage assessment. For example, the "SY / T 5358-2010 Reservoir Sensitivity Flow Test Evaluation Method" applies to clastic reservoirs with air permeability >1 mD and uses liquid phase core permeability testing. The "SY / T 5107-2005 Water-Based Fracturing Fluid Performance Evaluation Method" utilizes core displacement with fracturing fluid breakers, and the permeability test requires pressure and flow to stabilize for up to one hour. In summary, the existing technologies for evaluating tight gas reservoir damage have the following main problems: ① The test fluid is not representative. The existing technologies use liquid as the test medium. In tight gas reservoirs, the gas phase is the main mobile phase and production target. After gas reservoir damage, the main focus is on the change in gas phase permeability. Using liquid permeability as an evaluation parameter lacks specificity and representativeness. In addition, using liquid phase to test permeability has problems such as high test pressure, long experimental cycle, and large test data errors. ② The evaluation parameters are single, and the understanding of the essential laws of gas reservoir damage mechanism is not in-depth. The parameters of the existing core flooding experiments are mainly permeability reduction rate, which lacks in-depth evaluation of the reservoir damage mechanism, resulting in an unsystematic and incomplete understanding of the reservoir damage mechanism and the possibility of unreasonable implementation of reservoir protection measures. ③ The test results are affected by many factors, with large errors and poor repeatability. Core flooding experiments are highly random and have many influencing factors. The core cannot be reused, making it difficult to obtain accurate evaluation results. In particular, when conducting parallel testing and evaluation of multiple fracturing fluid systems, the randomness and non-repeatability of core samples make it difficult to reach a correct evaluation conclusion. Summary of the Invention

[0004] The purpose of the present invention is to provide a model and method for evaluating the damage of fracturing fluid to tight gas reservoirs. By establishing a reusable microscopic visual etching model to simulate tight gas reservoirs, and based on this model, a corresponding evaluation test program is established to evaluate the damage of fracturing fluid reservoirs, so as to solve the technical problems of the existing technology that the test fluid is not representative, the damage mechanism evaluation is insufficient, the error is large and the repeatability is poor.

[0005] A fracturing fluid damage evaluation model for tight gas reservoirs, including a microscopic etching matrix model and a microscopic etching crack model;

[0006] The micro-etching matrix model includes a first injection port, a first etching area, a second etching area, a third etching area, a second injection port, and a first sealing frame; the first etching area is located on one side of the first injection port of the micro-etching matrix model, the first etching area is connected to the first injection port, the depth of the first etching area is less than the depth of the first injection port, the second etching area is connected to the first etching area, the second etching area is etched according to a data file, and the etching depth is the average pore size of the rock sample; the third etching area is connected to the second etching area, and the depth increases linearly from the second etching area to the second injection port along the average pore size;

[0007] The microscopic etching crack model includes an injection port No. 1, an etching area No. 1, an etching area No. 2, an etching area No. 3, an injection port No. 2, a groove and a sealing frame No. 1. The etching area No. 1 is located on one side of the injection port No. 1, and the etching area No. 1 is connected to the injection port No. 1. The depth of the etching area No. 1 is less than the depth of the etching area No. 1. The etching area No. 2 is connected to the etching area No. 1. The etching area No. 2 is etched according to the data file, and the etching depth is the average pore size of the rock sample. The groove is etched in the etching area No. 2, connecting the etching area No. 1 and the etching area No. 3. The depth and width of the groove are both the average crack opening of the rock sample; the etching area No. 3 is connected to the etching area No. 2, and the depth of the etching area No. 3 increases linearly from the average crack opening from the etching area No. 2 toward the injection port No. 2.

[0008] Optionally, the micro-etched matrix model and the micro-etched crack model are made of transparent heat-resistant and pressure-resistant glass.

[0009] A method for evaluating damage of fracturing fluid to tight gas reservoirs comprises the following steps:

[0010] S1. Obtaining a damage evaluation model for fracturing fluid to tight gas reservoirs, including a microscopic etching matrix model and a microscopic etching crack model;

[0011] S2. Load the microscopic etched matrix model into a high-temperature and high-pressure visual displacement experimental device, inject nitrogen into the first injection port at a constant rate v1 until the pressure at the first injection port stabilizes, and record the injection pressure p0. Then, inject the fracturing fluid filtrate into the second injection port, and stop the injection after the fracturing fluid filtrate appears at the first injection port. Then, inject nitrogen from the first injection port at a constant rate v1 until the injection pressure at the first injection port stabilizes, and record the injection pressure p1. During the injection of different fluids, record the model image through a microscope, observe the filter cake thickness in real time, quantitatively evaluate the filter cake damage of the fracturing fluid, and calculate the damage degree of the fracturing fluid to the tight gas reservoir matrix based on the two recorded injection pressures.

[0012] S3. Clean the used micro-etched matrix model, inject fracturing fluid from the second injection port, record the injection pressure curve and model image of the second injection port, and evaluate the fracturing fluid invasion depth into the reservoir and the degree of filter cake damage based on the injection pressure curve and model image;

[0013] S4. Load the microscopic etched fracture model into a high-temperature and high-pressure visual displacement experimental device, inject nitrogen from injection port No. 1 at a constant rate v1 until the injection pressure at injection port No. 1 stabilizes, and record the injection pressure p2; then, inject fracturing fluid breaker from injection port No. 2; after standing, inject nitrogen from injection port No. 1 at a constant rate v1 until the pressure at injection port No. 1 stabilizes, and record the injection pressure p3; record the injection pressure curve during the fracturing fluid breaker injection process and the model images at each stage, and calculate the degree of damage caused by fracturing fluid residue to the cracks in the tight gas reservoir based on the two recorded injection pressures.

[0014] Optionally, a fracturing fluid damage evaluation model for tight gas reservoirs can be constructed by the following steps:

[0015] S11. Drill a rock sample from the target tight gas reservoir;

[0016] S12, preparing a casting thin section from the rock sample, and photographing the casting thin section through a microscope to obtain an image of the casting thin section;

[0017] S13. Converting the casting thin section image into a data file using image analysis software to obtain a distribution image of rock pores in a two-dimensional plane;

[0018] S14. A damage evaluation model for fracturing fluid to tight gas reservoirs is produced based on the data file and the distribution image by laser etching technology.

[0019] Optionally, in step S1, the method for determining the average pore size and the average fracture aperture of the rock sample is a nano-CT scanning method or a nuclear magnetic resonance method.

[0020] Optionally, in steps S2 and S4, the injection rate of nitrogen and fracturing fluid filtrate is 0.1 μL / min.

[0021] Optionally, in steps S2 and S3, the fracturing fluid filtrate is an anionic polyacrylamide solution filtered through a rock sample.

[0022] Optionally, in step S2, the damage degree η0 of the fracturing fluid to the tight gas reservoir matrix is Where p0 is the stable pressure after nitrogen injection, and p1 is the stable pressure after nitrogen injection after the fracturing fluid filtrate is injected and then allowed to stand.

[0023] Optionally, in step S4, the damage degree η1 of the fracturing fluid residue to the cracks of the tight gas reservoir is Among them, p2 is the stable pressure after nitrogen injection, and p3 is the stable pressure after nitrogen injection after the fracturing fluid and gel breaker are injected and left to stand.

[0024] A method for evaluating a fracturing fluid formulation comprises the following steps:

[0025] S21, conducting fault detection experiments on the fracturing fluids with different formulations and the tight gas reservoir damage evaluation model;

[0026] S22. Based on the degree of damage caused by the fracturing fluid to the matrix and cracks of the tight gas reservoir, the fracturing fluid with the least damage is selected as the optimal formula.

[0027] The beneficial effects of the present invention are as follows:

[0028] 1. Traditional experiments cannot directly observe the damage process of fracturing fluid to tight gas reservoirs, resulting in unclear damage mechanisms and the inability to take targeted remedial measures. The present invention can directly observe the dynamic process of fracturing fluid invasion and flowback of different types and components, and can analyze the reservoir damage mechanism of fracturing fluid at a microscopic level.

[0029] 2. The present invention provides a method for evaluating the damage of fracturing fluid to tight gas reservoirs based on a reusable micro-etching model, which solves the shortcomings of existing technologies such as core displacement, such as large randomness, multiple influencing factors, large errors, and poor repeatability. The evaluation results are more accurate and reliable, which is conducive to the implementation of parallel testing.

[0030] 3. The present invention designs two microscopic etching models and, by optimizing the test procedures, realizes the full-process evaluation of the damage of different types and components of fracturing fluids to the reservoir matrix, the damage of fracturing fluid filter cake to the reservoir, and the damage of fracturing fluid residue to the reservoir fractures. The evaluation results are more systematic and comprehensive. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 The present invention is a schematic flow chart of an embodiment of a method for evaluating damage of fracturing fluid to tight gas reservoirs.

[0033] Figure 2 It is a flow chart of an embodiment of a method for preparing a model for evaluating damage of fracturing fluid to tight gas reservoirs in an embodiment of the present invention.

[0034] Figure 3 This is a pore plane distribution diagram obtained after image analysis software processing in an embodiment of the present invention.

[0035] Figure 4 It is a planar structural diagram of the microscopic etching matrix model in an embodiment of the present invention.

[0036] Figure 5 1 is a side view of a microscopic etched matrix model in an embodiment of the present invention.

[0037] Figure 6 It is a planar structural diagram of the microscopic etching crack model in an embodiment of the present invention.

[0038] Figure 7 This is a partially enlarged image of the second etched area of ​​the micro-etched matrix model after the fracturing fluid filtrate test in an embodiment of the present invention.

[0039] Figure 8 This is the injection pressure curve during the injection of fracturing fluid into the micro-etched matrix model in the embodiment of the present invention.

[0040] Figure 9 This is a partially enlarged image of the second etched area of ​​the micro-etched matrix model after the fracturing fluid test in the embodiment of the present invention is completed.

[0041] Figure 10 This is a partially enlarged image of the second etched area of ​​the micro-etched fracture model after the fracturing fluid residue test in the embodiment of the present invention is completed.

[0042] Among them, 1-first injection port, 11-No. 1 injection port, 2-first etching area, 21-No. 1 etching area, 3-second etching area, 31-No. 2 etching area, 4-third etching area, 41-third etching area, 5-first sealing frame, 51-No. 1 sealing frame, 6-second injection port, 61-No. 2 injection port, 7-groove. DETAILED DESCRIPTION

[0043] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other.

[0044] The following detailed description is an exemplary description, which is intended to provide further detailed description of the present invention. Unless otherwise indicated, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art. The terms used in the present invention are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present invention.

[0045] like Figure 4 、 Figure 5 and Figure 6 As shown, a fracturing fluid damage evaluation model for tight gas reservoirs includes a microscopic etching matrix model and a microscopic etching crack model;

[0046] The microscopic etching matrix model includes a first injection port 1, a first etching area 2, a second etching area 3, a third etching area 4, a second injection port 6, and a first sealing frame 5. The first etching area 2 is located on one side of the first injection port 1 of the microscopic etching matrix model, and is connected to the first injection port 1. The depth of the first etching area 2 is less than the depth of the first injection port 1. The second etching area 3 is connected to the first etching area 2. The second etching area 3 is etched according to the data file, and the etching depth is the average pore size of the rock sample. The third etching area 4 is connected to the second etching area 3, and the depth increases linearly from the second etching area 3 to the second injection port 6 from the average pore size.

[0047] The microscopic etching crack model includes an injection port 11, an etching area 21, an etching area 31, an etching area 41, an injection port 61, a groove 7 and a sealing frame 51. The etching area 21 is located on one side of the injection port 11 and is connected to the injection port 11. The depth of the etching area 21 is less than the depth of the etching area 21 to the injection port 11. The etching area 31 is connected to the etching area 21. The etching area 31 is etched according to the data file, and the etching depth is the average pore size of the rock sample. The groove 7 is etched in the etching area 31, connecting the etching area 21 and the etching area 41. The depth and width of the groove 7 are both the average crack opening of the rock sample. The etching area 41 is connected to the etching area 31. The depth of the etching area 41 increases linearly from the average crack opening from the etching area 31 to the injection port 61.

[0048] like Figure 1 As shown, this embodiment provides a method for evaluating damage of fracturing fluid to tight gas reservoirs, comprising the following steps:

[0049] S1. Obtaining a damage evaluation model for fracturing fluid to tight gas reservoirs, including a microscopic etching matrix model and a microscopic etching crack model;

[0050] Specifically, if Figure 2 The steps for making the damage evaluation model of fracturing fluid to tight gas reservoirs are as follows:

[0051] S11. Drill a rock sample from a target tight gas reservoir; specifically, measure the average pore diameter and average fracture aperture of the rock sample using an experimental instrument. In this embodiment, a rock core sample from a tight gas reservoir in the Ordos Basin was drilled. CT scanning revealed that the average pore diameter of the rock sample was 69.5 nm, and the average fracture aperture was 162 μm.

[0052] S12, preparing a casting thin section from the rock sample, and photographing the casting thin section through a microscope to obtain an image of the casting thin section;

[0053] S13. Converting the casting thin section image into a data file using image analysis software to obtain a distribution image of rock pores in a two-dimensional plane;

[0054] S14. A damage evaluation model of fracturing fluid to tight gas reservoirs is obtained by laser etching technology based on the data file and distribution image. Figure 3 This is a pore plane distribution diagram obtained after image analysis software processing in an embodiment of the present invention.

[0055] In this example, a rock sample was prepared into a cast thin section, which was photographed using a microscope to obtain a cast thin section image. Subsequently, the cast thin section image was converted into a data file using image analysis software to obtain a two-dimensional distribution image of the rock pores. Figure 3 This is a pore plane distribution diagram obtained after image analysis software processing in an embodiment of the present invention. A fracturing fluid damage evaluation model for tight gas reservoirs is produced based on the data file using laser etching technology. The model material is glass.

[0056] like Figure 4 and Figure 5 As shown in the figure, as a specific example, the structure of the micro-etching matrix model includes: a first injection port 1, a first etching area 2, a second etching area 3, a third etching area 4, a second injection port 6 and a first sealing frame 5; the etching depth of the first etching area 2 is 1.0 mm; the second etching area 3 is etched according to the data file, and the etching depth is the average pore size of the rock sample; the etching length of the third etching area 4 is 0.5-2.0 cm, and the depth increases linearly from the average pore size to 1.0 mm from the second etching area 3 to the second injection port 6;

[0057] In the microscopic etching crack model, three grooves 7 are etched in the second etching area 31 to connect the first etching area 21 and the third etching area 41. The depth and width of the grooves 7 are both the average crack opening of the rock sample. The etching depth of the third etching area 41 increases linearly from the average crack opening to 1.0 mm from the second etching area 31 to the second injection port 61.

[0058] In this embodiment, the etched area within the micro-etched matrix model is 52 mm long and 20 mm wide. The first etched area 2 is etched to a depth of 1.0 mm and a length of 2.0 mm. The second etched area 3 is etched according to the etching file to a depth of 69.5 nm and a length of 30 mm. The third etched area 4 is etched to a length of 20 mm, with the depth increasing linearly from 69.5 nm to 1.0 mm from the second etched area 3 toward the second injection port 6. The etched model is packaged to obtain a micro-etched matrix model.

[0059] In this embodiment, three grooves 7 are etched in the second etching area 31 to connect the first etching area 21 and the third etching area. The depth and width of the grooves 7 are both 162 μm. The etching depth of the third etching area increases linearly from 162 μm to 1.0 mm from the second etching area 3 to the second injection port 61. The other parameters are consistent with the microscopic etching matrix model. The microscopic etching crack model is encapsulated, as shown in FIG. Figure 6 It is a planar structural diagram of the microscopic etching crack model in an embodiment of the present invention.

[0060] As a specific example, the method for determining the average pore size and the average fracture aperture of the rock sample can be a nano-CT scanning method or a nuclear magnetic resonance method.

[0061] As a specific example, the material of the micro-etching model is transparent heat-resistant and pressure-resistant glass.

[0062] In step S2, the microscopic etched matrix model is loaded into a high-temperature and high-pressure visual displacement experimental device, nitrogen is injected into the first injection port 1 at a constant rate v1 until the pressure of the first injection port 1 is stable, and the injection pressure p0 is recorded. Subsequently, the fracturing fluid filtrate is injected into the second injection port 6. The injection is stopped after the fracturing fluid filtrate appears at the first injection port 1. Nitrogen is injected from the first injection port 1 at a constant rate v1 until the injection pressure of the first injection port 1 is stable, and the injection pressure p1 is recorded. During the injection of different fluids, the model image is recorded through a microscope, the thickness of the filter cake is observed in real time, the filter cake damage of the fracturing fluid is quantitatively evaluated, and the degree of damage of the fracturing fluid to the tight gas reservoir matrix is ​​calculated based on the two recorded injection pressures;

[0063] The use of fracturing fluid filtrate in step S2 simulates the portion of the fracturing fluid that is absorbed by the formation upon injection. This portion of fluid may alter the permeability, porosity, pore throat structure, and other aspects of the formation, thereby affecting the flow conditions of oil and gas. By comparing the injection pressures before and after injection of the fracturing fluid filtrate, the degree of damage to the tight gas reservoir matrix by the fracturing fluid filtrate can be evaluated, and further, the degree of damage to the tight gas reservoir matrix by the fracturing fluid filtrate can be evaluated.

[0064] Specifically, the method for loading the microscopic etched matrix model into the high-temperature and high-pressure visual displacement experimental device is as follows: the microscopic etched matrix model is placed in a transparent temperature-resistant and pressure-resistant glass tube in the high-temperature and high-pressure visual displacement experimental device and fixed with sealant; the glass tube is then connected to two injection pumps, and connected to the first injection port 1 and the second injection port 6 respectively; finally, the glass tube is connected to the pressure sensor and the data acquisition instrument, and the glass tube is photographed with a camera.

[0065] In this embodiment, the fracturing fluid sample is an anionic polyacrylamide solution with a mass concentration of 0.2%. The microscopic etched matrix model is placed in a high-temperature and high-pressure visual displacement experimental device, and the first injection port 1 and the second injection port 6 are respectively connected to the fluid pipeline system of the high-temperature and high-pressure microscopic visual displacement experimental device.

[0066] Nitrogen was injected through the first injection port 1 at a rate of 0.1 μL / min until the injection pressure stabilized. The injection pressure p0 was recorded as 82 kPa. Subsequently, the rock sample-filtered fracturing fluid filtrate was injected through the second injection port 6 until the fracturing fluid filtrate appeared at the first injection port 1. Injection was stopped. After 2 hours of quiescence, nitrogen was injected through the first injection port 1 at a rate of 0.1 μL / min until the injection pressure stabilized. The injection pressure p1 was recorded as 107 kPa. Images of the model were recorded using a microscope during the injection of the various fluids.

[0067] In this example, the fracturing fluid filtrate used was an anionic polyacrylamide solution filtered through a rock sample, with a concentration of 0.1%. The nitrogen and fracturing fluid filtrate injection rates were both 0.1 μL / min. The degree of damage to the tight gas reservoir matrix caused by the fracturing fluid was calculated using the following formula: η0:

[0068]

[0069] Where p0 is the stable pressure after nitrogen injection, and p1 is the stable pressure after 2 hours of stabilization after injection of the fracturing fluid filtrate and then nitrogen injection. A greater damage degree η0 indicates more severe damage to the reservoir matrix by the fracturing fluid.

[0070] The fracturing fluid filtrate can enter the matrix, but the fracturing fluid residue cannot enter the matrix.

[0071] The calculated damage degree η0 of the fracturing fluid to the tight gas reservoir matrix is ​​23.36%. like Figure 7 This is a partially enlarged image of the second etched area 3 of the micro-etched matrix model after the fracturing fluid filtrate test in the embodiment. It can be seen that the fracturing fluid filtrate is retained in the control, which reduces the permeability of the model and causes damage to the reservoir.

[0072] S3. Clean the used micro-etched matrix model, inject fracturing fluid from the second injection port 6, and record the injection pressure curve and model image of the second injection port 6. Based on the injection pressure curve and model image, evaluate the fracturing fluid invasion depth into the reservoir and the degree of filter cake damage;

[0073] Specifically, the used micro-etched matrix model is cleaned and dried with high-speed nitrogen gas. The method is as follows: the micro-etched matrix model is removed from the high-temperature, high-pressure, and visualized displacement experimental apparatus and cleaned with alcohol; the surface of the micro-etched matrix model is then dried with high-speed nitrogen gas. The method for injecting fracturing fluid through the second injection port 6 is as follows: the cleaned and dried micro-etched matrix model is reinstalled into the high-temperature, high-pressure, and visualized displacement experimental apparatus and connected to a second injection pump; the fracturing fluid is then injected into the micro-etched matrix model through the second injection port 6, and the injection pressure curve and model image are recorded. In this embodiment, the fracturing fluid used is an anionic polyacrylamide solution with a concentration of 0.1%. The method for evaluating the depth of fracturing fluid invasion into the reservoir and the degree of filter cake damage is as follows: the extent of fracturing fluid invasion within the second etched area 3 is observed and measured based on the model image, as the depth of fracturing fluid invasion into the reservoir; and the filter cake resistance coefficient is observed and calculated at different time points based on the injection pressure curve, as the degree of filter cake damage. The larger the filter cake resistance coefficient, the more severe the filter cake damage.

[0074] In this embodiment, the fracturing fluid is injected from the second injection port 6 at a rate of 0.1 μL / min, and the injection is stopped when the injection pressure increases sharply. The injection pressure curve and model image during the injection process are recorded. Figure 8 This is the injection pressure curve of the micro-etched matrix model fracturing fluid injection process in the embodiment of the present invention. It can be seen that the fracturing fluid has high seepage resistance in the micro-etched matrix model and is difficult to enter the matrix. Therefore, the fracturing fluid mainly damages the matrix through the filtrate. Figure 9 This is a partially enlarged image of the second etched area 3 of the micro-etched matrix model after the fracturing fluid test in the embodiment of the present invention. Figure 9 The flow front of the fracturing fluid in the model can be clearly observed, and the flow position of the fracturing fluid in the model can be observed.

[0075] S4. Load the microscopic etched fracture model into a high-temperature and high-pressure visualized displacement experimental device, inject nitrogen from injection port 11 at a constant rate v1 until the injection pressure of injection port 11 stabilizes, and record the injection pressure p2; then, inject fracturing fluid breaker from injection port 61; after standing, inject nitrogen from injection port 11 at a constant rate v1 until the pressure of injection port 11 stabilizes, and record the injection pressure p3; record the injection pressure curve during the fracturing fluid breaker injection process and the model images at each stage, and calculate the degree of damage caused by the fracturing fluid residue to the cracks in the tight gas reservoir based on the two recorded injection pressures.

[0076] The purpose of using the fracturing fluid breaker in step S4 is to simulate the damage degree of the fracturing fluid residue to the cracks of the tight gas reservoir by comparing the injection pressure before and after the injection of the fracturing fluid breaker after fracturing is completed, and then evaluate the damage degree of the fracturing fluid to the cracks of the tight gas reservoir.

[0077] Specifically, the method for replacing the microscopic etched matrix model with the microscopic etched fracture model is as follows: the microscopic etched matrix model is removed from the high-temperature and high-pressure visual displacement experimental device and replaced with the microscopic etched fracture model; the microscopic etched fracture model is then connected to two injection pumps, and connected to the No. 1 injection port 11 and the No. 2 injection port 61, respectively. The method for injecting nitrogen and fracturing fluid breaker liquid through the No. 1 injection port 11 and the No. 2 injection port 61, respectively, is as follows: first, nitrogen is injected into the microscopic etched fracture model through the No. 1 injection port 11 until the injection pressure reaches a stable value p2, and the injection pressure and model image are recorded; then, the fracturing fluid breaker liquid is injected into the microscopic etched fracture model through the No. 2 injection port 61 until the injection pressure reaches a stable value, and the injection pressure and model image are recorded; finally, after standing for 2 hours, nitrogen is injected into the microscopic etched fracture model through the No. 1 injection port 11 again until the injection pressure reaches a stable value p3, and the injection pressure and model image are recorded. The fracturing fluid breaker used was an anionic polyacrylamide solution at a concentration of 0.1%, with 0.5% hydrochloric acid added as a breaker. Both nitrogen and the fracturing fluid breaker were injected at a rate of 0.1 μL / min. The degree of damage to tight gas reservoir fractures caused by fracturing fluid residue was calculated using the following formula:

[0078]

[0079] Where p2 is the stable pressure after nitrogen injection, and p3 is the stable pressure after injecting the fracturing fluid, leaving it to stand for 2 hours, and then injecting nitrogen. A greater damage level η1 indicates more severe damage to reservoir fractures caused by fracturing fluid residue.

[0080] In this embodiment, nitrogen is injected from the No. 1 injection port 11 at a rate of 0.1 μL / min until the injection pressure stabilizes, and the injection pressure p2 is recorded as 19 kPa; then, fracturing fluid breaker is injected from the No. 2 injection port 61 until it flows out of the No. 1 injection port 11 and the injection is stopped; after standing for 2 hours, nitrogen is injected from the No. 1 injection port 11 at a rate of 0.1 μL / min until the pressure stabilizes, and the injection pressure is recorded as 20 kPa; the injection pressure curve during the fracturing fluid breaker injection process and the model images of each stage are recorded; the damage degree η1 of the fracturing fluid residue to the tight gas reservoir cracks is calculated to be 5%, indicating that the damage degree of the fracturing fluid residue to the tight gas reservoir cracks is relatively small. Figure 10This is a partially enlarged image of the second etched area 31 of the model after the fracturing fluid residue test of the micro-etched crack model in the embodiment of the present invention. It can be seen that there is no fracturing fluid residue left in the groove 7 in the middle of the model.

[0081] This embodiment is also used to evaluate fracturing fluid formulations for tight gas reservoirs. The steps are as follows: fracturing fluids with different formulations are used to make microscopic etching models and conduct fault detection experiments, respectively. Based on the degree of damage to the matrix and cracks of the tight gas reservoir by the fracturing fluid, the fracturing fluid with the least damage is selected as the optimal formulation.

[0082] This method enables intuitive observation of the dynamic processes of fracturing fluid invasion and flowback, analyzing the reservoir damage mechanisms of fracturing fluids at a microscopic level. By designing two microscopic etching models and optimizing the testing procedures, it enables a comprehensive and systematic evaluation of damage to the reservoir matrix, damage to the reservoir caused by fracturing fluid filter cake, and damage to reservoir fractures caused by fracturing fluid residues.

[0083] It is understood from common technical knowledge that the present invention may be implemented by other embodiments that do not depart from its spirit or essential features. Therefore, the embodiments disclosed above are, in all respects, merely illustrative and not exclusive. All modifications within the scope of the present invention or equivalent to the scope of the present invention are intended to be encompassed by the present invention.

Claims

1. A method for evaluating damage of fracturing fluid to tight gas reservoirs, characterized in that: The following steps are involved: S1. Obtaining a fracturing fluid damage evaluation model for tight gas reservoirs, including a microscopic etching matrix model and a microscopic etching crack model; the microscopic etching matrix model includes a first injection port (1), a first etching area (2), a second etching area (3), a third etching area (4), a second injection port (6), and a first sealing frame (5); The first etching zone (2) is located on one side of the first injection port (1) of the microscopic etching matrix model, the first etching zone (2) is connected to the first injection port (1), the depth of the first etching zone (2) is less than the depth of the first injection port (1), the second etching zone (3) is connected to the first etching zone (2), the second etching zone (3) is etched according to the data file, and the etching depth is the average pore size of the rock sample; the third etching zone (4) is connected to the second etching zone (3), and the depth increases linearly from the second etching zone (3) to the second injection port (6) from the average pore size; The microscopic etching crack model comprises a No. 1 injection port (11), a No. 1 etching area (21), a No. 2 etching area (31), a No. 3 etching area (41), a No. 2 injection port (61), a groove (7) and a No. 1 sealing frame (51). The first etching area (21) is located on one side of the first injection port (11), the first etching area (21) is connected to the first injection port (11), the depth of the first etching area (21) is less than the depth of the first injection port (11), the second etching area (31) is connected to the first etching area (21), the second etching area (31) is etched according to the data file, and the etching depth is the average pore size of the rock sample, the groove (7) is etched in the second etching area (31), connecting the first etching area (21) and the third etching area (41), the depth and width of the groove (7) are both the average crack opening of the rock sample; the third etching area (41) is connected to the second etching area (31), and the depth of the third etching area (41) increases linearly from the average crack opening in the direction from the second etching area (31) to the second injection port (61); S2. Load the microscopic etched matrix model into the high temperature and high pressure visual displacement experimental device, and inject the first injection port (1) at a constant speed. v 1. Inject nitrogen into the first injection port (1) until the pressure is stable and record the injection pressure. p 0, then inject the fracturing fluid filtrate into the second injection port (6), stop injecting when the fracturing fluid filtrate appears at the first injection port (1), and inject nitrogen at a constant rate from the first injection port (1). v 1. Inject nitrogen into the first injection port (1) until the injection pressure is stable and record the injection pressure. p 1. During the injection of different fluids, the model images were recorded under a microscope, the filter cake thickness was observed in real time, the filter cake damage caused by the fracturing fluid was quantitatively evaluated, and the degree of damage to the tight gas reservoir matrix was calculated based on the injection pressures recorded twice; S3, cleaning the used micro-etched matrix model, injecting fracturing fluid from the second injection port (6), and recording the injection pressure curve and model image of the second injection port (6), and evaluating the fracturing fluid invasion depth into the reservoir and the degree of filter cake damage based on the injection pressure curve and model image; S4. Load the microscopic etched crack model into the high temperature and high pressure visual displacement experimental device, and inject it into the No. 1 injection port (11) at a constant speed. v 1. Inject nitrogen into the No. 1 injection port (11) until the injection pressure is stable and record the injection pressure. p 2; Subsequently, the fracturing fluid and gel breaking liquid are injected from the second injection port (61); after standing still, the fracturing fluid is injected from the first injection port (11) at a constant speed. v 1. Inject nitrogen until the pressure of injection port No. 1 (11) is stable and record the injection pressure. p 3. Record the injection pressure curve and model images of each stage during the fracturing fluid and gel breaker injection process, and calculate the damage degree of fracturing fluid residue to the cracks of tight gas reservoirs based on the two recorded injection pressures.

2. The method according to claim 1, characterized in that The damage evaluation model of fracturing fluid to tight gas reservoirs was developed according to the following steps: S11. Drill a rock sample from the target tight gas reservoir; S12, preparing a casting thin section from the rock sample, and photographing the casting thin section through a microscope to obtain an image of the casting thin section; S13. Converting the casting thin section image into a data file using image analysis software to obtain a distribution image of rock pores in a two-dimensional plane; S14. A damage evaluation model for fracturing fluid to tight gas reservoirs is produced based on the data file and the distribution image by laser etching technology.

3. The method according to claim 1, characterized in that In step S1, the average pore size and the average fracture aperture of the rock sample are determined by a nano-CT scanning method or a nuclear magnetic resonance method.

4. The method according to claim 1, wherein In steps S2 and S4, the injection rate of nitrogen and fracturing fluid filtrate is 0.1 μL / min.

5. The method according to claim 1, characterized in that In steps S2 and S3, the fracturing fluid filtrate is an anionic polyacrylamide solution filtered through a rock sample.

6. The method according to claim 1, characterized in that In step S2, the damage degree of the fracturing fluid to the tight gas reservoir matrix , ,in is the stable pressure after nitrogen injection, It is the stable pressure after injecting fracturing fluid filtrate, letting it stand and then injecting nitrogen.

7. The method according to claim 1, characterized in that In step S4, the damage degree of the fracturing fluid residue to the cracks of the tight gas reservoir is , ,in is the stable pressure after nitrogen injection, It is the stable pressure after injecting fracturing fluid and gel-breaking fluid, leaving it to stand and then injecting nitrogen.

8. The method according to claim 1, characterized in that The microscopic etching matrix model and the microscopic etching crack model are made of transparent heat-resistant and pressure-resistant glass.

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