Model and method for evaluating damage of fracturing fluid to tight gas reservoir

Abstract

The invention relates to the field of unconventional oil and gas field development, in particular to a model and a method for evaluating damage of fracturing fluid to a tight gas reservoir, the model for evaluating damage of fracturing fluid to the tight gas reservoir is manufactured and comprises a micro-etching matrix model and a micro-etching crack model, nitrogen and fracturing fluid filtrate are injected, injection pressure and model images are recorded, and the model is used for evaluating damage of fracturing fluid to the tight gas reservoir. Calculating the damage degree of the fracturing fluid to the tight gas reservoir matrix; replacing the microcosmic etching matrix model with a microcosmic etching crack model, injecting nitrogen and fracturing fluid gel breaking fluid, recording injection pressure and a model image, and calculating the damage degree of fracturing fluid residues to tight gas reservoir cracks. A reusable microscopic visual etching model is established to simulate a tight gas reservoir, and on the basis of the model, a corresponding evaluation test program is established to evaluate fracturing fluid reservoir damage, so that the technical problems that in the prior art, test fluid is not representative, damage mechanism evaluation is insufficient, errors are large, and repeatability is poor are solved.

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 the damage of fracturing fluid to tight gas reservoirs. Background Art

[0002] Tight gas reservoirs have well-developed nano- and micro-pore throats, poor pore throat connectivity, and low matrix permeability. During the development process, there are common problems such as low single-well production, short stable production period, rapid production decline, and low recovery degree. Therefore, tight gas reservoirs usually require fracturing and production-increasing measures to achieve economic development. However, during the fracturing construction process, the fracturing fluid will cause a certain degree of damage to the reservoir, reducing the fracturing and production-increasing effect. Especially for newly developed blocks, the damage of fracturing fluid to the reservoir has become a bottleneck restricting the efficient exploration and development of tight gas reservoirs, and has seriously affected the timely discovery, accurate evaluation, and efficient development of tight gas reservoirs. Therefore, it is crucial to scientifically, systematically, and accurately evaluate the degree of damage of fracturing fluid to tight gas reservoirs and clarify its damage mechanism.

[0003] At present, the damage evaluation of fracturing fluid to tight gas reservoirs is mainly based on the petroleum industry standards for conventional oil and gas reservoirs. However, these conventional evaluation methods and standards are not completely applicable to the damage evaluation of tight gas reservoirs. For example, "SY / T 5358-2010 Reservoir Sensitivity Flow Experiment Evaluation Method" is applicable to clastic reservoirs with air permeability>1mD, and uses liquid phase to test the permeability of cores; "SY / T 5107-2005 Water-based Fracturing Fluid Performance Evaluation Method" uses fracturing fluid degelling fluid to displace cores, and the permeability test requires the pressure and flow rate to be stable for up to 1h. In summary, the existing technology has the following main problems in evaluating tight gas reservoir damage: ① The test fluid is not representative; the existing technology uses liquid phase as the test medium. The gas phase in the tight gas reservoir is the main mobile phase and production object. After the gas layer is damaged, the main focus is on the change in the gas phase seepage capacity. The use of liquid phase permeability as an evaluation parameter lacks pertinence and representativeness, and the use of liquid phase to test permeability has problems such as high test pressure, long experimental cycle, and large test data error; ② The evaluation parameters are single, and the understanding of the essential laws of the gas layer damage mechanism is not in-depth; the parameters of the core displacement experiment of the existing technology are mainly the permeability reduction rate, and the in-depth evaluation of the reservoir damage mechanism is lacking, resulting in an unsystematic and incomplete understanding of the reservoir damage mechanism, and the implementation of reservoir protection measures may be unreasonable; ③ The test results are affected by many factors, large errors, and poor repeatability; The core displacement experiment is highly random, with many influencing factors, and the core cannot be reused, making it difficult to obtain accurate evaluation results, especially when it involves parallel testing and evaluation of multiple fracturing fluid systems. The randomness and non-repeatability of core samples make it difficult to obtain correct evaluation conclusions. 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. A reusable microscopic visual etching model is established to simulate the tight gas reservoir, and on the basis of 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 prior art 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 microscopic 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 microscopic 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 the 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 from 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, the etching area No. 1 is connected with the injection port No. 1, the depth of the etching area No. 1 is less than the depth of the injection port No. 1, the etching area No. 2 is connected with the etching area No. 1, the etching area No. 2 is etched according to a 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, and the depth and width of the groove are both the average crack opening of the rock sample; the etching area No. 3 is connected with 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 to the injection port No. 2.

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

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

[0010] S1. Obtaining a damage evaluation model of 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 visualized displacement experimental device, inject nitrogen into the first injection port at a constant speed v1 until the pressure of the first injection port is stable, record the injection pressure p0, then inject the fracturing fluid filtrate into the second injection port, stop the injection after the fracturing fluid filtrate appears at the first injection port, inject nitrogen from the first injection port at a constant speed v1 until the injection pressure of the first injection port is stable, record the injection pressure p1, record the model image through a microscope during the injection of different fluids, observe the thickness of the filter cake 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, cleaning the used microscopic etched matrix model, injecting fracturing fluid from the second injection port, and recording the injection pressure curve and model image of the second injection port, and evaluating the fracturing fluid invasion depth into the reservoir and the degree of filter cake damage according to the injection pressure curve and the model image;

[0013] S4. Load the microscopic etched fracture model into a high-temperature and high-pressure visualized 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 degelling liquid 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 injection of fracturing fluid degelling liquid and the model images at each stage, and calculate the degree of damage of fracturing fluid residue to the cracks of tight gas reservoirs based on the two recorded injection pressures.

[0014] Optionally, the fracturing fluid damage evaluation model for tight gas reservoirs is prepared according to the following steps:

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

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

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

[0018] S14. A damage evaluation model of 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 rate of injecting 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 left to stand.

[0023] Optionally, in step S4, the degree of damage of the fracturing fluid residue to the cracks of the tight gas reservoir is η1, Among them, p2 is the stable pressure after nitrogen injection, and p3 is the stable pressure after nitrogen injection after fracturing fluid and gel-breaking fluid 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 fluid with different formulations and the fracturing fluid damage evaluation model for tight gas reservoirs;

[0026] S22. According to the degree of damage of the fracturing fluid to the matrix and cracks of the tight gas reservoir, the fracturing fluid with the least degree of 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 mechanism and inability to take targeted solutions. The present invention can directly observe the dynamic process of fracturing fluid invasion and backflow 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, many 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 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 by optimizing the test procedure. The evaluation results are more systematic and comprehensive. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the 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 schematic diagram of a flow chart 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 It 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 a 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 a microscopic etching crack model in an embodiment of the present invention.

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

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

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

[0041] Fig.10 This is a partially enlarged image of the No. 2 etched area of ​​the model after the fracturing fluid residue test of the micro-etched fracture model 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 the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0044] The following detailed description is an exemplary description, which is intended to provide further detailed description of the present invention. Unless otherwise specified, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art to which the present application belongs. The terms used in the present invention are only for describing specific embodiments, and are not intended to limit 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, the first etching area 2 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 the etching area 21 is connected with the injection port 11. The depth of the etching area 21 is less than the depth of the etching area 21 with the injection port 11. The etching area 31 is connected with 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 with the etching area 31, and 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 the damage of fracturing fluid to tight gas reservoirs, comprising the following steps:

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

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

[0051] S11. Drill a rock sample of the target tight gas reservoir; specifically, measure the average pore size and average fracture opening of the rock sample by using an experimental instrument. In this embodiment, a rock core of a tight gas reservoir in the Ordos Basin is drilled, and the average pore size of the rock sample is 69.5 nm and the average fracture opening is 162 μm by CT scanning test.

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

[0053] S13, converting the casting thin section image into a data file by 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 It is a pore plane distribution diagram obtained after image analysis software processing in an embodiment of the present invention.

[0055] In this embodiment, the rock sample is made into a casting thin section, and the casting thin section is photographed by a microscope to obtain a casting thin section image. Subsequently, the casting thin section image is converted into a data file by image analysis software to obtain a distribution image of rock pores in a two-dimensional plane. Figure 3 It is a pore plane distribution diagram obtained after image analysis software processing in the embodiment of the present invention. A fracturing fluid damage evaluation model for tight gas reservoirs is produced by laser etching technology according to the data file, and the model material is glass.

[0056] like Figure 4 and Figure 5 As shown, as a specific example, the structure of 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 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 etching area inside the microscopic etching matrix model is 52 mm long and 20 mm wide, the first etching area 2 is etched with a depth of 1.0 mm and a length of 2.0 mm; the second etching area 3 is etched according to the etching file, with an etching depth of 69.5 nm and a length of 30 mm; the third etching area 4 is etched with a length of 20 mm, and the depth increases linearly from 69.5 nm to 1.0 mm from the second etching area 3 to the second injection port 6. The etching model is packaged to obtain a microscopic etching 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 a 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 may be a nano CT scanning method or a nuclear magnetic resonance method.

[0061] As a specific example, the material of the microscopic 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 visualized displacement experimental device, nitrogen is injected into the first injection port 1 at a constant speed v1 until the pressure of the first injection port 1 is stable, and the injection pressure p0 is recorded, and then the fracturing fluid filtrate is injected into the second injection port 6, and the injection is stopped after the fracturing fluid filtrate appears in the first injection port 1, and nitrogen is injected from the first injection port 1 at a constant speed 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, and the filter cake damage of the fracturing fluid is quantitatively evaluated. The degree of damage of the fracturing fluid to the tight gas reservoir matrix is ​​calculated according to the injection pressures recorded twice;

[0063] The use of the fracturing fluid filtrate in step S2 is to simulate the part of the fracturing fluid absorbed by the formation when it is injected into the formation. This part of the fluid may change the permeability, porosity, pore throat structure, etc. of the formation, thereby affecting the flow conditions of oil and gas. By comparing the injection pressure before and after the injection of the fracturing fluid filtrate, the degree of damage of the fracturing fluid filtrate to the tight gas reservoir matrix can be evaluated, and then the degree of damage of the fracturing fluid to the tight gas reservoir matrix can be evaluated.

[0064] Specifically, the method for loading the microscopic etching matrix model into the high-temperature and high-pressure visualized displacement experimental device is as follows: placing the microscopic etching matrix model in a transparent temperature-resistant and pressure-resistant glass tube in the high-temperature and high-pressure visualized displacement experimental device, and fixing it with a sealant; then connecting the glass tube to two injection pumps, and connecting them to the first injection port 1 and the second injection port 6 respectively; finally, connecting the glass tube to the pressure sensor and the data acquisition instrument, and photographing the glass tube with a camera.

[0065] In this embodiment, the fracturing fluid sample of this embodiment is an anionic polyacrylamide solution with a mass concentration of 0.2%. The microscopic etched matrix model is loaded into the 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 from the first injection port 1 at a rate of 0.1 μL / min until the injection pressure was stable, and the injection pressure p0 was recorded as 82 kPa. Subsequently, the fracturing fluid filtrate filtered by the rock sample was injected from the second injection port 6 until the fracturing fluid filtrate appeared at the first injection port 1 and the injection was stopped. After standing for 2 hours, nitrogen was injected from the first injection port 1 at a rate of 0.1 μL / min until the injection pressure was stable, and the injection pressure p1 was recorded as 107 kPa. The model image was recorded by microscope during the injection of different fluids.

[0067] In this embodiment, the fracturing fluid filtrate used is an anionic polyacrylamide solution filtered through a rock sample, and its concentration is 0.1%. The injection rate of nitrogen and fracturing fluid filtrate is 0.1 μL / min. The method for calculating the degree of damage of fracturing fluid to the matrix of the tight gas reservoir is as follows: the degree of damage of fracturing fluid to the matrix of the tight gas reservoir is calculated according to the following formula η0:

[0068]

[0069] Among them, p0 is the stable pressure after nitrogen injection, and p1 is the stable pressure after nitrogen injection after 2 h of standing after injection of fracturing fluid filtrate. The greater the damage degree η0, the more serious the damage of fracturing fluid to the reservoir matrix.

[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 model after the fracturing fluid filtrate test of the micro-etched matrix model 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, cleaning the used microscopic 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 according to the injection pressure curve and the model image;

[0073] Specifically, the used microscopic etching matrix model is cleaned and dried with high-speed nitrogen, and the method is: the microscopic etching matrix model is taken out from the high-temperature and high-pressure visual displacement experimental device, and cleaned with alcohol; then the surface of the microscopic etching matrix model is dried with high-speed nitrogen. The method of injecting fracturing fluid from the second injection port 6 is: the cleaned and dried microscopic etching matrix model is reloaded into the high-temperature and high-pressure visual displacement experimental device, and connected to the second injection pump; then the fracturing fluid is injected into the microscopic etching matrix model from 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 intrusion into the reservoir and the degree of filter cake damage is: observe and measure the intrusion range of the fracturing fluid in the second etching area 3 according to the model image, as the depth of fracturing fluid intrusion into the reservoir; observe and calculate the filter cake resistance coefficient at different time points according to the injection pressure curve, as the degree of filter cake damage. The larger the filter cake resistance coefficient, the more serious 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, and the injection pressure curve and model image during the injection process are recorded. Figure 8 : is the injection pressure curve of the microscopic 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 microscopic etched matrix model and is difficult to enter the matrix, so the fracturing fluid mainly damages the matrix through the filtrate. Fig. 9 This is a partial enlarged image of the second etched area 3 of the model after the microscopic etched matrix model fracturing fluid test in the embodiment of the present invention is completed. Fig. 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 the No. 1 injection port 11 at a constant speed v1 until the injection pressure of the No. 1 injection port 11 is stable, and record the injection pressure p2; then, inject fracturing fluid degelling liquid from the No. 2 injection port 61; after standing, inject nitrogen from the No. 1 injection port 11 at a constant speed v1 until the pressure of the No. 1 injection port 11 is stable, and record the injection pressure p3; record the injection pressure curve during the injection of the fracturing fluid degelling liquid and the model images at each stage, and calculate the degree of damage of the fracturing fluid residue to the cracks of the tight gas reservoir based on the two recorded injection pressures.

[0076] The use of the fracturing fluid degelling liquid 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 degelling liquid after fracturing, and then evaluate the damage degree of the fracturing fluid to the cracks of the tight gas reservoir.

[0077] Specifically, the method of replacing the microscopic etching matrix model with the microscopic etching fracture model is as follows: the microscopic etching matrix model is taken out from the high temperature and high pressure visual displacement experimental device and replaced with the microscopic etching fracture model; then the microscopic etching fracture model is connected to two injection pumps, and respectively connected to the No. 1 injection port 11 and the No. 2 injection port 61. The method of injecting nitrogen and fracturing fluid degelling liquid from the No. 1 injection port 11 and the No. 2 injection port 61 respectively is as follows: first, nitrogen is injected into the microscopic etching fracture model from 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 degelling liquid is injected into the microscopic etching fracture model from 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 etching fracture model from the No. 1 injection port 11 until the injection pressure reaches a stable value p3, and the injection pressure and model image are recorded. The fracturing fluid debonding liquid used is an anionic polyacrylamide solution with a concentration of 0.1%, and 0.5% hydrochloric acid is added as a debonding agent. The injection rate of nitrogen and fracturing fluid debonding liquid is 0.1 μL / min. The method for calculating the degree of damage of fracturing fluid residue to the cracks of tight gas reservoirs is as follows: The degree of damage of fracturing fluid residue to the cracks of tight gas reservoirs η1 is calculated according to the following formula:

[0078]

[0079] Among them, p2 is the stable pressure after nitrogen injection, and p3 is the stable pressure after the injection of fracturing fluid degelling fluid and then the injection of nitrogen after 2 hours of quiescence. The greater the damage degree η1, the more serious the damage of fracturing fluid residue to reservoir fractures.

[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 is stable, and the injection pressure p2 is recorded as 19 kPa; then, the fracturing fluid degelling liquid is injected from the No. 2 injection port 61 until it flows out of the No. 1 injection port 11 and then 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 is stable, and the injection pressure is recorded as 20 kPa; the injection pressure curve and model images of each stage during the injection of the fracturing fluid degelling liquid are recorded; the damage degree η1 of the fracturing fluid residue to the cracks of the tight gas reservoir is calculated to be 5%, indicating that the damage degree of the fracturing fluid residue to the cracks of the tight gas reservoir is relatively small. Fig.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 fracture model in the embodiment of the present invention is completed. 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 the formulation of fracturing fluid for tight gas reservoirs, and the steps are: using fracturing fluids of different formulations to make microscopic etching models and conduct fault detection experiments, respectively, and selecting the fracturing fluid with the least damage as the optimal formulation based on the degree of damage of the fracturing fluid to the matrix and cracks of the tight gas reservoir.

[0082] The present invention can directly observe the dynamic process of fracturing fluid invasion and backflow of different types and components, and can analyze the reservoir damage mechanism of fracturing fluid at the micro level. In addition, two microscopic etching models are designed, and by optimizing the test procedure, the full process evaluation of the damage of different types and components of fracturing fluid to the reservoir matrix, the damage of fracturing fluid filter cake to the reservoir, and the damage of fracturing fluid residue to the reservoir fracture is realized, and the evaluation results are more systematic and comprehensive.

[0083] It is known from common technical knowledge that the present invention can be implemented by other embodiments that do not deviate from its spirit or essential features. Therefore, the above disclosed embodiments are only illustrative in all respects and are not exclusive. All changes within the scope of the present invention or within the scope equivalent to the present invention are included in the present invention.

Claims

1. A fracturing fluid damage evaluation model for tight gas reservoirs, characterized in that: Includes microscopic etching matrix model and microscopic etching crack model; The microscopic etching matrix model comprises 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 first injection port (11), a first etching area (21), a second etching area (31), a third etching area (41), a second injection port (61), a groove (7) and a first 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 a data file, 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 fracture opening of the rock sample; the third etching area (41) is connected to the second etching area (31), the depth of the third etching area (41) increases linearly from the average fracture opening in the direction from the second etching area (31) to the second injection port (61).

2. The damage evaluation model of fracturing fluid to tight gas reservoir according to claim 1, characterized in that: The material of the microscopic etching matrix model and the microscopic etching crack model is transparent heat-resistant and pressure-resistant glass.

3. A method for evaluating damage of fracturing fluid to tight gas reservoirs based on claim 1, characterized in that: The following steps are involved: S1. Obtaining a damage evaluation model of fracturing fluid to tight gas reservoirs, including a microscopic etching matrix model and a microscopic etching crack model; S2, loading the microscopic etched matrix model into a high temperature and high pressure visualized displacement experimental device, injecting nitrogen into the first injection port (1) at a constant speed v1 until the pressure of the first injection port (1) is stable, recording the injection pressure p0, then injecting the fracturing fluid filtrate into the second injection port (6), stopping the injection after the fracturing fluid filtrate appears at the first injection port (1), injecting nitrogen from the first injection port (1) at a constant speed v1 until the injection pressure of the first injection port (1) is stable, recording the injection pressure p1, recording the model image through a microscope during the injection of different fluids, observing the thickness of the filter cake in real time, quantitatively evaluating the filter cake damage of the fracturing fluid, and calculating the damage degree of the fracturing fluid to the tight gas reservoir matrix based on the two recorded injection pressures; S3, cleaning the used microscopic etched matrix model, injecting fracturing fluid from the second injection port (6), recording the injection pressure curve and model image of the second injection port (6), and evaluating the penetration depth of the fracturing fluid into the reservoir and the degree of filter cake damage based on the injection pressure curve and the model image; S4. Load the microscopic etched fracture model into a high-temperature and high-pressure visualized displacement experimental device, inject nitrogen from the No. 1 injection port (11) at a constant speed v1 until the injection pressure of the No. 1 injection port (11) is stable, and record the injection pressure p2; then, inject fracturing fluid degelling liquid from the No. 2 injection port (61); after standing, inject nitrogen from the No. 1 injection port (11) at a constant speed v1 until the pressure of the No. 1 injection port (11) is stable, and record the injection pressure p3; record the injection pressure curve during the injection of the fracturing fluid degelling liquid and the model images at each stage, and calculate the degree of damage of the fracturing fluid residue to the cracks of the tight gas reservoir based on the two recorded injection pressures.

4. The method according to claim 3, characterized in that: The damage evaluation model of fracturing fluid to tight gas reservoirs was prepared according to the following steps: S11. Drill a rock sample of the target tight gas reservoir; S12, making a casting thin section from the rock sample, and photographing the casting thin section through a microscope to obtain a casting thin section image; S13, converting the casting thin section image into a data file by using image analysis software to obtain a distribution image of rock pores in a two-dimensional plane; S14. A damage evaluation model of fracturing fluid to tight gas reservoirs is produced based on the data file and the distribution image by laser etching technology.

5. The method according to claim 3, 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.

6. The method according to claim 3, characterized in that In steps S2 and S4, the rate of injecting nitrogen and fracturing fluid filtrate is 0.1 μL / min.

7. The method according to claim 3, characterized in that In steps S2 and S3, the fracturing fluid filtrate is an anionic polyacrylamide solution filtered through a rock sample.

8. The method according to claim 3, characterized in that In step S2, the damage degree of the fracturing fluid to the tight gas reservoir matrix is ​​η0, 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 left to stand.

9. The method according to claim 3, characterized in that: In step S4, the damage degree of the fracturing fluid residue to the tight gas reservoir fractures is η1, Among them, p2 is the stable pressure after nitrogen injection, and p3 is the stable pressure after nitrogen injection after fracturing fluid and gel-breaking fluid are injected and left to stand.

10. A method for evaluating a fracturing fluid formulation according to claim 3, characterized in that: The steps are, S21, conducting fault detection experiments on the fracturing fluid with different formulations and the fracturing fluid damage evaluation model for tight gas reservoirs; S22. According to the degree of damage of the fracturing fluid to the matrix and cracks of the tight gas reservoir, the fracturing fluid with the least degree of damage is selected as the optimal formula.

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