Acid fracturing bursting pressure prediction method considering rock mechanical property acid etching degradation

Abstract

The invention provides an acid fracturing bursting pressure prediction method considering rock mechanical property acid etching degradation. The method comprises the following steps: respectively carrying out a uniaxial compression experiment and a splitting experiment on a rock sample subjected to acid etching; according to the uniaxial compression experiment data and the splitting experiment data, obtaining an acid etching rock mechanical parameter-acid etching temperature-acid etching time model; and integrating the acid etching rock mechanical parameter-acid etching temperature-acid etching time model into a conventional formation fracture pressure model to obtain an acid fracturing fracture pressure prediction model considering rock mechanical property acid etching degradation. According to the method, the influence of the formation temperature and the acid etching time on the reservoir mechanical property is comprehensively considered, the acid fracturing bursting pressure is predicted, and a theoretical basis can be provided for field engineering parameter design.

Description

Technical Field

[0001] The present invention relates to a method for predicting acid fracturing pressure by taking into account the acid corrosion degradation of rock mechanical properties, and belongs to the technical field of oil and gas well engineering. Background Technology

[0002] Deep carbonate formations contain rich oil and gas resources and are one of the key areas of oil and gas exploration and development in the future. However, deep carbonate formations generally have low permeability and natural fractures and caves. Using hydraulic fracturing technology to establish diversion channels is one of the measures for reservoir transformation. Among them, acid fracturing is particularly suitable for carbonate reservoirs and is widely used in actual engineering.

[0003] Carbonate rock acid fracturing is quite different from ordinary hydraulic fracturing. Acid fracturing not only involves fluid-solid coupling, but also chemical reactions between carbonate rock and acid solution, which changes the internal structural characteristics of the reservoir rock matrix, ultimately affecting the changes in rock mechanical parameters and thus achieving the effect of reducing reservoir fracture pressure. Therefore, when predicting the initiation pressure of acid fracturing, it is necessary to take into account the degradation of the mechanical properties of the reservoir rock under the action of acid corrosion.

[0004] In the design of acid fracturing, if the degradation effect caused by rock acidification is not fully considered, the designed fracturing pressure will usually be greater than the actual fracturing pressure. Construction under excessively large design parameters is very likely to cause excessive extension of cracks, resulting in crack cross-layer. The harm of crack cross-layer in acid fracturing is more serious than that of ordinary hydraulic fracturing. The fracturing fluid used in acid fracturing contains a large amount of acid, which will cause significant changes in the physical properties of non-target reservoirs and cause pollution. Part of the oil and gas in the target reservoir will migrate to the cross-layer strata, making it difficult to exploit. If the crack crosses the layer to the groundwater layer, the acid and oil and gas will enter the formation water layer together, which will seriously pollute the formation water. As the formation water flows, the scope of pollution is difficult to control, and the exploitation and use of formation water will directly endanger the health of animals, plants and humans. Therefore, in order to safely and sustainably exploit formation oil and gas resources, the changes in the properties of the reservoir rock after acidification must be considered when using acid fracturing to transform the reservoir.

[0005] Therefore, developing a method to predict the fracture pressure of acid fracturing considering the acid corrosion degradation of rock mechanical properties has become one of the urgent problems to be solved in this field. SUMMARY OF THE INVENTION

[0006] To solve the above technical problems, the purpose of the present invention is to provide a method for predicting the acid fracturing pressure of acid fracturing considering the acid corrosion degradation of rock mechanical properties. This method comprehensively considers the influence of formation temperature and acid corrosion time on reservoir mechanical properties, predicts the acid fracturing pressure of acid fracturing, and can provide a theoretical basis for the design of field engineering parameters.

[0007] To achieve the above object, the present invention provides a method for predicting the fracture pressure of acid fracturing considering the acid-etching deterioration of rock mechanical properties, which comprises the following steps:

[0008] S1. Obtain rock samples of the target reservoir for acid fracturing, process the rock samples into standard uniaxial compression test rock samples and standard splitting test rock samples; and prepare acid solution according to the actual acid solution formula of the actual project;

[0009] S2. Design the acid-etching temperature according to the actual formation temperature of the target reservoir, and set the acid-etching time according to the acid injection and transformation time of the actual project;

[0010] S3. Under the designed acid-etching temperature condition, immerse the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid solution and reach the designed acid-etching time for acid-etching deterioration to obtain the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample, and conduct uniaxial compression tests and splitting tests on them respectively to obtain uniaxial compression test data and splitting test data;

[0011] S4. According to the uniaxial compression test data and splitting test data obtained in step S3, obtain the elastic modulus, Poisson's ratio and tensile strength of the rock sample after acid-etching under certain acid-etching time and certain acid-etching temperature conditions, and perform polynomial fitting on the elastic modulus, the Poisson's ratio and the tensile strength with the acid-etching temperature and the acid-etching time respectively to obtain the acid-etched rock mechanical parameter - acid-etching temperature - acid-etching time model, that is, E / μ / σ t = f(H,T);

[0012] wherein, T is the acid-etching time, min; H is the acid-etching temperature, °C; E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless quantity; σ t is the tensile strength, MPa;

[0013] S5. Integrate the acid-etched rock mechanical parameter - acid-etching temperature - acid-etching time model obtained in step S4 into the conventional formation fracture pressure model to obtain the acid fracturing fracture pressure prediction model considering the acid-etching deterioration of rock mechanical properties, as follows:

[0014]

[0015] wherein, P f is the fracture pressure, MPa; ξ 1 and ξ 2 are tectonic coefficients; P p is the formation pressure, MPa; α is the biot coefficient, σ z is the vertical stress, MPa; E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless quantity; σ t is the tensile strength, MPa; E, μ and σt It is an acid-etched rock mechanical parameter - acid etching temperature - acid etching time model;

[0016] S6. Predict the formation fracture pressure according to the acid fracturing fracture pressure prediction model considering the acid-etching deterioration of rock mechanical properties obtained in step S5.

[0017] In the above method, preferably, in step S1, the standard uniaxial compression test rock samples and the standard splitting test rock samples are processed according to the methods described in GBT50266-2013 "Standard for Engineering Rock Mass Test Methods".

[0018] In the above method, preferably, step S3 specifically includes: placing the acid solution in a constant temperature oven and preheating it to the designed acid etching temperature, then immersing the standard uniaxial compression test rock samples and the standard splitting test rock samples in the acid solution. When the immersion time reaches the designed acid etching time, take out the rock samples, wipe the acid solution on the surface and air dry naturally to obtain the acid-etched uniaxial compression test rock samples and the acid-etched splitting test rock samples. Then, conduct uniaxial compression tests and splitting tests on the acid-etched uniaxial compression test rock samples and the acid-etched splitting test rock samples respectively to obtain uniaxial compression test data and splitting test data. Specifically, the uniaxial compression test and the splitting test are carried out according to the methods described in GBT50266-2013 "Standard for Engineering Rock Mass Test Methods".

[0019] In the above method, preferably, in step S4, the elastic modulus and the Poisson's ratio are obtained according to the uniaxial compression test data.

[0020] In the above method, preferably, in step S4, the tensile strength is obtained according to the splitting test data.

[0021] In the above method, preferably, step S6 specifically includes: according to the acid fracturing fracture pressure prediction model considering the acid-etching deterioration of rock mechanical properties obtained in step S5, obtain the formation fracture pressure under different acid etching temperatures (i.e., formation temperatures) and acid etching time conditions.

[0022] In the above method, preferably, step S6 further includes: drawing an engineering layout of acid etching temperature (i.e., formation temperature), acid etching time and formation fracture pressure. The engineering layout can be used to guide the design of on-site engineering parameters.

[0023] The present invention provides a method for predicting the fracture pressure of acid fracturing considering the acid-etching deterioration of rock mechanical properties. In acid fracturing, the carbonate rock undergoes chemical dissolution with the acid solution, which changes the internal structural characteristics of the reservoir rock matrix, affects the change of rock mechanical parameters, and makes it difficult to predict the fracture pressure. The method of the present invention solves the above problems. This method comprehensively considers the influence of formation temperature and acid-etching time on the mechanical properties of the reservoir, predicts the fracture pressure of acid fracturing, and can provide a theoretical basis for the design of on-site engineering parameters. The method of the present invention starts from indoor experimental data, and gradually conducts experiments according to the steps of the present invention to obtain a prediction model for the fracture pressure of acid fracturing considering the acid-etching deterioration of rock mechanical properties, calculates the final predicted fracture pressure of acid fracturing, and guides the design of on-site engineering parameters through this model. In the prior art, acid fracturing does not consider the changes in rock elastic modulus, Poisson's ratio, and tensile strength under the action of acid solution. When predicting the formation fracture pressure, a conventional formation fracture pressure model is usually used, in which the elastic modulus, Poisson's ratio, and tensile strength are fixed values, which conform to the rock state of ordinary hydraulic fracturing but do not conform to the rock state of acid fracturing. The present invention creatively integrates the acid-etched rock mechanical parameter - acid-etching temperature - acid-etching time model into the conventional formation fracture pressure model, so that the elastic modulus, Poisson's ratio, and tensile strength in the obtained new model change with the acid-etching conditions of acid fracturing, which conforms to the rock state of acid fracturing. Therefore, the method of the present invention is applicable to the prediction of formation fracture pressure in acid fracturing, can simulate the initiation of acid fracturing cracks in carbonate rock formations under real formation conditions, improve the accuracy of fracture pressure prediction under acid fracturing conditions, improve the effectiveness of acid fracturing programs, thereby improve the construction success rate, reduce downhole complications, and thus reduce exploration and development costs.

[0024] The technical solution of the present invention has at least the following beneficial effects:

[0025] 1. This method fully considers the change in the mechanical properties of the rock after acid etching, and the prediction of the fracture pressure of acid fracturing is more accurate;

[0026] 2. This method uses formation temperature and construction acid-etching time as variables, which are easily obtained in on-site engineering;

[0027] 3. This method has a simple form and is convenient to calculate, and is very suitable for on-site personnel to quickly predict the fracture pressure of acid fracturing. Description of the Drawings

[0028] Figure 1 It is a diagram showing the rock samples in the specific embodiment of the present invention.

[0029] Figure 2 It is a diagram showing the acid solution in the specific embodiment of the present invention.

[0030] Figure 3The fitting diagram of elastic modulus - acid etching time - acid etching temperature of rock after acid etching in the specific embodiment of the present invention.

[0031] Figure 4 The fitting diagram of Poisson's ratio - acid etching time - acid etching temperature of rock after acid etching in the specific embodiment of the present invention.

[0032] Figure 5 The fitting diagram of tensile strength - acid etching time - acid etching temperature of rock after acid etching in the specific embodiment of the present invention.

[0033] Figure 6 The engineering layout diagram of formation fracture pressure - formation temperature - acid etching time in the specific embodiment of the present invention. Specific Embodiment

[0034] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solution of the present invention will be described in detail below, but it should not be construed as a limitation on the implementable scope of the present invention.

[0035] According to the specific embodiment of the present invention, the present invention provides an acid fracturing fracture pressure prediction method considering the acid etching deterioration of rock mechanical properties, which includes the following steps:

[0036] S1. Obtain rock samples of the acid fracturing target reservoir, process the rock samples into standard uniaxial compression test rock specimens and standard splitting test rock specimens; and prepare acid solution according to the actual engineering acid fluid formula.

[0037] S2. Design the acid etching temperature according to the actual formation temperature of the target reservoir, and set the acid etching time according to the acid injection transformation time of the actual project.

[0038] S3. Under the designed acid etching temperature condition, immerse the standard uniaxial compression test rock specimen and the standard splitting test rock specimen in the acid solution and reach the designed acid etching time for acid etching deterioration, obtain the uniaxial compression test rock specimen after acid etching and the splitting test rock specimen after acid etching, and perform uniaxial compression tests and splitting tests on them respectively to obtain uniaxial compression test data and splitting test data.

[0039] S4. According to the uniaxial compression test data and splitting test data obtained in step S3, obtain the elastic modulus, Poisson's ratio, and tensile strength of the rock specimen after acid etching under certain acid etching time and certain acid etching temperature conditions, and perform polynomial fitting on the elastic modulus, Poisson's ratio, and tensile strength with the acid etching temperature and the acid etching time respectively to obtain the acid etching rock mechanical parameter - acid etching temperature - acid etching time model, that is, E / μ / σ t = f(H, T);

[0040] Among them, T is the acid etching time, in min; H is the acid etching temperature, in °C; E is the elastic modulus, in MPa; μ is the Poisson's ratio, a dimensionless quantity; σ t is the tensile strength, in MPa;

[0041] S5. Integrate the acid-etched rock mechanical parameter - acid etching temperature - acid etching time model obtained in step S4 into the conventional formation fracture pressure model to obtain an acid fracturing fracture pressure prediction model considering the acid etching deterioration of rock mechanical properties, as follows:

[0042]

[0043] Among them, P f is the fracture pressure, in MPa; ξ 1 and ξ 2 are structure coefficients; P p is the formation pressure, in MPa; α is the Biot coefficient, σ z is the vertical stress, in MPa; E is the elastic modulus, in MPa; μ is the Poisson's ratio, a dimensionless quantity; σ t is the tensile strength, in MPa; E, μ, and σ t are the acid-etched rock mechanical parameter - acid etching temperature - acid etching time model;

[0044] S6. Predict the formation fracture pressure according to the acid fracturing fracture pressure prediction model considering the acid etching deterioration of rock mechanical properties obtained in step S5.

[0045] In some specific embodiments, in step S1, the standard uniaxial compression test rock samples and the standard splitting test rock samples are processed according to the methods described in GBT50266-2013 "Standard for Test Methods of Engineering Rock Mass".

[0046] In some specific embodiments, step S3 specifically includes: placing the acid solution in a constant temperature oven, preheating it to the designed acid etching temperature, then immersing the standard uniaxial compression test rock samples and the standard splitting test rock samples in the acid solution. When the immersion time reaches the designed acid etching time, take out the rock samples, wipe the acid solution on the surface, and air dry them naturally to obtain the acid-etched uniaxial compression test rock samples and the acid-etched splitting test rock samples. Then, conduct uniaxial compression tests and splitting tests on the acid-etched uniaxial compression test rock samples and the acid-etched splitting test rock samples respectively to obtain uniaxial compression test data and splitting test data. Specifically, the uniaxial compression test and the splitting test are carried out according to the methods described in GBT50266-2013 "Standard for Test Methods of Engineering Rock Mass".

[0047] In some specific embodiments, in step S4, the elastic modulus and the Poisson's ratio are obtained according to the uniaxial compression test data. Specifically, their calculation formulas are as follows:

[0048]

[0049]

[0050] Among them, E is the elastic modulus, in MPa; μ is the Poisson's ratio, a dimensionless quantity; σ a is the axial stress, in MPa; ε a is the axial strain, ε r is the radial strain.

[0051] In some specific embodiments, in step S4, the tensile strength is obtained according to the splitting test data. Specifically, its calculation formula is as follows:

[0052]

[0053] Among them, σ t is the tensile strength, in MPa; P max is the maximum load of the splitting test, in kN; D is the diameter of the rock sample, in mm; L is the thickness of the rock sample, in mm.

[0054] In some specific embodiments, in step S4, the fitting formulas adopted by the acid-etched rock mechanical parameter - acid-etched temperature - acid-etched time model include but are not limited to:

[0055] E = a + b×T - c×H - d×T 2 + e×H 2

[0056] μ = f + G×T + h×H

[0057] σ t =(i×T + j)×(k×H + l)

[0058] Among them, T is the acid-etched time, in min; H is the acid-etched temperature, in °C; E is the elastic modulus, in MPa; μ is the Poisson's ratio, a dimensionless quantity; σt is the tensile strength, in MPa; a, b, c, d, e, f, g, h, i, j, k, and l are all fitting parameters.

[0059] In some specific embodiments, in step S5, the conventional formation fracture pressure model is:

[0060]

[0061] Among them, P f is the fracture pressure, in MPa; ξ 1 and ξ 2 are formation coefficients; P pp is the formation pressure, MPa; α is the Biot coefficient, σz is the vertical stress, MPa; E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless; σt is the tensile strength, MPa.

[0062] In some specific embodiments, step S6 specifically includes: obtaining the formation fracture pressure under different acid etching temperatures (i.e., formation temperatures) and acid etching time conditions according to the acid fracturing fracture pressure prediction model considering the acid etching deterioration of rock mechanical properties obtained in step S5.

[0063] In some specific embodiments, step S6 further includes: plotting the engineering map of acid etching temperature (i.e., formation temperature), acid etching time, and formation fracture pressure. The field engineering parameter design can be guided by this engineering map.

[0064] Specifically, in one embodiment, the acid fracturing fracture pressure prediction method considering the acid etching deterioration of rock mechanical properties of the present invention includes the following steps:

[0065] S1. According to the engineering requirements, obtain the rock samples of the acid fracturing target reservoir, and process the rock samples into standard uniaxial compression test rock samples with a diameter of 25 mm and a height of 50 mm, and standard splitting test rock samples with a diameter of 50 mm and a height of 25 mm according to the method recorded in GBT50266-2013 "Standard for Engineering Rock Mass Test Methods"; the unevenness error of both end faces of the rock samples is less than 0.05 mm, the error along the height and diameter of the rock samples is less than 0.3 mm, and the maximum deviation of the end face perpendicular to the axis of the rock sample is less than 0.25°, as Figure 1 shown; and prepare the acid solution according to the actual acid solution formula of the actual project, such as gelled acid, and its formula is: 20.0% hydrochloric acid + 3.5% gelling agent + 3.5% corrosion inhibitor + 1.0% iron ion stabilizer + 1.5% corrosion inhibition synergist + 1.0% drainage aid + 1.0% composite multi-effect agent + the balance of water, as Figure 2 shown;

[0066] S2. Design the acid etching temperature according to the actual formation temperature of the target reservoir, and set the acid etching time according to the acid injection reform time of the actual project; taking a carbonate reservoir in Sichuan as an example, the reservoir temperature is usually below 100 °C, and the acid injection time is usually 30-90 min. Therefore, the acid etching temperatures for the experiment are designed as 20 °C, 60 °C, and 100 °C, and the acid etching times are designed as 30 min, 60 min, and 90 min. They are combined in pairs, and a total of 9 groups of experiments are carried out;

[0067] S3. Conduct a mechanical degradation test experiment on acid-etched rock: First, place the acid solution in a thermostatic oven and preheat it to the designed acid-etching temperature. Then, immerse the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid solution. When the immersion time reaches the designed acid-etching time, take out the rock samples, wipe the acid solution on the surface, and air-dry them naturally to obtain the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample. After that, conduct uniaxial compression tests and splitting tests on the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample respectively according to the methods described in the "Standard for Test Methods of Engineering Rock Masses" GBT50266-2013 to obtain uniaxial compression test data and splitting test data;

[0068] S4. Based on the uniaxial compression test data and splitting test data obtained in step S3, obtain the elastic modulus, Poisson's ratio, and tensile strength of the acid-etched rock sample under certain acid-etching time and certain acid-etching temperature conditions;

[0069] The elastic modulus and Poisson's ratio are obtained based on the uniaxial compression test data, and their calculation formulas are as follows:

[0070]

[0071]

[0072] Among them, E is the elastic modulus, MPa; μ is Poisson's ratio, a dimensionless quantity; σ a is the axial stress, MPa; ε a is the axial strain, ε r is the radial strain;

[0073] The tensile strength is obtained based on the splitting test data, and its calculation formula is as follows:

[0074]

[0075] Among them, σ t is the tensile strength, MPa; P max is the maximum load of the splitting test, kN; D is the diameter of the rock sample, mm; L is the thickness of the rock sample, mm;

[0076] Perform polynomial fitting on the elastic modulus, Poisson's ratio, and tensile strength respectively with the acid-etching temperature and the acid-etching time to obtain the acid-etched rock mechanical parameter - acid-etching temperature - acid-etching time model, that is, E / μ / σ t = f(H,T);

[0077] The fitting formulas adopted by the acid-etched rock mechanical parameter - acid-etching temperature - acid-etching time model include but are not limited to:

[0078] E = a + b×T - c×H - d×T 2 + e×H 2

[0079] μ = f + g×T + h×H

[0080] σ t = (i×T + j)×(k×H + l)

[0081] Wherein, T is the acid etching time, in min; H is the acid etching temperature, in °C; E is the elastic modulus, in MPa; μ is the Poisson's ratio, a dimensionless quantity; σt is the tensile strength, in MPa; a, b, c, d, e, f, g, h, i, j, k, and l are all fitting parameters;

[0082] Taking the on-site engineering mentioned in step S2 as an example, using the above fitting formulas to perform polynomial fitting on the elastic modulus, the Poisson's ratio, and the tensile strength with the acid etching temperature and the acid etching time respectively, the obtained fitting diagrams are as shown in Figure 3 、 Figure 4 and Figure 5 shown below, and the following models are obtained:

[0083] E = 52.61992 + 0.40693×T - 0.23962×H - 0.00634×T 2 + 8.60539×10 -4 ×H 2

[0084] μ = 0.22081 - 0.00136×T - 3.05167×10 -4 ×H

[0085] σ t = (-0.00943×T + 3.73)×(-0.01114×H + 3.87)

[0086] S5. Integrate the acid-etched rock mechanics parameter - acid etching temperature - acid etching time model obtained in step S4 into the conventional formation fracture pressure model to obtain an acid fracturing fracture pressure prediction model considering the acid-etching deterioration of rock mechanics properties.

[0087] Wherein, the conventional formation fracture pressure model is as follows:

[0088]

[0089] Wherein, P f is the fracture pressure, in MPa; ξ 1 and ξ 2 are formation coefficients; P pwhere \(P_p\) is the formation pressure, in MPa; \(\alpha\) is the Biot coefficient, \(\sigma_z\) is the vertical stress, in MPa; \(E\) is the elastic modulus, in MPa; \(\mu\) is the Poisson's ratio, a dimensionless quantity; \(\sigma_t\) is the tensile strength, in MPa;

[0090] The acid fracturing breakdown pressure prediction model considering the acid corrosion deterioration of rock mechanical properties is as follows:

[0091]

[0092] where \(P\) f is the breakdown pressure, in MPa; \(\xi_1\) 1 and \(\xi_2\) 2 are structure coefficients; \(P_p\) p is the formation pressure, in MPa; \(\alpha\) is the Biot coefficient, \(\sigma\) z is the vertical stress, in MPa; \(E\) is the elastic modulus, in MPa; \(\mu\) is the Poisson's ratio, a dimensionless quantity; \(\sigma\) t is the tensile strength, in MPa; \(E\), \(\mu\) and \(\sigma\) t are the acid-etched rock mechanical parameter - acid-etched temperature - acid-etched time model;

[0093] It should be noted that \(\xi_1\) 1 , \(\xi_2\) 2 , \(P_p\) p , \(\alpha\) (Biot coefficient), \(\sigma\) z are all formation environment parameters, and can all be obtained by those skilled in the art through logging interpretation based on the data measured in actual engineering according to the methods in the prior art;

[0094] Taking the acid-etched rock mechanical parameter - acid-etched temperature - acid-etched time model shown in step S4 as an example, the acid fracturing breakdown pressure prediction model considering the acid corrosion deterioration of rock mechanical properties is obtained as follows:

[0095]

[0096] A = 52.6199 - 0.2396H + 8.6053×10 -4 H 2 + 0.4069T - 0.0063T 2

[0097] B = 3.0517×10 -4 H + 0.0014T

[0098] S6. According to the acid fracturing breakdown pressure prediction model considering the acid corrosion deterioration of rock mechanical properties obtained in step S5, the formation breakdown pressure under different acid-etched temperatures (i.e., formation temperatures) and acid-etched time conditions is obtained, and the engineering layout of acid-etched temperature (i.e., formation temperature), acid-etched time and formation breakdown pressure is drawn, as Figure 6As shown, the engineering layout can be used to guide the design of on-site engineering parameters.

[0099] In the conventional formation fracture pressure model, the elastic modulus, Poisson's ratio, and tensile strength are fixed values, which are the original formation rock parameters. This model is applicable to ordinary hydraulic fracturing. In ordinary hydraulic fracturing, the fracturing fluid has no corrosive effect and has no impact on the mechanical properties of the formation rock, so its mechanical parameters are fixed values. However, when this model is applied to acid fracturing, a large error will occur. The main reason is that in acid fracturing, the acid fluid will react with the rock, and the mechanical properties of the rock will change and are not fixed values. In the method of the present invention, the acid-etched rock mechanical parameter - acid-etched temperature - acid-etched time model obtained based on experiments is embedded into the conventional formation fracture pressure model, so that the elastic modulus, Poisson's ratio, and tensile strength in this model become dynamic values related to the acid-etched conditions, making this model applicable to acid fracturing.

[0100] In this embodiment, the acid fracturing initiation model is verified through on-site data. The target reservoir of a certain carbonate rock reservoir in Sichuan has a vertical depth of 5905 m; the overburden pressure is 155.3 MPa; the structure coefficients are 0.7742 and 0.5538; the formation temperature is 100 °C, the formation pressure is 60.18 MPa, and the Biot coefficient is 0.8. The reservoir stimulation is divided into two steps. First, gelled acid is injected into the target reservoir for 45 minutes to acidify the reservoir, and then slickwater is used for fracturing until the fracturing is completed. The fracture pressure is 93.4 MPa.

[0101] Substitute the above parameters into the fracture pressure prediction model of this embodiment, and the predicted fracture pressure is 87.8 MPa. The difference from the actual fracture pressure is 5.6 MPa, and the error is 6.0%.

[0102] As a comparison, without considering the change of acid-etched rock mechanical parameters, but using the conventional formation fracture pressure model, with the elastic modulus, Poisson's ratio, and tensile strength being fixed values in the unacidified state, the predicted fracture pressure is 119.2 MPa, the difference from the actual fracture pressure is 25.8 MPa, and the error is 27.6%.

[0103] It can be seen from this that compared with the conventional method, the error between the predicted formation fracture pressure of the method of the present invention and the actual value is reduced by 20.2 MPa, and the accuracy is improved by 21.6%. Therefore, the prediction result of the acid fracturing fracture pressure prediction method considering the acid corrosion deterioration of rock mechanical properties provided by the present invention is more accurate.

Claims

1. A method for predicting fracture pressure of acid fracturing considering acid corrosion degradation of rock mechanical properties, comprising the following steps: S1. Obtain rock samples of the target reservoir for acid fracturing, and process the rock samples into standard uniaxial compression test rock samples and standard splitting test rock samples; And prepare the acid according to the actual engineering acid formula; S2. Design the acid etching temperature according to the actual formation temperature of the target reservoir, and set the acid etching time according to the acid injection and reconstruction time of the actual project; S3. Under the designed acid etching temperature condition, immersing the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid solution for the designed acid etching time to perform acid etching degradation, obtaining the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample, and performing uniaxial compression test and splitting test on them respectively to obtain uniaxial compression test data and splitting test data; S4, according to the uniaxial compression test data and the splitting test data obtained in step S3, the elastic modulus, Poisson's ratio and tensile strength of the rock sample after acid etching under certain acid etching time and certain acid etching temperature conditions are obtained, and the elastic modulus, the Poisson's ratio and the tensile strength are respectively fitted with the acid etching temperature and the acid etching time by polynomials to obtain the acid-etched rock mechanical parameters-acid etching temperature-acid etching time model, i.e., E / μ / σ t =f(H,T); Wherein, T is the etching time, min; H is the etching temperature, °C; E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless; σ t is the tensile strength, MPa; S5. Integrate the acid-etched rock mechanical parameters-acid-etching temperature-acid-etching time model obtained in step S4 into the conventional formation fracture pressure model to obtain an acid fracturing fracture pressure prediction model that takes into account the acid-etching degradation of rock mechanical properties, as shown below: Among them, P f is the bursting pressure, MPa; ξ1 and ξ2 are the structural coefficients; P p is the formation pressure, MPa; α is the biot coefficient, σ z is the vertical stress, MPa; E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless; σ t is the tensile strength, MPa; E, μ and σ t It is the model of acid-etched rock mechanical parameters-acid-etching temperature-acid-etching time; S6. Predicting the formation fracture pressure according to the acid fracturing fracture pressure prediction model taking into account the acid corrosion degradation of rock mechanical properties obtained in step S5.

2. The method according to claim 1, wherein: In step S1, the standard uniaxial compression test rock sample and the standard splitting test rock sample are processed according to the method described in GBT50266-2013 "Engineering Rock Test Method Standard".

3. The method according to claim 1, wherein: Step S3 specifically includes: placing the acid solution in a constant temperature box, preheating it to the designed acid etching temperature, and then immersing the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid solution, taking out the rock sample when the immersion time reaches the designed acid etching time, wiping the surface acid solution and naturally air-drying it to obtain the uniaxial compression test rock sample after acid etching and the splitting test rock sample after acid etching, and then performing uniaxial compression test and splitting test on the uniaxial compression test rock sample after acid etching and the splitting test rock sample after acid etching, respectively, to obtain uniaxial compression test data and splitting test data.

4. The method according to claim 1, wherein: In step S3, the uniaxial compression test and the splitting test are performed according to the method described in GBT50266-2013 "Engineering Rock Mass Test Method Standard".

5. The method according to claim 1, wherein: In step S4, the elastic modulus and the Poisson's ratio are obtained based on the uniaxial compression test data.

6. The method according to claim 1, wherein: In step S4, the tensile strength is obtained based on the splitting test data.

7. The method according to claim 1, wherein: Step S6 specifically includes: obtaining the formation fracture pressure under different acid etching temperatures and acid etching times according to the acid fracturing fracture pressure prediction model considering the acid etching degradation of rock mechanical properties obtained in step S5.

8. The method according to claim 7, wherein: Step S6 further includes: drawing an engineering map of acid etching temperature, acid etching time and formation fracture pressure.

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