An acidizing fracturing fracture pressure prediction method considering acid erosion deterioration of rock mechanical properties

Abstract

The application provides an acidification fracturing fracture pressure prediction method considering rock mechanics property acid erosion degradation. The method comprises the following steps: subjecting the acid-eroded rock sample to uniaxial compression experiment and splitting experiment respectively; obtaining an acid-eroded rock mechanics parameter-acid erosion temperature-acid erosion time model according to the uniaxial compression experiment data and the splitting experiment data; integrating the acid-eroded rock mechanics parameter-acid erosion temperature-acid erosion time model into a conventional formation fracture pressure model to obtain an acidification fracturing fracture pressure prediction model considering rock mechanics property acid erosion degradation. The method comprehensively considers the influence of formation temperature and acid erosion time on reservoir mechanics property, can predict the acidification fracturing fracture pressure, and can provide a theoretical basis for field engineering parameter design.

Description

TECHNICAL FIELD

[0001] The application relates to an acidification fracturing fracture pressure prediction method considering acid erosion degradation of rock mechanical properties, and belongs to the technical field of oil and gas well engineering. BACKGROUND

[0002] Deep carbonate rock formations contain rich oil and gas resources and are one of the fields of future key oil and gas exploration and development. However, deep carbonate rock formations generally have low permeability, natural fracture and cave development and the like, and the use of a hydraulic fracturing process to establish a flow channel is one of the measures for reservoir reconstruction, and acidification fracturing is particularly suitable for carbonate rock reservoirs and is widely applied in actual engineering.

[0003] There is a great difference between carbonate rock acidification fracturing and ordinary hydraulic fracturing. Acidification fracturing not only has a fluid-solid coupling process, but also has a chemical action of carbonate rock and acid liquid, so that the internal structure characteristics of the reservoir rock matrix change, and finally the change of rock mechanical parameters is affected to achieve the effect of reservoir fracture pressure reduction. Therefore, when the acidification fracturing initiation pressure is predicted, the degradation of the mechanical properties of the reservoir rock under the action of acid erosion must be considered.

[0004] In acidification fracturing design, if the degradation effect of rock acidification is not fully considered, the designed initiation pressure is usually greater than the actual initiation pressure, and under the design parameter of excessive extension, the fracture easily causes fracture layering. The damage of acidification fracturing to fracture layering is more serious than that of ordinary hydraulic fracturing. The fracturing fluid used in acidification fracturing contains a large amount of acid liquid, which can greatly change the properties of non-target reservoirs, form pollution, and part of the oil and gas in the target reservoir migrates to the layered formation, which is difficult to exploit. If the fracture layering reaches the underground water layer, the acid liquid and the oil and gas enter the formation water layer together, which can seriously pollute the formation water, and the pollution range is difficult to control with the flow of the formation water. The use of formation water can directly endanger the health of animals, plants and humans. Therefore, in order to safely and sustainably exploit the oil and gas resources in the formation, the change of the properties of the reservoir rock after acidification must be considered when the reservoir is reconstructed by using acidification fracturing.

[0005] Therefore, developing an acidification fracturing fracture pressure prediction method considering the acid erosion degradation of rock mechanical properties has become one of the problems to be solved in the field. SUMMARY

[0006] To solve the above technical problems, the purpose of the present application is to provide an acidification fracturing fracture pressure prediction method considering the acid erosion degradation of rock mechanical properties. The method comprehensively considers the influence of formation temperature and acid erosion time on the mechanical properties of the reservoir, predicts the acidification fracturing fracture pressure, and provides a theoretical basis for the design of field engineering parameters.

[0007] In order to achieve the above object, the present application provides an acidizing fracturing fracture pressure prediction method considering acid erosion degradation of rock mechanical properties, which comprises the following steps:

[0008] S1, obtaining rock samples of an acidizing fracturing target reservoir, processing the rock samples into standard uniaxial compression experimental rock samples and standard splitting experimental rock samples; and preparing acid liquid according to an actual engineering acid liquid formula;

[0009] S2, designing acid erosion temperature according to actual formation temperature of the target reservoir, and setting acid erosion time according to acid injection reconstruction time of the actual engineering;

[0010] S3, soaking the standard uniaxial compression experimental rock samples and the standard splitting experimental rock samples in the acid liquid under the designed acid erosion temperature and reaching the designed acid erosion time to perform acid erosion degradation, obtaining uniaxial compression experimental rock samples after acid erosion and splitting experimental rock samples after acid erosion, and respectively performing uniaxial compression experiment and splitting experiment on them to obtain uniaxial compression experimental data and splitting experimental data;

[0011] S4, obtaining elastic modulus, Poisson's ratio and tensile strength of the rock samples after acid erosion under certain acid erosion time and certain acid erosion temperature according to the uniaxial compression experimental data and the splitting experimental data obtained in step S3, and respectively performing polynomial fitting on the elastic modulus, the Poisson's ratio and the tensile strength with the acid erosion temperature and the acid erosion time to obtain an acid erosion rock mechanical parameter-acid erosion temperature-acid erosion time model, i.e. t =f(H,T);

[0012] Wherein, T is acid erosion time, min; H is acid erosion temperature, ℃; E is elastic modulus, MPa; μ is Poisson's ratio, dimensionless quantity; σ t is tensile strength, MPa;

[0013] S5, integrating the acid erosion rock mechanical parameter-acid erosion temperature-acid erosion time model obtained in step S4 into a conventional formation fracture pressure model to obtain an acidizing fracturing fracture pressure prediction model considering acid erosion degradation of rock mechanical properties, as shown below:

[0014]

[0015] Wherein, P f is fracture pressure, MPa; ξ1 and ξ2 are structure coefficients; P p is formation pressure, MPa; α is biot coefficient, σ z is vertical stress, MPa; E is elastic modulus, MPa; μ is Poisson's ratio, dimensionless quantity; σ t is tensile strength, MPa; E, μ and σ tAn acid-etched rock mechanical parameter-acid-etching temperature-acid-etching time model is provided.

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

[0017] In the method, preferably, 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 GBT 50266-2013 “Standard for Test Methods of Engineering Rock Mass”.

[0018] In the method, preferably, step S3 specifically comprises: placing the acid liquid in a thermostat, preheating to the designed acid-etching temperature, then immersing the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid liquid, taking out the rock samples when the immersion time reaches the designed acid-etching time, wiping the surface acid liquid and then naturally air-drying to obtain the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample, and then performing uniaxial compression test and splitting test on the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample respectively to obtain uniaxial compression test data and splitting test data. Specifically, the uniaxial compression test and the splitting test are performed according to the method described in GBT 50266-2013 “Standard for Test Methods of Engineering Rock Mass”.

[0019] In the 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 method, preferably, in step S4, the tensile strength is obtained according to the splitting test data.

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

[0022] In the method, preferably, step S6 further comprises: drawing an engineering map of acid-etching temperature (i.e. formation temperature), acid-etching time and formation fracture pressure. The engineering map can guide the design of field engineering parameters.

[0023] The application provides an acidizing fracturing fracture pressure prediction method considering acid erosion deterioration of rock mechanics properties. In acidizing fracturing, chemical action of carbonate rock and acid liquid causes changes in internal structure characteristics of a reservoir rock matrix, influences changes in rock mechanics parameters, and leads to difficulty in fracture pressure prediction. The method solves the above problems, comprehensively considers influences of formation temperature and acid erosion time on reservoir mechanics properties, predicts acidizing fracturing fracture pressure, and provides a theoretical basis for field engineering parameter design. The method starts from indoor experimental data, gradually obtains an acidizing fracturing fracture pressure prediction model considering acid erosion deterioration of rock mechanics properties according to steps of the application, calculates final predicted acidizing fracturing fracture pressure, and guides field engineering parameter design through the model. In the prior art, acidizing fracturing does not consider changes in rock elastic modulus, Poisson's ratio and tensile strength under the action of acid liquid, and a conventional formation fracture pressure model is usually used to predict formation fracture pressure, in which the elastic modulus, Poisson's ratio and tensile strength are fixed values, which are consistent with the rock state of ordinary hydraulic fracturing, but not consistent with the rock state of acidizing fracturing. The application creatively integrates the acid erosion rock mechanics parameter-acid erosion temperature-acid erosion time model into the conventional formation fracture pressure model, so that the elastic modulus, Poisson's ratio and tensile strength in the new model change with changes in acidizing fracturing acid erosion conditions, which is consistent with the rock state of acidizing fracturing. Therefore, the method is suitable for acidizing fracturing formation fracture pressure prediction, can simulate acid fracturing crack initiation of carbonate rock formation under real formation conditions, improve the accuracy of fracture pressure prediction under acid fracturing conditions, improve the effectiveness of acid fracturing schemes, thereby improve the success rate of construction, reduce downhole complexity, and reduce exploration and development costs.

[0024] The technical scheme of the application has at least the following beneficial effects:

[0025] 1. The method fully considers changes in mechanics properties of rock after acid erosion, and is more accurate for acidizing fracturing fracture pressure prediction.

[0026] 2. The method takes formation temperature and construction acid erosion time as variables, which are easy to obtain in field engineering.

[0027] 3. The method is simple in form, convenient to calculate, and very suitable for field personnel to quickly predict acidizing fracturing fracture pressure. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The figure is a rock sample in the specific embodiment of the application.

[0029] Figure 2 The figure is acid liquid in the specific embodiment of the application.

[0030] Figure 3A fitting graph of elastic modulus of rock after acid etching-acid etching time-acid etching temperature in the specific embodiment of the present application.

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

[0032] Figure 5 A fitting graph of tensile strength of rock after acid etching-acid etching time-acid etching temperature in the specific embodiment of the present application.

[0033] Figure 6 An engineering version graph of formation fracture pressure-formation temperature-acid etching time in the specific embodiment of the present application. DETAILED DESCRIPTION

[0034] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solutions of the present application are described in detail as follows, but cannot be understood as limiting the implementable scope of the present application.

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

[0036] S1, obtaining rock samples of an acidizing fracturing target reservoir, processing the rock samples into standard uniaxial compression experimental rock samples and standard splitting experimental rock samples; and preparing acid liquid according to an actual engineering acid liquid formula;

[0037] S2, designing acid etching temperature according to actual formation temperature of the target reservoir, and setting acid etching time according to acid injection reconstruction time of the actual engineering;

[0038] S3, under the condition of the designed acid etching temperature, soaking the standard uniaxial compression experimental rock samples and the standard splitting experimental rock samples in the acid liquid and reaching the designed acid etching time to perform acid etching deterioration, obtaining uniaxial compression experimental rock samples after acid etching and splitting experimental rock samples after acid etching, and respectively performing uniaxial compression experiment and splitting experiment on them to obtain uniaxial compression experimental data and splitting experimental data;

[0039] S4, obtaining elastic modulus, Poisson's ratio and tensile strength of the rock samples after acid etching under the condition of certain acid etching time and certain acid etching temperature according to the uniaxial compression experimental data and the splitting experimental data obtained in step S3, and respectively performing polynomial fitting on the elastic modulus, the Poisson's ratio and the tensile strength with the acid etching temperature and the acid etching time to obtain an acid etching rock mechanical parameter-acid etching temperature-acid etching time model, i.e. t E / μ / σ

[0040] Wherein, T is acid etching time, min; H is acid etching temperature, ℃; E is elastic modulus, MPa; μ is Poisson's ratio, dimensionless; σ t is tensile strength, MPa;

[0041] S5, the acid etching rock mechanics parameter-acid etching temperature-acid etching time model obtained in step S4 is integrated into a conventional formation fracture pressure model to obtain an acidizing fracturing fracture pressure prediction model considering acid etching deterioration of rock mechanics properties, as shown below:

[0042]

[0043] Wherein, P f is fracture pressure, MPa; ξ1 and ξ2 are tectonic coefficients; P p is formation pressure, MPa; α is biot coefficient, σ z is vertical stress, MPa; E is elastic modulus, MPa; μ is Poisson's ratio, dimensionless; σ t is tensile strength, MPa; E, μ and σ t is acid etching rock mechanics parameter-acid etching temperature-acid etching time model;

[0044] S6, the formation fracture pressure is predicted according to the acidizing fracturing fracture pressure prediction model considering acid etching deterioration of rock mechanics properties obtained in step S5.

[0045] In some embodiments, in step S1, the standard uniaxial compression test rock sample and the standard splitting test rock sample are processed according to the method recorded in GBT 50266-2013 "Standard for Test Methods of Engineering Rock Mass".

[0046] In some embodiments, step S3 specifically comprises: placing the acid liquid in a thermostat, preheating to the designed acid etching temperature, then soaking the standard uniaxial compression test rock sample and the standard splitting test rock sample in the acid liquid, taking out the rock samples after the soaking time reaches the designed acid etching time, wiping the surface acid liquid and naturally air-drying to obtain the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample, and then respectively performing uniaxial compression test and splitting test on the acid-etched uniaxial compression test rock sample and the acid-etched splitting test rock sample to obtain uniaxial compression test data and splitting test data. Specifically, the uniaxial compression test and the splitting test are performed according to the method recorded in GBT 50266-2013 "Standard for Test Methods of Engineering Rock Mass".

[0047] In some 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] E = a + b x T - c x H - d x T + e x H a is the axial stress, MPa; ε a is the axial strain, ε r is the radial strain.

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

[0052]

[0053] σt = (i x T + j) x (k x H + l) 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.

[0054] In some embodiments, in step S4, the fitting formula used by the acid-etched rock mechanics parameter-acid-etching temperature-acid-etching time model includes but is not limited to:

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

[0056] μ = f + G x T + h x H

[0057] σt = (i x T + j) x (k x H + l) t

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

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

[0060]

[0061] Pf = ξ1 x P + ξ2 x σz f is the fracture pressure, MPa; ξ1 and ξ2 are tectonic coefficients; P p 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 embodiments, step S6 specifically comprises obtaining the formation fracture pressure under different acidizing temperature (i.e. formation temperature) and acidizing time conditions according to the acidizing fracture pressure prediction model considering rock mechanical property acidizing deterioration obtained in step S5.

[0063] In some embodiments, step S6 further comprises drawing an engineering map of acidizing temperature (i.e. formation temperature), acidizing time and formation fracture pressure. The engineering map can guide the design of field engineering parameters.

[0064] Specifically, in one embodiment, the acidizing fracture pressure prediction method considering rock mechanical property acidizing deterioration of the present application comprises the following steps:

[0065] S1. According to engineering requirements, obtain rock samples of the acidizing fracture target reservoir, process the rock samples into standard uniaxial compression experimental rock samples with a diameter of 25 mm and a height of 50 mm and standard splitting experimental rock samples with a diameter of 50 mm and a height of 25 mm according to the method recorded in GBT 50266-2013 "Standard for Engineering Rock Mass Test Methods"; the error of the unevenness of the two end surfaces of the rock samples is less than 0.05 mm, the error of the diameter along the height of the rock sample is less than 0.3 mm, the maximum deviation of the end surface perpendicular to the axis of the rock sample is less than 0.25°, as shown in FIG. 1; and prepare acid according to the actual engineering acid formula, for example, gelled acid, whose formula is: 20.0% hydrochloric acid + 3.5% gelling agent + 3.5% corrosion inhibitor + 1.0% iron ion stabilizer + 1.5% corrosion synergist + 1.0% cleanup additive + 1.0% composite multi-effect additive + the balance of water, as shown in FIG. 2. Figure 1 Figure 2

[0066] S2. Design acidizing temperature according to the actual formation temperature of the target reservoir, and set acidizing time according to the actual acid injection time of the project; taking a carbonate reservoir in Sichuan as an example, the reservoir temperature is usually below 100℃, and the acid injection time is usually 30-90 min, so the acidizing temperature of the experiment is designed to be 20℃, 60℃ and 100℃, the acidizing time is designed to be 30 min, 60 min and 90 min, and two-by-two combinations are made, a total of 9 groups of experiments;

[0067] ​​S3, performing rock acid erosion mechanics deterioration test experiment: first, the acid is placed in a thermostat, preheated to the designed acid erosion temperature, then the standard uniaxial compression test rock sample and the standard splitting test rock sample are immersed in the acid, when the soaking time reaches the designed acid erosion time, the rock sample is taken out, the surface acid is wiped off and naturally dried, obtaining the acid-eroded uniaxial compression test rock sample and the acid-eroded splitting test rock sample, then the acid-eroded uniaxial compression test rock sample and the acid-eroded splitting test rock sample are respectively subjected to uniaxial compression test and splitting test according to the method recorded in GBT 50266-2013 "Engineering rock test method standard", obtaining uniaxial compression test data and splitting test data;

[0068] S4, obtaining the elastic modulus, Poisson's ratio and tensile strength of the rock sample after acid erosion under certain acid erosion time and certain acid erosion temperature according to the uniaxial compression test data and the splitting test data obtained in step S3;

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

[0070]

[0071]

[0072] Wherein, E is the elastic modulus, MPa; μ is the Poisson's ratio, dimensionless; 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 according to the splitting test data, and its calculation formula is as follows:

[0074]

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

[0076] The elastic modulus, the Poisson's ratio and the tensile strength are respectively polynomially fitted with the acid erosion temperature and the acid erosion time, obtaining the acid-eroded rock mechanics parameter-acid erosion temperature-acid erosion time model, i.e. E / μ / σ t =f(H,T);

[0077] The fitting formula used in the acid-eroded rock mechanics parameter-acid erosion temperature-acid erosion time model includes but is not limited to:

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

[0079] μ = f + g x T + h x H

[0080] σ t = (i x T + j) x (k x H + l)

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

[0082] Taking the field engineering mentioned in step S2 as an example, the elastic modulus, the Poisson's ratio and the tensile strength are respectively polynomially fitted with the acid etching temperature and the acid etching time using the above fitting formula, and the fitting graphs obtained are as shown in Figure 3 、 Figure 4 and Figure 5 The following models are obtained:

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

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

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

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

[0087] wherein the conventional formation fracture pressure model is as shown below:

[0088]

[0089] 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; σ t is the tensile strength, MPa;

[0090] The acid fracturing fracture pressure prediction model considering the degradation of rock mechanical properties by acid etching is shown below:

[0091]

[0092] Among them, P f ξ1 and ξ2 are the rupture pressure, MPa; P is the structural coefficient. p σ is the formation pressure, MPa; α is the biot coefficient, σ z σ is the vertical stress, MPa; E is the elastic modulus, MPa; μ is Poisson's ratio, dimensionless; σ t For tensile strength, MPa; E, μ, and σ t A model for the mechanical parameters of acid-etched rocks, acid etching temperature, and acid etching time.

[0093] It should be noted that ξ1, ξ2, and P p α (biot coefficient), σ z All of these are formation environment parameters, which can be obtained by those skilled in the art through well logging interpretation based on actual engineering measurement data using methods in the prior art.

[0094] Taking the acid-etched rock mechanical parameters-acid etching temperature-acid etching time model shown in step S4 as an example, the acid fracturing fracture pressure prediction model considering the acid etching degradation 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. Based on the acid fracturing fracture pressure prediction model considering the degradation of rock mechanical properties due to acid etching obtained in step S5, the formation fracture pressure under different acid etching temperatures (i.e., formation temperatures) and acid etching times is obtained. An engineering map of the acid etching temperature (i.e., formation temperature), acid etching time, and formation fracture pressure is then drawn, as follows: Figure 6 As shown, this engineering layout can 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. The model is applicable to ordinary hydraulic fracturing, in which the fracturing fluid has no corrosive effect on the mechanical properties of the formation rock, so its mechanical parameters are fixed values. However, the model will produce a larger error when applied to acid fracturing, the main reason being that in acid fracturing, the acid will react with the rock, and the mechanical properties of the rock will change, and are not fixed values. The method of the present application embeds the acid-etched rock mechanical parameter-acid-etching temperature-acid-etching time model obtained based on experiments into the conventional formation fracture pressure model, so that the elastic modulus, Poisson's ratio and tensile strength in the model become dynamic values related to the acid-etching conditions, so that the model is applicable to acid fracturing.

[0100] In the present embodiment, the acidification cracking model is verified by field data. The target reservoir of a certain carbonate reservoir in Sichuan has a vertical depth of 5905m; the overburden pressure is 155.3MPa; the tectonic coefficient is 0.7742, 0.5538; the formation temperature is 100℃, the formation pressure is 60.18MPa, and the biot coefficient is 0.8. The reservoir reconstruction is divided into two steps. First, gel acid is injected into the target reservoir for 45min to acidize the reservoir, and then slickwater is used for fracturing until the fracturing is completed, and the fracture pressure is 93.4MPa.

[0101] The above parameters are brought into the fracture pressure prediction model of the present embodiment to predict the fracture pressure as 87.8MPa. The difference from the true fracture pressure is 5.6MPa, and the error is 6.0%.

[0102] As a comparison, without considering the change of acid-etched rock mechanical parameters, the conventional formation fracture pressure model is used, and the elastic modulus, Poisson's ratio and tensile strength are fixed values under the un-acidized state, so the predicted fracture pressure is 119.2MPa, which is 25.8MPa different from the true fracture pressure, and the error is 27.6%.

[0103] It can be seen that compared with the conventional method, the error of the predicted formation fracture pressure and the true value is reduced by 20.2MPa, and the accuracy is improved by 21.6%. Therefore, the acidification fracturing fracture pressure prediction method provided by the present application considering the acid-etching deterioration of rock mechanical properties is more accurate.

Claims

1. A method for predicting fracture pressure of acid fracturing considering acid erosion degradation of rock mechanical properties, comprising the following steps: S1, obtaining rock samples of an acid fracturing target reservoir, processing the rock samples into standard uniaxial compression test rock samples and standard splitting test rock samples; and preparing acid according to the actual engineering acid formula; S2, designing acid erosion temperature according to the actual formation temperature of the target reservoir, and setting acid erosion time according to the acid injection reconstruction time of the actual project; S3, soaking the standard uniaxial compression test rock samples and the standard splitting test rock samples in the acid under the designed acid erosion temperature and reaching the designed acid erosion time to perform acid erosion degradation, obtaining acid-eroded uniaxial compression test rock samples and acid-eroded splitting test rock samples, and performing uniaxial compression test and splitting test on them respectively to obtain uniaxial compression test data and splitting test data; S4, obtaining the elastic modulus, Poisson's ratio and tensile strength of the rock sample after acid etching under certain acid etching time and acid etching temperature conditions according to the uniaxial compression test data and the splitting test data obtained in step S3, and performing polynomial fitting on the elastic modulus, the Poisson's ratio and the tensile strength respectively with the acid etching temperature and the acid etching time to obtain an acid-etched rock mechanics parameter-acid etching temperature-acid etching time model, that is , and ; wherein T is the acid etching time, min; H is the acid etching temperature, °C; E E is the elastic modulus, MPa; μ ν is the Poisson's ratio, dimensionless; σ t σ is the tensile strength, MPa; S5, integrating the acid-eroded rock mechanical parameter-acid erosion temperature-acid erosion time model obtained in step S4 into a conventional formation fracture pressure model to obtain an acid fracturing fracture pressure prediction model considering acid erosion degradation of rock mechanical properties, as shown below: wherein, p f is the fracture pressure, MPa; and is the construction coefficient; p p is the formation pressure, MPa; α is the biot coefficient, σ z is the vertical stress, MPa; E is the elastic modulus in the acid-etched rock mechanical parameter-acid-etching temperature-acid-etching time model, MPa; μ is the Poisson's ratio in the acid-etched rock mechanical parameter-acid-etching temperature-acid-etching time model, dimensionless; σ t is the tensile strength in the acid-etched rock mechanical parameter-acid-etching temperature-acid-etching time model, MPa; S6, predicting the formation fracture pressure according to the acid fracturing fracture pressure prediction model considering acid erosion degradation of rock mechanical properties obtained in step S5.

2. The method of claim 1, wherein, In step S1, the standard uniaxial compression test rock samples and the standard splitting test rock samples are processed according to the method recorded in GB / T 50266-2013 "Standard for Engineering Rock Mass Test Method".

3. The method of claim 1, wherein, Step S3 specifically includes: placing the acid in a thermostat, preheating to the designed acid erosion temperature, then soaking the standard uniaxial compression test rock samples and the standard splitting test rock samples in the acid, taking out the rock samples when the soaking time reaches the designed acid erosion time, wiping off the surface acid and naturally air-drying to obtain acid-eroded uniaxial compression test rock samples and acid-eroded splitting test rock samples, and then performing uniaxial compression test and splitting test on the acid-eroded uniaxial compression test rock samples and the acid-eroded splitting test rock samples respectively to obtain uniaxial compression test data and splitting test data.

4. The method of claim 1, wherein, In step S3, the uniaxial compression test and the splitting test are performed according to the method recorded in GB / T 50266-2013 "Standard for Engineering Rock Mass Test Method".

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

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

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

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

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