A method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data

Abstract

The present invention discloses a method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data, which relates to the field of oil and gas exploration and development. It is characterized in that: the circumferential image scanning, rock porosity testing and rock triaxial compression experiment are adopted to obtain the lateral fracture fractal dimension, porosity, elastic modulus and Poisson's ratio of the cylindrical core sample, and then a calculation model for the elastic modulus and Poisson's ratio of the rock is constructed according to the experimental data, and the model parameters are determined. Then, the porosity and fracture fractal dimension of the formation surrounding rock obtained by interpreting the logging data of the formation to be studied are substituted into the calculation model for the elastic modulus and Poisson's ratio of the rock to obtain the elastic modulus and Poisson's ratio of the formation rock. The present invention fully considers the influence of the development of formation fractures and pores on the elastic modulus and Poisson's ratio of the rock. The calculation model for the elastic modulus and Poisson's ratio of the formation rock constructed based on the experimental data is more reasonable, and the calculation results do not require a large amount of experimental data for verification, which can provide guidance for reservoir, oil production and drilling design.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas exploration and development, and particularly relates to a method for predicting the elastic modulus and Poisson's ratio of formation rocks using logging data. Background Technique

[0002] The elastic modulus and Poisson's ratio are important mechanical parameters of rocks and are widely used in the field of oil exploration and development. In the direction of reservoir engineering, the elastic modulus and Poisson's ratio of formation rocks are important parameters for establishing geological models and predicting reservoir properties, and their accurate evaluation plays an important role in determining the distribution of oil and gas reservoirs, designing development plans, and predicting production. In the direction of drilling engineering, the elastic modulus and Poisson's ratio of formation rocks are important parameters for calculating the in-situ stress state of the rock around the well, the collapse and fracture pressures of the wellbore, and their accurate evaluation is of great significance for the design of oil and gas drilling and the safe implementation of drilling operations. In the direction of oil production engineering, the elastic modulus and Poisson's ratio of formation rocks are closely related to the size of the fracture width, the level of the average sand-fluid ratio, whether the fracture is sanded out, the degree of crushing or embedding of the proppant, the selection of the proppant type, and the design and optimization of other process parameters, directly affecting the effect of fracturing and the increase of oil and gas production. It can be seen that the elastic modulus and Poisson's ratio of formation rocks are important parameters in the field of oil and gas exploration and development, and accurately and quickly evaluating the elastic modulus and Poisson's ratio of formation rocks has important value for all aspects of oil and gas exploration and development.

[0003] The evaluation of the elastic modulus and Poisson's ratio of formation rocks usually adopts seismic inversion methods, well logging interpretation methods and laboratory rock mechanics experiment methods. The seismic inversion evaluation method is usually based on the AVO (Amplitude variation with offset) theoretical model, and calculates and analyzes the elastic modulus and Poisson's ratio of formation rocks by using the longitudinal and transverse wave velocities of the formation obtained from seismic exploration. However, this method has a complex model, time-consuming calculation and low accuracy, and is generally used for the preliminary estimation of the elastic modulus and Poisson's ratio of formation rocks. The well logging interpretation method is based on the relationship model between rock acoustics and rock mechanics properties, and calculates the elastic modulus and Poisson's ratio of formation rocks by using well logging acoustic signals. Although this method can obtain rich one-dimensional continuous data of the elastic modulus and Poisson's ratio of formation rocks, the calculation accuracy depends on the adaptability and reliability of the calculation model, and its accuracy is generally not high. Moreover, in the evaluation of the elastic modulus and Poisson's ratio of formation rocks in heterogeneous and fractured formations, the evaluation effect is not ideal due to the propagation differences of acoustic waves in fractures and different lithologies. The well logging interpretation method for evaluating the elastic modulus and Poisson's ratio of formation rocks generally requires the calibration of laboratory rock mechanics experiment parameters. The laboratory rock mechanics experiment method for evaluating the elastic modulus and Poisson's ratio of formation rocks is calculated by using the stress and strain in the elastic stage obtained from the rock triaxial compression test, which is a direct measurement method for the elastic modulus and Poisson's ratio of rocks. Scholars generally believe that this method can accurately and truly reflect the rock mechanics properties. However, the laboratory rock mechanics experiment has the disadvantages of requiring valuable core samples downhole, being time-consuming and costly, and having few test data points. At present, it is still mostly used for the calibration of well logging interpretation and seismic inversion results.

[0004] To accurately, quickly and economically evaluate the elastic modulus of formation rocks and the Poisson's ratio of formation rocks, the present invention provides a method for evaluating the elastic modulus of formation rocks and the Poisson's ratio by using well logging data. According to the laboratory rock mechanics experiment data, this method constructs a more accurate and reliable calculation model for the elastic modulus and Poisson's ratio of formation rocks by considering the porosity and fracture development characteristics of formation rocks, and then substitutes the porosity of the surrounding rock of the study formation and the fractal dimension of the fractures in the surrounding rock of the study formation obtained from the interpretation of well logging data into the model for calculation, so as to realize the effective evaluation of the elastic modulus and Poisson's ratio of formation rocks. The theoretical basis for establishing the method for evaluating the elastic modulus of formation rocks and the Poisson's ratio by using well logging data is as follows:

[0005] 1. Boyle's law

[0006] Given the volume V2 of the rock chamber and the volume V1 of the standard volume chamber, when measuring, the gas is first filled into the standard volume chamber, and the pressure p1 is recorded. Subsequently, the helium gas source is cut off, and the gas in the standard volume chamber is released into the rock chamber containing the cylindrical rock sample to be measured through a valve, and after the gas is isothermally expanded to equilibrium, the pressure p2 is recorded. According to Boyle's law, we have:

[0007] V1p1 = V2p2+(V2 - V m )p2 (1)

[0008] Where: V1 is the volume of the standard volume chamber, cm 3 ; V2 is the volume of the rock chamber, cm 3 ; p1 is the pressure when the measured gas is first filled into the standard volume chamber, MPa; p2 is the pressure at equilibrium in the rock chamber, MPa; V m is the skeleton volume of the cylindrical rock core sample, cm 3 .

[0009] 2. Box dimension method

[0010] Cover the curve with boxes of side length r of a square grid, and count the total number of boxes N required to cover the curve; assume that at the i-th step, boxes of r i ×r i are used to cover the curve, and the number of boxes required is N i , and the boxes covered at the i + 1-th step are r i+1 ×r i+1 , then the number of boxes required is N i+1 ; there is the following relationship between the ratio of the number of boxes required at any two scales and the ratio of the yardsticks:

[0011]

[0012] It indicates that the curve has fractal characteristics, and its fractal dimension D can be expressed as follows:

[0013]

[0014] Where: D is the fractal dimension of the curve, dimensionless; r is the side length of the square grid, mm; N(r) is the number of boxes occupied by the statistical curve, a positive integer.

[0015] 3. Nonlinear curve fitting

[0016] (1) Bradley nonlinear curve fitting

[0017] Bradley curve fitting is a statistical method for fitting nonlinear data, usually used to establish a model adapted to the nonlinear relationship to analyze experimental data. This method is based on the least squares method to obtain a nonlinear model to better describe the fitting data relationship. The expression of the nonlinear model used in the present invention is as follows:

[0018] y = aln[-bln(x)] (4)

[0019] Where: y is the dependent variable, dimensionless; x is the independent variable, dimensionless; a and b are model parameters, positive numbers, dimensionless. b controls the slope of the logarithmic function, and a controls the scaling of the logarithmic function.

[0020] (2) Nonlinear curve fitting of ExpDec2

[0021] The ExpDec2 model is one of the nonlinear models used to fit data and is commonly used to analyze data containing an exponential decay component. The expression of this model is as follows:

[0022]

[0023] In the formula: y is the dependent variable, dimensionless; x is the independent variable, dimensionless; A2 and A3 are model parameters representing the amplitudes of two exponential decays, dimensionless; k1 and k2 are the rate constants of the two exponential decays, dimensionless.

[0024] (3) Least squares method

[0025] The core idea of the least squares method is to fit a nonlinear model by finding the parameter values that minimize the sum of squared residuals. Its goal is to find the value of parameter θ such that the objective function L(θ) reaches the minimum, thereby obtaining an optimal fitting function. The expression of the least squares method objective function is as follows:

[0026]

[0027] In the formula: L(θ) is the least squares method objective function, dimensionless; n is the number of observed data, dimensionless; y i is the i-th observed data, dimensionless; x i is the corresponding independent variable value, dimensionless; θ is the model parameter, dimensionless.

[0028] (4) Coefficient of determination R 2

[0029] The coefficient of determination measures the degree to which the model explains the variability of the dependent variable. It indicates the goodness of fit of the regression model to the data. The formula is as follows:

[0030]

[0031] In the formula: R 2 is the coefficient of determination, dimensionless; y i is the model predicted value, dimensionless; is the mean of the dependent variable, dimensionless.

[0032] By performing circumferential image scanning, rock porosity testing, and triaxial compression experiments on cylindrical core specimens, the lateral crack fractal dimension, porosity, elastic modulus, and Poisson's ratio data of the cylindrical core specimens are obtained respectively. Using Bradley nonlinear curve fitting for the elastic modulus experimental data and ExpDec2 nonlinear curve fitting for the Poisson's ratio experimental data, the fitting models for the rock elastic modulus and Poisson's ratio are obtained as follows:

[0033] E t = aln[-bln(D t φ t )] (8)

[0034]

[0035] Where: E t is the elastic modulus of the cylindrical core specimen, MPa; D t is the lateral crack fractal dimension of the cylindrical core specimen, dimensionless; φ t is the porosity of the cylindrical core specimen, %; ν t is the Poisson's ratio of the cylindrical core specimen, dimensionless; t1, t2, y0 are model parameters, dimensionless. Summary of the Invention

[0036] The object of the present invention is to solve the problem of evaluating the elastic modulus and Poisson's ratio of formation rocks, and for this purpose, a method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data is proposed.

[0037] The technical solution adopted by the present invention is as follows:

[0038] Step 1.1: Drill more than 5 cylindrical core specimens with a diameter of φ25mm and a length of 50mm from the full-size rock sample of the research formation, and then use the circumferential image scanning method to obtain the image of the lateral crack development characteristics of the cylindrical core specimen, and determine the lateral crack fractal dimension D t .

[0039] Step 1.2: Conduct rock porosity testing on the cylindrical core specimen to obtain the porosity φ t of the cylindrical core specimen.

[0040] Step 1.3: Conduct triaxial compression experiments on the cylindrical core specimen to obtain the elastic modulus E t and the Poisson's ratio ν t of the cylindrical core specimen.

[0041] Step 1.4: According to the porosity φ t of the cylindrical core specimen and the lateral crack fractal dimension D t, the elastic modulus E of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , a calculation model for the elastic modulus and Poisson's ratio of the rock is constructed, and the model parameters are determined.

[0042] Step 1.5: Use the acoustic logging data of the studied formation to interpret and obtain the porosity φ of the formation surrounding rock l , use the imaging logging data of the studied formation to interpret and obtain the fracture fractal dimension D of the formation surrounding rock l , and use them to replace the porosity φ of the cylindrical core sample t and the lateral fracture fractal dimension D of the cylindrical core sample t , substitute them into the calculation model of the elastic modulus and Poisson's ratio of the rock, and obtain the elastic modulus E of the formation rock f and Poisson's ratio ν f .

[0043] Furthermore, the specific steps of Step 1.1 are as follows:

[0044] Step 1.1.1: Drill on a full-size rock sample of the representative studied formation to obtain more than 5 cylindrical core samples with a size of φ25mm×50mm.

[0045] Step 1.1.2: Use an image acquisition tool to perform circumferential image scanning on the cylindrical core sample to obtain an image of the lateral fracture development characteristics of the cylindrical core sample.

[0046] Step 1.1.3: According to the image of the lateral fracture development characteristics of the cylindrical core sample, use the box-counting method to calculate the lateral fracture fractal dimension D of the cylindrical core sample t , and the formula is as follows:

[0047]

[0048] In the formula: D t is the lateral fracture fractal dimension of the cylindrical core sample, dimensionless; r′ i is the side length of the square grid of the image of the lateral fracture development characteristics of the cylindrical core sample in the i-th step, mm; N′ i (r′) is the number of boxes occupied by the statistical curve of the image of the lateral fracture development characteristics of the cylindrical core sample in the i-th step, a positive integer.

[0049] Furthermore, the specific steps of Step 1.2 are as follows:

[0050] Step 1.2.1: Use a helium porosity measuring instrument based on Boyle's law to conduct rock porosity tests on the cylindrical core specimens obtained by drilling. During the measurement, the gas is first filled into the standard volume chamber, and the pressure p1 is recorded. Subsequently, the gas source is cut off, and the gas in the standard volume chamber is released into the rock chamber containing the cylindrical core specimen through a valve. After the gas expands isothermally to reach equilibrium, the pressure p2 is recorded.

[0051] Step 1.2.2: Calculate the skeleton volume V of the cylindrical core specimen according to the rock porosity test results. m , and the formula is as follows:

[0052]

[0053] In the formula: V m is the skeleton volume of the cylindrical core specimen, cm 3 ; V1 is the volume of the standard volume chamber, cm 3 ; V2 is the volume of the rock chamber, cm 3 ; p1 is the pressure when the measured gas is first filled into the standard volume chamber, MPa; p2 is the pressure at equilibrium in the rock chamber, MPa.

[0054] Step 1.2.3: Calculate the total volume V of the cylindrical core specimen according to the circular cross-section diameter D0 and the length L of the cylindrical core specimen. b , and the formula is as follows:

[0055]

[0056] In the formula: V b is the total volume of the cylindrical core specimen, mm; π is the pi, generally taken as 3.14; D0 is the circular cross-section diameter of the cylindrical core specimen, mm; L is the length of the cylindrical core specimen, mm.

[0057] Step 1.2.4: Calculate the porosity φ of the cylindrical core specimen according to the skeleton volume V m and the total volume V b of the cylindrical core specimen. t , and the formula is as follows:

[0058]

[0059] In the formula: φ t is the porosity of the cylindrical core specimen, %.

[0060] Furthermore, the specific steps of Step 1.3 are as follows:

[0061] Step 1.3.1: Conduct a triaxial compression test on the cylindrical core specimen to obtain the stress-strain curve of the cylindrical core specimen.

[0062] Step 1.3.2: Calculate the elastic modulus E of the cylindrical core specimen according to the axial stress difference Δσ1 between two points in the elastic stage of the stress-strain curve of the cylindrical core specimen and the axial strain difference Δε1 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen t , and the formula is as follows:

[0063]

[0064] In the formula: E t is the elastic modulus of the cylindrical core specimen, in MPa; Δσ1 is the axial stress difference between two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, in MPa; Δε1 is the axial strain difference corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, in %.

[0065] Step 1.3.3: Calculate the Poisson's ratio ν of the cylindrical core specimen according to the axial strain difference Δε1 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen and the radial strain difference Δε3 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen t , and the formula is as follows:

[0066]

[0067] In the formula: ν t is the Poisson's ratio of the cylindrical core specimen, dimensionless; Δε3 is the radial strain difference corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, in %.

[0068] Furthermore, the specific steps of Step 1.4 are as follows:

[0069] Step 1.4.1: Construct a rock elastic modulus calculation model according to the porosity φ of the cylindrical core specimen t , the lateral crack fractal dimension D of the cylindrical core specimen t and the elastic modulus E of the cylindrical core specimen t :

[0070] E t = a ln[-b ln(D t φ t )]

[0071] In the formula: a is a model parameter, in MPa; b is a model parameter, dimensionless.

[0072] Step 1.4.2: Use the porosity φ of the cylindrical core specimen t , the lateral crack fractal dimension D of the cylindrical core specimen t and the elastic modulus E of the cylindrical core speciment , according to the rock elastic modulus calculation model, the model parameters a and b are determined by fitting.

[0073] Step 1.4.3. According to the porosity φ of the cylindrical core sample t , the lateral crack fractal dimension D of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , a rock Poisson's ratio calculation model is constructed:

[0074]

[0075] In the formula: A1, A2, t1, t2, y0 are model parameters, dimensionless.

[0076] Step 1.4.4. Using the porosity φ of the cylindrical core sample t , the lateral crack fractal dimension D of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , according to the rock Poisson's ratio calculation model, the model parameters A1, A2, t1, t2 and y0 are determined by fitting.

[0077] Furthermore, the specific steps of the said Step 1.5 are as follows:

[0078] Step 1.5.1. Using acoustic logging data interpretation to obtain the porosity φ of the formation surrounding rock l , the formula is as follows:

[0079]

[0080] In the formula: φ l is the porosity of the formation surrounding rock, %; DT is the formation acoustic travel time, μs / m; DT ma is the acoustic travel time of the rock skeleton, μs / m; DT sh is the acoustic travel time of the relatively thick mudstone near the formation, μs / m; DT f is the acoustic travel time of the mud filtrate, μs / m; V sh is the shale content of the formation, %; C p is the formation compaction coefficient, dimensionless.

[0081] Step 1.5.2. Using imaging logging data interpretation to obtain the image of the fracture development characteristics of the formation surrounding rock, and according to the image of the fracture development characteristics of the formation surrounding rock, the box-counting method is used to calculate the fractal dimension D of the fractures in the formation surrounding rock l , the formula is as follows:

[0082]

[0083] In the formula: D l is the fractal dimension of the fractures in the formation surrounding rock, dimensionless; ri ″ is the side length of the square grid of the formation imaging logging image box in the i-th step, in mm; N i ″(r″) is the number of boxes occupied by the statistical curve of the formation imaging logging image in the i-th step, a positive integer.

[0084] Step 1.5.3. Substitute the porosity φ of the formation surrounding rock and the fractal dimension D of the formation surrounding rock fractures obtained by interpreting the logging data l and the fractal dimension D of the lateral fractures of the cylindrical core sample l into the calculation models of the elastic modulus and Poisson's ratio of the rock respectively, replacing the porosity φ of the cylindrical core sample t and the fractal dimension D of the lateral fractures of the cylindrical core sample t to obtain the elastic modulus E of the formation rock f and Poisson's ratio ν f .

[0085] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows:

[0086] In the present invention, the fractal dimension of the lateral fractures of the cylindrical core sample, the porosity of the cylindrical core sample, the elastic modulus of the cylindrical core sample, and the Poisson's ratio of the cylindrical core sample are obtained by using circumferential image scanning, rock porosity testing, and rock triaxial compression experiments. Furthermore, a calculation model of the elastic modulus and Poisson's ratio of the rock based on the fractal dimension of the rock fractures and porosity is constructed according to the experimental data, and the model parameters are determined. Then, the porosity of the formation surrounding rock and the fractal dimension of the formation surrounding rock fractures obtained by interpreting the logging data of the research formation are substituted into the calculation models of the elastic modulus and Poisson's ratio of the rock to obtain the elastic modulus and Poisson's ratio of the formation rock. The present invention fully considers the influence of the development of formation fractures and pores on the elastic modulus and Poisson's ratio of the formation rock. The calculation model of the elastic modulus and Poisson's ratio of the formation rock constructed based on the experimental data is more reasonable, and the calculation results do not require a large amount of experimental data for verification. The present invention provides a scientific and reliable method for evaluating the elastic modulus and Poisson's ratio of the formation rock, and can provide guidance for the analysis of reservoir reserves, fracturing transformation, wellbore stability, etc. Description of the Drawings

[0087] Figure 1 is a flow chart of the method for evaluating the elastic modulus and Poisson's ratio of the formation rock using logging data

[0088] Figure 2 is an image of the development characteristics of the lateral fractures of the cylindrical core sample

[0089] Figure 3 is a calculation diagram of the fractal dimension of the lateral fractures of the cylindrical core sample using Avizo software

[0090] Figure 4 is the stress-strain curve of the cylindrical core sample

[0091] Figure 5 For the experimental test results and model parameters of cylindrical core specimens and the surrounding rock of the studied formation

[0092] Figure 6 For the model curve and model parameters of the rock elastic modulus calculation

[0093] Figure 7 For the model curve and model parameters of the rock Poisson's ratio calculation

[0094] Figure 8 For the residual analysis diagram of the rock elastic modulus calculation model

[0095] Figure 9 For the residual analysis diagram of the rock Poisson's ratio calculation model

[0096] Figure 10 For the image of the fracture development characteristics of the surrounding rock obtained from the interpretation of the imaging logging data of the studied formation

[0097] Figure 11 For the calculation diagram of the fracture fractal dimension Avizo software of the fracture development characteristics image of the surrounding rock of the studied formation

[0098] Figure 12 For the error analysis of the evaluation results of the rock elastic modulus and Poisson's ratio of the studied formation Specific implementation manner

[0099] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the following further illustrates the present invention in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0100] The flowchart of a method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data implemented by the present invention is shown in Figure 1 as follows, and the specific description is as follows:

[0101] 1. Drill 6 cylindrical core specimens with a diameter of φ25mm and a length of 50mm on the full-size rock sample of the studied formation, and then use the circumferential image scanning method to obtain the lateral fracture development characteristic image of the cylindrical core specimen, and determine the lateral fracture fractal dimension D of the cylindrical core specimen t . The specific process is as follows:

[0102] (1) Drill 6 cylindrical core specimens with a diameter of φ25mm and a length of 50mm on the full-size rock sample of the studied formation, and require that the flatness of the end face section is not greater than 0.05mm, and the errors of the specimen height and diameter are not greater than 0.03mm, and there are no defects, good uniformity and consistent texture observed by the naked eye.

[0103] (2) Use an image acquisition tool to perform circumferential image scanning on the cylindrical core specimen to obtain an image of the lateral crack development characteristics of the cylindrical core specimen. The specific method is as follows: Place the cylindrical core specimen in the center of the turntable, rotate the turntable to make the cylindrical core specimen rotate at a constant speed of 10° / s, take a photo of the side of the cylindrical core specimen with a high-definition camera every 3 s. After the cylindrical core specimen rotates one full circle, 12 photos are obtained, and each photo records a 30° image of the side of the cylindrical core specimen. Stitch the 12 photos describing the lateral crack characteristics of the cylindrical core specimen to obtain an image of the lateral crack development characteristics of the cylindrical core specimen( Figure 2 ).

[0104] (3) Import the image of the lateral crack development characteristics of the cylindrical core specimen into Avizo software for image processing. Use Avizo software to identify the lateral cracks of the cylindrical core specimen, obtain the lateral crack development characteristics of the cylindrical core specimen, and calculate the fractal dimension D of the lateral cracks of the cylindrical core specimen according to the following formula t

[0105]

[0106] In the formula: D t is the fractal dimension of the lateral cracks of the cylindrical core specimen, dimensionless; r′ i is the side length of the square grid of the image of the lateral crack development characteristics of the cylindrical core specimen at the i-th step, in mm; N′ i (r′) is the number of boxes occupied by the statistical curve of the image of the lateral crack development characteristics of the cylindrical core specimen at the i-th step, a positive integer.

[0107] 2. Perform rock porosity testing on the cylindrical core specimen to obtain the porosity φ t of the cylindrical core specimen. The specific method is as follows:

[0108] (1) Use a helium porosity measuring instrument based on Boyle's law to perform rock porosity testing on the drilled cylindrical core specimen. During the measurement, the gas is first filled into the standard volume chamber, and the pressure p1 is recorded. Then, the gas source is cut off, and the gas in the standard volume chamber is released into the rock chamber containing the cylindrical core specimen through a valve. After the gas expands isothermally to reach equilibrium, the pressure p2 is recorded.

[0109] (2) Calculate the skeleton volume V m of the cylindrical core specimen according to the volume V1 of the standard volume chamber, the volume V2 of the rock chamber, the pressure p1 when the measured gas is first filled into the standard volume chamber, and the pressure p2 at the equilibrium of the rock chamber obtained from the rock porosity testing. The formula is as follows:

[0110]

[0111] In the formula: Vm is the skeletal volume of the cylindrical core sample, cm 3 ; V1 is the volume of the standard volume chamber, cm 3 ; V2 is the volume of the core chamber, cm 3 ; p1 is the pressure when the measured gas is first filled into the standard volume chamber, MPa; p2 is the pressure at equilibrium in the core chamber, MPa.

[0112] (3) Measure the diameter D0 of the circular cross-section of the cylindrical core sample and the length L of the cylindrical core sample, and calculate the total volume V of the cylindrical core sample b , the formula is as follows:

[0113]

[0114] In the formula: V b is the total volume of the cylindrical core sample, mm; π is the pi, generally taken as 3.14; D0 is the diameter of the circular cross-section of the cylindrical core sample, mm; L is the length of the cylindrical core sample, mm.

[0115] (4) According to the skeletal volume V m of the cylindrical core sample and the total volume V b of the cylindrical core sample, calculate the porosity φ t of the cylindrical core sample, the formula is as follows:

[0116]

[0117] In the formula: φ t is the porosity of the cylindrical core sample, %.

[0118] 3. Conduct a triaxial compression test on the cylindrical core sample to obtain the elastic modulus E t and the Poisson's ratio ν t of the cylindrical core sample. The specific method is as follows:

[0119] (1) Place the cylindrical core sample into the pressure chamber of the rigid testing machine, and simultaneously apply the confining pressure and axial stress at a loading rate of 0.05 MPa / s until the designed confining pressure value of 60 MPa is reached. Keep the confining pressure unchanged, increase the axial stress at a loading rate of 0.05 MPa / , and when the cylindrical core sample enters the yield stage, adjust to a displacement-controlled loading method with a rate of 0.2 mm / min to increase the axial stress until the cylindrical core sample is completely damaged. During the experiment, synchronously collect and record data such as axial stress, displacement, axial strain, and radial strain in real time to obtain the stress-strain curve in the triaxial compression test of the cylindrical core sample.

[0120] (2) Calculate the elastic modulus E of the cylindrical core sample according to the axial stress difference Δσ1 between two points in the elastic stage of the stress-strain curve of the cylindrical core sample and the axial strain difference Δε1 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core sample. The formula is as follows: t , the formula is as follows:

[0121]

[0122] In the formula: E t is the elastic modulus of the cylindrical core sample, MPa; Δσ1 is the axial stress difference between two points in the elastic stage of the stress-strain curve of the cylindrical core sample, MPa; Δε1 is the axial strain difference corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core sample, %.

[0123] (3) Calculate the Poisson's ratio ν of the cylindrical core sample according to the axial strain difference Δε1 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core sample and the radial strain difference Δε3 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core sample. The formula is as follows: t , the formula is as follows:

[0124]

[0125] In the formula: ν t is the Poisson's ratio of the cylindrical core sample, dimensionless; Δε3 is the radial strain difference corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core sample, %.

[0126] 4. Construct a calculation model for the elastic modulus and Poisson's ratio of the rock and determine the model parameters according to the porosity φ t of the cylindrical core sample, the lateral crack fractal dimension D t of the cylindrical core sample, the elastic modulus E t of the cylindrical core sample, and the Poisson's ratio ν t of the cylindrical core sample. The specific method is as follows:

[0127] (1) Use Origin software to perform Bradley non-linear curve fitting on the porosity φ t of the cylindrical core sample, the lateral crack fractal dimension D t of the cylindrical core sample, and the elastic modulus E t of the cylindrical core sample to obtain the calculation model for the elastic modulus of the rock:

[0128] E t = aln[- bln(D t φ t )] (16)

[0129] where: a is a model parameter, MPa; b is a model parameter, dimensionless.

[0130] (2) Using the porosity φ of the cylindrical core sample t , the lateral fracture fractal dimension D of the cylindrical core sample t and the elastic modulus E of the cylindrical core sample t , according to the rock elastic modulus calculation model, the model parameters a and b are determined by fitting.

[0131] (3) Using Origin software for the porosity φ of the cylindrical core sample t , the lateral fracture fractal dimension D of the cylindrical core sample t , the Poisson's ratio ν of the cylindrical core sample t , perform ExpDec2 non-linear curve fitting to obtain the rock Poisson's ratio calculation model:

[0132]

[0133] where: A1, A2, t1, t2, y0 are model parameters, dimensionless.

[0134] (4) Using the porosity φ of the cylindrical core sample t , the lateral fracture fractal dimension D of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , according to the rock Poisson's ratio calculation model, the model parameters A1, A2, t1, t2 and y0 are determined by fitting.

[0135] 5. Using the formation acoustic logging data interpretation to obtain the formation surrounding rock porosity φ l , using the formation imaging logging data interpretation to obtain the formation surrounding rock fracture fractal dimension D l , respectively replace them with the porosity φ of the cylindrical core sample t and the lateral fracture fractal dimension D of the cylindrical core sample t , substitute them into the rock elastic modulus and Poisson's ratio calculation models to obtain the formation rock elastic modulus E f and Poisson's ratio ν f . The specific method is as follows:

[0136] (1) Using the formation acoustic logging data interpretation to obtain the formation surrounding rock porosity φ l , the formula is as follows:

[0137]

[0138] where: φ l is the formation surrounding rock porosity, %; DT is the formation acoustic travel time, μs / m; DT ma is the rock frame acoustic travel time, μs / m; DTsh is the acoustic travel time of the relatively thick mudstone near the formation, μs / m; DT f is the acoustic travel time of the mud filtrate, μs / m; V sh is the shale content of the formation, %; C p is the formation compaction coefficient, dimensionless.

[0139] (2) Use the imaging logging data interpretation to obtain the image of the fracture development characteristics of the formation surrounding rock, and calculate the fractal dimension D of the formation surrounding rock fractures by the box counting method according to the image of the fracture development characteristics of the formation surrounding rock l , and the formula is as follows:

[0140]

[0141] In the formula: D l is the fractal dimension of the formation surrounding rock fractures, dimensionless; r i ″ is the side length of the square grid of the box of the formation imaging logging image at the i-th step, mm; N i ″(r″) is the number of boxes occupied by the statistical curve of the formation imaging logging image at the i-th step, a positive integer.

[0142] (3) Substitute the porosity φ l of the formation surrounding rock obtained from the logging data interpretation and the fractal dimension D l of the formation surrounding rock fractures into the calculation models of the elastic modulus and Poisson's ratio of the rock respectively, replacing the porosity φ t of the cylindrical core sample and the fractal dimension D t of the lateral fractures of the cylindrical core sample, to obtain the elastic modulus E f and Poisson's ratio ν f of the formation rock.

[0143] Implementation case

[0144] The formation studied in this example is the Permian volcanic rock formation of Well YT-1, with a vertical depth of 5840 - 5880 m. First, 6 cylindrical core samples of φ25mm×50mm were drilled from the full-size rock samples of the studied formation. Subsequently, the circumferential image scanning method was used to obtain the image of the lateral fracture development characteristics of the cylindrical core samples, and the fractal dimension of the lateral fractures of the cylindrical core samples was calculated by Avizo software for the image of the lateral fracture development characteristics of the cylindrical core samples, as shown in Figure 3 . Then, the rock porosity test was carried out on the cylindrical core samples to obtain the porosity test data of the cylindrical core samples. Finally, the triaxial compression test was carried out on the cylindrical core samples to obtain the stress-strain curve, as shown in Figure 4 . According to the above method, the fractal dimension D t of the lateral fractures of the cylindrical core samples, the porosity φ t of the cylindrical core samples, and the elastic modulus E tand the Poisson's ratio ν of the cylindrical core sample t , a calculation model for the elastic modulus and Poisson's ratio of the rock was constructed, and the curve of the calculation model for the elastic modulus of the rock and the model parameters a and b are shown in Figure 6 , the curve of the calculation model for the Poisson's ratio of the rock and the model parameters A1, A2, t1, t2, y0 are shown in Figure 7 . The porosity φ of the formation surrounding rock was obtained by interpreting the acoustic logging data of the research formation l . The image of the fracture development characteristics of the surrounding rock obtained by interpreting the imaging logging data of the research formation is shown in Figure 10 . According to the image of the fracture development characteristics of the surrounding rock, the fractal dimension D of the fractures in the formation surrounding rock was calculated using Avizo software l , as shown in Figure 11 . According to the above methods and the fractal dimension D of the lateral fractures of the cylindrical core sample obtained from the experiment t , the porosity φ of the cylindrical core sample t , the elastic modulus E of the cylindrical core sample t , the Poisson's ratio ν of the cylindrical core sample t , the fractal dimension D of the fractures in the formation surrounding rock of the research l , the porosity φ of the formation surrounding rock of the research l , and the model parameters a, b, A1, A2, t1, t2 and y0 are shown in Figure 5 .

[0145] From Figure 6 and Figure 7 it can be seen that generally, as the product of the fractal dimension D t and the porosity φ of the cylindrical core sample increases t , the elastic modulus E of the cylindrical core sample t gradually decreases, while the Poisson's ratio ν of the cylindrical core sample t gradually increases. And from Figure 6 and Figure 7 it can also be intuitively seen that the calculation models for the elastic modulus and Poisson's ratio of the rock described in this invention patent can better describe the above variation trend.

[0146] Figure 8 and Figure 9They are respectively the residual analysis diagrams of the rock elastic modulus calculation model and the rock Poisson's ratio calculation model. It can be seen from the diagrams that: 1) The fitted Y values of the two calculation models and the conventional residuals show a uniform scatter distribution, without obvious trends or patterns. The residuals have little randomness and heteroscedasticity, showing independence; 2) The points in the conventional residual percentile diagrams of the two calculation models all fall on the 45° diagonal line, and the residuals follow a normal distribution; 3) The heights of the bars in the Counts bar charts of the two calculation models are close to zero and there are no obvious outliers, indicating a good fitting effect. Thus, it can be known that the rock elastic modulus calculation model and the rock Poisson's ratio calculation model meet the normality and independence tests, and the determination coefficients R 2 are 0.88 and 0.92 respectively, indicating a good fitting effect of the models.

[0147] Figure 12 For the error analysis of the evaluation results of the formation rock elastic modulus and Poisson's ratio. The model calculation values in the diagrams are the results calculated by using the method of the present invention based on acoustic logging data and imaging logging data; the experimental values in the diagrams are the triaxial compression experimental values of the core samples taken from the same depth of this well. It can be seen from the diagrams that, compared with the experimental values, the relative error values of the rock elastic modulus calculated by the model are 7.17% - 10.85%, and the relative error values of the rock Poisson's ratio are 3.3 - 10%, indicating a high degree of agreement between the formation rock elastic modulus and Poisson's ratio evaluated by the present invention and the experimental values. Thus, it can be seen that the present invention can be used for the evaluation of the formation rock elastic modulus and Poisson's ratio in the actual engineering of the oil and gas exploration and development field.

[0148] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data, characterized in that, The implementation steps are as follows: Step 1.1: Drill more than 5 cylindrical core specimens with a diameter of φ25mm and a length of 50mm from the full-size rock samples of the research formation. Then, use the circumferential image scanning method to obtain the lateral crack development characteristic images of the cylindrical core specimens, and determine the fractal dimension D of the lateral cracks of the cylindrical core specimens t ; Step 1.2: Perform rock porosity tests on the cylindrical core specimens to obtain the porosity φ of the cylindrical core specimens t ; Step 1.3: Conduct a triaxial compression test on the cylindrical core sample to obtain the elastic modulus E of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t ; Step 1.

4. According to the porosity φ of the cylindrical core sample t , the lateral fracture fractal dimension D of the cylindrical core sample t , the elastic modulus E of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , construct a calculation model for the elastic modulus and Poisson's ratio of the rock, and determine the model parameters; Step 1.5: Interpret the formation acoustic logging data to obtain the porosity φ of the formation surrounding rock l , interpret the imaging logging data to obtain the image of the fracture development characteristics of the formation surrounding rock, and calculate the fractal dimension D of the formation surrounding rock fractures by using the box counting method according to the image of the fracture development characteristics of the formation surrounding rock l , and replace the porosity φ of the cylindrical core sample with them respectively t and the fractal dimension D of the lateral fractures of the cylindrical core sample t , substitute them into the calculation models of the rock elastic modulus and Poisson's ratio, and obtain the elastic modulus E of the formation rock f and Poisson's ratio ν f ; The specific steps of step 1.4 are as follows: Step 1.4.

1. Based on the porosity φ of the cylindrical core sample t , the fracture fractal dimension D of the cylindrical core sample t and the elastic modulus E of the cylindrical core sample t , construct a calculation model for the elastic modulus of the rock: E t = a ln[-b ln(D t φ t )] In the formula: a is a model parameter, MPa; b is a model parameter, dimensionless; Step 1.4.2: Using the porosity φ of the cylindrical core sample t , the fracture fractal dimension D of the cylindrical core sample t and the elastic modulus E of the cylindrical core sample t , according to the rock elastic modulus calculation model, fittingly determine the model parameters a and b; Step 1.4.

3. According to the porosity φ of the cylindrical core sample t , the fracture fractal dimension D of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , construct a calculation model for the Poisson's ratio of rock: In the formula: A1, A2, t1, t2, y0 are model parameters, dimensionless; Step 1.4.4: Use the porosity φ of the cylindrical core sample t , the lateral fracture fractal dimension D of the cylindrical core sample t and the Poisson's ratio ν of the cylindrical core sample t , and according to the rock Poisson's ratio calculation model, fit and determine the model parameters A1, A2, t1, t2 and y0.

2. A method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data according to claim 1, characterized in that The specific steps of step 1.1 are as follows: Step 1.1.1: Drill and obtain more than 5 cylindrical core specimens with a size of φ25mm×50mm from the full-size rock samples of the representative research formation; Step 1.1.2: Use an image acquisition tool to perform circumferential image scanning on the cylindrical core specimens to obtain the lateral crack development characteristic images of the cylindrical core specimens; Step 1.1.3: Calculate the lateral fracture fractal dimension D of the cylindrical core sample by using the box-counting method according to the image of the lateral fracture development characteristics of the cylindrical core sample t , and the formula is as follows: where: D t is the lateral fracture fractal dimension of the cylindrical core specimen, dimensionless; r i ′ is the side length of the square grid of the lateral fracture development characteristic image of the cylindrical core specimen at the i-th step, mm; N i ′(r′) is the number of boxes occupied by the statistical curve of the lateral fracture development characteristic image of the cylindrical core specimen at the i-th step, a positive integer.

3. A method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data according to claim 1, characterized in that The specific steps of step 1.2 are as follows: Step 1.2.1: Use a helium porosity measuring instrument based on Boyle's law to perform rock porosity tests on the drilled cylindrical core specimens: When measuring, the gas is first filled into the standard volume chamber, and the pressure p1 is recorded. Subsequently, the gas source is cut off, and the gas in the standard volume chamber is released into the rock chamber containing the cylindrical core specimens through the valve, and after the gas isothermally expands to equilibrium, the pressure p2 is recorded; Step 1.2.

2. Calculate the volume V of the skeleton of the cylindrical core specimen according to the rock porosity test results. m The formula is as follows: Where: V m is the skeleton volume of the cylindrical core sample, cm 3 ; V1 is the volume of the standard volume chamber, cm 3 ; V2 is the volume of the core chamber, cm 3 ; p1 is the pressure when the measured gas is first charged into the standard volume chamber, MPa; p2 is the pressure at equilibrium in the core chamber, MPa; Step 1.2.

3. Calculate the total volume V of the cylindrical core sample according to the diameter D0 of the circular cross-section of the cylindrical core sample and the length L of the cylindrical core sample b , and the formula is as follows: Where: V b is the total volume of the cylindrical core sample, in mm; π is the pi, generally taken as 3.14; D0 is the diameter of the circular cross-section of the cylindrical core sample, in mm; L is the length of the cylindrical core sample, in mm; Step 1.2.

4. Calculate the porosity φ of the cylindrical core sample according to the skeletal volume V m of the cylindrical core sample and the total volume V b of the cylindrical core sample. The formula is as follows: t ​ where: φ t is the porosity of the cylindrical core sample, %.

4. A method for evaluating the elastic modulus and Poisson's ratio of formation rocks using logging data according to claim 1, characterized in that, The specific steps of step 1.3 are as follows: Step 1.3.1: Perform a triaxial compression experiment on the cylindrical core specimens to obtain the stress-strain curve of the cylindrical core specimens; Step 1.3.2: Calculate the elastic modulus E of the cylindrical core specimen based on the axial stress difference Δσ1 between two points in the elastic stage of the stress-strain curve of the cylindrical core specimen and the axial strain difference Δε1 corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen. The formula is as follows: t , as follows: where: E t is the elastic modulus of the cylindrical core specimen, in MPa; Δσ1 is the difference in axial stress between two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, in MPa; Δε1 is the difference in axial strain corresponding to the axial stress between two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, in %; Step 1.3.3: Calculate the Poisson's ratio ν of the cylindrical core specimen based on the axial strain difference Δη1 corresponding to two axial stresses in the elastic stage of the stress-strain curve of the cylindrical core specimen and the radial strain difference Δη3 corresponding to two axial stresses in the elastic stage of the stress-strain curve of the cylindrical core specimen. The formula is as follows: t , and the formula is as follows: where: ν t is the Poisson's ratio of the cylindrical core specimen, dimensionless; Δε3 is the difference in radial strain corresponding to the axial stress at two points in the elastic stage of the stress-strain curve of the cylindrical core specimen, %.

5. A method for predicting the Poisson's ratio and elastic modulus of formation rocks using logging data according to claim 1, characterized in that, The specific steps of step 1.5 are as follows: Step 1.5.

1. Obtain the porosity φ of the formation surrounding rock by interpreting acoustic logging data l , and the formula is as follows: where: φ l is the porosity of the formation surrounding rock, %; DT is the acoustic travel time of the formation, μs / m; DT ma is the acoustic travel time of the rock matrix, μs / m; DT sh is the acoustic travel time of the relatively thick mudstone near the formation, μs / m; DT f is the acoustic travel time of the mud filtrate, μs / m; V sh is the shale content of the formation, %; C p is the formation compaction coefficient, dimensionless; Step 1.5.

2. Interpret the imaging logging data to obtain the image of the fracture development characteristics of the formation surrounding rock, and calculate the fractal dimension D of the fractures in the formation surrounding rock by using the box-counting method according to the image of the fracture development characteristics of the formation surrounding rock l , and the formula is as follows: where: D l is the fractal dimension of fractures in the formation surrounding rock, dimensionless; r i ″ is the side length of the square grid of the box in the formation image logging image of the i-th step, mm; N i ″(r″) is the number of boxes occupied by the statistical curve of the formation image logging image of the i-th step, a positive integer; Step 1.5.

3. Substitute the formation surrounding rock porosity φ l and the formation surrounding rock fracture fractal dimension D l obtained from well logging data interpretation, respectively, for the porosity φ t of the cylindrical core sample and the lateral fracture fractal dimension D t of the cylindrical core sample, and substitute them into the calculation models of rock elastic modulus and Poisson's ratio to obtain the formation rock elastic modulus E f and Poisson's ratio νf.

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