Compressibility evaluation method based on post-compression coring nondestructive mechanical experiment

Abstract

The invention discloses a compressibility evaluation method based on a post-compression coring nondestructive mechanical experiment, and relates to the technical field of coring and testing. The method comprises the following specific steps: carrying out pressure-maintaining coring on a core layer position of a fractured well according to specific layer position requirements of a development block to obtain a high-quality fractured rock core sample; and performing high-energy CT ring scanning on the collected post-fracturing coring sample, and distinguishing by adopting a gray scale method to obtain a specific fracture three-dimensional distribution form of a rock core in a fracture development area, so as to realize quantitative evaluation of the development degree of the natural fracture before and after fracturing. According to the compressibility evaluation method based on the after-fracturing coring nondestructive mechanics experiment, natural fracture distribution, rock mechanics and ground stress characteristics after volume fracturing are evaluated are tested, key compressibility evaluation parameters can be obtained on the basis that rock cores are not damaged, and the compressibility effect is evaluated; and the method has important significance in guiding subsequent volume fracturing to carry out targeted fracturing parameter design.

Description

Technical Field

[0001] The present invention belongs to the technical field of coring and testing, and in particular relates to a compressibility evaluation method based on a non-destructive mechanical experiment of coring after compression. Background Art

[0002] In the development of unconventional oil and gas reservoirs such as shale and tight sandstone, hydraulic fracturing technology is the core means to achieve efficient extraction. As a key step in pre-fracturing design, hydraulic fracturing assessment directly determines the targetedness and effectiveness of fracturing operations. Although the currently widely used engineering hydraulic fracturing model in the industry has formed a multi-parameter comprehensive evaluation system, its core relies on basic parameters such as rock brittleness, the degree of natural fracture development, and the ground stress state. Furthermore, the weighting of each parameter is often dominated by human experience and is highly subjective, which can easily lead to deviations between the evaluation results and the actual reservoir stimulation potential.

[0003] From the perspective of data support, the construction of existing models mostly relies on indirect geophysical data such as well logging and seismic data. Field production profile data is only used as a means of later verification, lacking "first-hand evidence" that can directly reflect the characteristics of reservoir fracturing response. At the same time, although dynamic fracture monitoring technologies such as microseismic monitoring and distributed fiber optic sensing can capture fracture expansion trends in real time, they are limited by complex underground geological conditions and monitoring accuracy. The fracture network morphology inverted through the integrated geological and engineering numerical model often differs from the actual underground fracture distribution, making it difficult to use as a direct basis for compressibility assessment. Therefore, how to break through the existing technical bottleneck and establish a set of compressibility verification and evaluation methods based on intuitive and reliable data has become an important research direction for the oil and gas industry to improve fracturing efficiency.

[0004] The emergence of post-fracture coring technology offers a new solution to this problem. Using specially designed conformal coring tools, this technology can obtain core samples from the target interval after large-scale hydraulic fracturing, preserving the fracture morphology. This allows for a direct demonstration of the true effects of hydraulic fracturing. This contrasts sharply with the limitations of conventional coring, which destroys the original fracture structure during drilling. Internationally, the HTFS (Hydraulic Fracturing Test Site) project, conducted by several US oil companies and research institutions in the Wolfcamp Formation of the Permian Basin, has demonstrated the irreplaceable effectiveness of this technology in revealing the authenticity of hydraulic fracturing effects through large-scale post-fracture coring experiments in 11 horizontal wells and one highly deviated well.

[0005] In order to maximize the research value of precious post-fracture cores, non-destructive experimental technology has become the key; CT scanning technology can efficiently obtain the three-dimensional distribution characteristics of the core's internal structure and fracture connectivity through computer tomography. Combined with digital image processing and fractal theory (such as the box counting dimension method), it can achieve quantitative characterization of the complexity of the fracture network, providing support for the calculation of key parameters such as fracture permeability and porosity; and mechanical analysis methods such as rock surface scratch experiments and non-destructive stress testing can continuously obtain mechanical parameters such as internal friction angle, compressive strength, and ground stress without destroying the integrity of the core, providing direct data for evaluating the mechanical transformation effect of fracturing on the reservoir; the integrated application of these technologies has laid a solid foundation for the construction of a compressibility assessment system based on post-fracture coring.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a compressibility evaluation method based on non-destructive mechanical experiments of coring after compression, thereby solving the problems raised in the above-mentioned background technology.

[0008] In order to solve the above technical problems, the basic concept of the technical solution adopted by the present invention is:

[0009] A compressibility assessment method based on non-destructive mechanical testing of coring after compression includes the following steps:

[0010] S01: Based on the specific stratum requirements of the development block, pressure-maintained coring is performed in the core layers of the fracturing coring wells that represent structural, geological, and stress characteristics to obtain high-quality, full-diameter downhole core samples. Based on the formation properties, downhole temperature, and pressure conditions, the core claw structure is optimized, a low-friction inner barrel and a dedicated coring drill bit are selected, and high-quality post-compression coring samples are obtained using a long-barrel coring operation post-compression coring tool to ensure the success rate and quality of coring and reduce rock fragmentation and disturbance during the coring process.

[0011] S02: Perform high-quality CT ring scanning on the core sampled after fracturing to obtain the three-dimensional spatial distribution characteristics of the cracks, use the grayscale method to distinguish the specific three-dimensional distribution morphology of the cracks in the fracturing area, and identify the original natural cracks and post-fracturing cracks; Among them, use CT technology to perform three-dimensional ring scanning on the cored rock samples, so that sufficient data can be used to quantitatively describe the internal structure of the rock and natural cracks, reducing the uncertainty of natural crack characterization caused by differences in scanning positions. After the ring scanning is completed, a unified section is processed, and a continuous well section of 10 to 20 meters is selected for coring, so that the subsequent sample rock mechanics, CT ring scanning and other integrated analyses meet the requirements of parallel mechanics experiments; the rock samples are processed by high-energy CT ring scanning method, Considering the existence of various types of fracture structures inside the rock; at the same time, the grayscale differentiation principle is used to grayscale process the matrix, natural fractures, unsupported fractures, and supported fractures in the rock sample to distinguish various fracture types. First, the original natural fracture development density is analyzed to determine the development density of natural fractures. Secondly, after removing the natural fractures that have not been activated and disturbed inside the rock, the fracture network development morphology composed of hydraulic fractures and activated natural fractures after fracturing is obtained to achieve the characterization of the fracture network; based on the collected CT images, the distribution of natural fractures is quickly identified with the help of the optimized Frangi filter algorithm; when processing images, the algorithm can effectively retain the edge features of the fractures and smooth the background noise. At the same time, the algorithm uses the improved Hessian matrix eigenvalue discrimination method to enhance the recognition ability of small fractures in the image; finally, the algorithm combines the optimized Frangi filter with the automatic threshold segmentation technology to perform morphological processing on the image to filter out the smaller non-fracture areas, thereby achieving high-precision recognition of fractures;

[0012] S03: Use the surface scratch method to conduct mechanical scratch tests on the surface of the full-diameter rock core to obtain the rock mechanical properties. Among them, the surface scratch mechanical equipment is used to perform non-destructive scratch mechanical tests on the surface of the rock sample to obtain the basic rock mechanical properties of the rock, mainly the rock compressive strength and fracture toughness parameters. The rock surface needs to be mechanically polished and sandpaper is used to grind the surface to ensure the surface flatness.

[0013] S04: Carry out ground stress test using digital speckle-finite element method to obtain ground stress conditions;

[0014] S05: Based on fractal theory, a comprehensive fracture development index evaluation method for both pre- and post-fracturing fractures was established from the perspectives of fracture complexity and fracture reconstruction volume. Analytical theory was then used to quantitatively interpret fracture development characteristics. Fractal theory was used to simultaneously evaluate and calculate fracture complexity and its proportion in the entire core to determine a comprehensive fracture development index. Furthermore, a multifractal approach was used to reconstruct the degree of fracture development. The number of small cubes (Nk) required to cover the fracture space varied with cube size.

[0015] S06: Construct a compressibility assessment model that includes the development characteristics of natural fractures, rock mechanical properties, and stress conditions, and modify the weights through the comprehensive natural fracture development index to achieve correction of the compressibility assessment model, and use it for subsequent on-site compressibility assessment of fracturing wells; among them, a comprehensive fracture development index suitable for post-fracturing evaluation is constructed in combination with the fracture type for compressibility assessment, and the compressibility model includes key indicators of mechanical parameters, stress parameters, and natural fracture parameters.

[0016] Optionally, a digital speckle-finite element method non-destructive geostress test is performed on the obtained core after compression to obtain the geostress distribution characteristics.

[0017] After adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art. Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described below at the same time:

[0018] 1. The present invention proposes a technical route for conducting experiments and comprehensive evaluation of compressibility through post-compression coring parameters, which can intuitively evaluate the compressibility of the project;

[0019] 2. This invention constructs a comprehensive fracturing evaluation model based on post-fracturing coring data from four perspectives: fracture density, scratch strength, internal friction angle, and minimum horizontal principal stress. This model accurately determines the artificial fracturing status of high-quality intervals and targets precise development goals, thereby effectively improving the accuracy of artificial fracturing treatment in target areas.

[0020] 3. The present invention emphasizes the use of non-destructive experiments to obtain engineering compressibility parameters. On the one hand, it can ensure the parallel testing of mechanics and stress, and on the other hand, it can also realize the repeated use of the core after compression, thereby increasing the application value of the core;

[0021] 4. The present invention provides a reservoir fracturing evaluation method for unconventional oil and gas exploration effect evaluation and development plan design, which can improve the purposefulness and effectiveness of fracturing, thereby accelerating the progress of unconventional oil and gas exploration and greatly reducing exploration and development costs.

[0022] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings described below are only some embodiments. A person skilled in the art can derive other drawings based on these drawings without inventive effort. In the drawings:

[0024] Figure 1 This is a flow chart for evaluating the fracturing effect based on post-fracturing coring technology;

[0025] Figure 2 The experimental results of rock scratch mechanics tests (matrix area, crack development area);

[0026] Figure 3 Schematic diagram of the fractal identification results of cracks before and after compression;

[0027] Figure 4 This is a schematic diagram of the CT scanning identification results of the core fracture after compression;

[0028] Figure 5 Schematic diagram comparing on-site microseismic monitoring and compressibility assessment and prediction based on post-compression coring mechanical experiments.

[0029] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but rather to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0030] The present invention will now be described in further detail with reference to the accompanying drawings.

[0031] See also Figure 1-5 As shown, in this embodiment, a compressibility evaluation method based on a non-destructive mechanical experiment of coring after compression is provided, comprising the following steps:

[0032] According to the specific layer requirements of the development block, pressure-maintained coring is carried out on the core layers of the fracturing wells that represent the structural, geological and stress characteristics to obtain high-quality downhole full-diameter core samples;

[0033] A CT ring scan is performed on the core sampled after fracturing. Through a comprehensive comparative analysis of pre- and post-fracturing imaging logging, core photos, CT scans, and logging curves conducted at the same depth in adjacent wells in the same block, a grayscale method is used to distinguish the specific three-dimensional distribution morphology of fractures before and after fracturing. The fracture locations are identified by setting a specific grayscale prefabrication and combining regional growing and segmentation algorithms. Considering the problems of many noise points and poor micro-fracture identification accuracy in the CT ring scan of actual cores, the conventional threshold segmentation method is improved and a noise reduction method is used to process the CT images, thereby more accurately identifying fractures. Finally, the entire fracture data is spatially registered and superimposed to obtain the spatial distribution characteristics of the three-dimensional fracture morphology.

[0034] The surface scratch method is used to conduct mechanical scratch tests on full-diameter rock cores to obtain the mechanical properties of the rock. Before conducting the mechanical test, the rock sample test surface needs to be polished flat and smooth to ensure that the scratching tool head can evenly contact the scratched surface of the sample. The key mechanical parameters can be calculated using the scraping rock mechanics model as follows:

[0035] S a =F na / (wd) (1)

[0036] Where Sa is the scratch strength of the area before fracturing, MPa; w is the width of the scratch tip, mm; d is the scratch depth, mm; Fna is the normal component of the force on the friction surface of the tip, N;

[0037] The internal friction angle parameter of the rock can be further obtained from the scratch data. The calculation formula is as follows:

[0038]

[0039] Where, φa is the internal friction angle of rock, °; μ is the friction coefficient;

[0040] Based on statistics and fractal theory, a comprehensive fracture development index evaluation method is established from the perspectives of fracture complexity and fracture transformation volume. Based on the collected CT images, the distribution of natural fractures is quickly determined using a fracture identification algorithm based on an improved Frangi filter. Based on the preprocessed images, an improved Hessian matrix eigenvalue discrimination method is used to enhance the identification of small fractures in the image. Combined with an automatic thresholding method, small non-crack areas are filtered out to achieve high-precision extraction of fracture areas. Guided filtering is a filter that can perform a local linear transformation on the guided image while filtering, which can significantly reduce the time required for image processing while preserving edges and removing noise. For a given input image F, the specific derivation process of the guided filtering algorithm is shown in the formula.

[0041]

[0042] Where: i is a pixel; I is the guide image; O is the output image; wk is the neighborhood window centered on pixel k; ck and dk are constant linear coefficients in the square neighborhood window, obtained by minimizing the cost function, specifically representing the difference between the output image O and the input image F;

[0043]

[0044] Where: ε is the regularization parameter; f i Represents the grayscale value of the input image F at pixel i.

[0045] By calculating the mean grayscale value of all pixels in the target area in the local variance, the regularization parameter in the minimization cost function can be obtained; in order to prevent ck from being too large during the calculation process, the actual value of ε is 0.12;

[0046] Furthermore, according to the degree to which the responses of different regions in the image are affected by the eigenvalues ​​of the Hessian matrix, a method for determining the eigenvalues ​​of the Hessian matrix is ​​constructed.

[0047]

[0048] Where: β is the sensitivity coefficient, which reflects the degree of distinguishing the strip area and is positively correlated with the strength of the noise suppression effect; c is the smoothness coefficient, which reflects the smoothness of the image. The larger the c is, the higher the image smoothness is, and vice versa. R b′ represents the eigenvalue ratio, which is used to quantify the relative size of the eigenvalues ​​of the Hessian matrix; where λ1 and λ2 are the absolute minimum and maximum eigenvalues ​​of the Hessian matrix; S represents the eigenvalue norm, which is used to quantify the overall size of the eigenvalues ​​of the Hessian matrix.

[0049] Since the sizes of cracks are complex and uneven, a multi-scale data enhancement method is needed to ensure the comprehensiveness and diversity of crack data;

[0050] The distribution characteristics of the natural fracture development index were statistically analyzed, and a core-scale natural fracture development degree grading evaluation scheme was established based on the tertile statistical analysis method. The fracture development degrees were high, medium, and low, corresponding to fracture development indices ≥1 and 0.1 to 1≤0.1. The complexity was calculated using the fractal dimension method. The digital image of the fracture network can be calculated using the cube covering method to calculate its fractal dimension. The fracture development degree needs to be reconstructed using the multifractal method. The number of small cubes (Nk) required to cover the fracture space will vary with the size of the cube. The pixel values ​​are either 0 or 1, with a value of 1 indicating a fracture. Therefore, the maximum value of the pixels contained in the cube coverage area is greater than 0 and can be accumulated and counted towards the number of cubes required to cover the fracture space. The entire fracture space is traversed to obtain the number of cubes covering the target area after c equal divisions. The fracture image consists of a series of pixels, each with a corresponding grayscale. The space is divided into cubes of size m×n×h pixels. The bisection method is used to determine the side length of the small cube, which is calculated as follows:

[0051]

[0052] Where: δ k is the side length of the small cube; c is the number of equal divisions by bisection (a positive integer), indicating the number of times the image space is divided into equal parts; m, n, h are the total number of pixels of the image in the x, y, and z directions (such as the width direction).

[0053] The number of small cubes required to cover the crack space (N k ) will vary with the size of the cube; a binary crack image is obtained after image preprocessing; the pixel values ​​are 0 or 1, and a value of 1 indicates a crack; therefore, the maximum value of the pixels contained in the cube coverage area is greater than 0, which can be accumulated into the number of cubes required to cover the crack space; the entire crack space is traversed to obtain the number of cubes covering the target area after being divided into c equal times; for any value of c, the corresponding number of cubes can be calculated, and a series of data pairs (1 / c k-1 , N k ); In the double logarithmic coordinate system, (1 / c k-1 , N k ) can reflect the fractal characteristics, its slope is the fractal dimension of the crack, and the equation of the line is:

[0054]

[0055] Where: N k is the number of small cubes required to cover the crack space; D is the fractal dimension of the crack (dimensionless), which reflects the complexity of the crack network.

[0056] The digital speckle-finite element method is used to conduct geostress testing and obtain the stress conditions of the test sample. The digital speckle correlation method (DSCM) program is then used to analyze and calculate the small deformation of the rock. In addition, the combination of sub-pixel interpolation and correlation coefficient interpolation effectively improves the search efficiency and calculation accuracy of the digital speckle correlation method. In order to characterize the similarity between the reference image and the target image sub-region, a similarity function C is required. Digital speckle uses the correlation function as a judgment basis to search in the reference image and the target image to find the sub-region where the correlation function is the extreme value. The commonly used correlation functions are generally the following:

[0057] Normalized covariance correlation function:

[0058]

[0059] Normalized correlation function:

[0060]

[0061] Direct cross-correlation function:

[0062]

[0063] Where: f is the grayscale value of the reference image sub-area at the pixel point; g is the grayscale value of the target image sub-area at the pixel point; is the average gray value of the reference image sub-area (before deformation); The average grayscale value of the target image sub-region (after deformation).

[0064] During the correlation coefficient calculation process, the measurement point often moves to the sub-pixel position between the whole pixels. In order to obtain an accurate correlation coefficient, sub-pixel grayscale interpolation must be performed;

[0065] The correlation coefficient matrix of the integer pixel matching point and the eight points around it that can be obtained during the integer pixel search process can be used to obtain a quartic surface using the Lagrange interpolation method:

[0066] C(x,y)=Ax 2 y 2 +Bx 2 y+Cx 2 +Dxy 2 +Exy+Fx+Gy 2 +Hy+I (12)

[0067] The function C(x,y) should satisfy the following equation at the extreme point of the quartic surface:

[0068]

[0069] Where: C(x,y) is the correlation coefficient surface function after interpolation; x,y: are the coordinates of the sub-pixel position (continuous values).

[0070] A compressibility assessment model was constructed that incorporates the characteristics of natural fracture development, mechanical properties, and stress conditions. By directly comparing the weights of the natural fracture development comprehensive index between fracture-free zones (before fracturing) and fracture-developed zones (after fracturing), the compressibility assessment model was revised and used for subsequent on-site compressibility assessments of fractured wells.

[0071] The dimensionless method is used to perform dimensional processing on the obtained parameters such as natural crack development density, scratch strength, internal friction angle, and minimum horizontal principal stress:

[0072] Positive indicators:

[0073]

[0074] Negative indicators:

[0075]

[0076] Where S is the parameter standardization value; X max is the maximum value of the parameter; X min is the minimum value of the parameter; X is the parameter value;

[0077] Among them, natural crack development density, scratch strength, and internal friction angle are positive indicators, while minimum horizontal principal stress is a negative indicator;

[0078] In the engineering compressibility model, the key engineering sweet spot parameters are mainly natural fracture development density, scratch strength, internal friction angle, minimum horizontal principal stress and other parameters. Then, a post-compression core engineering sweet spot compressibility model considering the above parameters is established for compressibility evaluation, as shown in the following formula:

[0079]

[0080] Where, Fn is the compressibility evaluation index; W i The weights of the four main influencing factors determined by the hierarchical analysis method; X i are the values ​​of the four main influencing factors: natural crack development density, scratch strength, internal friction angle, and minimum horizontal principal stress.

[0081] It should be noted that all electrical devices involved in this application can be powered by batteries or external power supplies.

[0082] The present invention is not limited to the above-described embodiments. Any structural changes made under the guidance of the present invention, which have the same or similar technical solutions as the present invention, should be understood to fall within the scope of protection of the present invention. The technologies, shapes, and structural parts not described in detail in the present invention are all well-known technologies.

Claims

1. A compressibility assessment method based on non-destructive mechanical testing of coring after compression, characterized in that: The following steps are involved: S01: Based on the specific layer requirements of the development block, pressure-maintained coring is performed on the core layers of the fracturing coring wells that represent the structural, geological and stress characteristics to obtain high-quality downhole full-diameter core samples; S02: Perform high-quality CT circular scanning on the post-fracturing core to obtain the three-dimensional spatial distribution characteristics of the fractures. Use the grayscale method to distinguish the specific three-dimensional distribution morphology of the fractures in the fracturing area, and identify the original natural fractures and post-fracturing fractures. S03: Use the surface scratch method to conduct mechanical scratch tests on the full-diameter core surface to obtain the rock mechanical properties; S04: Carry out ground stress test using digital speckle-finite element method to obtain ground stress conditions; S05: Based on fractal theory, a comprehensive index evaluation method for fracture development before and after fracture is established from the perspectives of fracture complexity and fracture transformation volume, and analytical theory is used to quantitatively interpret fracture development characteristics; S06: Construct a compressibility assessment model that includes natural fracture development characteristics, rock mechanical properties, and stress conditions. Use the natural fracture development comprehensive index to correct the weights to achieve correction of the compressibility assessment model and use it for subsequent on-site compressibility assessment of fracturing wells.

2. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S01: Optimize the core claw structure according to the formation properties, downhole temperature and pressure conditions.

3. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S02: CT technology is used to perform three-dimensional circular scanning of cored rock samples, so that sufficient data can be obtained to quantitatively describe the internal structure of the rock and natural fractures, reducing the uncertainty in natural fracture characterization caused by differences in scanning positions. After the circular scanning, unified section processing is performed, and a continuous well section of 10 to 20 meters is selected for coring. This ensures that subsequent sample rock mechanics, CT circular scanning, and other integrated analyses meet the requirements of parallel mechanics experiments.

4. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S03: non-destructive scratch mechanics testing is performed on the surface of the rock sample using a surface scratch mechanics device to obtain basic rock mechanics properties of the rock.

5. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S02: rock samples are processed by high-energy CT ring scanning method, taking into account the existence of various types of fracture structures inside the rock.

6. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S02: based on the acquired CT image, the distribution of natural fractures is quickly determined by using the optimized Frangi filter algorithm.

7. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S05: Fractal theory is used to simultaneously evaluate and calculate the complexity of fractures and their proportion in the entire core to determine the comprehensive fracture development index.

8. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: The digital speckle-finite element method was used to perform non-destructive geostress testing on the cores obtained after compression to obtain the geostress distribution characteristics.

9. The compressibility assessment method based on non-destructive mechanical testing of coring after compression according to claim 1, characterized in that: In S06: A comprehensive fracture development index suitable for post-fracture evaluation is constructed based on fracture types to conduct compressibility assessment. The compressibility model includes key indicators such as mechanical parameters, stress parameters, and natural fracture parameters.

10. The compressibility evaluation method based on non-destructive mechanical experiment of post-compression coring according to claim 4, characterized in that: Basic rock mechanical properties include rock compressive strength and fracture toughness parameters.

Images