CN-121994835-A - Quantitative evaluation method for fracture complexity after compression fracture

Abstract

The invention belongs to the technical field of petroleum and natural gas geology and rock mechanics, and particularly relates to a quantitative evaluation method for fracture complexity after compression fracture. Compared with the existing statistical method, the method for evaluating the fracture length, density, shape, trend and the like from a single factor is more common, the method provided by the invention is used for calculating the fracture connectivity index and the fracture fractal dimension of the plunger sample after compression on the basis of an in-situ compression CT scanning experiment, and respectively obtaining the weight coefficients of the fracture connectivity index and the fracture fractal dimension by using an entropy weight method, so that the fracture complexity after compression damage is comprehensively evaluated. The method fully considers the fracture connectivity index and the fracture fractal dimension, and has comprehensive factor consideration, strong operability and high result reliability. The method is not only suitable for evaluating the development effect of shale oil and tight oil reservoirs, but also can be applied to other types of oil reservoirs and rock engineering.

Inventors

• XIN HONGGANG
• ZHAO TING
• FENG SHENGBIN
• LI CHENG
• MA WENZHONG
• ZHU LIWEN
• LI TAO
• WANG ZHITAO
• LI XIANG
• HAO BINGYING

Assignees

• 中国石油天然气股份有限公司

Dates

Publication Date

20260508

Application Date

20241107

Claims (9)

1. 1. A quantitative evaluation method for fracture complexity after compression fracture is characterized by comprising the following steps: Step 1) collecting a target layer drilling core sample of a research area, and recording the sampling depth and lithology of the sample; step 2) preparing a core sample into a plunger experimental sample; Step 3) utilizing CT to scan the plunger experimental samples, and selecting three similar plunger experimental samples after three-dimensional reconstruction, wherein the numbers of the three similar plunger experimental samples are S1, S2 and S3 respectively; Step 4) carrying out in-situ compression CT scanning experiments on the selected three plunger experimental samples until the plunger experimental samples are destroyed, loading the three plunger experimental samples with the same displacement, obtaining three-dimensional scanning data CT photographs, and obtaining a three-dimensional body of the experimental sample on the basis; Step 5) calculating a crack connectivity index I by utilizing a topological structure theory on the basis of the experimental sample three-dimensional body; step 6) calculating a crack fractal dimension C by utilizing a fractal analysis theory on the basis of a three-dimensional body of an experimental sample; Step 7), respectively carrying out normalization treatment on the crack connectivity index I and the crack fractal dimension C; And 8) on the basis of data normalization, calculating and obtaining weight coefficients p and q of a crack connectivity index I and a crack fractal dimension C by an entropy weight method, and further quantitatively evaluating the crack complexity F after compression fracture, wherein the F is in a range of 0-1, and the larger the value is, the more complex the value is.
2. 2. The method for quantitatively evaluating the complexity of a fracture after being broken under pressure according to claim 1, wherein the lithology in the step 1) is clastic rock or carbonate.
3. 3. The quantitative evaluation method of the fracture complexity after being broken under pressure according to claim 1, wherein in the step 2), the diameter of the experimental sample of the plunger is 6mm and the height is 12mm, and the upper end face and the lower end face are parallel by using a grinding machine or a lathe.
4. 4. The method for quantitatively evaluating the complexity of a fracture after being broken under pressure according to claim 1, wherein three similar plunger test samples selected in the step 3) are judged by porosity, permeability and pore structure.
5. 5. The method for quantitatively evaluating the complexity of a crack after being broken under pressure according to claim 1, wherein the three-dimensional reconstruction is performed by Avizo or Dragonfly software in the step 4) to obtain a three-dimensional body of the experimental sample.
6. 6. The quantitative evaluation method of fracture complexity after compression fracture according to claim 1, wherein the fracture connectivity index I is calculated by using a topological structure theory in the step 5), and is specifically calculated according to the following formula: wherein N I represents the number of I nodes, N Y represents the number of Y nodes, and N X represents the number of X nodes.
7. 7. The quantitative evaluation method of fracture complexity after compression fracture according to claim 1, wherein the step 6) is characterized in that the fractal dimension C of the fracture is calculated by using a fractal analysis theory and is specifically as follows: calculating the fractal dimension C of the crack by adopting an area coverage method: Where N is the number, S' is the unit area, and δ is the scale factor.
8. 8. The quantitative evaluation method of fracture complexity after compression fracture according to claim 1, wherein the normalization treatment in the step 7) removes the influence of units, and adopts a Max-Min method: Wherein X i represents normalized data, X max represents a data maximum, X min represents a data minimum, and X i represents any data.
9. 9. The quantitative evaluation method of fracture complexity after compression fracture according to claim 1, wherein the fracture complexity F in the step 8) is calculated by the following formula: Wherein j represents the number of a plunger experiment sample, I j represents the crack connectivity index normalization value of the j-th plunger experiment sample, C j represents the crack fractal dimension normalization value of the j-th plunger experiment sample, p j represents the weight coefficient of I j , and q j represents the weight coefficient of C j .

Description

Quantitative evaluation method for fracture complexity after compression fracture Technical Field The invention belongs to the technical field of petroleum and natural gas geology and rock mechanics, and particularly relates to a quantitative evaluation method for fracture complexity after compression fracture. Background Currently, commercial development of unconventional shale oil gas, dense oil gas and coal bed gas generally needs to be improved through fracturing, and therefore, the quantitative evaluation of fracture complexity after compression fracture is developed, and the quantitative evaluation has important practical significance. The patent of the invention of the issued publication number CN106769463B provides a quantitative characterization method for the complexity of the crack after the core is pressed, the inclination angle and the area of each crack are extracted from the image of the crack after the plunger-shaped core is pressed, the face fracture rate and the inclination angle dispersion of the crack of the core after the core is pressed are calculated, the face fracture rate and the inclination angle dispersion of the crack are normalized and then are added, the complexity of the crack after the core is pressed is obtained, and the capability of forming a fracture network in the fracturing process can be quantitatively evaluated. The patent application publication CN112304770a proposes a method and system for quantitatively analyzing the complexity of a post-fracture by quantitatively evaluating the complexity of the post-fracture by the number of peaks of the GdP/dG curve, the maximum slope of the first portion of the GdP/dG curve, and the standard deviation of the second portion, wherein the first portion refers to the portion of the curve from the origin to the first peak to the first trough in the GdP/dG curve, and the second portion refers to the portion of the curve from the first trough to the end in the GdP/dG curve. The invention patent of application publication number CN115808352A proposes a method and a device for extracting and characterizing complexity of a digital rock core and rock cracks, firstly, carrying out a simulated fracturing experiment on a cut rock core sample, acquiring CT scanning images of the rock core sample before and after the simulated fracturing experiment, respectively constructing digital rock cores before and after the simulated fracturing experiment according to the CT scanning images, extracting original crack characteristics and characteristics of cracks after pressing, further acquiring crack increment and crack morphology, and finally constructing a three-dimensional complexity characterization model of the cracks after pressing. The invention patent of application publication number CN111597671A provides a fracture network complexity determining method and system based on probability distribution, wherein a three-dimensional space containing all microseism events is established according to event information of microseism events formed during hydraulic fracturing, microseism event information in a preset three-dimensional space range at different positions of the three-dimensional space is selected according to a preset rule, and the fracture network complexity is determined according to the event information obtained by selecting the different positions of the three-dimensional space. The method is used for evaluating the complexity of the crack from a single factor or considering multiple factors but not considering the weight, so that the evaluation result has errors. Disclosure of Invention The invention aims to provide a quantitative evaluation method for fracture complexity after compression fracture, which overcomes the technical problems in the prior art. Therefore, the technical scheme provided by the invention is as follows: a quantitative evaluation method for fracture complexity after compression fracture comprises the following steps: Step 1) collecting a target layer drilling core sample of a research area, and recording the sampling depth and lithology of the sample; step 2) preparing a core sample into a plunger experimental sample; Step 3) utilizing CT to scan the plunger experimental samples, and selecting three similar plunger experimental samples after three-dimensional reconstruction, wherein the numbers of the three similar plunger experimental samples are S1, S2 and S3 respectively; Step 4) carrying out in-situ compression CT scanning experiments on the selected three plunger experimental samples until the plunger experimental samples are destroyed, loading the three plunger experimental samples with the same displacement, obtaining three-dimensional scanning data CT photographs, and obtaining a three-dimensional body of the experimental sample on the basis; Step 5) calculating a crack connectivity index I by utilizing a topological structure theory on the basis of the experimental sample three-dimensional body; step 6)