CN-121980656-A - Analysis method for formation mechanism and evolution mode of sand shale interbed cracks

CN121980656ACN 121980656 ACN121980656 ACN 121980656ACN-121980656-A

Abstract

The invention discloses a sand-shale interbed crack formation mechanism and evolution mode analysis method, which is characterized in that a reasonable rock sample loading rate and a reasonable rock sample shortening rate are set by using a particle flow numerical simulation technology according to a macroscopic structural style, and then rock mechanics experiments with variable loading rates are implemented, and by constructing fracture strength and stress-strain evolution patterns of sand/shale under different loading rate conditions, a sand-shale interbed crack development mechanism under a strong stress rapid extrusion environment is revealed, and angles similar to three aspects of physical composition, mechanical properties and interbed structure are utilized, an interbed artificial sample is prepared by utilizing an underground sand-shale core sample, and a loading synchronous CT scanning experiment and a three-dimensional acoustic emission source positioning experiment are developed, so that the visualization of the rock fracture process is realized, and a composite rock body fracture evolution process is revealed. The method has important theoretical and practical values for improving the success rate of fractured gas reservoir exploration, widening the exploration and development field and optimizing the reservoir transformation strategy.

Inventors

LIU JINGSHOU
SHI XIAN
FENG JIANWEI
JIANG SHU
YANG YADONG
QIAN CHAO
QU YONGQIANG
MA CUNFEI
JU WEI
YAN GUOLIANG
WU ZHONGHU

Assignees

中国地质大学（武汉）

Dates

Publication Date: 20260505
Application Date: 20260129

Claims (10)

1. A sand shale interbed crack cause mechanism and evolution mode analysis method is characterized by comprising the following steps: the first step is to determine the rock mechanics experiment loading rate and shortening rate based on the macroscopic structure style; The macroscopic structure pattern is closely related to the structure extrusion rate and the formation shortening rate, and the extrusion rate and the formation shortening rate range corresponding to various structure patterns are defined by using a macroscopic particle flow numerical simulation technology, so that the influence of the extrusion rate on the rock mechanical parameters, stress-strain states and rock breaking mechanisms is defined; Secondly, carrying out synchronous digital speckle monitoring of rock mechanics experiments at different loading rates; Collecting sandstone and mudstone samples at different intervals, carrying out synchronous digital speckle monitoring of rock mechanics experiments at different loading rates, and establishing evolution patterns of the mechanical parameters of the sandstone and the mudstone, the rock fracture patterns and the mechanical parameters at different loading rates; Thirdly, carrying out rock mechanics experiment numerical simulation based on particle flow, and clarifying the dependence of sand shale samples in different blocks on the loading rate; In order to avoid uncertainty of crack cause mechanism interpretation in rock mechanics experiments with single different loading rates, expand universality of loading rates and sandstone mechanics response models, based on a numerical simulation technology of particle flow rock mechanics experiments, the system reveals a dynamic mechanism of crack evolution under the action of rate-lithology mutual control; Fourthly, revealing a mechanism for forming a crack of the sand-shale interbedded; The crack main control factors are combined, the crack activity period is clarified, the diagenetic strength corresponding to different intervals is revealed, the influence rule of early cracks and the activity thereof on diagenetic strength and rock mechanical parameters is ascertained, the influence mechanism of loading rate and stratum shortening rate on the sand shale mechanical properties and fracture patterns is interpreted, and the sand shale interbedded crack development mechanism under the comprehensive control of the structure-fluid-diagenetic mechanical properties is clarified; fifthly, carrying out loading type synchronous CT scanning experiments and digital core modeling; Developing loading synchronous CT scanning experiments to find out the cracking evolution process of different lithology samples, revealing the evolution rules of rock porosities and tortuosity corresponding to different stress-strain stages based on digital core modeling, revealing the different cracking evolution rules of sand shale of different blocks from the angle of pore structure evolution in the loading process; Sixthly, carrying out a multi-scale fracture acoustic emission source positioning experiment of the composite rock mass under the control of a lithology interface; Under the constraint condition that three aspects of material composition, mechanical property and interbed structure are similar, preparing lamellar samples with different sand-mud proportions and different thickness proportions by adopting a core sample processing-manual sample preparation mode, carrying out a three-dimensional acoustic emission source positioning experiment, realizing transparent analysis of a crack development process under different loading rates, and revealing the vertical expansion of cracks and the development rule penetrating through the lithology interface from the angles of lithology interface and sand-mud rock thickness; the seventh step is to determine the crack evolution mode of the sand-mud rock interbedded under different extrusion rates; And carrying out numerical simulation by adopting a numerical simulation take over physical simulation method, carrying out numerical simulation of discrete elements of a large-scale lithology interface, comprehensively loading CT scanning experiments and acoustic emission source positioning experiments, determining the relation between a crack layering mode, a mechanical structure and energy gathering and scattering, quantitatively dividing the evolution stage of the crack, and establishing a crack evolution mode under extrusion rate-mechanical structure-stress strain control.
2. The analysis method of the formation mechanism and the evolution mode of the sand-shale interbed cracks is characterized by comprising the steps of firstly, establishing a basic structural geometric model by combining a typical structural style of a research area, secondly, setting a particle flow numerical simulation initial model parameter by referring to a triaxial mechanical simulation test result of different particles, establishing the initial structural model, and transposing the initial structural model into a three-dimensional discrete element elastoplastic model, setting the model and the deposited particles, simulating the erosion and roof removal process of a stratum in the structural evolution process, and finally, revealing the deformation and the extrusion rate of different types of structural units, so as to lay a foundation for exploring how the extrusion rate affects the rock mechanical parameter, the stress-strain state and the rock fracture mechanism.
3. The analysis method of the formation mechanism and the evolution mode of the sand-shale interbed cracks, which is disclosed in claim 1, is characterized in that in the second step, the step of establishing the evolution pattern of the sand-shale mechanical parameters, the rock fracture patterns and the mechanical parameters under different loading rates comprises the steps of respectively collecting a group of sand and shale samples according to the formation shortening rate and the stress intensity corresponding to different structural patterns, and polishing the surfaces of the samples; the method comprises the steps of determining the confining pressure condition of a rock mechanics experiment by combining a section ancient stress field simulation result, spraying a sample with speckles, spraying white matte priming paint, enhancing contrast, spraying black dispersion points, setting different loading rates for each group of n samples, carrying out rock mechanics experiments at different loading rates and synchronous digital speckle monitoring, reconstructing a light field in a specific three-dimensional space containing a target by calculating a simulated light propagation path in space by utilizing a digital holographic imaging technology, applying three-dimensional motion tracking of particles, combining the surface morphology of the rock sample to realize rock strain imaging, obtaining the Young modulus, poisson ratio, critical fracture threshold, energy release rate and fracture pattern of the rock corresponding to different rock samples at different loading rates, and establishing an evolution chart of sand shale mechanics parameters, rock fracture pattern and fracture mechanics parameters at different loading rates.
4. The analysis method of the mechanism and the evolution mode of the mutual layer cracks of the sand and the mudstone according to claim 3, wherein the analysis method is characterized in that a group of sand and the mudstone samples are collected, n samples are respectively arranged in each group, wherein n is more than or equal to 5, and the sample size is 25 50 The diameter of the spray-coated black dispersion point is set to be 1/50 of the size of sandstone and mudstone samples.
5. A sand shale interbed crack formation mechanism and evolution mode analysis method is characterized in that in the third step, rock mechanics experiment numerical simulation based on particle flow is achieved, wherein the rock mechanics experiment numerical simulation based on particle flow is achieved through the steps of firstly, adopting a parallel bonding model or a contact bonding model to represent cementing action among particles and endow micromechanics parameters including normal rigidity and tangential rigidity and bonding strength, secondly, obtaining macroscopic mechanical responses through sand shale indoor experiments of different blocks to serve as constraints, adjusting a microscopic parameter assignment scheme through a reverse iteration method, guaranteeing mechanical behavior consistency of a numerical model and a rock physics experiment, wherein the macroscopic mechanical responses comprise elastic modulus and peak strength, secondly, adopting a displacement control mode or a stress control mode, adjusting loading rate in real time based on a servo control system, guaranteeing that the experiment process meets quasi-static conditions, avoiding dynamic effect interference, determining the loading rate through deformation sensitivity analysis, guaranteeing numerical stability through strain rate form characterization, and assisting time step self-adaptive adjustment, finally, monitoring particle displacement, contact force chain and bonding rupture event in real time, recording stress-strain curve, rupture mode and energy dissipation characteristic, comparing the different block and energy dissipation coefficient, and comparing the different energy responses of different blocks to the loaded sand, and further clarifying the dynamic response to the friction coefficient by the different blocks.
6. The analysis method of the mechanism and the evolution mode of the mutual layer cracks of the sand shale according to claim 1, wherein in the fourth step, the mechanism for demonstrating the mutual layer crack development of the sand shale under the comprehensive control of the structure-fluid-diagenetic-mechanical properties is specifically realized by referring to the structural evolution of a region, the evolution of a stress field and the factors of main crack control, interpreting the crack development patterns and the differential evolution mechanism of mechanical properties of the sand shale under different loading rates from the angles of the development period and the diagenetic strength of the crack; firstly, collecting typical crack invasion rock vein or hot liquid vein samples at different intervals, observing micro-nano micro-crack geometric form, intersection restriction relation and diagenetic mineral type and structure, adopting fluid inclusion analysis technology in crack filler, performing lithology observation on fluid inclusion in typical crack vein, researching type and component, measuring uniform temperature and freezing point temperature of fluid inclusion, calculating fluid inclusion capture pressure in crack filler, revealing capture period of fluid inclusion, analyzing difference of capture temperature at each period, developing single well buried lifting-thermal evolution history simulation reconstruction, determining temperature and pressure evolution paths of different sand shale inter-layer sections, finding out period difference of crack activities of different sand shale sections, combining rock mechanics experiment results at different loading rates and activity period of different sand shale sections, selecting a shale sample to measure the parameters of the formation of the characteristics of the illite content, developing diagenetic strength evaluation, selecting typical slice, cathode luminescence and scanning electron microscope, counting microscopic pore size distribution, pore shape, ratio of contact point and number of contact particle number, representing the length of the contact particle, the method comprises the steps of combining the evolution of diagenetic minerals and pore-permeation parameters, establishing a mineral evolution sequence and a diagenetic evolution mode, analyzing the transformation effect of cracks on the mechanical properties of deep clastic rocks, finding out the inherent relationship among lithology, extrusion rate, mechanical parameters and the cracks, and clarifying the sand-shale interbedded crack development mechanism under the comprehensive control of the structure-fluid-diagenetic-mechanical properties.
7. A method for analyzing the formation mechanism and evolution mode of the crack of sand-mud rock interbedded according to claim 1 includes such steps as collecting sand-mud rock sample, preparing A, B sets of buried depth and rock features including lithology, mineral composition and non-uniformity, reasonably determining the size of sample according to the range of CT scan equipment and precision, setting up the size of sample to be 2.5-5 mm, and 5-10 mm, developing rock mechanical experiment on A sample, obtaining rock stress-strain curve, measuring the compressive strength sigma c of different samples, defining B sample scan points, using Zeiss X microscope, respectively selecting 0, 0.3 sigma c 、0.5σ c 、0.7σ c 、0.8σ c 、0.9σ c and sigma c as the stress points, setting up the stress-strain curve, setting up the stress-crack model, setting up the stress-crack state of different hole-phase, and setting up the two-dimensional model, from the angle of the pore structure evolution in the loading process, the differential cracking evolution rules of the sand shale of different blocks are revealed.
8. The analysis method of the mutual layer crack formation mechanism and the evolution mode of the sand shale according to claim 1 is characterized in that in the sixth step, transparent analysis of the crack development process under different loading rates is realized by the following steps that in view of the extremely difficult acquisition of the sand shale composite rock body meeting the experimental requirements in a natural state, the influence of lithology interfaces on the rock cracking and the crack expansion is difficult to analyze; the method comprises the steps of exploring the influence of a lithology interface on crack evolution in an artificial sample preparation mode, ensuring the similarity of the artificial sample preparation and an underground sample from three aspects of material composition, mechanical property and a interbedded structure respectively, firstly drilling core samples with different depths to form fragments of 0.8-1.5 cm in order to ensure that the artificial sample preparation and the underground sample have the similarity of the material composition, then placing rock sample fragments and steel balls into a piston container, carrying out high-speed rotary grinding by using a stirring drill to form a designated mesh number of the rock sample, mixing rock sample powder with epoxy resin glue, carrying out layer-by-layer lamination sample preparation by adopting a single-shaft test press, taking out the artificial core from a mould and placing the artificial core into a constant temperature box after compaction, drying the artificial core sample at the temperature of 80-150 ℃ for more than 12 hours, using a wire cutting machine to ensure that the artificial sample preparation and the underground sample have the similarity of the mechanical property, determining a layered sample preparation scheme with different sand mud proportion, taking the combination as constraint section, taking the preparation experience of the samples with different strength, reasonably setting single-shaft compression strength and time parameters, taking out the preparation of the interbedded mud, taking out the interlayer, and ensuring the mutual-layer-crack preparation with the macroscopic structure, and the macroscopic-sediment-bearing structure, and the macroscopic-sediment-bearing effects, and the mutual-sediment-bearing effects, and the crack-sediment-bearing effects, and the method, the method comprises the steps of determining an in-situ stress environment of a sand-mud interlayer three-type composite rock sample, utilizing a rock three-dimensional acoustic emission source positioning experiment to monitor acoustic emission signals of different samples, analyzing a sand-mud inter-layer structure, a lithology interface and crack propagation evolution rules under different rock layer thickness ratios, acquiring space positioning of microcrack events by calculating the waveform parameters of the same vibration source, dispersing the rock mass into a finite element system based on a rock local area related imaging algorithm, realizing multi-scale fracture acoustic emission data processing and visual reconstruction of damage distribution by constructing a quantitative relation between elastic wave velocity variation and rock damage, and finally, interpreting a composite rock mass crack evolution mode from the lithology interface, the thickness of the sand-mud and the angles of the inter-layer structure.
9. The analysis method of the formation mechanism and the evolution mode of the sand-shale interbedded cracks, which is disclosed in claim 1, is characterized in that in the sixth step, rock sample powder and epoxy resin glue are mixed, and a single-shaft test press is adopted to carry out layer-by-layer lamination of samples, specifically, the steps of mixing the rock sample powder and the epoxy resin glue according to the proportion of 100:9, carrying out manual rubbing and mixing uniformly, and carrying out layer-by-layer lamination of samples after 3 times of sieving.
10. The analysis method of the formation mechanism and the evolution mode of the sand-shale interbed cracks is characterized by comprising the steps of establishing a crack evolution mode under extrusion rate-mechanical structure-stress strain control in the seventh step, systematically analyzing a control mechanism of a large-scale lithology interface and a rock combination mode on crack expansion through a numerical simulation method taking over physical experiments, constructing a large-scale elastoplastic damage discrete element model containing the lithology interface by using discrete element software and a FISH compiler thereof based on loading synchronous CT scanning and different loading rate mechanical experimental results, realizing three-dimensional reconstruction of a single-well stress field, further revealing a regulation and control rule of geometric parameters of the large-scale lithology interface, an expansion path and stress accumulation release of the combination configuration, establishing a quantitative relation between rock fracture and lithology interface, an evolution stage and single-layer/interlaminar strength difference, and establishing a rock fracture-structure-stress interbed evolution rule under extrusion rate-structure-stress interbed stress crack interbed stress control by carrying out comparative analysis with a single-well discrete network model.

Description

Analysis method for formation mechanism and evolution mode of sand shale interbed cracks Technical Field The invention relates to the field of oil and gas field exploration and development, in particular to a sand shale interbed crack formation mechanism and evolution mode analysis method. Background The mechanism of sand-shale interbed fracture causes belongs to the category of composite rock mechanics research, and the composite rock strength theory can be divided into two main theories, namely a classical strength criterion and an empirical cracking criterion, wherein the former mainly comprises Drucker-Prager criterion and unified strength theory, and the latter mainly comprises Hoek-Brown empirical strength criterion and Yoshinaka power function type criterion (Dai Junsheng and the like, 2011; yin Guangzhi and the like, 2017;Zoback and Kohli, 2019; yang et al, 2023; xia Kaizong and the like, 2024). The fracture propagation mechanism of the layered rock mass is mainly divided into 3 types, ① Cook-Gordon stripping action, which is a common layering mechanism in composite materials (Larsen et al 2010), and in isotropic materials, when the rock is heterogeneous, the fracture may propagate along the weak face of the rock mass. ② Stress barriers, i.e. stress rotations at the interface, can act as a barrier to certain types of crack propagation, creating locally unfavorable stress states, resulting in crack propagation being blocked to end (Gudmundsson et al, 2006). ③ The toughness of a material, defined as the amount of energy absorbed per unit area of crack, continues to propagate through the lithology interface when the strain energy release rate of the crack reaches a critical energy value required for propagation, whereas the crack propagates along the interface in a deflected manner (Gudmundsson et al., 2010). Whether based on classical strength criteria or empirical fracture criteria, composite rock mass is dominated by hard (sandstone) cracks that develop, soft (mudstone) cracks that do not develop, and these cracks are mostly terminated or deflected at lithology interfaces, forming classical development patterns of sand-mudstone interbedded cracks (Lyu et al., 2019; zhang et al., 2020; xu et al., 2022; lei et al, 2023). Therefore, research on the mechanism and evolution mode of the sand-shale interbed fracture under the background of different extrusion rates is carried out, the traditional composite rock mass fracture cause theory can be perfected from the geomechanical angle, and meanwhile, the method has important application value for quantitative prediction of deep cracks and discovery of tight sandstone gas reservoir scale. Disclosure of Invention In order to solve the defects in the prior art, the invention aims to provide a sand shale interbed crack cause mechanism and evolution mode analysis method which can reveal the sand shale interbed crack cause mechanism and evolution mode based on physical and numerical simulation. The technical scheme of the invention is that the analysis method for the formation mechanism and the evolution mode of the sand-shale interbed cracks comprises the following specific implementation steps: the first step is to determine the rock mechanics experiment loading rate and shortening rate based on the macroscopic structure style; The macroscopic structure pattern is closely related to the structure extrusion rate and the formation shortening rate, and the extrusion rate and the formation shortening rate range corresponding to various structure patterns are defined by using a macroscopic particle flow numerical simulation technology, so that the influence of the extrusion rate on the rock mechanical parameters, stress-strain states and rock breaking mechanisms is defined; The method specifically comprises the following steps of firstly, establishing a basic structural geometric model by combining a typical structural style of a research area, secondly, setting particle flow numerical simulation initial model parameters by referring to triaxial mechanical simulation test results of different particles, establishing an initial structural model, and transposing the initial structural model into a three-dimensional discrete element elastoplastic model, thirdly, setting the model and deposited particles, simulating the erosion and roof removal process of a stratum in the structural evolution process, and finally, revealing deformation and extrusion rates of different types of structural units, so as to lay a foundation for exploring how the extrusion rates affect the rock mechanical parameters, the stress-strain states and the rock fracture mechanism. Secondly, carrying out synchronous digital speckle monitoring of rock mechanics experiments at different loading rates; And acquiring sandstone and mudstone samples at different intervals, carrying out synchronous digital speckle monitoring of rock mechanics experiments at different loading rates, and establishing evolutio