Numerical Study On The Mechanical Behavior And Scale Effect Of Conglomerate Based On Randomly Reconstructed Discrete Element Model

With the rapid development of the world economy,energy development is moving to the deep earth,and the exploration and exploitation of low porosity and permeability conglomerate reservoir has gradually become a hot spot in the global oil and gas industry.Influenced by sedimentary process and diagenesis,the lithofacies of conglomerate have obvious differences in mineral composition,gravel petrological characteristics,matrix content,cement content,gravel spatial distribution and other aspects,showing strong heterogeneity,which results in the significant differences in physical and mechanical properties of reservoir.The heterogeneous characteristics of conglomerate also have an important influence on the hydraulic fracturing effect of reservoir,which is not conducive to improving the oil and gas recovery of conglomerate.Therefore,it is of great scientific significance and engineering value to study the heterogeneity of deep-buried conglomerate reservoir for efficient development of conglomerate oil and gas resources and reduction of geological disasters caused by reservoir development.As a type of heterogeneous geomaterial,the conglomerate has a complex mesoscopic structure.The conglomerate reservoir raises many interests of scholars but the research on the heterogeneity of the geometric structure and the evolution law of multi-scale mechanical properties is rare,which limits the comprehension of the conglomerate mechanical properties and restricts the development and utilization of conglomerate reservoir in China.Therefore,it is urgent to study the heterogeneity of geometric structure and multi-scale mechanical mechanism of conglomerate.In this paper,the quantitative characterization and statistical description of gravel geometry are carried out through image recognition.Based on the statistical probability model of gravel geometry characteristics,the random generation and rapid placement technology of gravel is established.Combined with the discrete element method,the components of conglomerate were recognized and assigned by different contact types,with the microscopic parameters calibrated.Therefore,discrete element model characterizing geometric and mechanical heterogeneity of conglomerate is established.The mechanical behavior simulation and cross-scale fracture mechanism study of conglomerate were carried out to reveal the influence mechanism of gravel geometry and its spatial distribution on the fracture behavior of conglomerate,and the evolution law of cross-scale mechanical behavior controlled by heterogeneity was summarized.The main research contents are as follows:(1)The closed multi-segment line is used to accurately identify the gravel boundary in the outer surface image of the conglomerate.By analyzing the statistical characteristics of gravel particle size and slenderness ratio distribution,it is found that the maximum and minimum Ferret diameters of gravel follow the lognormal distribution,and the two are linearly correlated.The probability model is established to characterize the geometric characteristics of conglomerate.Based on the statistical characteristics of gravel,the algorithm of gravel random generation and rapid placement is developed,and the precise control of the petrological characteristics of conglomerate is realized by changing the characteristic parameters,so that the multi-scale gravel contour model of conglomerate that meets the real statistical characteristics can be generated quickly and in large quantities.(2)A series of gravel geometric profiles generated by the random generation algorithm were imported into the two-dimensional particle flow code(PFC2D),and the contact models between grains were divided into three groups according to the particle locations:"gravel","matrix" and "interface",and different components of the conglomerate were reconstructed.Based on the component mechanical parameters of each group obtained by nanoindentation test,the microscopic parameters of the above three groups of contact models are calibrated to ensure that the reconstructed discrete element model can accurately characterize the physical and mechanical characteristics of the conglomerate.(3)The discrete element models of conglomerate with different petrological characteristics are established to conduct numerical simulation of uniaxial compression test.The results show that the elastic modulus of conglomerate increases with the increase of various petrographic parameters(standard deviation,mean value,slenderness ratio and gravel content).The slenderness ratio and average value of gravel size have little effect on uniaxial compressive strength,while the increase of standard deviation and gravel content will weaken the uniaxial compressive strength.Moreover,the conglomerate with slenderer gravel,higher content,better sorting and smaller gravel size is more likely to form dispersed and complex fracture network.(4)A series of discrete element models with different sample sizes were designed based on the random generation algorithm,and the uniaxial compression simulation tests were carried out.The deformation and failure laws of the conglomerate with multi-scale meso-structures were investigated.The simulation results show that the uniaxial compressive strength of conglomerate decreases steadily with the increase of specimen size,which can be accurately predicted by calculating multi-scale failure probability.However,the mean values of elastic modulus and Poisson’s ratio hardly change with scale.Moreover,with the increase of specimen size,the failure ratio decreases gradually,and some of the cracks around gravel turn into the cracks through gravel.