Numerical Simulation Of Acoustic And Electrical Characteristics Of Shale Reservoir In Longmaxi Formation,sichuan Basin

China has abundant shale gas resources,and the efficient development of shale gas reservoirs is of great significance to the energy strategic security of our country.The complex pore structure and mineral composition of shale gas reservoirs bring great challenges to the exploration and development of shale gas reservoirs,so it is necessary to study the internal structure of shale and the relationship between mineral components and corresponding physical properties in detail.The conductivity and elasticity of shale are important parameters for evaluating reservoir saturation and fracturing ability.It is very difficult to carry out conventional rock physics experiments on shale samples to master the change law of rock conductivity and acoustic properties.Digital rock physics techniques,which use digital core images to characterize rock microstructure and conduct numerical simulations to study rock physical properties,are playing an increasingly important role in the study of complex reservoirs.In this paper,the core plunger sample of the Longmaxi Formation shale gas reservoir in the Weiyuan area of the Sichuan Basin is taken as the research object.According to the logging data of the study area,the influential factors of resistivity and acoustic logging are initially mastered.X-ray CT,large-area scanning electron microscopy(MAPS),and quantitative mineral evaluation(QEMSCAN)were used to scan core samples.Digital core models with different resolutions were established and mineral distribution images on the plunger surface were obtained.The pore structure and mineral distribution characteristics of shale gas reservoirs are mastered by analyzing CT images and MAPS images with different resolutions.The obtained QEMSCAN images are taken as labels,and the U-Net network in the deep learning algorithm is used to carry out automatic image segmentation training of the plunger CT.The results of U-Net image segmentation are compared with multi-threshold image segmentation and QEMSCAN image,which proves the reliability of this method.Then deep learning algorithm is used to segment the3 D CT image of the sample,and the multi-component 3D digital core model of the plunger is established.Based on the analysis of the QEMSCAN image and multicomponent 3D digital core model of samples,it is found that mineral distribution has obvious stratification characteristics.Therefore,a simple stratification model is proposed to simplify the core model,and the layer-representative volume element is used to replace the physical properties of corresponding layers.Prior to conducting the conductivity simulation,physical experiments were performed to obtain the resistivity of the plunger sample under dry and saturated brine conditions.When the finite element method is used to conduct the numerical simulation of core conductivity,the resistivity of the plunger sample under dry conditions is obtained by a simple stratified model.Compared with the physical experiment results,the equivalent conductivity value of pyrite in the plunger sample is determined by combining the mineral distribution characteristics in MAPS images with different resolutions.Using the equivalent conductivity value of pyrite,the conductivity value of the layerrepresentative volume element under saturated brine conditions is obtained.Based on the simple stratification model,the numerical simulation results of the representative volume element of the layer are fused,and the resistivity of the plunger sample core under saturated brine condition is obtained,which is in agreement with the physical experiment results.At the same time,the numerical simulation results of representative volume elements in multiple layers are analyzed.It is found that cracks can form a good conductive path,clay can effectively reduce the core resistivity,and the dispersion of pyrite has little effect on the conductivity.The finite element method is used to carry out elastic numerical simulation for the selected layer representative volume elements,and the results are compared with the theoretical model,and the numerical simulation results are in good agreement with the theoretical calculation results.The influence of clay and pyrite on the elastic parameters of the core is obtained by simple analysis of the simulation results of the representative volume elements of the bed.At the same time,interlayers of different components are added to the core,and the influence of interlayer composition and width on the anisotropy of core elastic parameters is simulated.It is found that the influence of cracks on the anisotropy parameters is severe,while the influence of clay and pyrite interlayers on the anisotropy of rock is not obvious,and the influence on the anisotropy can be ignored when the interlayer is small.