Study On Multi-field Coupling Seepage Model Of Shale Based On Dual Pore Distribution Characteristics

Dual porosity media widely exist in nature,such as underground shale reservoir containing oil and gas,water resources and geothermal energy.For porous formation,the pore and fracture system have fractal structural characteristics in space geometry,and with the progress of hydraulic fracturing and horizontal drilling technology,it provides natural convenient conditions for people to study the micro seepage mechanism of fluid in it and to extract shale gas from the matrix.At present,in order to accurately simulate the gas production of shale gas reservoir,not only the mass transfer and transportation between different media as well as the gas flow state and mode,but also the influence of structural parameters related to pore or fracture size distribution on gas production rate and production should be considered.In this paper,two complex network models for permeability of fractured rock and fractured porous media are derived according to the connection structure between matrix pores(microcracks),and a fractal power law seepage model of shale gas is established based on the fractal theory of porous media.Through the comparison and verification with the experimental data,the influence of the structural parameters of dual porosity medium on the effective permeability of complex fracture network is analyzed.At the same time,according to the elastic theory of porous media,combined with the general porosity and fractal permeability model of dual media,the mathematical dynamic model of shale gas under multi-field coupling is established.The fractal power law permeability model is simulated by COMSOL multiphysics.The main work and results are as follows:(1)According to the power law relationship of degree distribution of nodes in complex network,a new power law distribution relation of edge with node degree is proposed by corresponding the number of nodes to the number of edges of network.Then,a complex network model of fractured rock permeability is derived based on this relationship,which includes the maximum node degree,self-similarity index,fractional power index and network microstructure parameters,etc.Compared with the existing numerical simulation data,the correctness of the model is verified.Finally,the influence of model parameters on permeability is analyzed.(2)Based on the connection structure between pores and microcracks,the network composed of pores and microcracks is described as a hierarchical network with selfsimilarity.Through the application of complex network theory,a new dual porous media seepage model is constructed.According to the research results,the permeability of the model is a function of the network microstructure parameters of porous media and its self-similarity index.Compared with the available experimental data,the correctness of the model is verified,and the model can explain more seepage mechanisms than the traditional homogeneous model.By analyzing the influence of model parameters on permeability,it is found that the self-similarity index and the cluster coefficient of pore nodes are the main factors determining the permeability change of dual porous media.(3)The influence of volume strain and storage term on porosity and pore volume of dual porosity medium under fluid-solid coupling is analyzed by numerical simulation.The results show that with the decrease of pore pressure,the contribution ratios of adsorption and particle volume strain are greater,and the pore volume occupied by gas adsorption is much larger than that occupied by free gas in the storage term.Then the influence of adsorption volumetric strain and bulk modulus on reservoir permeability is analyzed.The results show that the Langmuir adsorption parameter is not sensitive to the change of pore shrinkage in the far region,and the desorption effect increases with the increase of the particle bulk modulus.Finally,the evolution process of fractal dimension of pore and fracture and hydraulic boundary permeability in time and space are analyzed.(4)The influence of fractal dimension and microstructure parameters of porous media on shale gas production rate and gas production in the dual fractal power law seepage coupling model are analyzed.The results show that when the fractal dimension of pore diameter increases,the cumulative gas production of shale reservoir will increase slowly when fractal dimension is greater than 1.3;when the fractal dimension of tortuosity increases,the cumulative production of shale gas decreases rapidly;the larger the linear length of the unit,the smaller the cumulative gas production;the larger the linear length of the unit,the smaller the cumulative gas production;the longer the maximum pore diameter,the more obvious the increase of cumulative gas production;reducing the fractal dimension of fracture length is helpful to improve the gas yield of shale;the maximum size of fracture length determines the contribution of gas production rate in different periods;by analyzing the spatial orientation of fracture network,it is found that the gas production rate of shale is positively correlated with the opening degree and negatively correlated with the dip angle of fracture.