Pore-structure Model Of Geo-materials And Micro-mechanics Model For Seepage

Seepage in geo-materials presents significant scale properties in pore size.In view of pore structure,pore-scale flow analysis can dynamically account for the micro-scale mechanism for multi-phase flow,retardation,dispersion,and fingering.This study reviews the previous research on seepage in porous media,and then proposed a theoretical multi-flow regimes model.Equivalent pore-network model was established to consider the connectivity,anisotropy,and porosity of different geo-materials.Furthermore,single phase seepage characteristic parameters were calculated for different geo-materials and extended to study the micro-mechanism of shale gas exploitation based on the proposed pore-network model.Finally,pore-network model was applied to study the two-phase flow in shale matrix and also the multi-component flow of CO₂ injected into shale formations.The following main conculsions can be drawn from the current research:A multi-flow regimes theoretical model was proposed to describe the gas flow in porous media with different pore-scales.The concept starts from the gas kinetic theory,and considers the variation of gas flow regimes due to the changes of gas molecular collision frequency.The proposed multi-flow regimes model was used to analyze shale gas exploitation,and a multi-physics model coupled with adsorption/desorption,deformation mechanisms was developed.An equivalent pore-network model for was established to simulate the complex pore-structures for geo-materials.The cahracteristics of pore structure of geo-materials was illustrated by specific parameters:porosity,pore size and pore throat size distribution,as well as pore space connectivity.The equivalent pore-network model was established based on these petro-physical parameters.The variation of flow regimes due to pore size in different geo-materials was incorporated in the model.The model has the flexibility to be applied to different porous media,accounting for the multi-directional connectivity and anisotropic property.Based on the pore-network model,the permeability for different geo-materials was investigated,especially for the low permeability media with micro-nano size.A micro-mechanics model of seepage was developed to characterize the gas flow in nano-scale pores in engineering cases.The physical model was extracted from engineering projects includes shale gas exploitation and CO₂ sequestration in shale formations.The pore-scale multi-flow regimes model was applied to analyze the dynamic variation of apparent permeability during shale gas exploitation.With the progress of hydraulic fracturing,the residual fracturing fluid may cause two-phase flow,hence the pore-scale two-phase flow model was developed.The dynamic relationship between saturation and capillary pressure was proposed,and the dynamic invasion of non-wetting phase was simulated.With the idea of enhancing shale gas production by injecting CO₂into shale formations,the pore-scale single-phase two-component flow model was established,and the competitive adsorption between two components and phase change was dynamically coupled at pore-scale.The proposed pore-structure model and micro-mechanics model of seepage for geomaterials may provide scientific analysis for seepage related engineering issues such as shale gas exploitation and CO₂ sequestration in shale formations.