Cross-scale Gas-water Two-phase Transport Simulation In Shale Gas Reservoir

Shale gas reservoirs are rich in nano-micron scale pores and microfractures,massive hydraulic fracturing causes large hydraulic fractures,which brings the multiscale characteristic in shale gas reservoirs.Fluid transport mechanisms are complex in multiscale shale spaces and fluid transport in nano-micron scale pores exhibits a microscale effect.In this case,the traditional linear transport law is not applicable.The difference in wettability of inorganic matter(IOM)and organic matter(OM)pore surface leads to the unclear understanding of corescale fluid transport behavior,and it is challenging to implement the core-scale experiments due to the ultra-low porosity and permeability of shale.During the flowback and production stage,the accurate considerations of microscale fluid transport mechanisms as well as OM distribution in macroscale numerical simulation of shale gas reservoir are desirable.Besides,establishing the connection between fluid transport simulations at different scales is still an unresolved problem.In this study,a numerical simulation method of gas-water two-phase transport in shale gas reservoir with respect to the pore,core,and field scale is proposed.The study is conducted based on the following four associated parts.First,the simulation method for shale gas transport is presented from pore-scale to core-scale.IOM and OM pore network gas transport models are developed based on electron microscope images from a specific shale basin in China.Subsequently,a core-scale model accounting for the OM distribution is constructed.Core-scale gas apparent permeability is derived by combining the calculated results of the IOM/OM pore network model through the Monte Carlo sampling method.The shale matrix representative elementary size is determined based on the core-scale model.The influences of different gas transport mechanisms and relative humidity are analyzed in detail.Second,as the direct laboratory measurements of shale gas-water transport properties are not available,a method of calculating gas-water two-phase transport parameters from pore-scale to core-scale is developed.The IOM pore network gas-water two-phase transport simulation and OM pore network single-phase gas/water transport simulation are investigated,considering the wettability difference between IOM and OM surface as well as microscale effects.Core-scale capillary pressure and gas-water relative permeability are then derived based on the representative elementary model by integrating the above gas-water pore-scale transport properties.Core-scale relative permeability variation with total organic carbon(TOC)content along with permeability difference between OM and IOM are analyzed.Third,the gas-water two-phase transport behavior assessment model in shale microfracture is investigated for the first time.The shale microfracture morphology is extracted based on the computed tomography(CT)image from a specific basin in China.The gas-water two-phase transport model in a microfracture is established by incorporating multiple gas/water microscale effects comprehensively.The microfracture critical aperture at which gas/water microscale effects and mobile water film should be considered are evaluated.Finally,the gas-water two-phase transport macroscale mathematical model coupling matrix,microfracture and hydraulic fracture is proposed,accounting for the pore-scale to core-scale simulation results.A macroscale gaswater two-phase transport simulation in shale gas reservoir considering comprehensive microscale effects is realized.Shale gas reservoir productivity during the flowback and production stage is investigated,and the effects of microscale effect,stress sensitivity and hydraulic fracturing parameters are evaluated extensively.This thesis presents a systematic cross-scale methodology to estimate gas-water two-phase transport properties in shale gas reservoir.It achieves gas-water two-phase transport stepwise simulation(from pore,core to filed scales)by incorporating complex gas/water microscale transport mechanisms,which provides effective guidance based on theoretical and technical perspectives.