Study On Multiple Transport Mechanisms Model Of Gas In Shale Reservoir

Shale gas,as a supplement to and replacement for conventional oil and gas resources,holds significant strategic importance.Due to the complex micro-pore characteristics and gas flow patterns in shale,the understanding of gas transport mechanisms within multi-scale porous structures in shale is not clear.Therefore,there is an urgent need to comprehensively understand the transport behavior of gas in different scale porous media based on shale reservoir characteristics and gas seepage mechanisms.Furthermore,it is crucial to establish a precise quantification of shale matrix apparent permeability models.This paper starts by investigating the micro-pore structure characteristics of shale reservoirs.It takes into account the synergistic effects of multiple gas transport mechanisms within shale pores and differentiates the characterization and modeling of gas transport in organic and inorganic pores.The study also examines various flow states of gas in multi-scale pores,such as slip flow,Knudsen diffusion,and surface diffusion.Additionally,it considers complex mechanisms including stress sensitivity,real gas effects,gas adsorption/desorption,and adsorbed water film.Based on these considerations,the paper establishes corresponding apparent permeability models using dual fractal theory,incorporating pore radius and tortuosity.Weighted average apparent permeability models for shale are obtained by combining the models for organic and inorganic matter based on total organic carbon(TOC)content.Finally,the reliability of the models is validated through experimental data comparison.The influence of different key characteristic parameters on gas transport capacity is analyzed,leading to the determination of multi-scale gas transport laws in shale.The research results indicate that gas transport in organic matter pores consists of three competing mechanisms: surface diffusion,slip flow,and Knudsen diffusion.Under constant confining pressure,the apparent permeability of gas exhibits a "V"-shaped variation pattern.Surface diffusion of adsorbed gas in small pores(r < 2 nm)contributes up to 80% to gas transport.As the pore radius increases to 50 nm,slip flow gradually strengthens and becomes the dominant mechanism for gas transport,while the contributions of surface diffusion and Knudsen diffusion become negligible.In inorganic matter pores,gas transport is mainly governed by slip flow and Knudsen diffusion,and the inorganic apparent permeability is inversely proportional to the relative humidity.As reservoir pressure decreases,relative humidity and water saturation rapidly increase.The thickness of the adsorbed water film on the walls of inorganic pores increases,reducing the effective pore diameter in real-time and resulting in a decrease in the inorganic apparent permeability.Based on the dual fractal theory of shale,this study objectively elucidates the flow and transport characteristics of gas in multi-scale porous media with co-developed organic and inorganic pores.It enhances the accuracy of predicting shale matrix permeability and provides theoretical guidance for the efficient and sustainable development of shale gas reservoirs in China.