Multi-scale Gas Transport Mechanism In Complex Pore-networks Of Shale Matrix

Nowadays,the world is facing the transformation in the field of energy pattern.With the gradual depletion of traditional fossil fuels,the unconventional reservoirs(e.g.,shale gas)are increasingly exposed in the historical stage and serve as the important supplement for the human's energy demands.The production of shale gas is the process that the gas is released from the shale matrix to fracture networks,and finally converging into production wells.The shale matrix is a typical micro/nano-porous medium with a larger number of micro/nano-pores,and thus the gas transport behavior in shale matrix is very complicated that should both consider the structural complexity and scale diversity,especially for gas transport in those nano-pores,where the gas transport characteristic is far away from the continuous flow assumptions with the significant gas-wall interfacial effects under such nano-confinement situations.Therefore,how to accurately depict the multi-scale gas transport behavior in complicated nano-porous structures of shale matrix is not only critical for the development of shale reservoir,but also fundamental for the mechanical field of micro/nano-confined mass transfer.This thesis performs a comprehensive investigation on the multi-scale gas transport in shale matrix,where the molecular dynamics(MD),lattice Boltzmann method(LBM),and pore network model(PNM)are successively applied to reveal the gas transport behavior and underlying mechanisms from nanoscale to pore-scale in shale formation,and the multi-scale gas transport modelling is established finally on the basis of the simulation results.The main contributions and new insights are summarized below.The MD simulations are firstly used to expound the formation pressure-dependent transport characteristic of shale gas at nanoscale.The results indicate that the gas exhibits a typical confined state within shale nano-pores,where the transport mechanism is dominated by the viscous flow(gas-gas interactions).As the pressure decreases,the diffusion effect(surface diffusion for adsorbed gas and Knudsen diffusion for free gas)becomes remarkable under frequent gas-wall interactions,which brings a significant increase in the gas slip velocity at the walls.Moreover,the further comparison on gas slip velocity at various walls demonstrates that the slip velocity is also strongly dependent on the microstructure of the walls,where the roughness factor plays a momentous role in gas-wall interactions.As the roughness factor rises,the gas diffusion effect would be weakened,resulting in a sharp drop of slip velocity.On this basis,an analytical model to depict the gas transport behavior in shale nano-pores is proposed under the combined action of system pressure and roughness factor,which is well verified against experimental and simulation results.Considering that the pores in shale matrix are multi-scale with numerous micro-pores that is scarcely possible to be described by MD simulations,a multi-scale gas transport modeling is consequently constructed by importing the gas-wall interfacial features acquired from MD simulations into LBM algorithm to correct the boundary conditions,and the gas transport behavior and mechanism under various scales are clarified systematically.For micrometer sized pores(H>1 ?m),the viscous flow is the dominated gas transport mechanism,in which the velocity profile is parabolic without the slip velocity at the walls.As the pore size decreases to the nanometer scale(10 nm<H<1 ?m),the slip velocity is no longer zero and increases with the reduction of pore size,where the gas transport is dominated by the viscous flow and Knudsen diffusion.When the pore size continues to decrease to lower than 10 nm(H<10 nm),the surface diffusion of adsorbed gas is non-ignorable and contributes significantly to gas transport in such narrower nano-pores.Consequently,a multi-scale gas permeability prediction model is derived based on the LBM results,which is applicable for the various flow regimes from continuous flow to transition flow.The multi-scale gas transport modeling is ulteriorly developed to describe the gas transport behavior in micro/nano-porous shale matrix,by coupling the multiple gas transport mechanisms into PNM,and the pore-scale transport characteristic of shale gas is explored in detail.The influence of pore structures on gas permeation capacity in shale matrix is revealed,and a theoretical model considering the shape factor of shale pores is proposed to estimate the gas transport capacity in various pore structures.The gas transport contribution of different pores from nano-pores to macro-pores under a series of exploitation stages is discussed,implying that the macro-pores are the main channels for gas transport in shale matrix at the initial exploitation stage(high formation pressure).But the contribution of micro-pores and nano-pores increases greatly,both of which become the dominated gas transport channels at the middle to late stages(low formation pressure),where the gas transport capacity is enhanced under the low-pressure conditions.Moreover,the heterogeneity(organic and inorganic matters)of shale matrix is considered in the gas permeability prediction models,finding that the gas slip velocity in organic pores is far higher than that in inorganic pores to form the fast gas transport paths in shale matrix,especially at lower-pressure and narrower pore size conditions,where the gas permeability is positively related with the content of organic matters in shale matrix.The multi-scale framework and models proposed herein are very critical for accurately understanding and quantitatively describing the gas transport behavior in shale matrix,which would provide essential theoretical guidance for the production prediction and exploitation strategy optimization of the shale formation.