Transporting Mechanisms Of Shale Gas In Nanopores And Well Testing Analysis

Shale gas is a kind of efficient and clean energy,which is stored in organic clay and shale rocks as well as their interlayers in the form of adsorbed and free gas state.The storage and seepage mechanism of shale gas is signicantly different from that of conventional oil and gas reservoirs,due to complex pore/throat structures and highly development of nanoscale pores in shale rocks.Shale formation belongs to typical multi-scale storage and seepage space,since micropores,mesopores,macropores and natural fractures are all developed in shale gas reservoirs.Adsorbed gas and free gas coexist in shale formations,which is another typical characteristic of shale gas reservoirs.Free gas is usually stored in fractures and macropores in compressed state,while adsorbed gas is stored on surface of nanopres and kerogen.Free gas in fractures and macropores can be described by Darcy’s law,while multiple seepage mechanisms need to be coupled to depict gas transporting in nanopores.In order to characterize gas transporting mechanisms as well as their effects on production dynamics,the following study was conducted in this thesis:(1)Taking microscale and real gas effects into account,free gas was described by real gas equation of state,according to the physical model of gas transporting in shale nanopores.The change of gas molecular mean free path and gas thermodynamics can be characterized by deviation factor.Meanwhile,the calculaition equation of real gas viscosity and concentration was given in the thesis.(2)Considering gas adsorption on nanopore surface,stress sensitivity of shale formation,the effects of gas volume on nanopore paths,dynamics adsorption and desorption and real gas effects,a new slip model was established for the description of gas transporing in shale matrix,where the effects of slip phenomenon,stress sensitivity and gas adsorption on gas transporting capacity were researched respectively.(3)According to multiple seepage mechanisms of shale gas in nanopores,a comprehensive apparent permeability model was established considering viscous flow,slippage effect,Knudsen diffusion and surface diffusion,while the contribution of each seepage mechanism was characterized in different pore and pressure conditions.(4)A new flow chart was ploted considering real gas effects,while diferent possible flow mechanisms in shale formations were given according to the flow chart.Different methods in describing gas transporting mechanisms were concluded and compared.Meanwhile,considering transient and pseudo-steady state diffusion of dissolved gas in kerogen as well as transient and pseudo-steady state interporosity from nanomatrix to fractures,four basic microscale seepage mechanism models were established and solved.(5)Considering multiscale storage space and multiple transporting mechanisms in shale gas reservoir,transient pressure behavior of a multiple fractured horizontal well in box-shaped formation was analyzed by adopting "pseudo-quadruple porosity" meadium model and source function method.The effects of Knudsen diffusion,surface diffusion,gas adsorption and desorption,dissolved gas diffusion and fracture parameters on type curves were studied,while two parameters were defined to weigh the amount of adsorbed gas and dissolved gas qualitatively.Gas transporting mechanisms in shale nanopores and gas seepage laws in multiscale formation were revealed in this thesis.The effects of slippage phenomenon,Knudsen diffusion and surface diffusion on microscale gas seepage laws were studied,while the influence of real gas,stress sensitivity,gas adsorption and desorption on permeability during shale gas production were also cleared.The effects of gas slippage,Knudsen diffusion,surface diffusion,adsorbed and dissolved gas on well testing curves were researched.There is important theoretical value and practical meanings in this study for effective development of shale gas reservoirs in our country.