Study On Shale Gas Migration Mechanism Under Water Film Evaporation And Gas Slippage Effect

Shale gas plays an increasingly important role in Chinese and the world’s energy structure.It is a kind of unconventional natural gas with abundant reserve and huge potential.Shale reservoir has the characteristics of ultra-low permeability and relatively low porosity.The shale gas migration in reservoir is a complex process with multi-scale,multi-component and multi-physics field coupling.Meanwhile,The transportation and exploitation of shale gas include the process from water film transient evaporation in inorganic nanopores to nonlinear transport in macro hydraulic fractures.It is difficult to exploit.Therefore,it is important to find out the shale gas migration mechanism and predict the gas production reasonably.In this paper,the effects of water film evaporation,gas slippage,TOM content,bottom-hole pressure and hydraulic fractures half-length on shale gas production are studied by the methods of experimental observation,theoretical analysis and numerical simulation.The main research contents are as follows:(1)TESCAN MAIA3 is used to observe the pore shape and size of inorganic nanopores.And,the pore shapes of rectangular or wedge-shaped are frequently occurred in inorganic nanopores.Water distribution characteristic influenced by surface wettability and disjoining pressure in slit-like nanopores is then analyzed in detail.We divide the water film attached to the surface of the inorganic nanopores into two layers:the high-viscosity water film and the bulk water film.Based on the Hertz-Knudsen equation and Moving Mesh Method in COMSOL Multiphysics,the influence of evaporation on gas production is discussed.Besides,The gas slip velocity is introduced to modify the model boundary conditions.It is found that The existence of water film evaporation and gas slippage in inorganic nanopores can greatly improve the shale gas mass flow rate.Larger pore size and more regular geometry are beneficial to shale gas production.(2)The QSGS method is used to construct the micron-scale shale matrix calculation model containing organic matter and inorganic minerals,and the effect of TOM on shale gas production is then discussed.The influence of water film evaporation,gas slippage and wedge coefficient in inorganic nanopores on gas production is investigated by modifying the matrix diffusion coefficient.The research shows that,with the increase of TOM,the shale gas production increases continuously when it reaches the stable state,but the effects of evaporation and slippage on gas production decrease slightly.(3)A productivity prediction model of the horizontal well in shale gas reservoir considering matrix,artificial fracture system and natural fracture network is established.The effect of water film evaporation,gas slippage and TOM on shale gas production is discussed by modifying matrix permeability coefficient.Furthermore,the influence of bottom-hole pressure and half-length of hydraulic fractures on cumulative gas production is also studied.It is found that the increase of pore size caused by evaporation and the existence of slippage make the migration of shale gas more smoothly and increase the gas production of shale gas.With the increase of TOM,the gas production is markedly increased.Besides,as time goes on,the effect of TOM content becomes more significant.Lower bottom-hole pressure and larger hydraulic fractures half-length are beneficial to shale gas production.There are 54 figures,13 tables and 123 references in this dissertation.