Research On The Transport Mechanisms Of Shale Gas At Micro/Nano Scale

With the rapid development of human society,the conventional coal-based and petroleum-based energy sources have been consumed dramatically.Due to the extensive use of these fossil fuels,the global environment has been seriously polluted.In this case,unconventional oil and gas resources have received more and more attention.Because of its numerous advantages such as wide distribution,abundant resource and low pollution,shale gas has become a hotspot of petroleum exploration and development.Shale gas,which is primarily composed of methane,is one type of natural gas trapped within shale formations.Different from conventional oil and gas reservoirs,shale matrix contains a large number of micro/nano pores and has the characteristics of ultra-low porosity and permeability.Under actual working conditions,shale gas has a variety of storage states(adsorption gas,free gas and solution gas)and multi-scale flow mechanisms(gas desorption,diffusion,non-Darcy flow and Darcy flow).Accordingly,the exploitation of shale gas involves a series of multi-scale and multidisciplinary problems.At present,there still exist several key problems during the exploitation.Though adsorption is the main storage method of shale gas,little correlative report about the adsorption mechanisms and adsorption structures of methane in the slit pores has been presented.Under actual working conditions,the adsorption of methane is influenced by many factors and accurate theoretical model for describing the adsorption capacity has not been obtained.Since large amounts of methane are adsorbed in nanopores,the exploitation is a process of desorption.For the observed desorption hysteresis,the underlying mechanisms are still unclear.Influenced by above desorption hysteresis,the efficiency of depressurization method is relatively low.To improve the recovery efficiency,a new method such as injection gases to displace the adsorbed methane is proposed.It is necessary to figure out the displacement mechanisms and processes.In addition,there is a microscale effect on the methane flow in micro/nano pores.To describe this flow behavior accurately,new flow equations are urgently needed.In order to solve these problems,a systematic investigation is carried out through Monte Carlo and molecular dynamics methods in this work.As the simulation results show,the storage capacity of the slit pore is higher than that of the same volume in bulk due to the intermolecular interaction between pore walls and methane.When methane molecules adsorb on pore walls,the potential energy of them decrease.With the increase of pore width,the structure of adsorbed methane transforms from single adsorption layer to four adsorption layers.Moreover,the adsorption isotherms of methane in different pores are simulated and compared.It is found that the small slit pore fills up quicker and can store more methane than the large one under relatively low pressure,which indicates that the adsorption capacity of small slit pore is stronger.The exploitation of shale gas is a process of desorption,obvious desorption hysteresis can be observed from the engineering and experiments.At present,there are two main mechanisms of this phenomenon:capillary condensation and variation of pore throat size.In this work,these two mechanisms are simulated and analyzed.It is found that desorption hysteresis caused by capillary condensation only occurs below the critical temperature of methane.Under actual working conditions,capillary condensation would not occur.Referencing previous works,a new mechanism about the variation of pore throat size is proposed.It is found that the adsorption of methane may lead to swelling of the shale matrix which further narrows the pore throats.During desorption,higher energy is required for the adsorbed methane to pass through the narrowed pore throats.Then,part of methane is trapped,which accounts for the desorption hysteresis.Based on this mechanism,the effect of pressure and temperature on desorption hysteresis is further studied.To displace the adsorbed methane and enhance the gas recovery,injection gases are employed in engineering.Carbon dioxide and nitrogen are two common choices of injection gases due to their advantages.The displacement mechanisms of these two gases are found to be different:carbon dioxide can replace the adsorbed methane directly while nitrogen works by decreasing the partial pressure of methane.Furthermore,the displacement processes of these two gases are compared.The simulation results show that injection of carbon dioxide gives slow breakthrough time,sharp front while injection of nitrogen gives fast breakthrough time,wide front.Shale matrix contains a large number of micro/nano pores,and there is a microscale effect on the methane flow in these pores.Using equilibrium molecular dynamics,the static properties including density distribution and self-diffusion coefficient of the confined methane are firstly analyzed.It is found that the adsorbed methane manifest higher density and lower self-diffusion.Using nonequilibrium molecular dynamics,the flow behavior of methane in nanopores ranging from 1 to 10 nm is investigated.The results show that velocity profiles manifest an obvious dependence on the pore width and they translate from parabolic flow to plug flow when the width is decreased.In relatively large pores,the parabolic flow can be described by the Navier-Stokes equation with appropriate boundary conditions.Based on this equation,corresponding parameters such as effective viscosity and slip length are determined.With the decrease of pore width,another mechanism of surface diffusion becomes more dominant.In these narrow pores,the velocity in the center is uniform while that near the wall is relatively large with linear increase.To describe this flow behavior accurately,a piecewise polynomial is proposed.Under this condition,it is found that surface diffusion remarkably enhances the total flux.In this work,the microscopic mechanisms about key problems of shale gas are revealed by molecular simulation.All these results are of great significance for the efficient exploitation of shale gas.