The Molecular Simulation Of The Diffusion And Permeation Of Shale Gas In Shale At Geological Depth

The demand on energy is rising and the lack of new clean energy.Shale gas as an important unconventional energy resource has received worldwide attention.It has shown a significant effect on the world's energy structure after the commercial exploitation of shale gas in the United States.However,shale gas is stored in tight shale under the geological conditions,which is extremely hard to explore largely and securely.Therefore,understanding diffusion and permeation of shale gas at geological depths is quite essential.But in the actual situation,the environment of shale gas storage is in the microscopic and mesoscopic scale,and it is difficult to understand the diffusion and permeation mechanism of shale gas in those scales.In this paper,we establishe the shale model and then discuss the diffusion and permeation mechanism of shale gas through the method of non-equilibrium molecular dynamics.It is very essential to understand the diffusion and permeation mechanisms of shale gas at geological depth for the efficient extraction of shale gas.In the first work,we first investigated the storage conditions of shale gas at geological depth,including temperature,pressure,and the pore size of shale.We calculated the mean free path and then obtained the Knudsen number according to the conditions.In this way,we know there is no mechanism could describe the permeation and diffusion of shale gas in shale,which means the condition is mostly in a blank region(0.01<Kn<10).For that case,we established a MMT model with a slit pore,using dual control volume-grand canonical molecular dynamics method to systematically investigate the permeation process of shale gas in montmorillonite at different geological depths.Results indicate that temperature,pressure,and pore size have an important effect on the permeability.On the basis of these simulated data,we further performed a quantitative analysis and propose a new mesoscale model to describe the permeability of shale gas at geological depths.The new mesoscale model shows extensive applicability and can excellently reproduce the extrapolation testing data,and it satisfactorily bridges the gap between microscopic and macroscopic permeation mechanism,which provides important fundamentals for exploitation of shale gas.In the second work,we studied the permeation of shale gas in organic shale.Firstly,numbers of carbon nanotube models with different pore sizes are established,then two graphene layers are placed on each end of the carbon nanotubes.A circle hole in the graphene layer with the same size as the pores of the carbon nanotubes is at the centre of the graphene layer.Shale gas can pass through carbon nanotubes.Similar to the previous simulation method,we used the DCV-GCMD simulation method to simulate the permeation process of shale gas in carbon nanotubes.The difference is that we ensure our simulations covers Kn between 0.01 and 10 by controlling the external conditions.By calculating the flux and the permeability,we find that the permeability of shale gas in carbon nanotubes decreases with the increasing pressure.The permeability of shale gas in carbon nanotubes monotonously decreases with increasing pore size.Besides,the permeability of shale gas fluctuates with the increasing temperature.The mesocale mathematical model we have previously proposed applies here roughly.Therefore,we could obtain the diffusion and penetration mechanism of shale gas in organic shale at geological depth,which has great significance for guiding the exploitation of shale gas.