Insight Mechanism Of Adsorption And Transport For Supercritical Carbon Dioxide In Shale Nano-channel

At present,with the increasing energy demand and the gradual depletion of conventional reservoirs,shale reservoirs have attracted wide attention as an alternative source with huge reserves.scCO₂ has huge application prospect in exploitation of shale reservoirs due to its low viscosity and strong diffusion ability.However,the insight mechanism of the adsorption and transport of scCO₂ in the shale nano-channel is not clear,and it is urgent to further research.Therefore,the molecular dynamics simulation method was utilized to consider the adsorption and transport behavior of scCO₂ in the shale nano-channel and the influence of CO₂ adsorption on its transport,which revealed the insight mechanism of adsorption and transport for scCO₂ in shale nano-channel.Thus,the insight mechanism would provide theoretical support for the application of scCO₂ in exploitation of shale reservoirs.Firstly,as the increasing of pressure gradient,the adsorption of CO₂ molecules on wall weakens and the overall diffusion capacity increases,indicating that the permeability increases and the viscosity decreases.At the same time,as the pressure gradient increases,the the slip phenomenon enhances non-monotonically,ultimately resulting in a non-linear change in the scCO₂ flow rate as the pressure gradient increases.The traditional Darcy’s law no longer applies to the migration of scCO₂ in shale nanopores.On the other hand,as the slit width increases,the overall diffusion capacity increases,indicating that the viscosity decreases;the adsorption density and strength decrease,resulting in a further increase in the permeability based on the increase in the volume of the channel.At the same time,as the slit width increases,the slip length increases firstly and then decreases,eventually leading to a fourth-order polynomial function relationship with the increase of slit width,which is different from the classical Poiseuille’s law,once again proving the speciality of scCO₂ transport in shale nano-channel.Then,due to the adsorption of CO₂ molecules on the wall surface,the slip phenomenon does not occur under a small pressure gradient.When the pressure gradient increases to a certain extent,the slip phenomenon occurs.The reason is that there is a critical shear force closed the wall for scCO₂.When the shear force near the wall is greater than the critical shear force,the slip phenomenon will occur.In addition,due to the interaction between CO₂ molecules and the wall,adsorption potential wells with a regular distribution corresponding to the hydroxyl sites on the wall exist,and there is a repulsive barrier in the region between potential wells.Thermal motion can’t drive the surface diffusion of CO₂ molecules,and there is a tendency of diffusion of CO₂ molecules on the wall.The surface diffusion of CO2molecules tends to be in different directions as the external driving force increases.