Abstract
A thorough understanding of supercritical CO2 (scCO2) transport through nanochannels is of prime significance for the effective exploitation of shale resources and the mitigation of greenhouse gas emission. In this work, we employed the non-equilibrium molecular dynamics simulations method to investigate the pressure-driven scCO2 transport behavior through silica nanochannels with different external forces and pore sizes. The simulations reveal that the capability of scCO2 diffusion enhances both in the bulk region and the surface adsorbed layer with the increasing of pressure gradient or nanochannel size, in addition, the slip length increases nonlinearly with the external acceleration or nanochannel width increases and finally reaches a maximum value. The negative slippage occurs at lower pressure gradient or within the narrower nanochannel. Overall, it is the combined effect of strong adsorption, surface diffusion and slippage that causes the nonlinear relation between flow rate and pressure gradient or nanochannel size. The present work would provide theoretical guidance for the scCO2 enhanced shale oil/gas recovery, CO2 storage, and mass transport in nanoporous materials.
Highlights
The behaviors of supercritical CO2 in narrow pores and con ned spaces have attracted extensive attention in the exploration and development of hydrocarbons stored in shale,[1,2] such as scCO2 fracturing[3,4] and injecting.[5,6]
The results indicate that a high pressure gradient dramatically affects the distribution of scCO2 inside the nanochannel
The shape of the density pro les remains almost unchanged under the lower pressure gradients indicating a little in uence on the distribution of scCO2, which is in agreement with the results obtained by Kasiteropoulou.[47]
Summary
The behaviors of supercritical CO2 (scCO2) in narrow pores and con ned spaces have attracted extensive attention in the exploration and development of hydrocarbons stored in shale,[1,2] such as scCO2 fracturing[3,4] and injecting.[5,6] On the other hand, CO2 injection into tight shales is one of the promising solutions for mitigating carbon emissions[7,8] and global warming.[9,10] The nanopores are widely present in shale reservoirs.[11,12,13,14] The basis for scCO2 geosequestration[15,16] and enhanced hydrocarbon recovery depends on its unique transport properties induced by the con nement of shale nanopores. The evaluation of scCO2 enhancing hydrocarbon recovery, enhancement of fracturing efficiency, and estimation of storage capacity require understanding of microscopic behaviors of scCO2 through shale nanopores, such as adsorption, diffusion and ow. Firouzi and Wilcox investigated the gas slippage and Klinkenberg effects of CO2 con ned in carbon slit pores.[23] They reported a plug ow occurs at pore size less than 2 nm, a parabolic velocity pro le at pore size larger than 10 nm, and the dramatic underestimation of the CO2 permeability using the bulk phase viscosity. The effects of ow driving pressure gradient and nanochannel size on scCO2 transport were determined by exploring the adsorption, diffusion and ow of scCO2 molecules. The velocity pro les, the slippage effect, and the ow rate as the function of pressure gradients or nanochannel sizes were applied to characterize the ow behavior
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