Molecular dynamics simulations of oil recovery from dolomite slit nanopores enhanced by CO2 and N2 injection

Shale oil reservoirs are dominated by micro- and nanopores, which greatly impede the oil recovery rates. CO2 and N-2 injection have proven to be highly effective approaches to enhance oil recovery from low-permeability shale reservoirs, and also represent great potential for CO2 sequestration. Therefore, a better understanding of the mechanism of shale oil recovery enhanced by CO2 and N-2 is of great importance to achieve maximum shale oil productivity. In this paper, the adsorption behavior of shale oil and the mechanism of enhancing shale oil recovery by CO2 and N-2 flooding in dolomite slit pores are investigated by performing nonequilibrium molecular dynamics simulations. Considering the shale oil adsorption behavior, mass density distribution is analyzed and the results indicate that a symmetric density distribution of the oil regarding the center in the slit pore along the x-axis can be obtained. The maximum density of the adsorbed layer nearest to the slit wall is 1.310 g/cm(3) for C8H18, which is about 2.0 times of that for bulk oil density in the middle area of slit pore. The interaction energy and radial distribution functions (between oil and CO2, and between oil and N-2) are calculated to display the displacement behavior of CO2 and N-2 flooding. It is found that CO2 and N-2 play different roles: CO2 has strong solubility, diffusivity and a higher interaction energy with dolomite wall, and the oil displacement efficiency of CO2 reaches 100% after 1 ns of flooding; however, during N-2 flooding, the oil displacement efficiency is 87.3% after 4 ns of flooding due to the lower interaction energy between N-2 and dolomite and that between N-2 and oil.

Automated summary

This article investigates the adsorption behavior of shale oil and the mechanism of enhanced recovery using CO2 and N-2 flooding in dolomite slit pores through nonequilibrium molecular dynamics simulations. The results show that CO2 has strong solubility and displaces oil efficiently, while N-2 has a lower displacement efficiency.