Molecular Understanding on Migration and Recovery of Shale Gas/ Oil Mixture through a Pore Throat

Shale gas/oil remaining in blind pores accounts for a large proportion in total reserves, which is urgent to be further exploited. Understanding the microscopic mechanism of gas/oil extraction from blind pores is critical to enhanced gas/oil recovery. In this study, we investigated the migration and recovery behavior of shale gas/oil mixture through a pore throat connecting blind pores and microfractures. Our simulations show that some oil molecules tend to migrate in the form of clusters, which increases their effective size and causes possible blockage of the pore throat. The recovery ratio of gas/ oil mixture has a strong dependence on the pore size. Below the critical size, which is smaller than the size of oil clusters, the pore throat would be blocked and the recovery ratio will decrease significantly. The heavy fractions of shale oil suppress the extraction of gas/oil molecules from blind pores by four inhibition mechanisms: edge adsorption, surface adsorption, throat blockage, and liquid bridge. The kerogen pore shows a stronger adsorption ability for gas/oil mixture than quartz pore, resulting in more residues in it. We also found that a higher gas/oil ratio can provide a larger driven force, leading to a higher recovery of methane and light fractions. These findings advance the understanding of the recovery mechanisms of remaining gas/oil in blind pores and provide a practical guideline on shale gas/oil exploitation.