Experimental measurements and mechanisms of selective hindrance of oil mixtures in Niobrara shale

In nanoporous rocks, potential size exclusion and fluid-rock interactions in nano-sized pores and pore throats may turn the rock into a semi-permeable membrane, hindering the transport of certain molecules while allowing other molecules to pass freely. In this work, to investigate whether Niobrara samples possess such selectivity to a liquid hydrocarbon mixture, we conducted core-flow tests by pumping a binary hydrocarbon mixture (decane, C10 and heptadecane, C17) into Niobrara samples and analyzing the produced fluid compositions. Molecular dynamics simulation of C10 and C17 adsorption on calcite surfaces using all-atom models was performed to help understand the mechanisms of the experiments. Experimental results show that the transport of heavier component (C17) was noticeably hindered, with a decrease of its mole fraction in the produced fluid and an increase in the upstream injection fluid, demonstrating the presence of selective hindrance. Simulation results indicate that, for a calcite surface in equilibrium with the binary mixture of C10 and C17, more C17 molecules adsorb on the calcite surface than C10. Compositional change of fluid upstream of the sample and a mole balance calculation suggest that size exclusion should also exist.

Automated summary

In nanoporous rocks, potential size exclusion and fluid-rock interactions can turn the rock into a semi-permeable membrane, hindering the transport of certain molecules. The study found that Niobrara samples exhibit selective hindrance to a liquid hydrocarbon mixture, mainly due to the hindered transport of heavier components.