Experimental and Molecular Insights on Mitigation of Hydrocarbon Sieving in Niobrara Shale by CO₂ Huff-n-Puff

In nanoporous rocks, potential size/mobility exclusion and fluid-rock interactions in nano-sized pores and pore throats can turn the rock into a semi-permeable membrane, blocking or hindering the passage of certain molecules while allowing other molecules to pass freely. In this work, we conducted several experiments to investigate whether CO₂ can mitigate the sieving effect on the hydrocarbon molecules flowing through Niobrara samples. Molecular dynamics simulations of adsorption equilibrium with and without CO₂ were performed to help understand the trends observed in the experiments. The procedure of the experiments includes pumping of liquid binary hydrocarbon mixtures (C₁₀ C₁₇) of known compositions into Niobrara samples, collecting of the effluents from the samples, and analysis of the compositions of the effluents. A specialized experimental setup that uses an in-line filter as a mini-core holder was built for this investigation. Niobrara samples were cored and machined into 0.5-inch diameter and 0.7-inch length mini-cores. Hydrocarbon mixtures were injected into the mini-cores and effluents were collected periodically and analyzed using gas chromatography (GC). After observing the membrane behavior of the mini-cores, CO₂ huff-n-puff was performed at 600 psi, a pressure much lower than the miscibility pressure. CO₂ was injected from the production side to soak the sample for a period, then the flow of the mixture was resumed and effluents were analyzed using GC. Experimental results show that CO₂ huff-n-puff in several experiments noticeably mitigated the sieving of heavier component (C₁₇). The observed increase in the fraction of C₁₇ in the produced fluid can be either temporary or lasting. In most experiments, temporary increases in flow rates were also observed. Molecular dynamics simulation results suggest that, for a calcite surface in equilibrium with a binary mixture of C₁₀ and C₁₇, more C₁₇ molecules adsorb on the carbonate surface than the C₁₀ molecules. Once CO₂ molecules are added to the system, CO₂ displaces C₁₀ and C₁₇ from calcite. The experimentally observed increase in the fraction of C₁₇ thus can be attributed to the release of adsorbed C₁₇. This study suggests that surface effects play a significant role in affecting flows and compositions of fluids in tight formations. In unconventional oil reservoirs, observed enhanced recovery from CO₂ huff-n-puff could be partly attributed to surface effects in addition to the recognized gas-liquid interaction mechanisms.