Abstract

In nanoporous rocks, potential size/mobility exclusion and fluid-rock interactions in nano-sized pores and pore throats may 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 Niobrara samples possess such sieving properties on hydrocarbon molecules. Molecular dynamics simulation of adsorption equilibrium was 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. To understand the potential effects of hydrocarbon-rock interactions on their transport, molecular dynamics simulations were performed to clarify the adsorption of C10 and C17 molecules on calcite surfaces using all-atom models. Experimental results show that the heavier component (C17) in the injected fluid was noticeably hindered. After the start of the experiment, the fraction of the lighter component (C10) in the produced fluid gradually increased and eventually reached levels that fluctuated within a range above the fraction of C10 in the original fluid; besides, the fraction of C17 increased in the fluid upstream of the sample. Both observations indicate the presence of membrane properties of the sample to this hydrocarbon mixture. 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 C10 molecules, providing a mechanism that directly supports the experimental observations. Some experimental observations suggest that size/mobility exclusion should also exist. This experimental study is the first evidence that nanoporous reservoir rocks may possess membrane properties that can filter hydrocarbon molecules. Component separation due to membrane properties has not been considered in any reservoir simulation models. The consequence of this effect and its dependence on the mixture and environmental conditions (surface, pressure, temperature) are worthy of discussions and further investigations.

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