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Selective hindrance during oil flow through nanoporous shale rocks
Zhu, Ziming
Zhu, Ziming
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2021
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Selective hindrance refers to the ability possessed by nanoporous shale rocks to hinder the transport of certain molecules through them while allowing other molecules to pass with fewer restrictions. Selective hindrance in tight rocks has long been hypothesized to exist due to size-exclusion and molecular selectivity of rock surfaces. However, only a few sets of experimental data exist. This study focuses on laboratory measurements of selective hindrance and the influences of gas injections.A specialized core-flow setup was designed and built to accurately and repeatably detect compositional changes during flows of binary liquid hydrocarbon mixtures and crude oils through nanoporous Niobrara core samples. Experimental results show that the transport of heavier components was noticeably hindered. Decreases in the fractions of these components were observed in the produced fluid. Molecular dynamics (MD) simulations of hydrocarbons in equilibrium with calcite surfaces, conducted by our collaborators, indicate that preferential adsorption of heavier components should be present. Compositional changes of fluid upstream of the samples and mole balance calculations suggest that size-exclusion should also exist. A 1-D finite-difference model was built based upon a modified continuity equation to simulate the core-flow process, preliminary simulation results successfully reproduced compositional changes of hydrocarbon components in the experiment.
Having confirmed the presence of selective hindrance, we investigated the influences of carbon dioxide and methane on the flows. Experiments were conducted to simulate huff-n-puff operations. After selective hindrance was observed, rock samples were soaked with carbon dioxide or methane for a certain time. Then, oil flow was resumed, and compositions and rates were monitored. Experimental results show that soaking core samples with carbon dioxide or methane noticeably stimulated the production of heavier components. MD simulation results show that carbon dioxide can release adsorbed hydrocarbon molecules by displacing them from rock surfaces into the bulk phase. Methane, on the other hand, did not show the ability to displace adsorbed hydrocarbons. Given the observed decreases in production rates after the injection of methane, the observed stimulation of heavier components production in methane experiments is likely due to the vaporization of lighter components.
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