Experimental and Molecular Studies of the Effect of CO₂ on Hindered Flows of Oil through Niobrara Shale

This paper presents experimental data on the hindrance effect of Niobrara samples on crude oil and demonstrates the effect of CO2 huff-n-puff in releasing stranded hydrocarbons. The experimental procedure includes the pumping of crude oil into Niobrara mini cores and compositional analysis of effluents using gas chromatography. Experimental results show that heavier components (C₁₁+) were noticeably hindered, lighter components (C₃ - C₆) were preferentially produced, and the fraction of intermediate components (C₇ - C₁₀) stayed about the same in the produced fluid. Soaking the core samples with CO₂ noticeably stimulated the production of heavier components, demonstrating the ability of CO₂ to mitigate hindrance. Molecular dynamics simulations show that calcite surfaces, which are abundant in our Niobrara samples, adsorbed more heavy molecules when in equilibrium with crude oil. By adding CO2 molecules into the equilibrium system, CO₂ displaced hydrocarbon molecules from rock surfaces into the bulk phase and adsorbed on calcite surfaces. This study provides evidence that crude oil can experience compositional change as it flows through a nanoporous reservoir and that CO2 can mitigate this selective hindrance. Mitigation that likely occurs through a surface mechanism has not been considered in current CO₂ huff-n-puff EOR studies for tight reservoirs. This study highlights component separation due to selective hindrance as a potential mechanism affecting the production and retention of hydrocarbons in nanoporous media, which has not been considered in current reservoir simulation models.