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.
According to US Energy Information Administration, the primary recovery from unconventional reservoirs is predicted to be less than 10%, which implies a greater potential for these resources by implementing suitable enhanced oil recovery (EOR) techniques. However, the mainstream EOR techniques and technologies nowadays are primarily developed for conventional reservoirs where flow and retention of hydrocarbons are not necessarily governed by the same mechanisms as in the unconventional reservoirs. Oil produced from unconventional reservoirs is predominantly light, implying selective hindrance, which may be caused by a combination of size exclusion and preferential adsorption in matrix nano-pores/throats. If so, most of the remaining oil could be heavier than that produced, and the EOR approach for unconventional reservoirs could focus on alleviating the hindrance.