Molecular sieving of hydrocarbon mixtures through nano-porous membranes

Abstract

That the majority of the pore sizes of nanoporous unconventional reservoirs are on the order of the molecular dimensions of large hydrocarbons of crude oils implies the possibility of molecular sieving during the flow of crude oil in these formations. The objective of this study is to provide evidence for the filtration of hydrocarbon mixtures in nanoporous media and to support the hindered transport models for flow in nanoporous, unconventional reservoirs. The hindered transport of hydrocarbons in nanoporous media may be contributed by a variety of mechanisms, such as molecular sieving, size exclusion, selective adsorption, and retention in dead-end pores. Core experiments conducted by Zhu et al. (2019) intended to examine filtration and hindered transport in its entirety. The experimental setup used in this research, on the other hand, was designed to focus on the molecular sieving component of filtration. Therefore, although hindered transport in porous media resembles depth filtration, where adsorption and retention in dead-end pores and tight pore-throats may be significant contributors of filtration, in this study, thin polymeric membranes simulated surface filtration due to molecular sieving only. Two versions of the experimental setup have been used: The first setup was a regular dead-end cell to study the effects of temperature, pressure, and CO₂ on filtration. The second setup adopted a stirred cell to alleviate concentration polarization and eliminate accumulation of heavy hydrocarbons on the filter surface. The results of both experiments have been compared and interpreted. The filtration of hydrocarbon molecules by polymeric membranes was verified. The retention efficiency of a hydrocarbon is proportional to its molecular weight. The effect of concentration polarization was significant in extended period tests using regular dead-end cell. It reduced the retention of large hydrocarbon molecules due to increased higher chemical potential from higher concentration. The effect of concentration polarization was alleviated significantly using stirred cell. The retention of large hydrocarbon molecules was higher in this scenario.CO₂ treatment of hydrocarbon mixture increased the total flux and reduced retention of large hydrocarbon molecules using regular dead-end cell. However, during a stirred cell test, CO₂ treatment of hydrocarbon mixture did not reduce retention of heavy or medium hydrocarbons and only affected the retention and flux of light hydrocarbons. The retention of large hydrocarbon molecules was proportional to filtration pressure and inversely proportional to temperature. That is, in general, the higher the pressure and the lower the temperature, the higher the retention of large molecules.An improved understanding of the factors affecting the retention of large hydrocarbon molecules in nanoporous media will facilitate developing models for the filtration behavior and shed light on potential methods to reduce retention of large hydrocarbon molecules in shale reservoirs.