Hydrocarbon mixture phase behavior in multi-scale systems in relation to shale oil recovery: The effect of pore size distributions

Abstract

Shale media contains a large amount of nano-scale pores, with their total pore volume comparable to that of the connected macropores and natural/hydraulic fractures (bulk). Previous work largely neglected the interplay between nanopores and bulk region as well as the effect of pore size distribution (PSD), where fluids can freely exchange between nanopores and bulk region. To accurately predict production and ultimate oil recovery, PSD effect should be taken into consideration. In this work, engineering density functional theory (DFT) is used to study phase behaviors of hydrocarbon mixtures in multi-scale nanoporous media with PSD effect during constant composition expansion (CCE) and constant volume depletion (CVD) processes. We found that under the PSD effect, due to the chemical equilibrium between various nanopores and connected bulk as well as competitive adsorption in nanopores, the interplay between nanopores and bulk region influences phase behaviors and properties of fluids in the multi-scale system. Phase transitions first occur in the bulk region, then the larger pores followed by the smaller pores. The bulk bubble point pressure increases as the volume ratio of the smaller pores in the system increases, while the bulk dew point decreases. When fluids in one specific pore begin to vaporize, in other pores, the heavier component would be adsorbed, while the lighter component would be released, which suppresses the phase transitions in the smaller pores because of the heavier component accumulation. The higher volume ratio of the smaller pores suppresses the heavier component production, when pressure is below the bulk dew point.