Effect of pore size distribution on hydrocarbon mixtures adsorption in shale nanoporous media from engineering density functional theory

Unlike the conventional reservoir, shale can have an extensive amount of pores ranging from a few to hundreds of nanometers. In the past, a number of research works have been applied to study the properties and phase behaviors of nanoconfined hydrocarbons based on a single-pore model, while ignoring pore size distribution (PSD). In this work, we use engineering density functional theory (DFT) to study the effect of PSD and volume partitioning on the hydrocarbon mixture recovery from shale nanoporous media. By adopting the actual shale play PSDs, we use the constant volume depletion (CVD) method to simulate shale gas recovery. The equilibrium properties at given pressure conditions are determined by the chemical equilibrium between nanopores and bulk region as well as materials balance. We find that as pressure drops, while the average densities of the lighter components (i.e. C-1 and C-2) in nanopores decrease, those of C-3 and nC(4) first increase, then decrease. It also shows that with more large-nanopores, while the residual ratios of hydrocarbons in nanopores are higher, the overall recovery factors are higher due to more significant volume expansions in the bulk region. PSD strongly influences the released fluids, bulk fluid compositions, and hydrocarbon mixture adsorption in nanopores, especially for the heavier components. It also influences residual ratios, compositions of produced fluids, and recovery factors. Our work should provide fundamental understandings about the effect of PSD on hydrocarbon mixture adsorption in shale nanoporous media and important insights into the optimization of shale gas recovery.