Effect of Pore Size Heterogeneity on Hydrocarbon Fluid Distribution, Transport, and Primary and Secondary Recovery in Nano-Porous Media

Abstract

In this paper, we investigate the effect of pore size heterogeneity on fluid composition distribution of multicomponent-multiphase hydrocarbons and its subsequent influence on mass transfer in shale nanopores. The change of multi-contact minimum miscibility pressure (MMP) in heterogeneous nanopores was investigated. We used a compositional simulation model with a modified flash calculation, which considers the effect of large gas–oil capillary pressure on phase behavior. Different average pore sizes for different segments of the computational domain were considered and the effect of the resulting heterogeneity on phase change, composition distributions, and production was investigated. A two-dimensional formulation was considered here for the application of matrix–fracture cross-mass transfer and the rock matrix can also consist of different segments with different average pore sizes. Both convection and molecular diffusion terms were included in the mass balance equations, and different reservoir fluids such as ternary mixture syntactic oil, Bakken oil, and Marcellus shale condensate were considered. The simulation results indicate that oil and gas phase compositions vary in different pore sizes, resulting in a concentration gradient between the two adjacent pores of different sizes. Given that shale permeability is extremely small, we expect the mass transfer between the two sections of the reservoir/core with two distinct average pore sizes to be diffusion-dominated. This observation implies that there can be a selective matrix–fracture component mass transfer as a result of confinement-dependent phase behavior. Therefore, the molecular diffusion term should be always included in the mass transfer equations, for both primary and gas injection enhanced oil recovery (EOR) simulation of heterogeneous shale reservoirs.

Phase Behavior and Composition Distribution of Multiphase Hydrocarbon Binary Mixtures in Heterogeneous Nanopores: A Molecular Dynamics Simulation Study

“…Whereas, for C1, as shown in Figure 21 a, the molecules are homogenously dispersed. This observation suggests that nano-confinement can lead to phase transition, in addition to compositional alteration and fluid property change, not only inside the pores, but also near the pore entrance in the bulk (in agreement with previous macro-scale studies [ 25 , 26 ]).…”

“…Since in C2, gas and liquid are present, the pore surface preferentially adsorbs the liquid phase, and since the liquid phase is mostly composed of heptane, the concentration of heptane molecules increases more efficiently than the other conditions. Our results here agree with our larger scale simulation results, stating that heavier molecules tend to migrate from the bulk (fracture) to the matrix (nanopore) [ 25 ]. This implies that, in the field scale, it is a challenge to recover heavier hydrocarbon molecules from the shale nanopores, as they have the tendency to remain in the confined space and not migrate to the fracture high permeability zone.…”

“…In previous sections, the effect of nano-confinement on density distribution and phase behavior has been identified. In a previous study, when the nanopore effect is considered, such as capillary pressure, the fluid-phase compositions inside nanopores and in bulk does differ, which in turn results in a different overall composition in the bulk compared to the nanopore, as a result of molecular diffusion [ 25 ]. In addition, it was observed that there is a tendency of an increase in heavier molecules’ composition in the nanopores (shale matrix) compared to the bulk (fracture) [ 25 ].…”

“…In a previous study, when the nanopore effect is considered, such as capillary pressure, the fluid-phase compositions inside nanopores and in bulk does differ, which in turn results in a different overall composition in the bulk compared to the nanopore, as a result of molecular diffusion [ 25 ]. In addition, it was observed that there is a tendency of an increase in heavier molecules’ composition in the nanopores (shale matrix) compared to the bulk (fracture) [ 25 ]. Recently, Zhao and Jin used a density functional theory to quantify the role of pore size distribution when nanopore confinement effects on the phase equilibrium are present.…”