A New Model of Compositional Transport in Sorbed Layers for Multicomponent Multiphase Simulation of Tight Reservoirs

Abstract

Summary The main challenge in modeling compositional transport in tight formations is to include the complex interaction of chemical species with rock surfaces. Such interactions result in compositional variations near the surfaces, which can significantly affect the compositional diffusive transport through tight pores. This paper presents the implementation of a new framework for modeling multicomponent diffusive transport in the central and near-wall sorbed layers of pores. The importance of the inclusion of sorption and capillary pressure is demonstrated in case studies. The new method in this paper iteratively computes the fluid compositions in the central and sorbed layers subject to a wall chemical potential by a rapid approximation of the Multicomponent Potential Theory of Adsorption (MPTA). This calculation uses a nested loop of flash calculations to minimize the Helmholtz free energy including the effect of capillary pressure for each fluid layer in a pore. The in-house simulator computes the mass transfer driven by fugacity gradient based on the dusty gas model for the central and near-wall layers of pores. This simulator uses a partially implicit formulation to solve the multiphase multicomponent mass transfer equations including the sorbed layer, where only the driving forces are computed implicitly. The two-region approximation of MPTA is validated against the MPTA calculation using 100 regions discretizing the pore volume between the pore center and the pore wall. The multicomponent Langmuir model is the most often used for computing the sorbed excess in micropores. However, results show that the Langmuir model becomes physically inconsistent at high pressures. The new approach developed in this research is sufficiently flexible and reasonably accurate for modeling the multicomponent sorption at high pressures. The diffusion simulation with the rapid MPTA showed that the sorption and capillary pressure caused the compositional segregation between the central and sorbed layers. The segregation enhanced the rates of methane injection and n-decane production in their counter-current diffusion. Methane (the lightest) is transported deep into the reservoir by diffusing through the central layer while n-decane (the heaviest) is diffused primarily through the sorbed layer. n-Butane (the intermediate) did not show preferential partitioning into either layer, resulting in relatively inefficient transport. In the absence of sorption and capillary pressure, the countercurrent diffusion occurred between methane (the lightest) and n-butane (the intermediate) while n-decane (the heaviest) remained nearly immobile. That is, whether the simulation considers the surface-fluid interactions, such as sorption and capillary pressure, can substantially affect the compositional transport (e.g., produced fluid composition) through tight porous media.