Affiliation:
1. The University of Texas at Austin
Abstract
AbstractSmall pore sizes influence the mass transport in gas enhanced oil recovery (EOR) in tight formations through complex phenomena such as diffusion and sorption. However, conventional simulators use relatively simple models for diffusion and sorption that do not properly represent non-ideal fluid interactions at high pressures. This paper presents a newly developed simulator for advanced compositional modeling for tight formations. A case study highlights the impact of diffusion and sorption on compositional transport in tight porous media.The simulator uses multicomponent diffusion based on the dusty-gas model (DGM), which uses the fugacity gradient as the driving force. The adsorption model used is an approximate solution based on the Multicomponent Theory of Adsorption (MPTA) using the sorbed and bulk regions. The capillary pressure is included both in the flow equations and the flash calculations by minimizing the Helmholtz free energy using the Peng-Robinson equation of state at a fixed temperature, pore volume, and overall composition.The simulator is applied to model CO2 injection into a 1-D tight porous medium with a ternary mixture of methane, n-butane, and n-decane as the initial oil. Before CO2 injection, n-decane is most attracted to the pore wall and contained at a high concentration in the sorbed region. During CO2 injection, CO2 displaces n-decane from the pore wall, causing the counter-current transport of CO2 and n-decane.Simulation results also show that the mixing of reservoir oil with CO2 in small pores results in local pressure changes, which tend to drive both the diffusion and convection. In all cases, convection enhances multicomponent transport by dissipating these pressure changes. When CO2 is strongly attracted to the pore walls, a large pressure change on mixing accelerates the CO2 diffusion into the reservoir through the sorbed region and also the counter-current transport of oil components through the central region.