Simulation of slip flow and phase change in nanopores

Abstract

Unlike conventional oil and gas reservoirs, unconventional petroleum systems are dominated by nanometer pores. At the nanoscale, laws that govern fluid flow and phase transition are different from those established for micrometer-sized pores, and research is needed to develop new data, methods, and correlations. For this dissertation, we developed pore-scale models to simulate fluid slip flow through complex porous media and phase transition in nanopores with a pore size distribution. Slip flow is a well-recognized phenomenon that enhances the rate of liquid or gas flow at micrometer and nanometer scales. However, the ability to conduct porous media flow simulation with the slip boundary condition using the lattice-Boltzmann method is not well established. The traditional LB method simulates the no-slip boundary condition, and therefore only predicts the intrinsic permeability of a porous medium; by modifying the commonly adopted bounce-back scheme, a new LB model that captures the general first-order slip boundary condition has been derived and developed. The new LB slip flow model was validated by the analytical solutions for 1D channels and asymptotic solutions for 2D square arrays of cylinders. With this new model, slip flows through simple cubic (SC), body-centered cubic (BCC) and face-centered cubic (FCC) arrays of spheres were simulated for the first time, and correlations for the apparent permeability were established. This new LB slip model, built on an existing parallel LB framework, is capable of high-performance computing. In nanopores, phase transition is affected by capillary pressure and fluid-surface interactions. Although models that relate phase behavior to the dimension of the pores have been developed, understanding of phase transition in unconventional reservoirs is still hindered by lack of experimental data and proper account of the effect of pore size distributions. Through nanofluidic experiments, we observed that phase change should follow a sequence dictated by the pore size, and phase changes that are earlier in the sequence will change the composition of the remaining fluids and their phase transition points. A vapor-liquid equilibrium calculation procedure that accounts for the effect of capillary pressure was developed to model depressurizations of a light oil and a retrograde gas confined in nanoporous media. This procedure allows us to quantitatively predict the state of phases in a nanopore system with a pore size distribution for given pressure, temperature, fluid composition, and depressurization process.