Experimental Study of Gas/Liquid Diffusion in Porous Rocks and Bulk Fluids To Investigate the Effect of Rock-Matrix Hindrance

Abstract

Summary The design of oil-recovery processes by gas injection or vapor solvent relies on the knowledge of diffusion coefficients to enable meaningful production predictions. However, laboratory measurements of diffusion coefficients are often performed on bulk fluids, without accountability for the hindrance caused by the pore-network structure and tortuosity of porous media. As such, our ability to predict effective diffusion coefficients in porous rocks is inadequate, and additional laboratory work is needed to investigate the impact of the medium itself on transport by diffusion. A vast number of studies focus on measurements of diffusion coefficients on simple binary systems and a few heavy-oil systems. This study proposes an experimental methodology, based on the pressure-decay technique, to measure diffusion of gas (methane) in crude oil within a porous rock. A diffusion experiment of gas into bulk oil (without porous medium) provides an upper-limit estimation of the gas/liquid-diffusion coefficient. Diffusion experiments using Indiana Limestone and Bakken Shale provide insight into different degrees of rock-matrix hindrance. Two analytical models and one numerical model were implemented to estimate the diffusion coefficients from time-dependent experimental pressure-decay data. These diffusion coefficients were found to be in agreement with literature on corresponding data, demonstrating the validity of the modeling approaches used. Results indicate considerable hindrance to diffusion in porous media relative to bulk oil and relate to the tortuosity of the rock matrix. The diffusion coefficient of methane in bulk oil was found to be 3.8×10−9 m2/s. In the limestone sample, this diffusion coefficient dropped by one order of magnitude, ranging between 1.5 and 6.5×10−10 m2/s, and decreased by another order of magnitude in the Bakken Shale sample to 2.0×10−11 m2/s. By comparing the influence of tortuosity with increasing the initial pressure, results show that tortuosity has a more-significant impact on the effective diffusion coefficient than the initial pressure.