Study of gas diffusion in liquid-saturated porous media for oil recovery and carbon dioxide sequestration

Gas injection into oil and gas reservoirs to enhance hydrocarbon recovery has been widely practiced in the oil and gas industry since the beginning of the last century. More recently, CO2 geological sequestration has been considered as one of the most promising options to mitigate the dramatically increasing CO2 concentration in the atmosphere due to the booming fossil energy consumption since the last century. Understanding the gas migration and distribution within the geological media in gas injection projects for both the EOR (Enhanced Oil Recovery) process and CO2 storage process requires reliable data of gas effective diffusion coefficients in liquid-saturated porous rocks under practical reservoir conditions (high pressures and elevated temperatures). To date, no method has been reported for gas effective diffusion coefficient measurement in porous rocks under reservoir conditions.; This thesis proposes a novel method for measuring the gas effective diffusion coefficient in water- or oil-saturated porous media under reservoir conditions. Porous rock columns with the two end faces sealed are used as the test samples so that the gas diffuses into the porous column only through the radial direction. The effective diffusion coefficient measurement is conducted in a high-pressure diffusion cell containing a liquid-saturated porous column in the middle and gas in the annulus between the rock column and the cell wall. A small-pressure decay technique is employed to record the pressure change of the gas phase in the diffusion measurement. Mathematical models are derived to describe the mass transfer process for two representative systems: a CO2/water-saturated porous column system and a CO2/oil-saturated porous column system. For the CO2/water-saturated porous columns, the swelling of the liquid phase caused by gas dissolution is negligible and, thus, an analytical solution is obtained in analogy with the available radial diffusion model. For the CO2/oil-saturated porous columns, the oil phase swells significantly as diffusion proceeds. Therefore, a new mathematical model (non-linear diffusion-convection equation) is derived to describe the swelling-induced convection. A numerical solution, along with a computer code, is developed for the diffusion-convection equation. An effective diffusion coefficient is determined by matching the experimental pressure drop with the corresponding mathematical model. To test the method developed, measurements are conducted for CO2/brine-saturated, CH4/oil-saturated, and CO 2/oil-saturated porous rock columns at different pressures. The measured pressure drop versus time plot shows good agreement with the theoretical predictions using the best-fitted effective diffusion coefficient. With the measured effective diffusion coefficient and the diffusive tortuosity of the rock sample, the diffusion coefficients of CO2 in bulk liquids are also obtained. The derived method can be readily implemented in most laboratories that can handle high-pressure fluids; thus, it provides a tool for routine measurements of gas effective diffusion coefficient in liquid-saturated porous rocks.