The pore-level investigation of phenomena affecting the recovery of oil with gas assisted gravity drainage

Gas Assisted Gravity Drainage (GAGD) is an effective method of oil recovery that can be implemented injecting gas in the crest of reservoirs and producing oil from lower zones. GAGD is controlled by the interaction between capillary, gravitational and viscous forces, which depend on parameters of the operation, porous medium, and fluids. In this research, the performance of GAGD under various conditions was investigated by visualizing the flow of fluids at the pore-level to understand phenomena affecting the recovery of oil. A new pore network micromodel with an improved capillary continuity was developed that provides a transparent porous medium for studying the interplay between capillary, gravitational and viscous forces. The visualization of fluids’ interfaces helped with the characterization of GAGD performance under various conditions. Results of the studies imply that the porous medium heterogeneities caused the gas-front to bypass oil in smaller pores surrounded by larger pores. The bypassed oil could flow in the form of thick films in fine capillaries of porous media upon a subsequent enhancement of the gas-oil capillary pressure due to the effect of gravity on the gas-oil differential density. In the presence of mobile water, a better GAGD performance was obtained under oil-wet conditions as the hydraulic continuity of oil under water-wet conditions can be arrested by the residual water in small pores and fine capillaries of a porous medium. Although the recovery of oil at earlier times after a gas-breakthough was higher under oil-wet conditions, extending the duration of GAGD resulted in a high oil recovery under water-wet conditions upon an effective reduction of the residual water saturation. In post-waterflood GAGD, increasing the production rate resulted in the instability of the gas-front and the reduction of oil recovery at gas-breakthrough because of viscous pressure drops and capillary pressure associated with the flow of oil and water from trailing zones toward leading zones of the gas-front. Experimental results suggest that both oil-wet and water-wet reservoirs are excellent candidates for the implementation of post-waterflood GAGD. However, higher rate of oil drainage with less water production can be expected from early stages of the process in oil-wet reservoirs. Under water-wet conditions, although the production rate of oil is initially low, a very low residual oil saturation can be obtained after an effective reduction of the water saturation