Analytical and Simulation Study of Sweep Efficiency in Gas-Injection EOR

Abstract

WAG (Water-Alternating-Gas injection) is a non-thermal EOR process, which was proposed to improve the volumetric sweep efficiency and consequently the oil recovery during a gas injection project. Though miscible gas injection gives fantastic displacement efficiency due to its miscibility with oil, it usually shows very poor volumetric sweep efficiency due to the high mobility of gas phase. Alternate injection of gas and water significantly reduces the gas relative mobility, and therefore leads to less gas fingering and/ or tonguing of gas. Aside from fingering and channeling, gravity segregation is another major effect that leads to the deterioration of sweep efficiency in gas-injection EOR processes. After the injected gas and water travels a certain distance in the reservoir, they completely segregate from each other under gravitational forces. Gas goes to the top of reservoir forming an override zone, and water goes to the bottom forming an under-ride zone. In Chapter one, fractional-flow theory is applied to provide insight into the advantages of HWAG. The fractional-flow method describes the flooding process in 1-D homogeneous reservoirs, which can be applied to a wide range of EOR processes. The method is accurate, when its assumptions are satisfied, in reflecting the saturation, front position and relative mobility of the agents injected, from which an optimal injection strategies can be determined. The main focus of Chapter two is the simulation study of gravity segregation in non-horizontal reservoirs. First, extensive simulations are done to examine the accuracy of Namani’s model for the segregation distance in dipping reservoirs. Second, it is equally important to understand all the dynamic processes during the process of gravity segregation.