A comprehensive review on Enhanced Oil Recovery by Water Alternating Gas (WAG) injection

Water-Alternating-Gas (WAG) injection is a relatively mature oil recovery technique in hydrocarbon reservoirs that has long attracted the interest of the oil and gas industry due to its successful performance. The main goal of the WAG projects is to control the mobility and to decrease the problem of viscous fingering, leading to improved oil recovery by combining the benefits of Gas Injection (GI) and Waterflooding (WF). Implementation of a new EOR/IOR project requires a comprehensive knowledge of previous successful and failed experiences, and adequate understanding of the technical and non-technical aspects of the recovery process. This knowledge may be attained from reviewing similar projects that were reported in the literature. Despite great applications of the WAG injection in hydrocarbon reservoirs and extensive studies, the last comprehensive review goes back to 1998, focusing on the field applications only. There are a few review papers that are more updated; however, they are either dedicated to a particular aspect of the WAG (such as CO2 abnormalities), or applications in a specific geographical region (such as North Sea). An updated comprehensive study, covering recent experiences and lessons that are learnt from previous studies, seems to be imperative. This paper reviews the WAG theory, applications, governing mechanisms of fluid displacement and oil production from pore to field scale. It also addresses the most common challenges and operational problems along with the remedies during WAG projects. The effects of important variables such as reservoir properties, fluid properties, and operating conditions on the performance of WAG are studied from experimental, simulation and modeling, and pore-scale investigations.