Analysis of interactions between hysteresis and viscous fingering in compositional water-alternating-gas injection

Extensive research has been published on both water-alternating-gas (WAG) injection and viscous fingering as independent phenomena, under both immiscible and miscible conditions. However, their combined effects have received less attention. Specifically, little attention has been given to the interaction between hysteresis and channeling/fingering effects during WAG displacement. This study investigates the physics governing the interaction between hysteresis and viscous fingering under WAG injection at different miscibility levels via numerical simulation of synthetic two-dimensional models. Simulations were performed using a commercial compositional simulator in which three-phase hysteresis was accounted for using the Larsen-Skauge model, a correlated random permeability field was employed to seed the gas fingers, and gas-oil miscibility was controlled through the binary interaction coefficients of the equation of state. In all miscibility scenarios, WAG displacement improved oil recovery compared to either gas or water flooding alone. Simulation results demonstrated that while recovery gains for the immiscible and near-miscible scenarios resulted from both improved microscopic displacement efficiency and enhanced areal sweep by the injected phases, the fully miscible case benefited primarily from the latter. The interaction between hysteresis effects and gas fingering provided novel insights into the physics of WAG processes. Gravity segregation of the gas phase was shown to be a major contributor to gas trapping along the gas fingers. For the immiscible and near-miscible cases, oil crossflow into gas fingers also played a significant role in trapping the gas phase. Such "crossflow trapping" during WAG may be of particular interest for CO 2 storage projects. • Efficient methodology for evaluating the influence of gas-oil miscibility in oil recovery • Main factor that improved recovery in non-immiscible cases was improved areal sweep • Trapping of gas in the fingers were mainly due to gravity segregation and oil crossflow