Simulation of Immiscible WAG Injection in a Stratified Reservoir - Characterization of WAG Performance

Abstract

Summary Water-Alternating-Gas (WAG) is a well-established EOR process where gas and water are injected in alternating fashion. Good volumetric sweep is achieved as water and gas target both the oil residing in low and high portions of the reservoir, respectively. Other important features in three phase hysteretic flow include phase trapping which is believed to be more strongly associated with the gas phase. With these aspects in mind a vast simulation study has been performed investigating immiscible WAG injection focusing on mechanisms such as mobility, gravity, injected volume fractions, reservoir heterogeneity, gas entrapment and relative permeability hysteresis. A horizontally layered reservoir is considered where oil is displaced by water and gas alternately injected towards a producer. The model is a modified blackoil type, where hysteresis in the gas phase is modeled using Land's model for trapping and Carlsens model for relative permeability hysteresis. It is seen that gravity segregation in uniform models and increased heterogeneity in no-gravity models both lead to lower oil recovery. However, in heterogeneous models, gravity can divert flow from high permeable layers into low permeable layers and improve recovery. Hysteresis lowers gas mobility and hence improves gas-oil mobility ratio and reduces gravity segregation. The first effect is always positive, but the second is mainly positive in more uniform reservoirs where gravity segregation has a negative effect on recovery. In heterogeneous reservoirs, reducing gravity segregation can lead to that the oil in low permeable layers remains unswept. In addition to demonstrating the role of the various parameters we derive a characteristic dimensionless number to scale these mechanisms and predict recovery trends. This number is effectively a WAG mobility ratio, termed M expressing how well the injected fluid mixture is able to displace oil whether it is due to fluid mobilities, heterogeneity or other effects. At a value of M near 1 optimal recovery is achieved, while logaritmic increase of M reduces recovery.