Gravity Override and Vertical Sweep Efficiency in Dipping Reservoirs

The model of Stone (1982) and Jenkins (1984) predicts the extent of gravity override at steady state during gas-liquid co-injection in an enhanced oil recovery (EOR) process. The model is accurate for WAG injection if the slug sizes are sufficiently small. The model is exact for homogeneous reservoirs if the standard assumptions of fractional-flow theory apply (Rossen and Van Duijn, 2004). Stone and Jenkins relate the distance to complete gravity segregation to total mobility in the mixed zone and the thickness of the override zone beyond this point to mobilities in the override and underride zones. Namani et al. (2012) and Jamshidnezhad (2009) have proposed extensions to Stone and Jenkins' model for dipping reservoirs. The accuracy of these correlations were tested in this study for a wide range of conditions and angle of reservoir dip using 2D computer simulations. Both correlations are approximately correct, but deviate from observed behaviour at large dip angle. Specifically, gravity override occurs at shorter distances than predicted by Namani et al. for up-dip injection, but longer distances for down-dip injection at moderate dip angles. Jamshidnezhad's correlation works reasonably well for down-dip injection except at large dip angles, for which segregation occurs at much shorter distances than predicted. Much of the sweep of gas occurs not at steady-state, however, but during the transient period before steady state is attained. During the period of three-phase flow, mobilities are lower; as suggested by Stone's approach, this temporarily extends the mixed zone beyond that at steady state. In up-dip injection, the override zone is extended much deeper into the reservoir during the period when gas first enters than at steady state. The oil swept during this period can greatly exceed that represented in the mixed zone in any of these models: even though the mixed zone is reduced, overall sweep can be greatly increased by this effect. This extension of the override zone during transient flow follows the logic of Jenkins' derivation of the thickness of the override zone based on mobilities. Unfortunately, there is no single exact equation for gravity segregation in dipping reservoirs as for horizontal reservoirs, even at steady state. Therefore behaviour varies somewhat from case to case.