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We investigate the optimal water-alternating-gas (WAG) injections into petroleum reservoirs. When water and CO2 are injected to displace oil, then the less denser gas rises to the roof of the reservoir, whereas the denser water phase sinks to the base of the reservoir. Thus, some oil can be bypassed by the injected fluids causing reduction in the WAG efficiency. The gravity override also influences the optimal completion intervals of the injection and producing wells. For example, if injection well is completed only at the roof of the reservoir, then CO2 flows to the producing well along the roof and the sweep efficiency is lower.To determine the most profitable strategy of WAG, we couple a 2-D cross-section reservoir model with a simple economic model of WAG and aim at the net present value (NPV) maximization [1,2]. At the initial moment of time the reservoir 0≤X≤L, 0≤Z≤H is saturated with oil at connate water saturation, where X and Z are the horizontal and vertical coordinates. The oil displacement is simulated by placing two point sources at X=0 (Injector) and X=L (Producer), which are opened at t=0. We consider various depths for the Injector and Producer. Either of the point sources can be located near the upper boundary Z=0 (i.e. near the roof of the reservoir), at Z=H/2 and Z=H (i.e. at the base of the reservoir). The Injector is operated at a constant volume injection rate at reservoir conditions for any displacing fluid. The Producer is operated at a constant bottom-hole pressure that is equal to the initial reservoir pressure. We consider only two types of displacing fluid, namely water and CO2. Both continuous and alternating injection scenarios are examined. The water and hydrocarbon (HC) mixture flow is simulated by a system of the mass conservation equations for each component of the HC mixture and water and Darcy`s law.We propose two dimensionless similarity parameters characterizing the influence of the gravity override on the WAG efficiency. The first parameter characterizes the reservoir anisotropy and associated pressure distribution. The second parameter characterizes the influence of the gravity. We investigate the optimal gas flooding strategies in the space of these parameters and present the maps showing the optimal strategy and NPV against the dimensionless parameters. Also we investigate the influence of the Injector and Producer completion intervals on optimal WAG scenario. For a fixed dimensionless parameters and injection strategy, we present a map showing optimal completion intervals for both Injector and Producer.The authors acknowledge funding from the Russian Science Foundation under grant # 19-71-10051.[1] Afanasyev A., Andreeva A., Chernova A. Influence of oil field production life on optimal CO2 flooding strategies: Insight from the microscopic displacement efficiency // Journal of Petroleum Science and Engineering. 2021. V. 205. P. 108803[2] Afanasyev A., Andreeva A., Chernova A. Numerical optimisation of CO2 flooding using a hierarchy of reservoir models // Advances in Geosciences. 2021. V. 56. P. 19-31 |
