The numerical simulations required for the robust optimization (RO) of the alternating injection of water and CO2 in hydrocarbon reservoirs are computationally expensive due to engineering, geological, and economic uncertainties. Using approximation models of the desired objective function(s) can significantly decrease the cost associated with the optimization routines while providing an adequate sampling of the input variables or uncertainties. In this study, we optimized the value of a water-alternating-gas (WAG) injection project in a Niger-Delta oil reservoir by applying the Markowitz classical theory to a suitable approximation model of the objective function. Our RO methodology incorporated significant geological and economic uncertainties—such as uncertainties due to the upscaling of the coarse-scale reservoir model and those due to the lack of other relevant geological and economic data—to the optimization routine to create better operating strategies for the projects that are risk-quantified. To compute the objective function, a novel economic model for the CO2 sequestration processes in the Niger-Delta hydrocarbon basin was applied. The result obtained in the presented case showed that a net present value (NPV) of at least USD 65.98 million was derived from the project depending on the engineer’s or user’s confidence level. Overall, the applicability of proxy models to RO routines was demonstrated. As a result, investment decisions that accounted for uncertainties and any potential variation in the field’s operation and development were made.

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