This paper presents an optimization methodology for CO2 enhanced oil recovery in partially depleted reservoirs. A field-scale compositional reservoir flow model was developed for assessing the performance history of a CO2 flood and optimizing oil production and CO2 storage in the Farnsworth field unit (FWU), Ochiltree County, Texas.

A geological framework model constructed from geophysical, geological and engineering data acquired from FWU was used for the reservoir modeling. A laboratory fluid analysis was tuned to an equation of state and subsequently used to predict the thermodynamic minimum miscible pressure (MMP). An initial history calibration of primary, secondary and tertiary recovery are conducted as the basis for the study.

After a good match was realized, an optimization model with proxy was constructed with an objective function that maximized both oil recovery and CO2 storage. Experimental design was used to link uncertain parameters to the objective function. A reduced order proxy model was necessary to reduce computational cost. Control variables considered in this study included: CO2 purchase, recycled CO2, water alternating gas cycle and ratio, infill wells and bottomhole pressure of injectors and producers. Polynomial response surface methodology was used to create the proxy model based on training simulations. This involved an iterative process until a validated surrogate model was achieved. A sensitivity analysis was first conducted to ascertain which of these control variables to include in the reduced order model. A genetic algorithm using a mixed-integer capability optimization approach was employed to determine the optimum developmental strategy to maximize both oil recovery and CO2 storage.

The proxy model reduced the computational cost significantly. The validation of the reduced order model ensured accuracy in the dynamic modeling results. The prediction outcome showed the robustness and reliability of the genetic algorithm in optimizing oil recovery and CO2 storage.

The reservoir modeling approach used in this study showed an improved way of optimizing oil production and CO2 storage within partially depleted oil reservoirs such as FWU. This study serves as a benchmark for potential CO2–EOR projects in the Anadarko basin and/or geologically similar basins throughout the world.

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