(147f) Optimization of CO2-Enhanced Oil Recovery with CO2 Storage in a Mature Oil Field

This paper presents an optimization methodology for CO2 enhanced oil recovery in mature oil reservoirs. A field-scale compositional reservoir flow model was developed for assessing the performance history of an active CO2 flood and for optimizing both oil production and CO2 storage in the Farnsworth Unit (FWU), Ochiltree County, Texas. A geological framework model constructed from geophysical, geological, and engineering data acquired from the FWU was the basis for all reservoir simulations and the optimization method. An equation of state was calibrated with laboratory fluid analyses and subsequently used to predict the thermodynamic minimum miscible pressure (MMP). Initial history calibrations of primary, secondary and tertiary recovery were conducted as the basis for the study.

After a good match was achieved, an optimization approach consisting of a proxy or surrogate model was constructed with a polynomial response surface method (PRSM). A sensitivity analysis was first conducted to ascertain which of these control variables to retain in the surrogate model. The PRSM utilized an objective function that maximized both oil recovery and CO₂ storage. Experimental design was used to link uncertain parameters to the objective function. Control variables considered in this study included: water alternating gas cycle and ratio, production rates and bottom-hole pressure of injectors and producers. Other key parameters considered in the modeling process were CO₂ purchase, CO₂ recycle and adding infill wells and/or patterns as well as compressor capacity. The PRSM proxy model was ‘trained’ or calibrated with a series of training simulations. This involved an iterative process until the surrogate model reached a specific validation criterion. A genetic algorithm with 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 criteria of the reduced order model ensured accuracy in the dynamic modeling results. The prediction outcome suggested robustness and reliability of the genetic algorithm for optimizing both oil recovery and CO2 storage. The reservoir modeling approach used in this study illustrates an improved approach to optimizing oil production and CO2 storage within partially depleted oil reservoirs such as FWU.

This study may serve as a benchmark for potential CO2–EOR projects in the Anadarko basin and/or geologically similar basins throughout the world.