Modeling CO2 Recovery Using CO2 Solubility Adjusted Saturations and a Two Flow Regime Geological Model
Authors: Masoner L.O. 
Published: December 2007

Abstract
The need for a CO2 recovery strategic planning model induced the development of alternative simulation approaches for both CO2 recovery and geological heterogeneity. The miscible CO2 recovery mechanism functions by altering interfacial tensions, reducing oil viscosity and swelling oil. The simulation approach used in this paper tracks the impact from CO2 solubility on the oil saturation changes (swelling) and oil viscosity while ignoring changes to the interfacial tensions.
The swelling of reservoir oil by solubilized CO2 acts to increase oil relative permeability and lower the waterflood residual oil saturation independently from any reduced interfacial tension due to CO2 first or multi-contact miscibility. Further, the solubilized CO2 also acts to reduce oil viscosity similarly to dissolved natural gases. The combination of oil swelling and reduced oil viscosity results in increased oil production rates and recovery. Ignoring changes in interfacial tension makes this simulation approach suitable for predicting immiscible CO2 displacement.
This simulation approach also incorporates a simplified geological heterogeneity model defined by separating the rock into two pore volumes. These pore volumes are distinguished as high and low transmissibility. This permits rapidly creating and history matching multiple geological visualizations and predicting general conformance improvement impacts.
The modeling approach outlined in this paper appears to capture the dominant displacement and sweep efficiency impacts while dramatically reducing history match and prediction run times. It can utilize spreadsheet technology enabling decision analysis software to automatically render expected value liquid forecasts with geological uncertainty included. The approach readily incorporates financial and technical performance measures and operational constraints reflecting real-world WAG management decision drivers. Application to the Rangely Weber Sand Unit (RWSU) CO2 flood has generated some of the best history matches to date, suggesting that swelling and viscosity reduction displacement mechanisms and geological heterogeneity have dominated CO2 performance for the first 8 years. It has helped to substantiate conformance improvement problems, yield insights into potential benefits from conformance improvement activities, and address full field WAG management issues.

Introduction
Chevron U.S.A. Production Co. operates the Rangely Weber Sand Unit (RWSU) CO2 flood located in northwest Colorado. The RWSU has utilized finite-difference compositional, pattern element, and Hybrid (combination 2D finite-difference and streamtube) simulation approaches to predict CO2 recovery. These approaches posed two problems. The size of the field &gt;600 active wells covering 30 square miles) and WAG management complexities made these approaches cumbersome or impractical for investigating full field decisions and uncertainties. These approaches also did not appear to properly predict the water-alternating-gas (WAG) impacts to the total mobility changes as observed from actual field performance.
Formulating a full field strategic model that could address the key future decisions and uncertainties of the RWSU CO2 miscible flood constituted the primary objective of this revised modeling approach. The model needed significant flexibility regarding the decision driven activities that dominate the field's performance:
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