Rock-Fluid Characterization for Miscible CO2 Injection: Residual Oil Zone, Seminole Field, Permian Basin
Authors: Honarpour M. M. , Nagarajan N. R. , Grijalba A. C. , Valle M. , Adesoye K. 
Published: February 2011

Low to moderate remaining oil saturation exists in deeper zones of several Permian Basin oil reservoirs. The Residual Oil Zone (ROZ) in the Seminole San Andres Unit (SSAU) is found below its traditional producing oil-water contact and contains significant (20-40%) immobile oil. ROZ production is not technically or economically feasible through primary or secondary oil recovery mechanisms. However, miscible flooding with carbon dioxide, CO2, has emerged as a viable enhanced oil recovery mechanism with a high potential to mobilize significant portions of this resource. Reliable rock and fluid characterizations are critical to evaluate the CO2 flood potential and an optimum design of CO2 flood strategy.

The critical rock-fluid parameters and processes influencing the feasibility of producing the remaining oil in the ROZ are:

■ Reliable estimates of Remaining Oil Saturation (ROS) - obtained from pressure core, sponge cores and laboratory waterflood tests at reservoir conditions ■ Characterization of formation anisotropy and scale-dependency of permeability through whole core analysis and potential formation damage during CO2 flood ■ Residual oil saturation to miscible CO2 flood (Sorm)- obtained from miscible CO2 flood tests at reservoir conditions using live oil and field observation ■ Reservoir oil/ CO2 PVT properties - obtained by basic oil PVT tests, oil swelling and multi contact tests ■ Minimum miscibility pressure to CO2 - obtained through slim tube and coreflood laboratory CO2 flow tests ■ Equation of state compositional fluid characterization to capture key mechanisms of CO2 miscible flooding ■ Compositional CO2 flood simulation using geocellular models and the above rock and fluid data and models

Significant variation in remaining oil saturation of 20-40% were observed field-wide in ROZ, but through data integration, it was shown that the most probable average value of the remaining oil saturation is 28-32%. The residual oil saturation to waterflood from centrifuge imbibition capillary pressure tests were shown to be in the range of 20-28% with an average of 25%. Residual oil saturation to miscible CO2 flood was measured in 6.0" to 12.0" long core composite found to be moderately low, ~12%. Coreflood tests showed no formation damage due to CO2 injection after water flooding but significant porosity and permeability enhancement during WAG process. Laboratory measurements showed that the MMP of ROZ oil with CO2 is below the current reservoir pressure and the oil swelling is nearly 75%. This result indicates that miscibility development is achievable under current operational conditions. However, there is a possibility of a second liquid phase, rich in CO2 appearing over a small pressure window, that may complement the miscible recovery process through liquid-liquid extraction. Compositional simulation with reliable rock and fluid data input was used in the design and implementation of the CO2 flood in Seminole ROZ.

Significant improvement in data quality and reduction in rock and fluid data uncertainty for miscible CO2 flood were achieved by adopting best practice methodologies for characterization of rock and fluid properties.