The technical success of an Enhanced Oil Recovery (EOR) project depends on two main factors - first, the reservoir remaining oil saturation (ROS) after primary and secondary operations, and second, the recovery efficiency of the EOR process in mobilizing the ROS. These two interrelated parameters need to be estimated prior to embarking on a time-consuming and costly process for designing and implementing an EOR process. The oil saturation can vary areally and vertically within the reservoir, and the distribution of the ROS will determine the success of the EOR injectants in mobilizing the remaining oil.

There are many methods for determining the oil saturation (Chang et al. 1988, Pathak et al. 1989) and these include core analysis, well log analysis, Log-Inject-Log (LIL) procedures (Richardson 1973 and Reedy 1984), and Single Well Chemical Tracer Tests (SWCTT). These methods have different depths of investigation and different accuracies, and they all provide valuable information about the distribution of ROS. There is no single method that provides the best estimate of ROS, and a combination of all these methods is essential in developing a holistic picture of oil saturation and in assessing whether the oil in place is large enough to justify the application of an EOR process. As Teletzke et al. (2010) have shown, EOR implementation is a complex process, and a staged, disciplined approach to identifying the key uncertainties and acquiring data for alleviating the uncertainties is essential. The largest uncertainty in some cases is the remaining oil saturation in the reservoir.

This paper presents the results from a field-wide data acquisition program conducted in a West Texas carbonate reservoir to estimate ROS as part of an EOR project assessment. The Means field in West Texas has been producing for more than the past 75 years and the producing mechanisms have included primary recovery, secondary waterflooding and the application of a CO2 EOR process. The Means field is an excellent example of how the productive life and oil recovery can be increased by the application of new technology. The Means story is one of judicious application of appropriate EOR technology to the sustained development of a mature asset. The Means field is currently being evaluated for further expansion of the EOR process and it was imperative to evaluate the oil saturation in the lower, previously-undeveloped zones. This paper briefly outlines the production history, reservoir description and reservoir management of the Means field, but this paper concentrates on the Residual Oil Zone (ROZ) that underlies the Main Producing Zone (MPZ), and describes a recent data acquisition program to evaluate the oil saturation in the ROZ. We discuss three major methods for evaluating the ROS - core analysis, LIL tests and SWCTT tests.

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