Evaluation and Comparison of Residual Oil Saturation Determination Techniques

Abstract

Chang, M.M., Natl. Inst. for Petroleum and Energy Research Maerefat, N.L., SPE, Natl. Inst. for Petroleum and Energy Research Tomutsa, L., SPE, Natl. Inst. for Petroleum and Energy Research Honarpour, M.M., SPE, Natl. Inst. for Petroleum and Energy Research Summary. The amount and distribution of residual oil saturation (ROS) are critical parameters for determining whether to apply an EOR process to a reservoir. A brief review of available ROS techniques is presented, indicating advantages, limitations, problems, and possible improvements of each technique. Advantages and disadvantages of each ROS-determination technique are summarized. Screening criteria for determining the best ROS technique under certain wellbore or reservoir conditions are presented. This paper also presents results from comparisons of ROS measurements obtained from the literature as calculated from resistivity logs, pulsed neutron capture (PNC) logs, pressure coring. single-well tracer tests, nuclear magnetism logs (NML), carbon/oxygen (CIO) logs, and electromagnetic propagation tool (EPT) measurements. In this study, the ROS measured by each method is compared with that determined by other methods conducted in the same well. The comparison shows that average values of ROS determined by CIO log, PNC-LIL (log-inject-log), and single-well tracer test do not differ statistically when compared with other methods. The resistivity log tends to give higher than average [2 saturation units (s.u.)] ROS measurements, while pressure coring tends to give lower than average (4 s.u '.) ROS values. EPT and NML show deviations of about 8 s.u. of ROS values from other methods, which indicates a statistically significant difference. ROS vertical profiles obtained by two different methods from the same well were compared to eliminate the ROS variation resulting from formation depth. The vertical profiles based on ROS zoning and foot-by-foot measurements were studied to provide more "resolution" for comparisons. The results show that discrepancies in measurement methods are more pronounced when vertical profiles are divided into different zones. This could mean that the discrepancies are much greater for some zones than for others. This approach offers the possibility of studying ROS-method discrepancies as a function of different ROS values. Introduction ROS is the oil saturation remaining in the reservoir after extraction by conventional recovery methods, such as waterflowing. The amount and distribution of residual oil in a reservoir are significapt factors in deciding whether EOR methods are suitable for economic exploitation of a reservoir. Variation of ROS may be caused by heterogeneities in the rock or by recovery processes such as waterflooding. Many different ROS measuring techniques are available, but they do not necessarily produce the same results. Therefore, understanding the limitations, accuracies, and sources of error of the different measurement techniques is important in selecting a technique to measure ROS. Because there is no absolute, correct method of measuring ROS, and because every method is subject to some errors. a comparison of results from different ROS techniques is an alternative for ROS evaluation. The Interstate Oil Compact Commission (IOCC) conducted a comprehensive study of different ROS techniques in 1978. Since then, substantial improvements of existing measurement methods and development of new methods have been reported. An updated comparison of methods and a better understanding of ROS-measurement errors are essential for evaluating new improvements in ROS techniques. In this paper, available ROS techniques are reviewed, and the updated results of the IOCC study are presented. The relatively new EPT. NML, and C/0 log are included in this study. ROS profiles obtained from different techniques are analyzed for changes in ROS with changes in formation depth resulting from reservoir heterogeneity. The vertical ROS profiles were digitized to provide ROS values at every foot of depth. With these data, various tool responses at a given well depth can be evaluated. More than one method is often recommended for ROS determination. The cost of the measurement could become substantial, depending on the method used at a given geographic location. Review Of ROS-Measurement Techniques Available techniques have been classified as single-well, interwell, and material-balance ROS measurements. Advantages and disadvantages of each technique are listed in Table 1. On the basis of this review and our judgments, screening criteria for determining the best ROS technique under certain wellbore or reservoir conditions are presented in Table 2. Single-Well ROS Measurements. Core analyses, backflow tracer tests, well logs, and single-well transient tests are considered single-well ROS techniques because only one well is involved in the measurement. Core Analysis. Core analysis is a direct method for measuring ROS in the laboratory. Based on the core-retrieval tool, routine core analyses for saturation measurements can be classified by three categories: conventional, pressure, and sponge coring. ROS values determined by conventional core analysis are substantially less than in-situ values obtained from logging methods. The most severe change in oil saturation is caused by expulsion (bleeding) and associated shrinkage of the quantity of oil in the core as pressure decreases when the core is lifted to the surface. Attempts have been made to correct oil-saturation measurements obtained by conventional coring analysis, but with unreliable results. Pressure coring solves expulsion and shrinkage problems by maintaining the core specimen at bottomhole pressure (BHP) until the core fluids can be immobilized by freezing. Experience has shown excellent accuracy in ROS profile from pressure coring. Pressures from several hundred to more than 6,000 psi [41 MPa] have been processed. Core recoveries varying from 51 % for very soft to soft formations to 70% for consolidated formations have been reported. Special techniques for analyzing cores recovered by pressure-coring methods have also been developed. Sponge coring uses a sponge-sleeve modification to a conventional core barrel. The sponge sleeve is made of a porous, oil-wet, polyurethane sponge. The oil bleeding from the core is collected in the sponge and reconstituted back into the core porosity to correct the oil saturation for bleeding. SPEFE P. 251^