Introduction The 1984 Natl. Petroleum Council (NPC) evaluation of domestic EOR potential stands as the most extensive analysis of the U.S. potential EOR resource ever completed. potential EOR resource ever completed. Publication of the study culminated a 2-year, Publication of the study culminated a 2-year, 50-person-year effort. EOR experts from industry, universities, and government participated. The findings of this study have been participated. The findings of this study have been widely reported, critiqued, and used throughout the industry. L.F. Elkins previously discussed the series of SPE articles on the NPC results by offering his alternative interpretations of the NPC methods and results. He concluded that the methods used in the study were in error; ". . the permeability variations used in the NPC study are anomalous and exceedingly high." He also states that" . . the analyses made by the NPC task force overstate the amounts of remaining oil in place that are trapped in lower-permeability parts of sandstone reservoirs that are within the active dynamic waterfloods. "He concludes this on the basis of his interpretation that the volumetric estimates used in the analysis overstated initial oil in place." These three objections provide the core of his criticism of the NPC analysis. This reply clarifies the NPC's approach and also attempts to clarify several points raised by Elkins concerning the study procedures, models, and data. procedures, models, and data. NPC Data Sources and Procedures The NPC assembled detailed reservoir data and engineering-based models to project possible EOR production under a variety of possible EOR production under a variety of cases reflecting the oil price and the state of technology development. The data-collection effort started with the information available from existing sources within the U.S. DOE, but was significantly expanded and enhanced during the study. Additional material was obtained from questionnaires completed by 18 different companies on 1,300 reservoirs. Table R-1 lists the data elements collected and used in the models. This information was assembled, to the extent available, for each reservoir in the data base. Once collected, all data were thoroughly screened and evaluated for in ternal consistency and were cross-checked with other reservoir properties, companyprovided data, and average values for other reservoirs in the region. For data elements that could not be readily and consistently determined from available data sources, the committees chose to develop default procedures. Where possible, these values were estimated from other data by use of correlations. After all changes were complete, the entire updated data base was again thoroughly screened by committees assigned to assess the potential for the specific EOR methods evaluated. The three committees were composed of renowned experts on miscible flooding, chemical processes, and thermal recovery. Each processes, and thermal recovery. Each committee, in addition to the coordinating committee, included representatives from the oil industry, universities, and government agencies. This effort resulted in the most complete, usable data base possible while guaranteeing internal consistency. The resulting data base contained rock, fluid, geologic, and production information on more than 2,500 reservoirs originally containing more than 330 billion bbl [52.5 × 10–9) M3] Of oil, more than 70% of the national total estimated by the API in 1980. Because of time constraints, the data base was pared to consider only reservoirs with original oil in place (OOIP) estimated at more than 50 million bbl [7.9 × 106 M3]. As a result, the evaluation considered just over 1,000 reservoirs, accounting for more than two-thirds of the total domestic OOIP, a total of around 309 billion bbl [49.1 × 10–9 M3]. Because results were not extrapolated beyond the reservoirs specifically analyzed in this study, they are conservative estimates of the true national EOR potential. The miscible and chemical flooding prediction models required an estimate of the prediction models required an estimate of the permeability variation for each analyzed permeability variation for each analyzed reservoir. Fewer than 50 reservoirs in the data base had values reported for the coefficient of permeability variation. The supplied values were subjective because how they were obtained from core analysis data was not known-e.g., whether the permeabilities were arranged in sequence or averaged permeabilities were arranged in sequence or averaged by position. Because reservoir heterogeneities are unique to each reservoir and directly affect the waterflood as well as the EOR process performance, it was decided to process performance, it was decided to estimate the permeability variation from the demonstrated waterflood recovery performance. performance. The approach used for estimating the Dykstra-Parsons coefficient, VDP, considered the demonstrated waterflood performance in each reservoir. The ultimate performance in each reservoir. The ultimate recovery was estimated by adding seven times the current annual production to the cumulative production (an assumed reserves-to-production ratio of 7 years), as long as the cumulative production was greater than 80% of the estimated ultimate recovery. The recovery efficiency is the estimated ultimate recovery divided by the OOIP. If the cumulative recovery was less than 80% of the ultimate, then the sum of the recovery factors for primary and secondary recovery reported by the operators was used as the ultimate recovery efficiency, if the information was available. The volumetric sweep efficiency was calculated from the recovery efficiency, FVF'S, initial oil saturation, and waterflood residual oil saturation (ROS). The endpoint mobility ratio was calculated from data-base or default values of viscosities and relative permeabilities. A pseudo-VDp was determined on the basis of pseudo-VDp was determined on the basis of the calculated sweep efficiency and mobility ratio for each reservoir in the data base that had sufficient data to perform these calculations. The pseudo-VDp correlations were based on results from a HigginsLeighton streamtube model of a five-spot pattern with 100 permeability layers, pattern with 100 permeability layers, assuming that the economic limit was reached at a producing WOR of 25. The median of all the calculated values was 0.72. The reservoirs for which insufficient data were available to calculate a value were assigned the median value of 0.72. If the pseudo-VDp value was calculated to he less than 0.5, it was given a default value of 0.5. This pseudo-VDp for each reservoir in the data pseudo-VDp for each reservoir in the data base was used in the predictive models for miscible and chemical EOR processes. The methodology the NPC used to calculate pseudo-VDp scales the EOR production to the waterflood volumetric sweep performance because it assumes that the geological performance because it assumes that the geological features that affect the waterflood will also affect the EOR process. Related papers: SPE 13239, SPE 13240, SPE 13241 Related discussions and replies:SPE 18397, SPE 20007, SPE 20009