The recovery efficiency or the percentage of stock-tank oil in place in the reservoir recoverable by natural expulsion mechanisms is known to vary over a wide range. Under the most favorable conditions recovery efficiencies as high as 85 percent to 90 percent of the oil in place have been repower At the other end of the scale recoveries as low as 10 percent of the oil in place have been experienced. The principal reason for such differences is in the displacement mechanism itself specifically whether the oil is displaced by invading water (water drive), by the expanding gas coming out of solution (solution gas drive) or by an expanding gas cap, or by gravity segregation.
This paper is a continuation of the study, made by the API subcommittee on recovery efficiency, which culminated in Nov., 1967, with the publication of API Bulletin D14, "A Statistical Analysis of Recovery Efficiency". For this study the sub-committee used case histories on some 312 producing oil reservoirs from which certain empirical relationships were derived by regression analysis. For the water-drive cases the following regression., equation for the recovery efficiency was found:
The basic data used for this equation came almost exclusively from sand and sandstone reservoirs. A comparison of the observed and computed values of the recovery factor for 70 water drive reservoirs is reproduced here as Fig. 1.
For the reservoirs under a solution-gas-drive mechanism the subcommittee established for the recovery efficiency below the bubble point the following regression equation:
The basic data used for this equation came from a variety of rock types, including carbonate rocks. A comparison of observed and computed values of the recovery factors from the 80 solution-gas-drive reservoirs below bubble point used in this study is shown as Fig. 2. For the units and nomenclature, reference may be made to the list of symbols at the end of this paper.
The cumulative distribution of the recovery efficiencies for four drive mechanisms as determined by this same subcommittee study is shown here as on the log-normal graph of Fig. 3. It may be noted that the recovery efficiencies both for water drive and for solution-gas drive below the bubble point follow a typical log-normal distribution as indicated by their straight-line trends on these charts.