A new single-well tracer method has been developed to measure residual oil saturations of watered-out formations within a precision of 2 to 3 PV percent. This in-situ method makes an average measurement over a large percent. This in-situ method makes an average measurement over a large reservoir volume by using trace chemicals dissolved in formation water. The technique is applicable in both sandstones and limestones for a wide range of conditions. Introduction Residual oil saturation is a basic item of data for many aspects of reservoir engineering. This number is required for normal material-balance calculations. Residual oil saturation is also extremely important in determining the economic attractiveness of a planned waterflood or a proposed tertiary recovery operation. Finally, in some areas proration is related to attainable residual oil saturation. Core analysis and well logging, the two most widely used methods for measuring residual oil saturations, are subject to a variety of well known limitations. One principal common fault is that both methods yield values that are averages over very small reservoir volumes. The chemical tracer method described in this paper samples a much larger volume of reservoir around a single well, The residual oil saturation measured represents an average over as much as several thousand barrels of pore space. Because this method makes an in-situ measurement, additional limitations of other methods are also avoided. In the single-well tracer technique, a primary tracer bank consisting of ethyl acetate tracer dissolved in formation water is injected into a formation that is at residual oil saturation. This bank is followed by a bank of tracer-free water. The well is then shut in to permit a portion of the ethyl acetate to hydrolyze to permit a portion of the ethyl acetate to hydrolyze to form ethanol, the secondary tracer. Finally, the well is produced and the concentration profiles of the two tracers are monitored. Ethyl acetate is soluble in both the water and oil phases, but ethanol is, for all practical purposes, phases, but ethanol is, for all practical purposes, soluble only in the water phase. As a result, the ethanol travels at a higher velocity and returns to the wellbore earlier than does the ethyl acetate. The difference in arrival times can be used to determine the residual oil saturation through the use of computer programs that simulate the tracer test (the greater the programs that simulate the tracer test (the greater the oil saturation, the greater the difference in arrival times). Field tests have demonstrated the reliability and applicability of this technique. This paper describes the tracer method, gives results of field experience, and presents a mathematical description of the process. One field application is described in detail, followed by a discussion of the scope and limitations of the technique. General Description of the Tracer Method Theoretical Basis The chemical tracer method depends on chromatographic retardation of a tracer chemical that is soluble both in formation water and in oil. Considering a system in which the oil is stationary (or at residual saturation) and the formation water is flowing at a-> velocity V w, the local velocity of a typical tracer molecule is-> -> JPT P. 211

Published in Petroleum Transactions, AIME, Volume 219, 1960, pages 24–30. Abstract Procedures have been developed to study the performance characteristics of unconfined pilot water floods using scaled laboratory models. The effects of operating conditions on pilot performance for various well patterns have been investigated. The limitations of the laboratory model studies are outlined. Relationships between the oil recoveries of pilots and large-scale floods have been established. The applications of model studies to the prediction of large-scale water flood performance and pilot flood design are discussed. Introduction Pilot water floods are undertaken to evaluate operational procedures and to gain advance information about the performance of an extensive water flood. In general, however, the performance of a pilot flood is not directly indicative of what can be expected from a large-scale pattern water flood. This difference stems from the fact that the over-all flow configurations are not the same for the two systems. A water flood normally consists of an array of identical well patterns. In such an array, the perimeters of the well patterns are axes of symmetry and act as impermeable boundaries. Thus, an extensive, pattern water flood can be visualized as a repetition of "confined" floods. In contrast, a pilot water flood involving only one or a few well patterns is "unconfined". Here the well patterns are not balanced by other flood units; hence, the perimeter of the pilot area does not act as an effective boundary. Accordingly, only a portion of the fluids injected (and also of those originally contained within the pilot area) is captured by the producers of the pilot. The rest escapes into the surrounding reservoir. This usually causes the amounts of oil and water produced in the pilot to be different from those recovered by a producer in an extensive water flood. Moreover, the recovery observed in the pilot can be expected to be greatly influenced by operating conditions, i.e., by the bottom-hole pressures at the injection and producing wells. The above considerations show that full utilization of pilot test results and reliable prediction of large-scale water floods require a knowledge of the relationship between the recovery performance of confined and unconfined well patterns.