Recent Applications of the Single Well Tracer Method for Measuring Residual Oil Saturation
Authors: Bragg J.R. , Carlson L.O. , Atterbury J.H. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., March 22–24, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Eleven successful single-well tracer tests have been conducted by Exxon in nine reservoirs to measure residual oil saturations. These tests were run to evaluate tertiary recovery potential and to aid in determining waterflood potential and to aid in determining waterflood conformance. This paper presents field data and interpreted results for several of the more recent tests, including the test sensitivity to the measured residual oil saturation. Also, two examples are given to illustrate how the residual oil saturations measured in these tests have been combined with other reservoir data to better evaluate waterflood conformance.

Introduction
Details of the theory and the test procedure of the single-well tracer method for procedure of the single-well tracer method for measuring residual oil saturation have been previously described. Briefly, the test is previously described. Briefly, the test is conducted by injecting a bank of primary tracer dissolved in formation water into a reservoir zone that is at residual oil saturation. The primary tracer can be one of several organic acid esters having a significant solubility in both formation water and crude oil. The bank of primary tracer is displaced into the formation by primary tracer is displaced into the formation by injecting additional water that contains no primary tracer. All of the injected water is tagged primary tracer. All of the injected water is tagged with methanol, which serves as a non-reactive material balance tracer. Following injection, the well is shut in to allow partial hydrolysis of the ester to form an alcohol, which is the secondary tracer. Finally, the well is produced and the concentrations of all tracers are measured in the wellhead fluid.
Principles of chromatographic separation are used to relate the residual oil saturation to the difference in arrival times at the wellbore of the primary and secondary tracers. The alcohol (secondary tracer) is almost insoluble in oil, so it travels at a velocity nearly equal to the velocity of the formation water. The ester moves more slowly, since it is partially soluble in the oil and spends a portion of its flow time in the immobile oil phase. The interstitial velocity of each tracer i, vi. is related to the water velocity, vw, by:
(1)