The Use of the Conventional Gamma-Ray Log to Locate Saltwater Invasion

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 The conventional gamma-ray log has proved useful in locating behind-the-pipe salt-water invasion in a number of Texas oil fields. In the fields discussed in this paper, we found that the natural gamma-radiation level increased substantially in zones invaded by salt water. Consequently, gamma-ray logs are currently being used in these fields to track behind the-pipe salt-water movement and to locate bypassed oil zones. The principal advantage of the gamma-ray log over other techniques for locating salt-water invasion is its relatively low cost. Even though the gamma-ray log may not prove to be as definitive in locating salt prove to be as definitive in locating salt water as other more sophisticated logging techniques, its low cost should allow more frequent use of this log, resulting in the location of more potential bypassed oil zones. The applicability of this technique to a particular reservoir must be established through particular reservoir must be established through field observation. This paper does not intend to suggest that the technique can be necessarily applied to all or even to most reservoirs; however, in the six reservoirs studied to date the technique does work. Introduction Increases in radioactivity have long been found to occur opposite zones from which significant amounts of water have been produced. In addition, published work indicates that channels may often be located by observing an increased response on the gamma-ray log. The increased radioactivity has been explained to be the result of a radioactive crust being deposited by the salt water around the wellbore. We have found that a simple, low-cost technique for locating this salt-water invasion is to use the conventional gamma-ray log. This technique appears to work satisfactorily in at least six Texas oil fields, and it may have even more widespread applicability. Although we have been using the technique primarily in clean sands, there is no apparent reason to expect it not to work in shaly sands. The technique has a number of limitations, which arise from the fact that phenomena other than salt-water invasion of a zone can cause an increase in radioactivity. For example, we have observed increases in radioactivity opposite perforations that have produced salt water. perforations that have produced salt water. Also, channels have been found to produce increased radioactivity.