Chlorine Logging in Cased Holes

Abstract

Abstract A new chlorine logging tool, developed primarily for the detection of oil saturation behind casing, is described. The principle involved is the selective detection of gamma radiation from neutron capture in chlorine. The tool is relatively insensitive to porosity, responding mainly to the concentration of chlorine in the interstitial liquid rather than to the total chlorine content in the formation. A brief review of the fundamentals of chlorine logging is given. This includes a discussion of the partition of neutron captures in clean sand and limestone formations, and the spectra of gamma rays obtained. The effect of neutron capture in iron is shown to be important. Following a short description of the logging tool, the laboratory-determined response of the instrument in cased sandstone formations is presented. It is shown that better results can be obtained by logging in empty casing than in casing filled with oil, fresh water, or salt water. Effects of neutron capture in the casing, casing liquid, cement annulus and formation matrix are discussed. A method of interpreting the Chlorine log, based on the use of the response curves, is explained. This is illustrated with several field examples which also show comparisons of electrical and Chlorine logs. In Gulf Coast formations, the Chlorine log often bears a marked resemblance to the SP curve, and in some cases water-oil contacts have been identified by a visual comparison of the two curves. Comparison of the Chlorine log with a neutron log is also valuable in interpretation. The role of the neutron log is to give a porosity (which need be approximate only), to indicate formation shaliness and, most important, to identify gas which is generally indistinguishable from oil on the Chlorine log. The Chlorine log should prove most useful in old wells where adequate open-hole logs do not exist, or where saturation conditions in the producing zone have changed. The combination of the Chlorine log with a Cement Bond log and the wireline formation tester should aid materially in successful reworking of such wells.