Chlorinity and Porosity Determination in Cased and Open Holes by Nuclear Logs

Abstract

Permian Basin Oil Recovery Conference, 7–8 May, Midland, Texas Introduction For a number of years radio activity logging devices have been used to define lithology, bed thickness, permeable intervals, and porosity both behind casing and within open holes. The need has long existed of defining oil and water saturation either qualitatively or quantitatively within intervals already cased off from the borehole. It is now feasible to differentiate oil bearing from salt water bearing zones behind well casing with a combination neutron curve and a chlorinilog curve, The two curves fall along the same line opposite oil filled or fresh water filled zones. Opposite salt water filled intervals the two curves depart from each other in recorded response and thus offer a graphic means of distinguishing hydrocarbon bearing from nonproducible horizons. Within the article below the logging device will be briefly described and a background of basic theory will be given. It will be pointed out that several factors influence the respective response of the standard neutron curve and the chlorinilog curve and that it is necessary to consider essentially similar zones, with particular reference to such considerations as cement thickness, borehole size, casing weight, borehole fluid type, particularly when attempting quantitative evaluation. The manner of presentation and a suggested method of evaluating water saturation will be demonstrated. Various current applications and uses as well as promising future applications will be discussed. The instrument used to record both the "H" curve and the" CL" curve is of the scintillation type. In order to give consistent results, the equipment is designed for maximum stability and efficiency. It is refrigerated to allow operation at high temperatures without detrimental effects. There are a number of methods that may be used to determine the presence of chlorine. Among these are the neutron-neutron method, the neutron-gamma method, and the spectral analysis method. The principle of operation of any of these methods depends upon the relatively large thermal neutron capture cross section of chlorine. Table 1 gives the abundance and the neutron capture cross section of the elements that are of any importance in nearly all types of formations.