A Theoretical and Laboratory Evaluation of Carbon Logging - Laboratory Evaluation

Results we presented from a laboratory experimental evaluation of a carbon well-logging tool based on the detection of the 4.43-MeV gamma rays produced by the inelastic scattering of 14-MeV neutrons. The success of a carbon logging tool is dependent primarily upon the gamma-ray detection scheme used, which in this investigation is a two-crystal pair spectrometer. Using such a device in a simulated reservoir with and without casing and a cement annulus, semiquantitative detection of carbon was accomplished for a fluid-filled packed sand of 35-percent porosity. Analysis of the spectral results show that the log would be sensitive to lithology, saturation and porosity as well as carbon. With the resolution of this particular gamma-ray detector scheme, the presence of the casing using seen, but did not interfere with the carbon signal. Likewise the 1-in. annulus of concrete had no effect on the carbon detection. Comparison of our results with those previously published show that the oxygen and silicon interference encountered i, other proposed logging schemes is eliminated by the two-crystal pair spectrometer. As presently envisaged, between 5 to 10 minutes per pay zone to be evaluated is required to per pay zone to be evaluated is required to accumulate raw data. However, the growth potential offered by the new Ge(Li) gamma-ray counters may well remove this restriction. Introduction Theoretical results presented in Ref. 1 have indicated that liquid hydrocarbons should be detectable in noncarbonaceous reservoirs. Although the interference problem posed by energy degraded gamma rays from omnipresent oxygen is severe, it is not severe enough to prohibit detection of hydrocarbons by nuclear means. The theoretical data also showed thermal neutron effects caused by energy degradation of gamma rays from thermal capture in silicon, chlorine and iron could be eliminated by proper choice of a neutron source gamma detector gating scheme. Indeed, this has been demonstrated experimentally. The crux of the hydrocarbon detection problem lies with finding a gamma detector system with sufficient resolution to pick out 4.43-MeV gamma rays (carbon) from the background provided by oxygen. Previous investigations have shown that single NaI(T1) crystal detectors did not have sufficient energy resolution to accomplish the task. However, energy resolution is not the whole answer when it comes to extracting a monoenergetic signal from a continuum background. In addition to the interfering gamma rays produced through Compton collisions external to the detector system, there is an in-crystal Compton background in single-crystal spectrometry. The second source of interference arises because gamma rays entering the crystal at energies higher than the discrete energy of interest will produce Compton collisions within the crystal generating electrons that have precisely the same energy as that of the "desired signal". This second source of interference significantly reduces the carbon sensitivity of a single-crystal detector. Hence, elimination of the in-crystal background from oxygen produced gamma rays would go far in improving the in-situ carbon detection picture. In searching for an acceptable detector system, this point was kept foremost in mind. It was known a three-crystal pair spectrometer would virtually eliminate the in-crystal Compton background, but limitations imposed by borehole tool size ruled out its use. However, from studying the working principle of this device the concept of the principle of this device the concept of the two-crystal pair spectrometer emerged. A description of such a device was later found in Ref. 4 although no evidence has been found that such a device has been used to record spectra in a borehole or simulated borehole environment. This report presents experimental results obtained with such a presents experimental results obtained with such a two-crystal spectrometer in various simulated reservoir conditions. SPEJ P. 129