Carbon/Oxygen (C/O) Logging Instrumentation (1974)

Abstract An instrument system has been developed for field measurement of the relative carbon content of reservoir rocks. Gamma rays produced by inelastic scattering of 14-MeV neutrons are detected and analyzed in a sophisticated scintillation spectrometer system adapted for routine commercial well logging. A pulsed-neutron source similar to those in commercial use for other nuclear logs is employed. A van de Graaff generator provides the accelerating voltage to initiate the deuterium-tritium reaction, which produces the neutrons in a specially designed ion acceleration tube. Pulses from the radiation detector are transmitted to the surface oiler a conventional seven-conductor logging cable without degrading the spectrum. The log is available on a limited commercial basis and is recommended for identifying oil-saturated zones in cased wells where formation water is too fresh for reliable application of the Neutron Lifetime Log. Introduction A practical method of identifying the element carbon and assaying its abundance in cased-off formations has long been sought as a means for finding oil in situ. Since oil is largely composed of carbon, a measurement of carbon concentration is a direct method of evaluating the oil content of a formation. A nuclear logging technique whereby such a measurement can be made has been known since about 1950, and much effort has been expended in many laboratories to develop a system that could be used commercially to evaluate the hydrocarbon saturation of potentially productive reservoir rocks. Such a system is now available and has been field tested with gratifying results. This paper concerns the general features of the system, which is presently in commercial use on a limited basis. FIELD INSTRUMENTATION The subsurface instrument is 3-5/8 in. in diameter by 14 ft long. It is rated for a maximum temperature of 350 degrees F and pressure of 15,000 psi. It is equipped with a collar locator to enhance the accuracy of depth correlation and it can log to 7 ft above bottom. One or more neutron-type curves can also be run to assist in correlation with available open- or cased-hole logs. The instrument operates on a standard seven-conductor logging line, and curves are recorded on a conventional strip-chart recorder. Measurement of carbon-to-oxygen ratio can be recorded as a continuous curve vs borehole depth when desired. In usual practice, a continuous curve is of little value because statistical fluctuations in the measurement tend to obscure the true character of me log. For best results, stationary measurements are made at preselected points of interest. These are made for a time interval of 1 to 10 minutes each, depending on the accuracy desired and the number of zones to be surveyed in the time available for the logging operations. Typically, a logging operation comprises a continuous "correlation curve" over all zones of interest plus a station measurement at each of 10 to 50 specific points. PRINCIPLES OF OPERATION PRINCIPLES OF OPERATION The logging system comprises a subsurface instrument equipped to initiate and detect radiation from carbon, and surface gear to analyze and record the amount of such radiation in a format that facilitates formation evaluation. To initiate the emission of radiation, a source of 14-MeV neutrons is required. To detect the radiation, a scintillation spectrometer is used. And to assay the abundance of formation carbon by analyzing the detected radiation, a multichannel pulse-height analyzer system is employed. SPEJ P. 463