Use of Carbon/Oxygen Measurements Under Conditions Different From Those Calibrated, Cabinda, Angola, Africa
Authors: Neuman C.H. 
Published: December 2007

Abstract
A method is presented that enables estimation of oil in place using the gamma rays due to inelastic scattering of high-energy neutrons under conditions significantly different from those for which the tool was calibrated. Previous methods for interpretation of the spectra recorded through casing have been shown to reflect saturations when wellbore conditions are near those for which the tools were calibrated. Conditions in the reservoir for which measurements are reported, the Vermelha formation in the Takula field in the Cabinda province of Angola, Africa, are necessarily very different from those for which service companies calibrate tools. Furthermore, significant details of the environment in which the measurements must be made cannot be determined accurately. Thus, a method is proposed for interpreting spectral data in a manner that is less influenced by the wellbore conditions than the existing windows or spectral stripping methods and for calibrating the response insitu. The proposed interpretation method consists of fitting a background curve to parts of the spectrum nearly insensitive to oil saturation and then adjusting the carbon/oxygen ratio calculated from ratios of counts in energy windows that represent those elements by the ratio of counts through the same energy windows calculated from the fit curve. Logged carbon/oxygen response from the method proposed as well as that from the windows and spectral stripping methods presented commercially are calibrated based on log readings opposite wet formations with varying quantities of dolomite as determined from openhole logs calibrated to core measurements. Logged carbon/oxygen ratios from formations evaluated are corrected for the carbon in dolomite. Examples are presented to show that the adjusted windows method is less sensitive to variations in downhole conditions than the other two methods and that insitu calibration enables practical use of any of the three methods under conditions significantly different from available test pit measurements.

The Physical Basis for Calculation of Saturations from Logged Carbon/Oxygen Ratio
Carbon/oxygen (C/O) logging is based on the fact that high-energy neutrons are scattered inelastically by carbon and oxygen as well as by a few other elements. The word inelastic implies that the sum of the energy of the neutron and object scattering the neutron are less after the collision than before by the energy of a resulting gamma ray. Logs based on this measurement method were proposed before 1960 and have been used since 1974 for through-casing measurement of saturations when the more conventional pulsed neutron capture (PNC) measurements are not useful because formation brine is either fresh or has unknown salinity. Many instances of successful identification of bypassed oil and - in a few cases - accurate measurements of oil saturation have been published, but ambiguous results have discouraged widespread use of the method.
The basis for determining carbon and oxygen abundance from the gamma rays produced by inelastic scattering is that interaction of a high-energy neutron with carbon causes a 4.433 MeV (Million electron Volt) gamma ray and with oxygen causes a 6.130 Mev gamma ray. A few gamma rays at other energies from other elements are released as well.
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