A carbon/oxygen calculation method to improve the response sensitivity of carbon/oxygen logging

Carbon/Oxygen logging is an effective method to perform oil layer recognition and oil saturation calculation, which plays an important role in the evaluation of remaining oil after casing. At present, there are two main methods to calculate the ratio of carbon to oxygen (C/O). Compared with the energy window count method, the element yield method can avoid the influence of background count in the gamma spectrum and calculated carbon/oxygen value has the higher sensitivity and better accuracy, but it is still greatly affected by carbon and oxygen elements in the formation skeleton. Therefore, a new carbon/oxygen calculation method is proposed in this paper to overcome the influence of formation skeleton and improve the response sensitivity of Carbon/Oxygen logging. Based on Monte Carlo method, the inelastic gamma spectrum of pure sandstone and pure limestone skeletons are obtained. Based on spectrum analysis technology, the ratio relationship of carbon, oxygen yield and skeleton mark element (Si, Ca) yield in two skeletons are obtained. Using these ratio relationships, the carbon and oxygen yields from the sandstone and limestone reservoir skeletons are deducted from the total carbon and oxygen yield. A new carbon/oxygen parameter called the residual carbon/oxygen value (C/O)_R is calculated to perform oil-water recognition and a set of oil saturation calculation model suitable for the residual carbon/oxygen value is proposed. The environment application of the residual carbon/oxygen value is also analyzed. The study shows that the residual carbon/oxygen value has higher sensitivity than original carbon/oxygen value in oil layer recognition and has a great accuracy in oil saturation calculation. In environment application aspect, the residual carbon/oxygen value is basically unaffected by formation water salinity and is affected by wellbore fluid and wellbore size obviously. This new carbon/oxygen value calculation method has an important significance to improve the application effect of Carbon/Oxygen logging.