Novel Approach to Estimate Water Saturation Behind Casing Using Carbon Oxygen Data: Challenges and a Case Study in Abu-Dhabi Onshore Reservoirs

Abstract

Carbon-Oxygen ratio measurements are advantageous for calculation of water saturation behind casing in situations where the normal pulsed neutron sigma log fails to do so. Examples include reservoirs stimulated by acids or those having fresh water. Carbon steel casing commonly used to inhibit corrosion processes. In most cases, the carbon content in the steel material is an unknown parameter that can contaminate the accuracy of the C/O measurement. Another challenge, particularly in Abu Dhabi National Oil Company (ADNOC) reservoirs, is the high salinity of the formation water (typically 200 Kppm NaCl) where Chlorine makes more gamma rays per neutron compared to other elements and can interfere with the oxygen accuracy. In this paper, we propose methods and workflows to assure the accuracy of oil saturation from C/O measurements. To lower the dependence on carbon, examples of a process that uses the ratio of the Oxygen window to the total inelastic gamma counts are demonstrated. The responses of other inelastic data including the total inelastic count rate and the ratio of inelastic to capture can be scaled as water saturation as a secondary quality control method. Examples of C/O processing with chlorine corrections are also demonstrated. Data from two wells with consistent reservoir parameters (porosity, rock, wellbore, etc.) are used to demonstrate the proposed techniques. This data is augmented with computer models and analysis to understand the various steps in the workflow. The proposed technique has been applied with acceptable saturation results behind carbon steel in acid stimulated reservoirs, with high salinity water. This paper provides a solution to estimate water saturation behind carbon steel casings in acid stimulated reservoirs and with high salinity water. We have developed workflows as best practices to assure the accuracy of Carbon/Oxygen oil saturation measurements.