Abstract

CO₂ flooding was applied to a mature field to increase oil recovery in West Texas. Accurately determining residual oil saturation (ROS) is very important for evaluating project economics. In addition, constant surveillance of the CO₂ migration during flooding is critical so that the injection pattern can be modified to optimize oil sweeping efficiency.

Due to field operation procedure, wells are cased and cemented after initial open hole logging. Future ROS evaluation must be performed with logging tools suitable for cased-cemented wells. In an oil-water two components system, ROS determination using carbon oxygen (C/O) data from a pulsed neutron log (PNL) is a proven technology for this situation. When CO₂ is in the field, the gas phase saturation can be determined by using a Thermal Multigate Decay 3 Detectors (TMD-3D™) tool’s SATG log which is based on the hydrogen index response in a time window in the long detector decay response. SATG log is also used to correct for the CO2 effect on C/O log to accurately determine oil component saturation and water component saturations. A case study from a selected well is presented to demonstrate the feasibility of this technology. A petrophysics model and operational best practices are also discussed.

This method is different from a typical sigma log and is not limited by the sigma contrast between oil, saline water, and gas. This method is also different from typical resistivity based open hole log method which is dependent on the Archie parameter selection (a, m, n, and Rw).