Post-Waterflooding Oil Reservoir Surveillance Using Pulsed Neutron Well Logging in Upper Burgan Formation, Northern Kuwait

Abstract

Operators commonly adopt waterflooding as a secondary recovery method to maintain reservoir pressure and displace remaining oil for production enhancement. Effluent and seawater have been injected into the Upper Burgan formation, which contains multiple layers of sand reservoirs, in the North Kuwait Raudhatain field. Well-based surveillance to understand post-waterflooding hydrocarbon distribution is essential for new perforation additions. Formation saturation monitoring for cased wells is widely performed with pulsed neutron well logging techniques. Pulsed neutron well logging provides time-based thermal neutron capture cross-section (i.e., sigma log) and energy-based element-specific ratios (i.e., carbon/oxygen (C/O) logs). Formation water salinity must be known and high to use sigma data to quantify formation fluids. When formation water salinity becomes a variable due to effluent and seawater injection, sigma log-based saturation analysis is not applicable. A salinity-independent measurement that distinguishes between oil and water is required; consequently, a C/O log must be used to obtain saturation profiles in mixed-water salinity reservoirs. The Upper Burgan formation’s initial water salinity in the Raudhatain field is high (i.e., approximately 220-240 kppm NaCl equivalent); thus, water saturation computation was performed with a sigma log. After the injection of effluent and seawater (mixed-water salinity ranges from 50 kppm to 170 kppm) was started, formation oil volumes must be evaluated using C/O logging. A well-specific Monte Carlo Neutron Particle (MCNP) model and two-detector-balanced C/O data sets were combined to compute oil saturation. We demonstrate multi-well case examples delineating well-based formation saturation profiles in post-injection reservoir conditions. A comparison of sigma- and C/O-based saturation analyses revealed water-flooded zones. Time-lapse sigma data sets highlighted how the water injection impacted thermal neutron capture cross-section measurements. Additionally, multi-detector, time-based nuclear attributes were used to evaluate formation properties and the presence of hydrocarbon-bearing sands. Following pulsed neutron log interpretation, subsequent add-perforation activities were performed; consequently, by-passed or remaining hydrocarbon was successfully produced. Evaluation of current formation fluid distribution in areas of the field where mixed-water salinity exists is challenging. Integrating sigma, C/O, and auxiliary pulsed neutron logs determined the remaining formation oil distribution and volume. The optimized perforation strategy to maximize oil production from existing wellbores was executed.