Life of Field Reservoir Surveillance and Optimization Using Cased-Hole Log Measurements

Abstract Reservoir management is about optimizing reservoir performance to achieve long-term recovery goals. This can be accomplished through a systematic application of integrated, multidisciplinary technologies to positively impact financial performance and achieve synergy. Multidisciplinary integration leads to better understanding of reservoir performance for proper decisions on remedial actions such as water shut-off, additional perforations, stimulation; for long-term planning and enhanced oil recovery to optimize production at the well and field levels. The case of the As-Sarah field in Libya shows how cased-hole logging and production-monitoring contributed to successful reservoir management. Cased-hole production and pulsed-neutron-logging have been used extensively since the early life of As-Sarah field for reservoir surveillance and to identify opportunities for remedial actions. A significant number of thermal--decay-time, reservoir saturation and production logging campaigns have been performed. The aims of these campaigns were to estimate the propagation of the oil/water contact (OWC), determine the residual oil saturation profile, identify watered-out zones for isolation, and define zonal contribution. A re-interpretation project of all As-Sarah field cased-hole logs was completed and presented in a comprehensive time-lapse display. The results of this project guided the fluids movement during history-matching in the dynamic model. Using reservoir saturation measurements along with the production logging data ensured data consistency and reliability. This helped the operator to be certain that there was no movable-oil left in the formation, enabling confidence in the decision to plug-back the water-entry zones. This integration was used for tar-mats mapping, characterization of the low-productivity D and E units at the top of the reservoir, re-evaluating the recovery factor, and anticipating the OWC for infill-well planning. Building time-lapse water saturation and flow profiles is not straightforward. The water and remaining oil saturations must be calculated for each well to evaluate the fluids' mobility. The project included detailed saturation processing taking into account the field-wide reservoir residual oil saturation. Presenting the results in this unique format provides quick and easy access to a huge amount of data, thus providing an at-a-glance understanding of the reservoir behavior and aquifer strength. It also contributed to optimizing liquids' off-take, minimizing bypassed reserves and selecting wells for further monitoring. The log results were correlated with production history to calibrate and validate the dynamic model that is being used for field development.