Water movement controls in an Early Cretaceous reservoir: An integrated analysis from a large offshore field, Abu Dhabi, United Arab Emirates

Abstract

A Thamama carbonate reservoir in a major offshore field has experienced early water breakthrough with rapidly increasing water cuts. Concern over this production challenge and its impact on water handling, wellbore completion, sweep efficiency and ultimate recovery prompted a study to improve the understanding of the geologic controls over reservoir quality variations, how to properly capture them in static and dynamic reservoir models and mitigate their impact on reservoir performance. A multi-disciplinary, integrated study of relevant geoscience and engineering data has been undertaken to address this issue. The study focused on several geological components including depositional facies, diagenesis and tectonic factors. A deepening upward succession of carbonate ramp facies have been identified and characterized from examination of slabbed core, thin sections and SCAL data from over 35 wells in the field. Diagenetic overprints have impacted the original porosity and permeability resulting in inter-beds of moderately to highly porous zones, which exhibit significant variations in permeability. The diagenetic overprint has, in part, been correlated to major faults zones in the field. These faults and diagenetic effects have resulted in the formation of fractures within the reservoir. Cased-hole log and production data has been instrumental in the identification of the spatial and stratigraphic distribution and timing of water movement. Results indicate that water breakthrough is controlled by variation in permeability due to depositional texture and diagenetic overprints and fracture enhancement on the original reservoir character. These variations are stratigraphically controlled near the top and base of the reservoir, resulting in a potential lack of sweep and recovery from the middle portion.