Unlocking the Hidden Potential of an Upper Jurassic Carbonate Reservoir After 46 Years of Field Discovery: A Case Study in Offshore Abu Dhabi

Abstract

After a long history of unsuccessful appraisal wells, a new phase of reservoir appraisal focusing on data gathering for reservoir and fluid characterization led to positive results. Fluid sampling, acid recipe, formation pressure, and horizontal drilling were key factors for the successful appraisal. This reservoir is now a significant upside for the field development plan. During the early phase of field development, careful data gathering plan was designed to characterize the reservoir. The plan included coring, logging, reservoir formation pressure, downhole fluid analysis, fluid sampling, conventional Pressure, Volume, and Temperature (PVT) studies, and asphaltene and flow assurance studies. After collecting downhole oil samples, a compatibility study with acid recipe was performed and many chemical additives were tested to find the optimal one. A horizontal drain was drilled to maximize the reservoir contact. The well was tested with drill stem test (DST). Reservoir formation pressure acquired in 4 pilot holes at locations covering the reservoir confirmed fluid mobility, initial reservoir pressure, and possible oil pool limits. Downhole fluid analysis and sampling allowed the characterization of the reservoir fluid properties. Conventional PVT, asphaltene and flow assurance studies confirmed light oil with good flow potential. However, the compatibility study with existing acid recipe showed a high increase in fluid viscosity. This could prevent the well from flowing after matrix acidization. Naphta, among many tested chemical additives, proved to be the best to resolve the viscosity increase. The horizontal drain was successfully acidized with the new acid recipe and the well flowed oil during DST for the first time, 46 years after the field discovery. The well was tested through separator at different chokes before the main pressure build-up (PBU). The well was shut-in for 78 hours. BU analysis showed that reservoir permeability is in line with previously collected cores. Although earlier appraisals were successful in upper reservoirs, a classic approach to reservoir appraisal of this thin oil reservoir failed. Our approach of carefully planning the data gathering sequence, testing acid and oil compatibility, proved essential to understand the past failures, correct the shortcomings, and carry on a successful appraisal.