The oil-bearing Upper Jurassic Arab reservoirs of an offshore Abu Dhabi fractured carbonate field (Abu Al Bukhoosh) have been producing for more than thirty years. All the available informations indicate that the producing layers, subdivided into Upper and Lower Arab, are fractured to varying extents. As a result, a better understanding of the fracture networks and their relationship with major and sub-seismic faults in this field is now critical to optimize infill drilling and produce the remaining reserves.
Formation pore pressure and near-wellbore mobility are key parameters for reservoir description. Traditionally, these data are acquired with wireline formation pressure tester. However, today a large number of wells are being drilled at high deviation, as producers and injectors, and wireline can be a time-consuming operation, as the tools must be conveyed by drillpipe. Acquiring this type of formation evaluation data with a logging while-drilling tool represents a significant time and cost savings opportunity, simultaneously improving efficiency and safety while reducing risk and non-productive time. This paper discusses the planning phase, the operational results and the lessons learned from a well drilled utilizing formation pressure testing in combination with a full logging while-drilling data suite. The formation fluid pressures, mobility and pressure gradients were taken after drilling to the well total depth during a cleanout trip. Thirty-eight successful tests were taken over several carbonate reservoirs with mobility ranging from 0.1 to 100mD/cp, achieving 100% seal success rate. A fully automated testing sequence, time optimized to 5 minutes, with variable rate, volume and number of drawdowns downhole, was used. Seal integrity and tight tests could be readily identified in real time from 6-bits per second mud-pulse telemetry. The logging while-drilling service proved it could save two days of valuable rig time by eliminating one dedicated wireline pressure-measurement run and one cleanout trip. In addition, this improvement in operational efficiency minimized wellbore exposure time, thus reducing the risks during the casing run. Introduction Cost reduction is a driving factor in the development of brownfield wells. Better planning and deployment of new technology can lead to better execution in the drilling phase with less non-productive time and reduced costs. This paper discusses how the application of advanced drilling and measurement technologies, including formation pressure, has enhanced answers through acquiring comprehensive formation evaluation data in a single logging while drilling (LWD) run. The well is located in a field offshore Abu Dhabi, which had been produced since 35 years. The field development projects today aim at producing the last by-passed oil pockets with conventional and unconventional recovery methods. Reservoir Overview The field is a large NE-SW trending anticline which is affected by a network of faults. The stratigraphic sequence comprises a thick calcareous platform succession ranging in age from Permian to recent. Depositional environments range from shallow marine to supratidal. Limestone, dolomite and anhydrite are the most common minerals encountered. The well in the 8 ½in. BS section crossed one main reservoirs of the Upper Jurassic Age and one secondary reservoir of the Lower Cretaceous Age.Lower Cretaceous reservoirs (Thamama IV) - 70m zone of pure-limestone reservoirs separated by tight, impermeable layers. Core permeabilities range from a few millidarcies up to 50mD depending on the reservoir.Upper Jurassic reservoirs (Upper Arab) - 110m multilayered zone of dolomite reservoirs separated by anhydritic barriers, calcite can still be found in one or two layers. Core permeabilities range from a few millidarcies up to 60mD. Each layer behaves as an independent reservoir whose thickness varies from 1 to 5m. Reservoirs underwent early and late diagenesis. This is especially true for the Upper Jurassic for which diagenesis completely modified the structure of the porous network, where dolomitization degrades permeability.
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