| Popis: |
The Upper Jurassic formation in Offshore Abu Dhabi is formed of regressive cycles of sedimentation divided into four highly heterogeneous reservoirs (labelled Zones A through D) made up of dolomites/limestones/anhydrytes alternations. In these reservoirs, a fairly dense fault network can also affect fluids movements. Down-dip gas injection is one of the options currently being screened to sustain long term plateau duration of a major offshore Abu Dhabi field. In order to reduce uncertainties related to sweep efficiency in this complex environment, a gas injection pilot, made up of 2 gas injectors and 2 observers, was initiated late 2004. The Pilot objectives were to: Quantify the impact of tertiary gas injection in lowest Zone D water flooded sub-reservoirs,Evaluate at microscopic scale the remaining oil saturation after gas FRONT,Evaluate the vertical and areal sweep efficiencies,Assess performance of oil producers associated with down-dip gas injection,Evaluate gas injectivity in the pilot area. A full integrated monitoring and data gathering plan covering all formation reservoirs in the area of the pilot was implemented at project start, including: Extensive data acquisition on newly drilled wells: CMR, FMI, MDT, cores,Tracer injection and analysis,Regular time lapse Cased Hole Neutron and Saturation Logs on observer wells,Interference test between injectors and producers,Regular shut-in bottom-hole pressure data,Frequent production tests including gas sampling for tracer detection and chromatography analysis. A full integration of all available data led to build a comprehensive and consistent understanding of gas movement and impact over pilot life: Tracer confirmed that gas has been detected in all upper sub-zones (A – D), suggesting vertical communication within the pilot area.Significant vertical differential pressure depletion has been observed between zones.Gas movements are mostly governed by heterogeneities in a multi-layer reservoir that showed the influence of the related fault system and differential pressure depletion. RST time lapse logs show that microscopic sweep efficiency has been improved by gas injection in water swept zones. This will be confirmed by coring of the observer side track hole planned soon. |
