Managing the start-up of a fractured oil reservoir: development of the Clair field, West of Shetland

Abstract The Clair oilfield was discovered in 1977 and began production in 2005. It is a heterogeneous fractured sandstone reservoir with an estimated STOIIP of c . 1.5 billion barrels in the Phase 1 development area. An extended appraisal programme was required to assess reservoir deliverability, which is controlled by the distribution of natural fractures. Development drilling increased the understanding of the fracture system, through acquisition of conventional core, open-hole logging, drilling mud-loss recording, production logging and well test transient analysis. These data revealed a complex fracture system of conductive faults and background joints. Post-production formation pressure measurements showed greater lateral and vertical connectivity than would be expected from matrix and fluid properties alone. However, large pressure differences were encountered close to or across features mappable at a seismic scale. Closed fractures or sealing faults can form baffles between compartments, and conductive faults or fractures allow rapid pressure communication. In some cases the connection path or barrier can be identified with seismically mapped features. When the field was put onto water injection, one year after start-up, a key concern was the degree of imbibition of water from the fractures to the matrix. Despite the observed pressure connectivity, water breakthrough did not take place until 2007. Most producers have completions designed to permit zonal water shut-off. A successful intervention was carried out in early 2008, after a production logging run had demonstrated water entry at the toe of a high-angle well. The crestal Core segment of the development benefits from a permanent 4D seismic array of ocean-bottom cables. In 2007 the first permanent monitor survey was acquired and processing gave indications of a 4D response. These 4D data are improving the dynamic reservoir understanding and providing valuable spatial context to the well results.