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.

The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland

“…This architecture reflects the long-term persistence of the Rona Ridge as an emergent topographic high from Triassic to Cretaceous times. Regional subsidence and burial occurred in the Late Cretaceous, blanketing the ridge in deep marine mudstones that are thought to form the regional seal (Ritchie et al 2011;Witt et al 2011;Trice 2014). Most cores of the basement-cover unconformity reveal little evidence of deep surface weathering of the exposed crystalline rocks, unlike other basement plays such as the Utsira High in the Norwegian North Sea (Riber et al 2017;Trice et al 2019).…”

Hydrocarbon exploration and exploitation West of Shetlands

“…The first of these -Foinaven (Cooper et al 1999) and Schiehallion (Leach et al 1999) -were developed rapidly with first oil flowing less than seven years after discovery. Clair moved rather slower, with the need to delineate the complex reservoirs and optimize hydrocarbon recovery techniques in what was seen for many years as a marginal project, leading to a delay of more than 27 years from discovery to first oil (Witt et al 2010).…”

Structural geology and well planning in the Clair Field

“…2). The trap itself is a four-way closure extending over approximately 220 km 2 (Witt et al 2010). The top seal is Upper Cretaceous mudstone.…”

“…Detailed planning of the early Clair Phase II production wells started in 2009. A more detailed discussion of the development of the Clair Field is given by Witt et al (2010).…”

“…More details are given by Coney et al (1993), Nichols (2005), Barr et al (2007) and Witt et al (2010).…”