The Clair oilfield is a large fractured sandstone reservoir lying 75 km west of Shetland on the UK continental shelf. Fracture analysis and modelling was carried out in preparation for the phase 1 development, which started production early in 2005. Fracture clusters and discrete fluid inflows observed in wells are associated with faults and other localized deformation features tens or hundreds of metres apart. The reservoir has moderate to good matrix permeability, but well flow rates and profiles are fracture-dominated. Full-field geological models were built using conventional object modelling approaches for matrix and discrete fracture networks for fractures, and upscaled to populate a reservoir simulation grid. Dual-porosity, dual-permeability dynamic modelling (full-field and well-test) was undertaken to understand the fracture and matrix flow contributions and their interaction. Fracture models were conditioned to wells and to seismic data, including coherency and multi-azimuthal velocity information from a four-component, ocean bottom cable three-dimensional seismic survey. At this early stage in field development, there is insufficient calibration to select a single fracture model. Instead, well and depletion plans have been tested against multiple fracture models chosen to encompass a wide range of plausible outcomes.

Clair field came on production in February 2005.Clair is a large oil reservoir (>4 billion bbl STOIIP), 75 km west of Shetland.It is the largest naturally fractured reservoir developed in the UK, and is recovered by waterflood.Its 28 year appraisal period reflects the high reservoir complexity, relatively poor quality conventional seismic image, and uncertain impact of the conductive fractures. This paper compares early Clair production with pre-drill expectations.Its primary focus is on reservoir uncertainty, and how the drilling sequence and data acquisition are managed to allow rapid and flexible response to learning, given an expectation of surprises.Clair is not pre-drilled, and the preferred well locations and design will change.A condition of responding rapidly is a hopper of well options, kept filled with diverse well types and locations, from which appropriate wells are selected using current knowledge. Key decision points in the sequence are identified in advance and options at each point evaluated against reservoir scenarios. This process allows planning for potential outcomes, generates the required hopper wells and identifies data requirements for future decisions. Clair well-planning requires integration with reservoir surveillance and modelling.Early surveillance focuses on reservoir pressure response.All wells (including injectors) have a permanent down-hole pressure gauge, RFT data acquisition and a baseline production log. These data establish reservoir connectivity, injection response, and controls on productivity and injectivity.Also important is seismic data calibration against log and core information, to refine models of the natural fracture distribution.Water movement through fractures is expected and will become a surveillance focus. Clair reservoir uncertainty is represented in sets of static and dynamic reservoir models which express the full uncertainty range consistent with data.At start-up, a wide range of fracture models, with diverse outcomes, can be consistent with the limited dynamic data.As new data arrive, the objective is to condense the range of models, which continue to be diverse, but must also be consistent with all historic data at the end of every month.These models inform drilling programme decisions, reflecting both the continuing uncertainty and rapid learning. The Clair Phase 1 Development Clair is a naturally fractured Devonian sandstone reservoir with 600 m reservoir interval, containing 23 deg. API oil.It lies 75 km west of Shetland (Fig. 1) in 150 m of water.It was discovered in 1977 and appraised by a series of vertical wells with generally disappointing rates.The potential of the field was finally demonstrated by the drilling of high angle appraisal wells 206/8–9z and 206/8–10z, culminating in an extended well test (EWT) of 10z in 1996, which produced 500,000 stb of oil and demonstrated communication with a minimum of 500 million stb.The EWT led to sanction of the Clair Phase 1 development in 2001 and first oil in February 2005. The Clair Phase 1 development comprises the Core, Graben and Horst fault blocks (Fig. 2), which are believed to contain approximately 1.5 million stb of oil.Appraisal of the remaining areas of the field is continuing and may lead to subsequent phases of development.Phase 1 is a single fixed platform development.The first step was the recompletion of 10z as a production well, but otherwise the field is not pre-drilled, and will be developed by the drilling of 22 additional wells over a 3 year period.In part, this is a deliberate strategy to maximise learning during field development.The principal recovery mechanism is waterflood, with a planned ratio of approximately 2 producers per injector.Sweep is expected to be predominantly lateral rather than gravity stable, and is seen in terms of pattern flooding rather than purely peripheral drive.