Reservoir characterization in the Captain Field: integration of horizontal and vertical well data

Abstract

The Captain Field lies in UKCS Block 13/22. It is a large viscous oil field with an excellent quality Lower Cretaceous sandstone reservoir. Development of this high viscosity crude (88 cp at the in situ temperature, 87°F) in the offshore environment was possible only after the development of reliable horizontal drilling technology, combined with high volume down-hole electric pumps, and the ability to run long pre-packed sand control screens. As the first development of this type within Texaco, the development decision was helped by the extremely favourable reservoir conditions present in the Captain Field. The Captain reservoir is encountered directly beneath the chalk and consists of Late Aptian aged turbidite sandstones, with high porosity (av. 31%) and permeability (av. 7 D). The sandstones were deposited on the feather edge of a large turbidite fan system derived from the East Shetland Platform to the north and which thins onto the Captain Ridge. The reservoir sequence consists of two principal sand bodies, the Upper and Lower Captain Sandstones with contrasting reservoir geometry. The Upper Captain Sandstone is developed in a continuous sheet that systematically thins and pinches out to the southeast whereas the Lower Captain Sandstone is concentrated in a sand fairway, interpreted to be a back-filled submarine canyon cutting across the Captain ridge. Core description show the sandstones to be predominantly massive suspension fall out sediments (S3 deposits, Lowe 1982 ) with rare intra-reservoir claystones. In this paper the reservoir geometry and sedimentology of the Captain Sandstone Member is described with particular attention being drawn to how horizontal drilling has tested and enhanced the geological model. Key reservoir parameters (e.g. connectivity, claystone distribution, grain size distribution, porosity, horizontal and vertical permeability) are discussed and their contribution to the success of the project evaluated.