Development of the offshore extension of the 700 mmSTB Sherwood reservoir, Wytch Farm Field, Dorset, UK, commenced in 1993 with 4 to 6 km extended reach (ERD) wells drilled from an onshore site. Initial focus was on reserves from the Central Horst where oil columns of 70 to 100 m thickness were anticipated.
Ability to reach 8 km departure was soon demonstrated, giving access to the eastern rim. However this required wells to be aimed at targets often less than 50 m thick in the more layered Upper Sherwood. Reservoir sections of up to 2.3 km have been drilled, often developing two geological targets via one wellbore Optimal trajectory design must consider reach, directional control, cross-cutting of different producing layers and later well interventions. RFT measurements have been made at 250 locations in 10 wells at departures up to 7400 m, and production logs have been acquired in two wells. These dynamic data, together with 21 km of formation evaluation logs obtained whilst drilling, reveal that although the reservoir is layered and faulted, it is not compartmentalised. This aids oil recovery, but also presents a threat of water ingress by upwards movement of the oil-water contact and along open fault planes.
Such understanding of fluid flow and recovery mechanisms has been used to refine trajectory and completion design in the Upper Sherwood. Combined with uncertainty analysis of reservoir structure and quality, a simple tool has been developed to evaluate reserves associated with a given trajectory, and convey reservoir risk across disciplines. The result has been a clear demonstration of the value of surveillance data, and a strategy aimed at maximising length of productive wellbore whilst minimising trajectory complexity.
The Wytch Farm Field was discovered beneath Poole Harbour, Dorset, UK in 1974. It is the largest onshore oilfield in western Europe, with initial reserves estimated at 434 mmboe (200 mmboe remaining). The field is currently on plateau and delivers about 110 Mbbls of oil, 17.6 mmSCF of gas and 725 tonnes of LPG per day. The principal reservoir is the Triassic fluvio-lacustrine Sherwood Sandstone Group (Figure 1). It lies at about IS8Sm true vertical depth sub-sea (TVDSS) and contains about 85 % of Wytch Farm reserves. The accumulation is bound by faults to the north and south, and by dip-closure to the east and west. Original oil water contact (OWC) is at 1623.5 mTVDSS throughout.
Approximately one half of Sherwood reserves are contained in an offshore extension to the reservoir. This acreage is being developed by a series of record-breaking wells using Extended Reach Drilling (ERD) technology. Stepouts of >8 km have been achieved from an onshore well site within Poole Harbour. The geotechnical planning, drilling and completion of Wytch Farm ERD wells has been extensively documented.
Quality of the Sherwood Sands generally deteriorates up-section, from Lower to Upper reservoir. Mudstone horizons also occur with increasing frequency, and some are correlated over large areas. This presented a dilemma for ERD trajectory design as development moved to deepened structural areas. The question was whether to remain high in the section away from the oil-water contact, or drill down at high angle to intersect all layers.
Considerable effort had been focused on static reservoir description prior to offshore development. With a clear frame of reference thus provided an extensive set of formation pressure data were acquired and analysed in conjunction with production logs, surface measurements and data from permanent downhole gauges and flow meters. The result was increased understanding of reservoir connectivity and recovery mechanisms. Uncertainty analysis was then embedded in the well design process; ranges in interpreted reservoir depth, internal architecture and quality were described. Thus a basic philosophy for well trajectory design was developed for application in the poorer quality Upper Sherwood.
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