Don - A Cost-Effective Approach to Subsea Design

Abstract

Summary Marginal developments in the U.K. sector of the North Sea require anapproach that reduces project development costs and risks project developmentcosts and risks while providing safe and reliable operation. This paperillustrates how the Don development satisfied these criteria, although theproject was characterized by significant uncertainties. At the time of projectapproval, Don was the deepest and farthest subsea development from the hostplatform in the U.K. sector. Introduction Location and Reservoir. The Don field is situated in the northeast corner of Block 211/18a of the U.K. sector of the North Sea at a latitude ofapproximately 61.5 degrees N (Fig. 1). The field is some 9.3 miles [15km] northof Thistle Platform A. The seabed in the area is generally flat, and waterdepth varies between 525 and 560 ft [160 and 170 m]. The reservoir lies in the Brent group of sands at about 11,155 ft [3400 m] subsea. It is geologicallycomplex and extensively faulted. The discovery well, Well 211/18–12, wasdrilled in 1976 and penetrated the Don southwest accumulation. A second well, Well 211/18–13, confirmed the discovery of Don northeast in 1977. Furtherdrilling during 1977–85 identified a number of potential satellites adjacent tothe two main accumulations, indicating increased complexity and leavinguncertainty about the degree of faulting and the extent of reservoircontinuity. The Don northeast fault block, considered the more prolific area, is separated from the adjacent hydrocarbon-bearing fault blocks by sealingfaults. The Don southwest area is of similar areal extent, but its productivityis poorer. Reservoir pressure and temperature are about 7,300 psia and 265degrees F [50 MPa and 130 degrees C], respectively. Phased Development. A variety of development Phased Development. A varietyof development options were considered before selection of the current plan, which is based on a phased subsea development of Don as a satellite to Thistle. A phased development was essential because of the complexity and uncertainpotential of the reservoir, the uncertainty potential of the reservoir, theuncertainty about the fluid properties in the Don northeast area, the distanceof the Don satellite from the host platform, and the water depth. A phasedapproach also provided an opportunity phased approach also provided anopportunity to confirm the reservoir characteristics before finalizing aflexible plan to develop the area fully in the most cost-effective manner. In Phase 1, an initial performance appraisal of Don northeast was conducted fromtwo producers supported by a water injector. If further phases do not proceed, Phase 1 could recover up to 24 million STB [3.81 × 10(6) stock-tank m3] oil foran estimated development cost of 56 million ($105 million U.S.). Peak designrates in this phase are 15,000 B/D [99.4 m3/h] of oil and 10,000 B/D [66.2m3/h] of water injection. The Phase 1 development facilities (Fig. 2) comprisea simple subsea manifold surrounded by a tight cluster of three subsea wells;trees, control modules, and associated hardware for he three wells; two subseaumbilicals linking the subsea facilities at Don with the platform-mountedcontrol system and chemical-injection facilities; two 8-in [20-cm]-diameterpipelines, one in production service and one for water production service andone for water injection; a 30-in [76-cm]-diameter multifunction caisson riserat Thistle, containing pipeline risers and umbilical conduits; platform-mountedelectrohydraulic control equipment, including the master control station; and Thistle topsides modifications and tie-ins to handle Don production and waterand chemical injection. Development Philosophy. The development philosophy for the field, developedphilosophy for the field, developed before front-end engineering work began, covered many aspects, including drilling and workover operations, reservoirdevelopment. Thistle platform operations and maintenance, reservoir monitoring, and subsea design control and maintenance. This philosophy defined theprinciples upon philosophy defined the principles upon which the developmentshould be based and the extent of the facilities to be provided. The mainprinciples were as follows.Downhole reservoir monitoring on both productionwells for Phase 1 to maximize production wells for Phase 1 to maximizeacquisition of reservoir performance data.Use of field-proven equipment.Adoption of a tight cluster well manifold configuration centered around theexisting appraisal Well 211/18–13 as the first producer, PN1. producer, PN1.Avoidance of complicated protective structures.Positioning of keyequipment, likely to require maintenance and intervention, on a retrievablevalve package mounted within the tree structure/frame.Provision offacilities to allow wireline intervention with a ship-shaped vessel and subsealubricator. JPT P. 386