The Ivanhoe/Rob Roy Fields: Operational Innovation on a Major Subsea Development

Abstract

Summary The Ivanhoe/Rob Roy development consists of a floating production facility(FPF) serving two remote subsea production/injection production/injectionmanifolds and well clusters. These clusters, about 2 miles apart, consist ofnine production and six injection wells depleting five separate hydrocarbonaccumulations. The field development philosophy has centered on usingstate-of-the-art hardware and techniques to achieve optimum exploitation ofreserves. A strong emphasis on safety is reflected in all aspects of theproject from design to operation. This paper describes the development after 1year in operation. The approach taken will be of interest to other North Seaoperators contemplating marginal subsea tiebacks or stand-alonedevelopments. Introduction The lvanhoe and Rob Roy fields are located in the U.K. Sector, Block 15/21a, about 110 miles northeast of Aberdeen. Monsanto Oil Co. was awarded the blockas operator during the fourth U.K. licensing round in March 1972. Ivanhoe wasdiscovered in 1975 with Well 15/2 1a-3 and was considered uncommercial untildiscovery of the Rob Roy accumulation 9 years later. Discovery Well 15/21a-11located roughly 2 miles east of Ivanhoe provided sufficient reserves forcommercial development of both fields through a single facility. Developmentplans for the field were already under way when, in late 1985, Amerada Hess Ltd. acquired Monsanto's U.K. interests and, consequently, operatorship of Block 15/21a. Annex B approval for the single development of both fields wasgiven in early 1986. The dramatic fall in the price of oil during 1986 led to areappraisal of the project economics, which delayed construction. Conversion ofthe semisubmersible Sedco Philips SS began in April 1987, and Philips SS beganin April 1987, and development drilling began in July 1987. First oil from thefields was achieved on July 7, 1989, ahead of schedule and within the 1986revised budget. As development of the Ivanhoe and Rob Roy fields progressed, exploration in the Block 15/21 area continued, resulting in the discovery ofthe Hamish field with Well l5/21b-21. This field was appraised, developed, andput on production 2 years after its discovery using the existing Ivanhoe/Rob Roy infrastructure. Figs. I and 2 show the location of Block 15/21 and the Ivanhoe/Rob Roy facilities. Reservoirs The Ivanhoe and Rob Roy reservoirs, located in the Upper Jurassic Pipersandstone, were formed as a result of an upthrown fault closure. Figs. 3 and 4show a plan and cross-sectional view of the reservoirs, respectively. Eachreservoir contains two main sand bodies separated by a continuous shale member. Classed as the Main and Supra Piper sands, they are highly porous and have Piper sands, they are highlyporous and have very high permeabilities and very low connate watersaturations. The Main Piper sands are thicker and generally of better qualitythan the Supra Piper sands. The reservoirs are located at roughly 8,000-ft truevertical depth subsea (TVDSS) and are normally pressured. Virgin reservoirpressure in both reservoirs was found to be the same (3,510 psia at 7,700-ftTVDSS), and the reservoir temperature of 175 degrees F was normal for the depthof burial. The field has three distinct crude types. The Ivanhoe crude in both Piper sands is the same, but the crudes in the Rob Roy Main and Supra Pipersands differ. During appraisal testing, the reservoirs were found to have asmall but significant CO2 content (=1% in gas) and a trace of H2S. Table Isummarizes the reservoir and crude properties. properties. The formation waterin both the Ivanhoe and Rob Roy fields is essentially the same and much moresaline than North Sea seawater. Total dissolved solids are around 100,000 mg/Lwith a chloride content of about 60,000 mg/L. Barium and bicarbonate are high(150 and 300 mg/L, respectively), indicating a potential for barium sulfate andcarbonate scaling in injection and production wells. production wells. A simplelinedrive water-displacement pressure-maintenance system was adoptedpressure-maintenance system was adopted as part of the reservoir developmentscheme. The Main and Supra Piper sands in both reservoirs were considered to bepressure isolated by the intervening middle shale member. Therefore, thestrategy to deplete the Main and Supra Piper sands in both fields independentlywas incorporated into the completion design described later. Total stock-tankoil initially in place for the fields has been estimated at 199 MMSTB, with areserve of 106 MMSTB estimated over the 10-year field life. After 12 months ofproduction, about 20% of total reserves has been produced. As predicted, nowater breakthrough has occurred to date and no major reservoir managementproblems have been encountered. Carter and problems have been encountered. Carter and Morel1 described the reservoir performance during the first year ofproduction. performance during the first year of production.