Design and Installation of a Subsea Multiwell Drilling Template at Garden Banks Block 189

Abstract

Summary This paper describes the economics, design, and installation of a simple drilling template that was used to retain or save subsea exploratory and delineation wells in Garden Banks Block 189 (GB 189) in the Gulf of Mexico. Introduction One way an operator can reduce project development costs is to drill multiple delineation wells through a drilling template to investigate a prospect. A template provides a means to space future wells accurately in relation to each other. If the prospect is commercial, a production facility (typically a platform) can be aligned and installed over the predrilled wells, which can be tied back to the surface and used as predrilled wells, which can be tied back to the surface and used as production wells. Cost savings are achieved in this development scheme production wells. Cost savings are achieved in this development scheme because there are no expendable wells. The first prerequisite for a drilling-template application is that the template site must be a suitable platform site. This location typically is not known after one exploratory well; two or more delineation wells are normally required to pinpoint the platform site. For this reason, Texaco and other companies have not routinely benefited from using drilling templates. In May 1989, a nine-slot template was installed over a discovery well (Well No. 1) in 718 ft of water at GB 189. The circumstances led to the use of a unique template design and an unconventional installation procedure. Since installation, two delineation wells have been drilled procedure. Since installation, two delineation wells have been drilled through the template and temporarily abandoned. Tiebacks of the three wells are scheduled to occur immediately after platform installation in Sept. 1991. Project History Project History In Fall 1988, Texaco drilled one successful subsea exploratory well in 718 ft of water at GB 189, located in the Gulf of Mexico 185 miles southeast of Houston. The straight-hole discovery well penetrated 169 ft of oil and 126 ft of gas pay from six intervals ranging from 5,000 to 8,500 ft true vertical depth. Two of the oil zones were flow-tested at 739 and 2,037 B/D, respectively. Well No. 1 was drilled through a seismic bright-spot area in the southwest quadrant of GB 189 (Fig. 1). The well is positioned 1,200 ft south of an east/west shipping fairway that obstructs the entire northern half of the block. The southeast and southwest quadrants are geologically separated by a salt diapir. Severe seafloor irregularities exist throughout the south-central region of the block over the salt diapir. Potential platform sites were limited to the southwest quadrant because of the confinements imposed by the fairway, seafloor irregularities, the salt diapir, reserve locations, and lease lines. Well No. 1's location was determined to be a suitable platform location on the basis of preliminary mapping, a shallow-hazard study, and a drilling-reach study. The site was operationally suitable because it would permit future platform wells to exploit the discovered bright-spot region fully and would platform wells to exploit the discovered bright-spot region fully and would allow directional reach into a potential wildcat bright-spot region to the northeast. The limited number of alternative platform locations in GB189 led to the investigation of using a drilling template. The final decision to use a drilling template was justified by a favorable economic analysis. In addition to saving future delineation wells, the drilling-template strategy had to incorporate the previously drilled discovery well. Therefore, a project goal was to finish the delineation program and develop GB189 with project goal was to finish the delineation program and develop GB189 with no expendable wells.