Subsea Template Drilling in the North Sea-Experience Gained During the Initial Stages of the Balmoral Development

Abstract

Summary. Drilling offshore in hostile environments, such as the North Sea, presents a unique challenge, which is intensified when working over a presents a unique challenge, which is intensified when working over a subsea template. Operations, procedures, and experience relating to drilling through the Balmoral template are described. Special equipment and techniques-including cuttings disposal, cementing, and safety-are detailed, along with directional drilling, surveying, and cuttings disposal. An expensive low-toxicity oil-based mud was used for drilling operations, and a specially developed oil recovery system with a high-speed decanting centrifuge was constructed and installed to reduce costly losses. The system is described and its operational features are highlighted. Introduction The Balmoral field is a Paleocene reservoir lying some 7,000 ft [2100 m] below sea level in U.K. Block 16/21 in the central North Sea. The field was first discovered in 1975 and proved to be a marginal prospect after four appraisal wells were drilled. Because of the field size and the desire to ensure early oil production, a drilling/ production template, along with satellite wells, flexible risers, production template, along with satellite wells, flexible risers, and a floating production facility, were chosen as the optimum method of developing the field. Water depth at the location is 480 ft [146 m]. Plans to drill and to complete 10 template and 5 satellite wells over a 3-year period, in order to bring first oil ashore in early 1987, have been successfully implemented. Four existing satellite wells were also recompleted and all nine satellites tied into the template within the same time frame. This paper focuses on the first three template wells, Wells B1 through B3, and the 30-in. [76-cm] template conductors. The achievements discussed took place with the semisubmersible Western Pacesetter 1 from July 1984 to Feb. 1985 and constitute a considerable learning curve for the operator and contractors. Operations are currently under way on the fourth template well. Cuttings disposal, safety, directional surveying, rig movements between slots, securing of the template by the 30-in. [76-cm] conductors, blowout preventer (BOP) handling procedures, and tophole drilling-all related specifically to template drilling operations-are discussed. These topics and others peculiar to the Balmoral development, such as the casing program and the oil recovery system, are dealt with in their relevant sections. Conductors To expedite drilling operations and to ensure that the template was firmly secured in place, 30-in. 176-cm] conductors were batch set in all 14 template slots before the first well was begun. Permission to do this was granted by the U.K. Dept. of Energy. The operations were conducted over a 30-day period during Summer 1984. Careful consideration was given to the sequence of conductor placement so that the leveling of the template, which had previously been set to within 1/2 deg. [8.7 × 10 rad] of horizontal, would not be adversely affected. The four corner slots were cased initially, followed by alternate slots on each side of the template, beginning at the northeastern end. The pattern was varied slightly to ensure that the placement of the final conductor coincided with the first well to be drilled. The average time to drill, case, and cement was 36 hours. When moves between well bays-achieved by simply winching along the anchor chains-are included, the average time per slot was 51 hours. Navigation between slots was by visual observation of numbers painted on the well-bay funnel areas. Slot spacing is 14 ft [4.27 m] between centers, the overall template being 110 × 105 ft [33.5 × 32 m]. A schematic of the template layout is shown in Fig. 1. At the outset, concern was expressed regarding the centralization of the 36-in. [91-cm] bit and 9 1/2-in. [24-cm] bottom-hole assembly (BHA) inside the 40-in. [102-cm] -diameter well bay. Poor centralization could lead to crooked drilling of the top holes, particularly in face of potential cement-pan buildup as successive conductors potential cement-pan buildup as successive conductors were cemented in place. Even with 14-ft [4.27-m] spacing between well bays, it was essential to have a vertical or a slight outward lean on the conductors to facilitate later directional operations. In practice, all holes drilled easily in the soft sediments; cement buildup from adjacent casings presented no problem. A centralizer bushing on the 9 1/2-in. [24-cm] drill collars (DC's) was made available but was never needed. Throughout all 14 slots, the 30-in. [76-cm] bottom survey ranged from 0. 12 to 0.6 deg. [3.7 × 10 to 10.5 × 10 rad], giving departures between 0.04 and 1.25 ft [1.2 and 38 cm]. JPT P. 331