Deepwater Subsalt Development: Directional Drilling Challenges and Solutions

Abstract

Gulf of Mexico deepwater drilling activity has increasingly discovered economic hydrocarbon deposits lying below salt formations. Subsalt reservoir development is becoming one of the major challenges for operators, with the salt providing challenges in both seismic interpretation and drilling.1 This paper looks at how one operator successfully undertook a subsalt development in Mississippi Canyon. The project covered the completion of an exploratory well and the directional drilling of two development wells which produced from a state-of-the-art subsea system of wells, manifold and flowlines. The Gemini development was one of the first deepwater subsalt projects in the Gulf of Mexico and provided unique challenges for the directional drilling operations. These challenges included kicking off the well riser less in large-diameter hole, controlling wellbore trajectory through more than 3000 ft of salt and executing sidetracks to revised bottom hole targets after initial geological evaluation Success was achieved through detailed planning from a directional drilling aspect that considered the geological, engineering and economic requirements and novel application of technology. In this paper we describe how these challenges were met from the standpoint of wellbore trajectory, bottom hole assembly design and operational procedures, and we review the performance of the directional drilling operations. Introduction The Gemini field is a joint subsea development between Texaco (60%) and Chevron (40%) in the Gulf of Mexico in Mississippi Canyon Block 292. The development is approximately 90 miles southeast of New Orleans in a water depth of 3400 ft (Fig. 1). Gemini is a subsalt field that was discovered in 1995. In May 1996 the interval to be developed was tested in the No. 1 exploratory well at a rate of 32 MMcf/D of natural gas and 627 B/D of condensate. The subsea development consists of three production wells tied into a four-slot cluster manifold; each well was approximately 50 ft from the manifold. The development plan called for the drilling of two additional wells and completion of all three wells during the period of January 1999 to September 1999. The Diamond Offshore Ocean Star semisubmersible drilling rig (Fig. 1 inset) was utilized for the drilling operations and was also used concurrently to install the subsea system components (manifold, trees, and jumpers). The production from the field flows through two 12-in. flowlines back to the Chevron VK 900 platform located 27.5 miles northwest of MC-292 in 340 ft of water (Fig. 1). Initial Well Planning Initial targets allocated for the two new wells were based on results from the No. 1 exploratory well. These two wells would become the No. 3 and No. 4 development wells. Surface locations were dictated by the need to tie into the four-slot cluster manifold and by the anchor pattern of the Ocean Star. Maximizing operational efficiency, the surface locations were spaced at 50-ft intervals because the rig was required to move from each wellhead without pulling anchors. The targeted production zone was in the Allison sand at approximately 11,300 ft TVD RKB. Both wells included additional appraisal drilling for deeper targets, namely the Dean sand and the Erin sand at a depth of 15,000 ft TVD RKB. These targets lay directly below the Allison sand, and reaching them would dictate an S-shape directional profile for each well. Fig. 2 illustrates the target and wellhead locations.