Hoover-Diana is a deepwater Gulf of Mexico oil and gas development. The reservoir development plan is a hybrid drilling and completion program encompassing remote subsea wells (Diana) connected by flowlines to a host DDCV (deep draft caisson vessel) and platform-type wells (Hoover) on the DDCV with surface trees connected to the seafloor by production risers.The Diana subsea wells were batch drilled with a dynamically-positioned drillship and then completed with a new-build, moored semisubmersible. These subsea wells were designed as horizontal open-hole gravel packs to deplete a relatively thin oil rim that lies between an aquifer and a large gas cap. The horizontal sections of each well were drilled and completed with the subsea tree in place, and all wells were flowed back to the drilling rig for clean-up and hydrate prevention prior to installation of the DDCV and flowlines. At the time, these were the deepest water-depth horizontal wells in the world. Numerous challenges were encountered in the design and construction of these wells, including the scheduling aspects associated with the two deepwater drilling rigs and pipeline installation vessels.The Hoover drilling program required significant upgrades of an existing API-type platform rig combined with unique modifications to accommodate DDCV motions and riser systems. The Hoover drilling and completion program is a mixture of cased-hole frac pack wells and open-hole, horizontal gravel pack wells. Furthermore, it marks the first time a drilling rig would set subsea wellheads from a caisson vessel and perform all of the drilling and completion operations including the riserless operations.
A large number of subsea production wells have been drilled, but to date there has been limited industry experience of redrilling subsea wells, whereas, redrills have become routine in platform and surface wellhead applications. The focus of this paper is to highlight some of the key aspects of redrilling a subsea production well to extend field life.The paper will describe how the unique challenges of redrilling three subsea wells were overcome in old and declining fields where the original wells were designed and drilled with limited consideration for redevelopment of the fields through reuse of existing wells. The paper will define the problems of and describe the solutions to:• The mooring and BOP handling constraints in the crowded subsea infrastructure of a subsea field development.• The riser and wellhead analysis for redrilling a well through a subsea tree, and the operational practices required to maintain tree and wellbore integrity.• The analysis of the additional loads put on a subsea well through the extended period of riser and BOP loading.• The casing exit and redrill constraints unique to operations from a semi-submersible rig; casing recovery operations, the management of window milling debris in a subsea BOP system.There has been limited industry experience to date of subsea production well redrills, and this paper will share the significant lessons learned which will be of wider application, as the number of subsea wells increase along with the requirement to maximize recovery and extend field life.
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