Hoover-Diana Deepwater Drilling and Completions
Authors: Moyer M.C. , Barry M.D. , Tears N.C. 
Published: October 2008

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 completionoperations including the riserless operations.

Introduction

The Diana basin is an intraslope salt withdrawal basin located 160 miles south of Galveston, Texas with blocks in East Breaks and Alaminos Canyon in water depths that range from 4600 to 4800 ft, refer to Figure 1. The hydrocarbon accumulations lie primarily in two reservoirs, Diana and Hoover1. The lease holders are ExxonMobil and BP. The Diana #1 discovery well was drilled in 1990 and 3D seismic was recorded in 1991. The Hoover field was discovered in 1997 and is located ~12 miles east of the Diana reservoir.

The Diana reservoir is predominantly a gas reservoir with a narrow oil rim in the Pleistocene horizon, referred to as the A- 50 sand located at ~10,500 ft sstvd. The field is approximately seven miles long and two miles wide. The oil column height is about 250 ft and the gas column height is about 1000 ft with a large aquifer in communication with the reservoir. Gross reservoir thickness ranges from 100 ft in the north to approximately 50 ft in the south with a corresponding degradation in reservoir quality. The Phase I development plan calls for depletion of the oil rim. The wells are designed to flow through 16 miles of un-insulated flowlines to the host DDCV; therefore only modest water cut can be tolerated before hydrate formation becomes unmanageable. The design team selected open-hole horizontal wells for the Diana oil rim in order to minimize drawdown and water coning while ptimizing oil recovery.