Shell Offshore Inc. (SOI) as operator, with Joint Venture participation of Amoco Production Company and Sun Exploration and Production Company, spudded the first of several planned East Coast deepwater wells in a world record 6,448' of water during August 1983. The well, in Wilmington Canyon Block 587, was finished in December 1983. The well planning for this venture is described, including generalized geologic setting, pore pressure/fracture gradient estimations, the wellhead, casing, and hole programs, and the drilling fluids and cementation programs. Particular features and experience unique to the deepwater environment are emphasized.
The deepwater drilling environment offers some unique well design challenges, namely low fracture gradients, low temperatures, and the possible outfall of drilling-with a highly tensioned riser from a dynamically positioned drillship.
The offshore Mid-Atlantic setting presents three other challenges:
a carbonate reef objective which could be so porous and permeable (possibly cavernous) that massive lost returns are a definite possibility,
appreciable ocean currents arising from warm core rings spun off by the Gulf Stream, and
weak ocean floor soils.
Finally, the objectives are of Cretaceous and Jurassic age, both of which have frequently been found elsewhere to contain hydrogen sulfide in the produced fluids. This, then, is the setting for the well design for the first shelf margin well drilled on the East Coast Continental Slope.
Pore pressure was predicted (and found) to be normal to the depth drilled from analyses of the geologic setting and from inferences drawn from other East Coast wells, e.g., the COST B-3.
The fracture gradient estimation was made using an equation for horizontal stress (fracture pressure) by Hubbard and Willis:
(Mathematical equation available in full paper)
Also shown in Table 1 are the predicted and actual values for the first well drilled in the deepwater program, Wilmington Canyon Block 587, No.1. Note that especially for the deeper casing point, the prediction of Equation 1 is still conservatively low.
Casing, hole size, and drilling strategy had each to be paced to the drilling environment, especially the abnormally low near mud line fracture gradient associated with the deepwater and the possibility for lost returns which were expected to complicate drilling and well control in the objective zone. The pore pressure and fracture gradient prediction for the well is shown in Figure 1. Note that sufficient fracture gradient to enable circulating minimum mud weights back to the rig (9.1–9.5 ppg mud, 9.6–10.0 ppg fracture gradient) is not reached until 2,000' below the mud line.
