logic formations to create a design that included elements such as drilling procedures, drilling mud, drill bits, casing design, cement, and testing.
The original plan, shown in Figure 2-1, called for eight casing strings and liners (each consisting of steel casing segments that were screwed together), but the plan was modified to react to conditions that were encountered during drilling. Drilling ceased at 18,360 feet (a shallower depth than planned) and involved the use of a total of nine casing strings and liners, rather than the planned eight, including the final 9 ⅞- × 7-inch tapered production casing (sometimes referred to as a “long string”) as shown in Figure 2-2. The well was to be temporarily plugged and abandoned after the production casing was set and then completed for production at a later date.
The Macondo well presented a number of technical challenges to the drilling and completion teams, including the deep water, high formation pressures, and the need to drill through multiple geologic zones of varying pore and fracture pressures. In general, many of these problems can be anticipated, but some, such as pore and fracture pressure, are difficult to estimate in advance of drilling the well. This is especially true for the first well drilled in a new area, as was the case for Macondo. Thus, adaptation of the original well plan to the changing conditions encountered with depth when the well is drilled is not unusual. It is critical that the design be adapted to changing conditions with sufficient margins of safety to allow for further uncertainties that may be encountered during the operation.
Wellbore events that necessitated changes to the Macondo well plan included the following (BP 2010, 17-22):
1. Measurements showed that pore pressures were increasing at a faster rate than anticipated, combined with a period of lost circulation of drilling mud at 12,350 feet, indicating that the well could not be continued without setting protective casing. The 16-inch liner was set at 11,585 feet to seal off this section of the well. The setting depth of this liner was 915 feet shallower than planned.
2. In the course of drilling at 13,250 feet, a kick occurred, and the lower annular blowout preventer (BOP) was closed in response. During well control operations, the drill string became stuck and was severed at 12,147 feet. The drill string and hole below 12,147 feet were abandoned, and subsequent well drilling deviated slightly to go around the abandoned materials left in the original hole. The 13 ⅝-inch liner was run at 13,145 feet, which was shallower than planned, to allow the well to be drilled safely past the higher-pressure reservoir that had been encountered. The 11 ⅞-inch liner was used at 15,103 feet to seal the reservoir and allow for the use of higher mud weights than had been anticipated. Mud weight was to be kept between the curves for pore pressure and fracture pressure, as shown in Figure 2-3.