RIASSUNTO
Abstract
Shell Exploration and Production successfully deployed an automated bottom hole pressure control system developed by Shell International R&D called Dynamic Annular Pressure Control (DAPC) to solve lost circulation and hole instability problems in a deep water Gulf of Mexico well on the Mars Tension Leg Platform, after two failed sidetrack attempts. Dynamic Annular Pressure Control is a system designed to apply controlled annular surface pressure, with the goal of maintaining constant bottom hole pressure. Bottom hole pressure (BHP) can be maintained at any desired over/under balance by use of a lower mud weight than typical, and adjustment of surface backpressure as the circulation rate is varied.
In Shell's maturing deep water developments, rock stress redistribution due to reservoir depletion is causing drilling problems on a scale not seen in the past. High mud weights are still required for hole stability, but reduced fracture gradients have been observed both in reservoir and non-reservoir rock. DAPC was used in the third attempt to sidetrack the Mars A-14 well, to keep Equivalent Circulating Density within the shrinking pore pressure / fracture gradient window.
The DAPC system was fabricated, installed and tested on a fast track basis after a review of the technology indicated a high likelihood of enabling the well to be drilled without problems. Cased hole tests indicated that BHP could be maintained within the required limits. The mud weight was reduced for the problematic section, and the well reached planned total depth without experiencing lost circulation or hole instability problems. The production liner was run and cemented without incident.
DAPC has been demonstrated as a promising new tool to solve drilling problems associated with reservoir depletion. As the technology matures, it has the potential for broad application to effectively widen the pore pressure / fracture gradient window in deep water, HPHT, extended reach and other challenging drilling environments by continuously and automatically maintaining a desired bottom hole pressure.
Introduction
The Mars Tension Leg Platform is located in the Gulf of Mexico, approximately 130 miles southeast of New Orleans, in 3,000 feet of water (Figure 1). The A-14 well is an updip oil producer in the waterflooded M1/M2 (Upper and Lower Green) reservoir, and is expected to recover the majority of the reserves. The well was originally drilled and completed in the M2 sand in 1996. The M1 sand was added during a 2000 workover for a sand control failure. A-14 was shut in again in May 2003 because of sand production and decreasing production rate. In 2004 it was decided to redrill the well to the original M1/M2 objective after recovering the slot. The completion tubing and production tie-back were recovered, a whipstock was set, and the well was sidetracked out of the existing 11-7/8?? casing. An intermediate 9-5/8?? x 11-7/8?? expandable liner was successfully set at 14,330' MD. Sidetrack 1 reached total depth of 21,144' (16,340' TVD), at a maximum inclination of 56° and departure of 10,116', when lost circulation was encountered which, in combination with subsequent hole instability problems, led to loss of the bottom hole assembly (BHA). After unsuccessful attempts to fish the BHA, the open hole was plugged back and a second sidetrack was attempted.
The plan was to sidetrack from the previous casing shoe, and to set a second expandable drilling liner, in order to be able to run 7-5/8?? production liner across the M1/M2 objective. Hole instability and lost circulation problems prevented the expandable liner from reaching planned depth, and during subsequent remedial operations, a breach was discovered in the 9-5/8?? x 11-7/8?? expandable liner near where the whipstock had been set to exit the 11 ¾?? casing. Well operations were terminated and a detailed After Action Review was carried out.