Shell Exploration and Production successfully deployed an automated bottomhole 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 bottomhole pressure. Bottomhole 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.