Proactive planning to mitigate drilling hazards

W&T Offshore planned to re-enter well SS 359 A5 through a window milled in 7 3/4" and 97/8" casing, and drilling a 6 1/2" hole to access hydrocarbon reserves in the target “P" sand. It was expected that the margin between the pore pressure and fracture gradient was expected to be only 1.2 ppg.

Automated MPD system provides near constant bottom hole pressure to avoid losses and swabbing in depleted reservoir

Knowing the challenges that could be faced drilling through such a narrow mud weight window, W&T Offshore chose to employ a proactive managed pressure drilling approach by using the DAPC dynamic annular pressure control system to automatically manage the wellbore annular profile.

Early start up to operations

MPD operations were started earlier than planned as a result of gains and losses observed early in the hole section. Rigging up and pressure testing was done offline in order to minimize the impact on critical path rig time. Only minimal tuning and calibration were performed prior to starting MPD operations, operator experience and hydraulics modeling were able to maintain bottom hole pressure within a window of +/- 0.25 ppg.

Avoiding swabbing

One of the concerns drilling the well was the risk of swabbing while tripping out of the hole. In order to mitigate this risk, a fixed back pressure was applied to the well and maintained by using a back pressure pump. Optimal selection of the back pressure and mud weight required for tripping required consideration of many factors, including bottom hole pressure would allow tripping in without surging and not exceeding fracture gradient while the pipe is stationary with back pressure.

Effective planning and monitoring allowed the bottom hole pressure to be controlled within a window of +/- 0.1 ppg while tripping out of the well.

Equipment and services provided

Providing near constant bottom hole pressure required several key components. The HOLD rotating control device (RCD) allows for closed loop circulation by diverting returns through the flow control system. The RCD is remotely operated from the rig floor promoting a safe working environment and ease in changing out sealing elements. The RCD is accompanied by a hydraulic power unit (HPU) to ensure adequate lubrication is provided to the bearing assembly, high pressure seals, and to maintain a positive pressure differential across the high pressure seal section.

The dynamic annular pressure control (DAPC) system provided control of bottom hole pressure within pre-set drilling windows through independent and/or simultaneous use of redundant chokes. The system actively controls the choke position. The system can use its own dedicated pump to circulate mud and actively supply back pressure.

A coriolis type flow meter is connected to the low pressure side of the choke manifold and provides flow out data that is continuously monitored and compared to flow in data for kick and losses detection.

Drilling engineering services provided a complete MPD drilling program and assistance in addressing the requirements of NTL 2008-G07 as required for MPD wells in the Gulf of Mexico. These services included hydraulics modeling, hazard identification and operability (HAZID/HZOP) workshops, and well site training to assure the competency of involved personnel.