During initial redevelopment of the Amberjack field in the Gulf of Mexico (GoM), Stone Energy used managed pressure drilling (MPD) to avoid the high nonproductive time (NPT) and unplanned sidetracks experienced by a previous operator in earlier wells.
Discovered in 1983, the Amberjack field lies 50 km (32 miles) offshore Louisiana in Mississippi Canyon Block 109 in 314 m (1,030 ft) water depth. By 1992, 38 wells had been drilled, and in 1999 a redevelopment program was initiated to boost production. The first two sidetrack development wells encountered severe trouble above the target sands due to uncertainties in pore pressure and in the target sands due to narrow margins between pore pressure and fracture strength. These conditions caused severe lost circulation with associated annular pack-off, stuck drillpipe, gas influx, and ultimately two unplanned additional sidetracks. There were no viable drilling alternatives the operator could use at that time to mitigate these conditions and avoid excessive NPT.
Proactive planning
When Stone Energy took over the field, the company looked for drilling methods to avoid these problems. During the pre-planning phase, drilling engineers concluded that the solution to the majority of the trouble was continuous control of bottomhole pressure (BHP). Stone engineers investigated the use of different drilling fluids, bottomhole assembly (BHA) designs, and alternate well plans to maintain borehole stability, reduce equivalent circulating density, avoid mud losses, and minimize NPT. In the end, they also chose to include MPD because it provided an effective means of controlling the BHP.
Stone Energy and Schlumberger drilling engineers started working together in the planning phase to apply standard practices in developing fit-for-purpose hydraulics and drilling plans. Early preparation for the first Amberjack well began with a rig survey by an experienced MPD supervisor, who mapped out and planned the rig-up ahead of time. Preparation and planning stages also included a drilling engineering review of relevant well design elements and development of BHP management and drilling contingency plans.
One of the final steps in the planning process involves the standard practice of drilling the well on paper (DWOP) to identify drilling risks and develop mitigation plans. Pre-well DWOP, HAZID, and HAZOP workshops were held to prepare operator, rig, and service company personnel for the planned well.
An important objective while drilling any well is to maintain the BHP above formation pore pressure and below the fracture pressure. In the Amberjack field, the difference between these two pressure limits is narrow, and accurate pressure control was essential. DWOP allowed engineers to test possible situations and assure themselves that the equipment and personnel were ready.
Rig personnel on the Amberjack platform were familiarized with MPD services during practical lessons in what are now routine procedures and actions for planned and unplanned events.
Service setup and execution
Four Amberjack wells were drilled with MPD using a dynamic annular pressure control (DAPC) system. The primary components of this system consisted of a modular manifold, backpressure pump, Coriolis flow meter, and control modules. Once it is set up, the system actively monitors flow-in and flow-out, annular pressure, depth, and other drilling parameters. The controller uses drilling and real-time hydraulics data to calculate the required amount of backpressure to keep the BHP at the desired level and at the designated depth.
When DAPC senses a drop in flow rate, the system begins adjusting the choke to increase backpressure. It continues to adjust the choke until it reaches the programmed set point and then maintains it after the rig pumps are turned off. These functions are performed while drilling, during connections, and when tripping out of the hole for a bit change.
Together, the Coriolis meter and DAPC system proved to be a useful combination not only for kick detection but also for pore pressure determination. The technologies allowed engineers to perform a series of static flow checks in formations known to have uncertain pore pressure. During the checks, the DAPC system was used to step down the static BHP in controlled increments until the mud column was at hydrostatic pressure. While the BHP was being stepped down, the system monitored the flow-out measurements from the Coriolis meter. These tests were useful in helping the engineers determine the correct mud weight for drilling and tripping.
During the planning phase, engineers determined that the narrow size of the pressure window in the Amberjack wells would require an additional level of backpressure control. They decided the system would have to do more than just trap pressure; it would have to be capable of adding backpressure when the rig pumps were turned off. A very useful feature of the DAPC system is its ability to control an auxiliary pump and add backpressure independent of the rig pumps. That feature proved indispensible to managing BHP more than once. On one such occasion the rig performed an unannounced fire drill during which the driller hard-stopped the pumps in 30 seconds. Within 20 seconds of the initial change in pump strokes, the MPD system automatically responded by simultaneously closing the choke and turning on the auxiliary pump. After another 20 seconds, the system stabilized the BHP at the programmed set point, reversing a drop of 0.15 pounds per gallon caused by the sudden change.
Drilling benefits
MPD also proved beneficial when the drillstring was being pulled out of the hole. Tripping out with constant backpressure allowed the engineers to protect the well against swab and surge effects and during rollover operations when the density of the mud was increased from drill weight to trip weight.
However, the most valuable benefit MPD provided on Amberjack can be seen in a comparison of MPD and conventional drilling NPT. Total NPT for three of the four MPD wells was less than half a day, while for the earlier conventionally drilled wells it was more than a month.
Using MPD on Amberjack helped the company drill historically difficult geomechanical objectives more efficiently and with significantly less NPT. The preparation paid off with near-flawless drilling execution.
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