A strategically aligned managed pressure drilling (MPD) system incorporating interval-specific protocol aimed at maintaining constant bottomhole pressure (CBHP) enabled the safe and effective drilling of the deepest HP/HT exploratory well ever constructed in the Mediterranean Sea.

The landmark well, which was drilled from a jackup in the Egyptian Nile Delta to 7,207 m (23,639 ft) total depth with static bottomhole temperature of 208 C (406 F), would have been prohibitively difficult if not impossible to drill with a conventional open-to-atmosphere circulating fluids system. Thus, the success of this deep exploratory well established that a properly planned and executed MPD strategy is the accepted benchmark for the future delivery of equally challenging wells in the area.

Indeed, the CBHP MPD system not only provided early kick/loss detection but was instrumental in dealing with the pore pressure uncertainties intrinsic in ultradeep exploratory wells. Strictly speaking, the automated MPD system supported the evaluation of real-time pore pressure and dynamic formation integrity tests, thereby allowing the proactive optimization of drilling parameters including mud weights to sustain control over changing equivalent circulating densities (ECDs) and head off costly drilling issues including severe losses, differential sticking and formation fracturing, among others.

One of the keys to the success of the project was the engineering of distinctive operational guidelines for each hole interval taking into account the respective pressure profile ambiguities, specific casing points and the narrowing drilling window expected in the deeper hole sections. Moreover, drilling largely in MPD mode for the entirety of the well allowed wellsite personnel to sequentially enhance familiarity with the system—an imperative as the well progressed and the pore pressure/fracture gradient narrowed.

The practical advantage of this approach was accentuated when two micro influxes between 3,299 m and 3,828 m (10,820 ft and 12,556 ft) were successfully mitigated and the go-forward mud densities were optimized based upon the results of subsequent pore pressure tests.

Having familiarity with the MPD system and its capabilities proved invaluable in dealing with similar events in the more critical deeper hole sections. Moreover, the early detection of ½-bbl fluid gain—which would have been impossible in a conventional scenario—avoided unnecessary shut-ins and nonproductive time.

Strategy behind MPD planning

The core of the system was the MPD-standard rotating control device (RCD), which provides a pressure-tight barrier in the annulus and, in combination with the closed-loop MPD system, enhances safety by diverting returning fluids from the rig floor. Importantly, the Microflux intelligent control unit provided automatic surface control of the system, delivering instantaneous response to wellbore signals to ensure incidents were contained before deteriorating into uncontrollable events. The system also included the requisite auxiliary pump to maintain CBHP during static periods or while performing dynamic flow checks.

Planning the operation began eight months before the MPD system was rigged up, with all of the equipment undergoing an exhaustive certification process conducted in accordance with applicable API procedures. Once certified, the equipment was assembled at the Weatherford Cairo base yard, where certified closed-loop fingerprinting flow exercises were performed to help define flow tolerance bands, choke pressure at various flow rates and provide the means of calibrating sensors and the hydraulics programming of the MPD system.

In addition, as part of the strategy aimed at fully immersing relevant personnel on the intricacies of the MPD system, a complete fingerprinting analysis with decision-makers was designed to further understanding of the communication protocol for each procedure as well as the inherent risks. During planning, pertinent rig personnel also were introduced to the MPD system via the KCA Deutag simulator in Aberdeen.

The development of an interval-specific strategy included comprehensive daily reviews of MPD equipment, procedures and personnel throughout the operation. To circumvent the issues typically associated with conventional drilling, in the deeper hole sections the application-specific strategy called for the use of MPD to proactively reduce the overbalance margin.

Further, since the high temperature likely would render the pressure-while-drilling (PWD) tool inoperable, compressive modeling and configuration of the hydraulic software for the automated MPD system were used to obtain negligible discrepancy between the model and PWD measurements, ensuring the maintenance of CBHP throughout the operation.

As it would be impossible to monitor the flow difference during back-reaming operations within the prescribed time tolerance, the strategy included the mitigation of swabbing effects prior to commencing MPD or making conventional connections. The swabbing calculations would be compared with the real-time ECD readings delivered by the PWD tool.

During the operation, additional procedures performed included dynamic formation integrity tests while drilling to evaluate actual pressure integrity of the formations being drilled, dynamic flow checks to evaluate well conditions, pore pressure tests with incremental reductions in surface backpressure to evaluate the appropriate mud density for drilling, and the mud weight displacement during tripping operations.

Importantly, early kick detection (EKD) flow charts were prepared that specified how to decide whether a micro influx can be circulated out using MPD equipment or if the well should be shut in using conventional well control practices.

MPD recap

As with every interval, fingerprinting exercises were performed prior to initiating drilling in MPD mode in the underreamed 14¾-in.-by-17½-in. section at 2,997 m (9,830 ft). Drilling commenced with 3.5-lb/l (13.4-lb/gal) oil-based mud at a flow rate of 3,028 l/min (800 gal/min) and a surface backpressure (SBP) of 60 psi, necessary to obtain an ECD of 3.6 lb/l (13.66 lb/gal). After circulating out the aforementioned micro influxes, a dynamic pore pressure test was run to refine the mud density followed by a dynamic formation integrity test. Throughout the well, the dynamic formation integrity test and dynamic pore pressure test were crucial in refining the ever-narrowing drilling window to not only optimize drilling but also gather valuable data for future planning with minimal interruption to the well construction process.

The detection and remediation of total losses notwithstanding, the intermediate 12¼-in.-by-14¾-in. hole was drilled otherwise uneventfully to the planned 14-in. casing depth at 3,711 m (12,173 ft). Notably, the capacity of MPD to deliver continual surveillance of drilling parameters to optimize performance was underscored in the 9⅝-in. section, which was drilled relatively trouble-free to a new programmed setting depth of 5,632 m (18,472 ft) tied back to the 11¾-in. liner at 4,755 m (15,598 ft).

In the doubly challenging 8½-in hole at 6,422 m to 7,207 m (21,064 ft to 23,639 ft), early loss detection by the MPD system prompted an immediate reduction in BHP and the pumping of lost circulation material, thereby averting potentially total loss returns. While drilling the section, the mud density was adjusted from a beginning weight of 4.36 lb/l (16.5 lb/gal) to maintain an ECD of 4.48 lb/l (16.96 lb/gal) to 4.44 lb/l (16.8 lb/gal) with a flow rate of 1,817 l/min (480 gal/min) and ECD of 4.58 lb/l (17.35 lb/gal).

In addition, the SBP was alternately reduced and increased to evaluate background gas while drilling, which reached a high of 29.4% but dropped quickly to 10% upon circulation through the mud-gas separator. Furthermore, performing dynamic flow checks before tripping and displacement with heavy mud and efficient calculation of steel volume displacement of each stand ensured the well remained under control and that ECD management was preserved during trips.

Overall, the well was drilled successfully to a final depth of 7,207 m, meeting all objectives and reinforcing MPD as the standard for addressing the uncertainties intrinsic to deep exploratory prospects. Further, by employing MPD the operator obtained valuable formation data that will help in future well planning and optimization of drilling operations in the area.

Comments