Hybrid system taps benefits of both MPD, underbalanced drilling, opens new horizons in southern Pakistan

Application of hybrid system allows operator to drill through, case off complicated sequence of formations without need for additional casing string

By Qasim Ashraf, Weatherford International

The northern Pothohar plateau of Pakistan is known to bear significant oil and gas reserves. Due to severe geological uncertainties, the area exhibits the most challenging drilling conditions in the region. The subject field is located in the southern part of the Pothohar plateau. Drilling challenges of severe loss circulation, high-pressure water influxes, low rate of penetration (ROP), simultaneous loss/gain issues and depleted reservoir formations are the most common problems experienced in the area.

The subject well was a reentry well where the objective of the operator was to drill and produce the deeper Permian (glacial deposit) and Cambrian (sandstone) formations. This well was producing from an Eocene (limestone) formation, which had become depleted over time. The reentry well was to be sidetracked from a Miocene formation, which was notorious for formation pressure reversals in the area.

A sidetracked 8 1/2-in. hole was initiated from 2,022 m in the Miocene formation with a mud weight (MW) of 1.50 specific gravity (SG). After reaching 2,080 m, a high-pressure gas kick was encountered, which was not experienced in the mother hole. It was deducted that an unexpected sand body was encountered, which was not observed in the original wellbore. The well was subsequently killed with a MW of 1.75 SG.

The drilling operation continued with this increased MW, but continuous high background gases required the MW to be further increased to 1.78 SG. The forthcoming formations in the section were also hydrocarbon bearing, and nearby wells had been producing from them. These formations had become depleted over time and required a relatively low bottomhole pressure (BHP) for successful drilling. The paradox thus presented to the operator was to either control the unexpected gas kick from the shallower formation by keeping a high MW/BHP or to keep a lower MW/BHP to avoid losses and formation damage in the deeper formations. To add to these challenging drilling conditions, a fault was expected in the middle of the section, which was expected to present a total lost circulation scenario.

Initially, it was planned to use managed pressure drilling (MPD) to precisely and efficiently manage the bottomhole pressure as per requirement. MPD would have helped to ascertain the precise pressure of the kick zone and keep an ECD just marginally above the pore pressure. The hurdle was the deeper (Eocene) depleted formations, which might not have withstood the higher BHP even with MPD.

Another option to address this situation was to go completely underbalanced using underbalanced drilling (UBD), but using UBD throughout is a tough call as oil/water was expected to flow from the Eocene formation. That could adversely affect the water-sensitive clay beds of the Miocene formation. Hole stability was also a serious concern while drilling in an underbalanced mode through the deeper Eocene formations. This situation demanded a solution that could handle the two contrasting conditions simultaneously.

The idea of hybridizing MPD and UBD drilling methodologies was thus innovated. It was decided to design and engineer the equipment in such a way that the benefits of both MPD and UBD could be combined. The rotating control device (RCD) was installed as the first piece of equipment that diverts the return from the well toward the MPD choke. The MPD choke was the most critical component of this setup that was used to maintain the required SBP.

Downstream of the choke, a mass/volume flow meter was installed that works as an early kick detection system (EKDS). The EKDS has the ability to detect the slightest change in flow out from the well and alert the operator of any downhole influxes early on. Ahead of the EKDS, a four-phase separator was installed that would be used to handle and separate the various fluids returning from the wellbore and ship them off to their appropriate destinations.

The plan of succession while drilling was to start drilling with the MPD system using a low MW and manage the required BHP with the application of surface backpressure. MPD would be used to drill across all the formations, which can withstand the high BHP. As soon as a depleted formation was encountered with a risk of severe losses, the system would be changed over to underbalanced drilling. The BHP would be adjusted in such a way that the annular pressure profile would be underbalanced against the Miocene gas pocket and marginally overbalanced against the Eocene formations. Underbalanced drilling would then be continued until section target depth.

Prior to deployment of the hybrid system, the operator had cemented the entire open hole. The hybrid system was rigged up and calibrated with the rig system. Before commencing with drilling with an MPD system, the hole was displaced to a 1.55 SG MW from the original 1.78 SG MW present in the hole. Drilling was started in MPD mode, maintaining an equivalent circulating density (ECD) of 1.78 SG from 2,073 m. Drilling continued until 2,182 m, where partial losses were observed. The SBP was reduced to maintain an ECD of 1.76 SG.

Drilling continued in MPD mode until 2,201 m, when heavy losses were observed and drilling shifted to UBD mode. Drilling then continued, maintaining an ECD of 1.56 SG until 2,254 m, where complete losses were observed. The presence of the expected fault was confirmed at this depth; hence, it was decided to drill this fault zone without returns from 2,254 m to 2,257 m.

During the drilling of this fault zone, the well was under continuous pressure due to an influx from the gas zone in the Miocene formation. After passing this fault, the thief zone was cured with heavy loss circulating material (LCM) and cement plugs. Drilling then continued until 2,459 m, where section target depth (TD) was declared. The hole was displaced to a 1.62 SG mud, two wireline logs were performed, and the hole was cased off with a 7-in. liner.

By the application of a hybridized MPD/UBD system, the operator was able to drill through and case off this complicated sequence of formations without the need for an additional casing string. The MPD mode of the system allowed the operator to conduct a dynamic pore pressure test and ascertain the pressure of the unexpected gas pocket. The operator, with this knowledge, drilled across this gas pocket and the forthcoming formations with the least possible MW, and reduced formation invasion damage expected with high overbalance pressures in conventional drilling.

The UBD mode of the system allowed an immediate switchover to underbalanced drilling, when severe losses were encountered against the thief zone in the exposed formation fault. The well was allowed to flow without stopping operations, and drilling continued across the fault. The thief zone was then cured in a live well with numerous bridging materials that were pumped through the drillstring, all the while keeping the string in motion and preventing it from getting stuck due to the presence of the hybrid system. The drillstring became plugged on multiple occasions, and to replace the bypass sub the drillstring was pulled out and run back in a live well without needing to kill the well. Continuously producing from the unexpected gas zone significantly depleted it, as it proved to be a small but high-pressured pocket.

The benefits provided by this hybridized system could not have been achieved with a conventional mud system. This application has opened a new horizon for drilling in the region and encourages further exploration. DC