In 1998, Lasmo Oil began drilling the Badhra #2 well, targeting the Lower Cretaceous Goru and Sembar sands. However, a high-pressure sequence forced the operator to stop. Available casing strings were consumed at a measured depth (MD) of 11,483 ft (3,500 m); furthermore, managing the high levels of background gas and fluid losses in the 6-in. hole proved impossible.

Ten years later, Eni Pakistan decided to target the Lower Goru (11,483 ft MD) and Chiltan (16,076 ft or 4,900 m MD) formations, which had never been explored in the Kirthar Fold Belt. Eni Pakistan believed managed pressure drilling (MPD) techniques, coupled with the proprietary continuous circulating valves (E-CD), would enable drilling through the HP/HT intervals in the Lower Goru formation to a deeper carbonate sequence in the Chiltan formation.

The Microflux display

The Microflux display shows an influx that is being circulated out of Eni’s Bado Jabal well during the drilling of the 81?2-in. section. The increase in the outward flow rate (red) shows the influx. The annular backpressure has been increased to compensate, as has the standpipe pressure. The system allowed Eni to continue drilling while handling up to 50% gas cut in the annular flow. (Images courtesy of Weatherford International)

The projected MD for the Bado Jabal well was 17,060 ft (5,200 m). Originally, drilling was planned in two phases – Phase 1: surface to 14,436 ft (4,400 m) MD; and Phase 2: 14,436 to 17,060 ft (4,400 m to 5,200 m) MD, to overcome the limitations imposed by the pressure capacity (5,200 psi) and certified maximum drilling depth (16,404 ft or 5,000 m) of the rig’s pressure mud system. Further assessment of the drilling conditions allowed the drillers to tap the Chiltan formation at 16,404 ft (5,000 m) MD using the same 2,000-hp drilling unit selected for the first section of the well.

The Lower Goru formation consists of hydrocarbon-rich sandstone and a mixture of limestone and interbedded shale. The limestone is weak and vuggy, often leading to costly mud losses. Furthermore, pockets of highpressure gas in the Goru formation can require days to circulate out, resulting in excessive nonproductive time (NPT); a gas slug in the first well precipitated six days of NPT.

The window between pore pressure and the fracture gradient in the Lower Goru formation is narrow. As such, a slight drop in equivalent circulating density (ECD) during pipe makeup can lead to kicks, well-control issues, and borehole instability. Conversely, the use of overbalanced drilling techniques to prevent gas influxes tends to yield fractures, lost circulation, formation damage, and a low ROP. The narrow gradient window has proven impossible to navigate using conventional drilling technology.

Beneath the Goru lies the Chiltan pisolitic limestone; this formation had never been explored in the Kirthar Fold Belt due to its depth and the difficulty of reaching it through the overlying Goru formation.

Eni Pakistan overcame the obstacles of drilling the Bado Jabal well using MPD techniques and two successful technologies, the Microflux control system and Eni’s E-CD sub. The Microflux system monitors annular backpressure with a high degree of precision, making it possible to drill within a narrow pore-fracture window. On the Bado Jabal operation, the system proved capable of detecting and controlling an influx of less than 1.5 bbl within two minutes.

Mounted to the top of pipe stands, Eni’s E-CD hub allows circulation to continue while pipe connections are made. This helps prevent changes in ECD that can cause a kick. It also circulates cuttings out of the hole continuously, keeping the hole clean and preventing stuck pipe.

By maintaining continuous circulation and controlling annular and bottomhole pressure in the difficult section 11,811 to 16,404 ft (3,600 to 5,000 m) MD of the Bado Jabal well, Eni and Weatherford were able to maintain an average ROP of 8 ft/hr (2.5 m/hr). The system effectively controlled background gas from the Lower Goru sandstone/limestone without causing lost circulation. Drilling continued while circulating out as much as 50% gas cut.

Operational detail
The Microflux system features a rotating control device (RCD) used to maintain a closed and pressurized annular

The well schematic shows the plan for the Bado Jabal #1 well.

The well schematic shows the plan for the Bado Jabal #1 well.

environment. It also is equipped with sensors that monitor annular pressure and other drilling variables in real time and an automated surface choke. During the Bado Jabal operation, an auxiliary mud pump was used to increase back pressure as needed.

Rigup for the MPD operation entailed installing the RCD, adjustable choke manifold, and control unit for the Microflux system and topping the last set of stands with the E-CD valves.

The platform incorporated a topdrive system. During drilling, mud flowed normally through the system and across the top of the sub (E-CD valve). When the sub reached the table, a mud hose was connected to its side port; the side flapper valve opened to allow mud to enter, and the flapper valve on the top of the sub closed so the top drive could be disconnected.

Pumping continued through the sub’s side port until the next stand of pipe was connected. Mud flow again was routed through the top-drive system from the top of the new stand. The auxiliary hose was disconnected from the bottom sub, the side port valve closed, and the sub was run in as part of the drillstring.

The Microflux system helped the operators in several ways during drilling. Combined with an auxiliary pump, it enabled them to maintain a constant level of annular backpressure, which ultimately contributed to increasing ROP. It also helped them manage transient conditions such as pumpoff, displacement of heavier mud pills, deployment of mud caps, circulation of high gas cut, plugging of nozzles at bit, and swab/surge. Furthermore, it detected two major partial-loss events followed by influxes and allowed the influxes to be circulated out without impacting drilling activities.

Maximum drilling mud weight in the 81?2-in. section of the well was 2.07 kg/cm2. The maximum recorded circulating temperature was 250°F (121°C) – static temperature above 329°F (165°C).

The advanced technologies used on this operation supplanted conventional mud logging-based flowout monitoring and trip/active tank recordings.

Benefits of the two new technologies provided on the Bado Jabal operation include minimized NPT; the ability to continue drilling while circulating out up to 50% gas cut; and the prevention of kicks, other well-control issues, lost circulation, and differential sticking. Eni used these technologies to reach total depth under extremely challenging circumstances.