Reliance Industries Limited (RIL) India has drilled the well (GS01-M1) using Managed Pressure Drilling Technology (MPD) as part of its exploration program. Well GS01-M1 was the fourth exploratory well drilled in the GS – OSN – 2000/1 Mumbai offshore basin with the objective to test thehydrocarbon potential of the carbonate reservoirs of Late Oligocene and Mid Eocene. Since a narrow operational window between the formation pore and fracture pressures was expected, the used of two variants of the Managed Pressure Drilling technique were planned for this well to be utilized depending on the formation requirements.

The Constant Bottom Hole Pressure (CBHP) technique, one of the MPD variants, was selected for the primary purpose of controlling the drilling fluid Equivalent Circulating Density (ECD) and to be capable of maneuvering the drilling fluid’s density through the narrow formation pore pressure andfracture pressure window. In addition, as operational window between formation and fluid losses pressure could get closed, leading to the boundary where these losses are uncontrollable by mechanical means, the drilling mode could be switched to the Pressurized Mud Cap Drilling (PMCD) technique which will allow drilling with no returns on surface injecting down the drill pipe and into the formation sacrificial fluid whilst the annulus is closed at the MPD choke and maintained full of high viscosity mud. This technique takes the advantage of the natural ability of the fractured formation to take both, injection fluid and drilling cuttings.

The Managed Pressure Drilling technology used to drill the 12-1/4" and 8-1/2" hole sections of well GS101 – M1, accurately allowed to control the downhole pressure profile according to the real time Pore Pressure calculations performed while drilling based on MWD logging tools thus, the balance between the limits of the fluid losses and the formation influx pressure was ascertained and by means of surface back pressure adjustments, this balanced bottom hole pressure condition was maintained. Operations were conducted from a semi submersible drilling rig equipped with a 10K psi sub-sea BOP stack. In order to increase the pressure rate in the marine riser from 500 to 2000 psi to enable the MPD operations, a 13-3/8" casing was used as a concentric riser hanged from a flange connector which was nippled up onto the marine riser expansion joint. This assembly allowed to rig-up the surface MPD stack for flow and pressure control. The 12-1/4" hole was drilled from 2168 m to 3805 m in 27 days since the 13-5/8" casing shoe was drilled out until the 9-5/8" liner was run and set. Another 6 days were used to drill the 8- 1/2" hole section from 3805 m to 4326 m. In both cases, the ECD was managed combining the friction loss pressure in the annulus and the back-pressure produced by the MPD choke; 500 psi of kick tolerance was used as safety margin for the maximum allowed casing pressure in dynamic conditions as Reliance and the drilling contractorwell control policies. The application of the CBHP technique reduced the NPT experienced in the drilled off-set wells in the same basin by controlling the mud losses and formation influxes events associated with the drilling of fractured limestones.

This paper describes the Front End Engineering Design, Project Management, Risk Mitigation, Detailed Engineering & Design, Operational Results and Lessons Learned from this project.

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