Managed Pressure Drilling (MPD) has become a well-recognized and common technique used to drill across hole sections with a small margin between the pore and fracture pressures of the exposed formations. In many applications worldwide, conventional drilling is not viable in extreme narrow drilling window sections, where well control incidents are numerous and lost circulation is severe. Rather than tirelessly trying to tweak mud weight (MW) to handle these troubles while drilling conventionally, the use of surface backpressure (SBP) along with lower density drilling fluids allow for maintaining the bottom-hole pressure (BHP) within the drilling window. The MPD surface choke, equipped with hydraulics modelling software, exerts SBP automatically to maintain a stable BHP while drilling and thus results in an integrated drilling operation with minimal risk and proactive approach (Medley et al. 2008).

This paper highlights the drilling operation of an onshore HPHT gas field where an over-pressured zone is encountered, requiring extreme mud gradients. Wells in the field required varying mud densities from as low as 147 pcf and up to 157 pcf. This high density mud along with the narrow drilling window increase the risk of lost circulation and stuck pipe when drilling conventionally. MPD was utilized successfully in several wells, providing improvement in the time required to drill and reduced drilling complications. It enabled drilling with lower MW and enhanced control of BHP, thereby avoiding losses resulting from the high equivalent circulating density. Specific procedures were developed for MPD drilling across this high pressure zone. Manganese tetroxide mud was used to enhance the rheological properties of the drilling fluid. The resulting mud has proven to have significantly lower viscosity and less tendency to settling compared to barite mud systems. The combination of manganese tetroxide mud and MPD drilling proved to be an excellent option to drill through tight mud windows with ultra-high mud weight systems.

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