As the industry pushes the boundary of technology to drill narrow margin wells, combining safe drilling practices with drilling efficiency is becoming ever more challenging. The practice of drilling with Managed Pressure Drilling (MPD) using statically underbalanced mud weight is gaining increasing acceptance in High Pressure High Temperature (HPHT) well construction. This paper describes the planning and execution of using mud cap fluid in the drilling of an ultra-narrow margin (0.50ppg window at the planning stage) HPHT well from a Jack Up rig.

Drilling Equivalent Circulating Density (ECD) with overbalanced mud weight at acceptable flow rate would have exceeded the formation fracture gradient and resulted in a loss of well integrity. To avoid this outcome, the HPHT section of the well was drilled with statically underbalanced mud weight. Displacing well to kill weight fluid at acceptable flow rates prior to any trip out of hole was not viable. Planning focused on how to maximise operational efficiency by using a cap fluid to trip in and out of hole without compromising open hole integrity and well safety.

In this paper, we discuss the design of the mud cap fluid, selection of Changeover Depth (CoD), risks associated with use of the cap fluid, determination of available window for mud cap placement and removal, planning and execution of mud cap placement and removal, challenges of running and displacing the cap fluid with a liner and, lessons learnt from repeated use of the technique throughout the well construction phase, including coring and wireline logging under MPD conditions.

Significant operational efficiency was gained from the use of cap fluids, making it possible to drill a well which would otherwise have been near impossible to drill with minimum lost time.

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