This paper describes the planning and execution of use of mud-cap fluid in the drilling of an ultranarrow-margin (0.50-lbm/gal window at the planning stage) high-pressure/high-temperature (HP/HT) well from a jackup rig. Significant operational efficiency was gained from the use of cap fluids, making it possible to drill a well that otherwise would have been nearly impossible to drill with minimum lost time.
Introduction
The IM field is located in Block MLHP-7 in the Rio Del Rey basin offshore Cameroon in a water depth of 58 m (Fig. 1 above). At the target reservoir level, Well IM-5 was predicted to be a marginal HP/HT well with an estimated reservoir temperature of 181°C and a reservoir pressure of 9,971 psi (16.80 lbm/gal) at a depth of 3475-m true vertical depth subsea (TVDSS).
The Middle Isongo sand, intersected by Well IM-3, was the primary gas/condensate target, consisting of deepwater sands with interbedded shales. The Middle Isongo target package consisted of three reservoir sands, the deepest two of which had not been penetrated in any of the earlier offset wells; this was the primary target of Well IM-5. A secondary target for Well IM-5 was the Intra Isongo, occurring between the Upper Isongo and the Middle Isongo main target.
The well design allowed the 12¼-in.-hole section to be drilled conventionally with statically overbalanced mud weight (MW), but it was impossible to drill the 8½-in.
