This paper summarises experiences gained and highlights some of the challenges posed by the recent Brunei Shell High Pressure High Temperature drilling campaign, the exploration well design adopted, and discusses some of the issues raised. The aim is to capture the learning points gained during the drilling of the six wells for future campaigns.
A series of deep, high pressure plays was identified, in water depths of less than 50m, in the late 1980's as being key to Brunei Shell's exploration effort in the 1990's and beyond. Following experiences gained from drilling Brunei Shell's first HP well, Champion Ultra Deep (Peragam-1) in 1990, a conventional 10k psi jack-up rig was upgraded in 1992 with 15k psi equipment, and state-of-the-art drilling instrumentation for the high pressure drilling campaign (Ref. 1). Required modifications to the rig were based on industry experience at the time, mainly from the North Sea sector (Ref. 2) and developed for local conditions. All six wells in the campaign were drilled with this rig.
A summary well schematic of the wells is shown in Figure 1. The wells fall just outside the strict definition of what constitutes HPHT, in that the maximum temperatures experienced exceeded 150degC in only one well, Kalagiau-1, however the maximum mud gradients in all wells exceeded 21.5kPa/m (0.95psi/ft) with the highest at 23.3kPa/m (1.03psi/ft). Typical alonghole depths reached were in excess of 4,000m (13,124ft) with average durations of some 160 days.
30" conductor. Driven. Verticality is critical to mitigate later casing wear, so the conductor is gyro'd once settled on the sea bed and centered with the rig prior to piling. 36" conductor is used where the well is to be suspended as a freestanding conductor - typically in water depths less than 15m.
20" casing. Set between 500m and 1000m. In one well, Enggang-2, this casing was set at 1462m in an effort to push the casing as deep as possible to give maximum flexibility for a planned sidetrack. Although successfully cemented, there was a recognised risk of,
Casing collapse during cementing,
Losses during the cement job due to bridging or formation breakdown at the 20" shoe.
However, setting 20" above 1000m is considered optimum for Brunei operations, since additionally, Rate of Penetration (ROP) in the 26" hole section slows significantly below this depth.
16" casing. Has been used only when absolutely necessary e.g. Kalagiau-1 and Peragam-2 to cope with inflated pressures above depleted zones. Its use is not considered optimum due to the requirement for underreaming to accommodate both the 16" casing, and then below in order to accommodate the 13.3/8" casing. Underreaming has been extremely problematic and time consuming due to poor equipment performance and should be avoided for long intervals if possible.
13 3/8" casing. Set as deep as possible, generally limited by the formation fracture gradient at the 20" shoe, often just above the shallowest prognosed hydrocarbons and the onset of mild overpressures. Drilling very deep 26" and 17.1/2" holes requires careful Bottom Hole Assembly (BHA) design to maximise hydraulics and minimise the various modes of drill string vibration. In the deviated well, Enggang-2, several BHA failures occurred that were attributed to these effects. In Tara Tara-1, after optimising BHA's in consultation with Koningklijke Shell Exploration and Production Laboratory (KSEPL - Shell E&P Research), no failures occurred, and ROP increased with noticeably less drill string vibration.