Significant Performance Improvement with MPD in HPHT Narrow Drilling Window Campaign in the North Sea

Abstract

Significant depletion in reservoir pressure, huge uncertainties in pore and fracture pressure, high overburden pressure on top of reservoir, Narrow Mud Weight Window (NMWW) and Partial/Total losses whilst entering the reservoir made these HPHT (High Pressure High Temperature) wells conventionally un-drillable. Due to these substantial challenges these wells were considered not only costly but also carry a high probability of failure to reach well TD (Total Depth). MPD (Managed Pressure Drilling) is a safer and more effective drilling technique as compared to conventional drilling, especially in wells with NMWW and downhole hazards. The precise determination and dynamic downhole pressure management was imperative to complete these wells without well control incidents. The Constant Bottom Hole Pressure (CBHP) variant in combination of automated MPD system was deployed with a mud weight statically underbalanced while dynamically managed above formation pore pressure to minimize the overbalance across the open hole. MPD enabled the operator to efficiently navigate Equivalent Circulation Density (ECD) through the pore and fracture pressure window, allowed significant improvements throughout the entire campaign.

This paper discusses the challenges faced during the last three wells drilled in the campaign which includes equipment issues, commissioning delays, losses whilst drilling, Managed Pressure Cementing (MPC), 7" drill-in-liner and plugged/blocked lines due to weather and mud conditions. The paper describes HPHT infill drilling experience, specific techniques, practices as well as lessons learned from each well during the campaign were implemented to address challenges and to improve performance. The MPD system commissioning was optimized by repositioning the lines which saved significant critical rig time. The blowdown points were added on the lines that were not operational continuously therefore a procedure was developed for flushing to avoid plugging. Optimized drilling strategy was also developed where MW was further reduced to avoid losses as observed in previous wells and CBHP was maintained by manipulating Surface Back Pressure (SBP) from surface.

This paper also discusses continuous improvements /upgrades in MPD operating software which assisted the operator in accurate monitoring of flow, SBP and BH-ECD to save significant rig cost in terms of invisible Non-Productive Time (NPT). MPD is a drilling enabler and performance enhancer which saved 80 days of Authorization for Expenditure (AFE) on this challenging HPHT campaign.