Case Study

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In-Fill Drilling in Depleted Reservoirs - Brent Field

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Background

• Production started from 1976; accompanied by initial depletion and then pressure maintenance.

• In 1992 a £1.3 Billion depressuriation re-development was approved, which was implemented in 1998.

• Through-Tubing infill drilling wells were planned to access compartmentalised and isolated isolated oil contained in the listric structures on the eastern flank of the Brent reservoir.

• These low-cost in-fill wells started experience severe losses: The Infill drilling was about to be stopped.

 

Technical Challenge

• Pre-1998 – no major issues,

• 1998-1999 – major mud losses on several infill wells and £6.5 MM NPT on one well,

• Infill wells: high angle sidetrack from existing wells,

• April 1999: Review of losses resulted in the identification of lower Fracture Propagation Pressures (FPP) and Fracture Breakdown Pressures (FBP),

• Intra-reservoir shales present which probably had original reservoir pressures, and were subject to instability and overgauge hole.

 

Solution: ECD Management 

• The reduction of the fracture gradients accompanying the depletion was recognised as the major cause of the mud losses. This was one of the first fields in the North Sea to experience this. 

• An in-house calculator was built for drillers to optimise mud weights useable on any new wells, based on the anticipated depletion level (dPp = 500 psi/yr),

• Drill-string was re-designed to minimum ECDs ,

• Integrated hole-cleaning models used to minimise the bottom hole pressures,

• Underbalanced drilling limits for the intra-reservoir shale established,

• Subsequent continual improvement programme adopted which included graphite mud additives and & well re-design.