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A North Sea operator was working on a large gas field with difficult drilling conditions and a high cost per well. Very rapid pressure increases over a short depth interval were a major challenge in drilling the 12-¼-in. bit sections. The operator’s objective was to drill these as deep as possible before setting a liner, yet ensure there was still a safe standoff from reservoir pressures of up to 13,500 psi.

The drilling target for the bottom of each 12-¼-in. section was in the Tuxen Limestone Member, a short distance above the Top Reservoir at a depth of approximately 14,000 ft (4,267 m). This target would permit logging while drilling measurements and biostratigraphy to confirm the operator had entered the formation and reached the optimum casing point.

However, the range of uncertainty in predicted depths from surface seismic data and other sources showed considerable overlap in the estimated depths of the Tuxen Member and the Top Reservoir. Therefore, in order to reduce the risk and costs of achieving the drilling target, more accurate and reliable depth predictions were needed.

Because vertical seismic profiling (VSP) services have receivers in the well close to the formations they image, and they record data with shorter travel paths, they can produce better resolution and depth prediction accuracy than surface seismic.

A simulated “look ahead” survey was performed by Baker Hughes, a GE company (BHGE), using a subset of the shallower portion of an existing VSP survey from a nearby well. This confirmed the capability to provide considerably better depth estimates and survey plans were then tailored for each development well.

The operator chose the Walkalong vertical incident service (WAVI-VSP) from BHGE. The WAVI-VSP service provides high-quality seismic imaging with its high source-receiver density and uniform ray path coverage. The service offered significant benefits in comparison to standard vertical incidence or walk-above VSP methods. These included clearer and more detailed structural images with no increase in survey time.

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Challenges & Results
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Challenges

  • Formation pressure hazard
  • High individual well cost (>$200 million USD)
  • Hard-to-interpret surface seismic
  • Formation depth uncertainties
  • Excessive safety risk

Results

  • Pre-survey modeling verified borehole seismic would reduce risk
  • WAVI-VSP service provided superior data over standard VSP
  • Survey operation plan and operation were performed smoothly
  • Results were available 12 hours before drilling operations restarted
  • Subsequent drilling confirmed depth formation estimates with six times less error
  • All well liners set in the target formation with safe standoff from high reservoir pressure