Borehole instability related problems due to massive formation caving in micro-fractured shale resulted in substantial non-productive times (NPT) and accounted for as much as 25% of the total programmed drilling time.
Geomechanics and image log analyses clearly showed the natural/micro fractures were responsible for large cavings. Drilling optimization strategies to mitigate formation caving were focused on:
Reducing mud weight to decrease fluid/pore pressure penetration in fracture planes
Use of mud additives to seal-off natural fractures
Drilling with downhole motor and stabilizers to minimize lateral vibrations.
Average drilling time prior to implementing these strategies was 14.75 days. The drilling time improved to 8.18 days after the implementation, an improvement of 44.5% with no occurrence of major instability events.
This paper will present a case study from an oilfield in Southern Argentina demonstrating how the assimilation of geomechanics modeling and drilling optimization practices is critical to improving the drilling curves.