Wellbore stability refers to the mechanical integrity of the borehole during and after drilling. When stresses around the wellbore exceed formation strength, the hole can collapse, fracture, or wash out—leading to stuck pipe, lost circulation, and significant non-productive time.
Failure Mechanisms
Three primary failure modes threaten wellbore stability:
- Shear failure (breakout): Occurs when mud weight is too low to support formation. Studies show breakouts remain manageable up to 90° width in vertical wells but only 30° in horizontal sections
- Tensile failure (fracturing): Occurs when mud weight exceeds fracture gradient, creating induced fractures and lost circulation
- Chemical instability: Reactive shales absorb water from drilling fluids, causing swelling and sloughing
Economic Impact
Wellbore stability issues represent major drilling costs:
- NPT contribution: 10% of all drilling non-productive time
- Per-well cost: Averaging $2 million in North Sea operations across 93 wells studied (SPE Norway 2020)
- Global losses: Estimated $6 billion annually from wellbore instability (MDPI Minerals, 2024)
Advanced Solutions
Autonomous drilling systems maintain ECD within stability windows through precise parameter control, navigating narrow margins that would challenge conventional operations.
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