Chris Henderson (Directional Driller)
Wellbore Stability may be defined as the prevention of brittle failure or plastic deformation of the rock surrounding the wellbore due to mechanical stress or a chemical imbalance therein.
Before we start drilling, the mechanical stresses in the formation are less than the strength of the rock. The chemical action is also balanced, or occurring at a rate relative to geologic time (millions of years). Rocks under this balanced or near-balanced state can be considered as stable.
After drilling, the rock surrounding the wellbore undergoes changes in tension, compression, and shear loads as the rock forming the volume of the hole is removed. Chemical reactions also occur with exposure over time to the drilling fluid.
Under these conditions, the rock surrounding the wellbore can become unstable, begin to deform, fracture, and cave into the wellbore or dissolve into the drilling fluid.
Excessive rock stress can further collapse the hole resulting in a stuck pipe situation.
Hole squeezing from mobile formations (ie – Halites) can produce tight hole problems and stuck pipe.
Cavings from failing and caving formation makes hole cleaning more difficult and increases mud and cementing costs accordingly.
Understanding the conditions that cause hole stability problems can produce:
- More effective planning.
- Earlier and easier detection of warning signs.
- Contingency plans to avoid the progression of the problem.