Changes

==Hole instability==

==Hole instability==

Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are shales and salt beds.

Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are [[shale]]s and [[Evaporite|salt beds]].

Instability can result from the following phenomena:

Instability can result from the following phenomena:

* Water absorption, swelling, or dispersion

* Water absorption, swelling, or dispersion

Instability occurs when the relief of overburden pressure exceeds the yield strength of the drilled formation, resulting in flow of the formation (plastic flow). Abnormally high pore pressures can cause blowouts when present in highly [[permeability]] formations. If the pressure differential between the wall of the hole and the fluid in the hole is large, the formation may slough off. Structural stresses can also cause hole instability.

Instability occurs when the relief of overburden pressure exceeds the [http://dictionary.reference.com/browse/yield+strength yield strength] of the drilled formation, resulting in flow of the formation ([http://dictionary.reference.com/browse/plastic+flow plastic flow]). Abnormally high pore pressures can cause [[Wikipedia:Blowout (well drilling)|blowouts]] when present in highly [[permeability|permeable]] formations. If the pressure differential between the wall of the hole and the fluid in the hole is large, the formation may slough off. Structural stresses can also cause hole instability.

Problems associated with hole instability include the following:

Problems associated with hole instability include the following: