differential sticking

Diagram of differential sticking
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1. n. [Drilling]
A condition whereby the drillstring cannot be moved (rotated or reciprocated) along the axis of the wellbore. Differential sticking typically occurs when high-contact forces caused by low reservoir pressures, high wellbore pressures, or both, are exerted over a sufficiently large area of the drillstring. Differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost. It is important to note that the sticking force is a product of the differential pressure between the wellbore and the reservoir and the area that the differential pressure is acting upon. This means that a relatively low differential pressure (delta p) applied over a large working area can be just as effective in sticking the pipe as can a high differential pressure applied over a small area.
2. n. [Drilling Fluids]
A situation in which the drilling assembly (pipe, drill collars and bottomhole assembly) is stuck in filter cake that was previously deposited on a permeable zone. The pipe is held in the cake by a difference in pressures between the hydrostatic pressure of the mud and the pore pressure in the permeable zone. The force required to pull the pipe free can exceed the strength of the pipe. Methods used to get the pipe free, in addition to pulling and torquing the pipe, include: (1) lowering hydrostatic pressure in the wellbore, (2) placing a spotting fluid next to the stuck zone and (3) applying shock force just above the stuck point by mechanical jarring, or (4) all the above. The most common approach, however, to getting free is to place a spot of oil, oil-base mud, or special spotting fluid.