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Flow channels

These are generally approximately 2D features (failure planes) in particular orientations and result from loading in tension or shear. Such flow channels are often formed deliberately in order to improve inflow conditions (hydraulic frac). In contrast, such failures must be avoided if a section of the rock is to act as a seal.

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Failure zones

Large zones of rock can fail under shear loading or collapse under high all-round compressive loading. The possible effects include migration of particles into wellbores during production of fluids (sanding), emission of strong seismic waves, and subsidence.

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Uncased wellbores and perforations

Their stability depends not only on the relevant in situ loadings, deformation behaviour and strengths, but is also influenced by technical factors.

These include the drilling direction (inclination and azimuth), the mud pressure, the rate of infiltration into the rock and the temperature difference between the rock and the mud. Operations can be optimized using analyses of the effects of the technical options on the level of stability.

Perforations can be evaluated in the same way as uncased wellbores, except that close to the wellbore the secondary stresses from the cased or uncased wellbore itself are the relevant ones.

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Cased wellbores

The presence of a casing suppresses, at least partially, any radial and tangential deformations of the wellbore wall resulting from changes of loading. This modifies the secondary loading controlling the stability in the zone close to the wellbore.