Stability Rock Mechanics Consultancy Dr. Roland Braun

Stability

The stability of a rock mass, or of underground openings within it, is guaranteed as long as the loadings do not exceed the rock strength. To evaluate the stability the interactions of these loadings with the load-dependent behaviour of the rock and its strength must be considered. The 3D software package BOREHOLE uses numerical analysis for uniform or heterogeneous (fissures, strata boundaries etc.) rock zones, and can take account of the different drilling options and conditions encountered in oil and gas exploration and development.

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Stability under primary loading

In situ stability under primary loading is important in two economically significant contexts.

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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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Stability under secondary loading

Secondary stresses are developed around underground openings and depend on the opening shape and on the in situ primary stresses. Using BOREHOLE it is possible to analyse many different wellbore conditions.

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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.

Instability in the zone of secondary loading can threaten the safety of a wellbore and may also result in solids being transported into it by fluids

For further information on wellbore stability see the following publications:

Braun, R.; Tauber, F.; Stromeyer, D.: Methodik zur Einschätzung der Bohrlochstandsicherheit. Erdöl Erdgas Kohle,
Vol 108, No.9, S. 345-347, 1992

Braun, R.: Optimizing mud pressure for horizontal wells. OIL GAS European Magazine, Vol 21, No.4, pp 26-28, 1995
Braun, R.: Spülungsdruckoptimierung für Horizontalbohrungen. Erdöl Erdgas Kohle, Vol 111, No.10, S. 406-408, 1995
Braun, R.: Analysis of stability and sand production in deviated wells. OIL GAS European Magazine,
Vol 23, No.2, pp 8-9, 1997
Braun, R.: Analyse der Standsicherheit und Sandproduktion in abgelenkten Bohrungen. Erdöl Erdgas Kohle,
Vol 113, No.4, S. 176, 1997
Braun, R.: A Commonly Neglected Factor in Rock Mass and Borehole Stability. OIL GAS European Magazine, 2/2007, pp. OG79 - OG82

Braun, R.: Consideration of 3D Rock Data for Improved Analysis of Stability and Sanding. OIL GAS European Magazine, 2/2008, pp. OG64 - OG68

Braun, R.: Reservoir Pressure and Mechanical Integrity of the Overburden. Erdöl Erdgas Kohle, Vol. 125, No.11, S. 427-432, 2009

Braun, R.: Critical Borehole Orientations - Rock Mechanics Aspects. OIL GAS European Magazine, II/2010, pp. OG75 - OG79