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Analyses
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BOREHOLE
BOREHOLE can be used to evaluate the stability of wellbores and of perforations extending from these.
It is based on the interactions of the primary in situ loadings, and those around the wellbores/perforations,
with the rock's deformation behaviour and strength. As the input parameters can easily be varied BOREHOLE can
be used not only for individual analyses but also for optimization of drilling parameters, or for back-analyses
of unstable conditions to ascertain the controlling parameter values.
Also with BOREHOLE, the effects on the in situ stability in the primary stress zone of changes of pore pressure
and/or of specific pressure conditions in weak zones (bedding planes, fissures, cracks etc.) can be evaluated.
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Input data
The input data include drilling parameters and their variations as well as information on the in situ conditions.
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Drilling parameters
For the analysis of the stability of wellbores/perforations information is required on the depth,
the alignment (inclination and azimuth) and the internal pressure. If the wall is permeable then the
possible depth of penetration or radius of fluid drawdown should be considered. It can also be
distinguished between uncased (free deformation) and cased (limited deformation) conditions.
Significant temperature differences in the zone around a wellbore can also be considered.
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In situ conditions
The input parameters defining the in situ conditions are the
3D effective stresses, or the 3D total stresses, the pore
pressure and the 3D pore pressure effectivenesses (see Stresses
). In addition, the deformation properties and the strength
parameters of the rock are prerequisites for the stability
analyses. These can be derived from RACOS®
analyses and special laboratory
measurements
. If thermal loadings are to be considered then the expansion coefficient of the rock is also required.
In the case of disturbed rock the 3D orientations and specific strengths of up to three different weak zones
(e.g. bedding planes, fissures, cracks etc.) can be input.
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Procedure
In BOREHOLE the 3D stability analyses are made using the numerical finite element approach to
calculating the stress-strain behaviour coupled with appropriate analytical solutions. The results of
this approach are identical with those obtained with a 3D mesh of elements, but with considerably
reduced computing effort (especially with parameter variations). This enables quick trend and optimization
analyses to be made.
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Results
For each element in the mesh the calculated stress condition is compared with the relevant rock strength.
This may be the peak strength, but residual strength or creep limits can also be considered. In an element
the relative distance of the 3D stress condition from the strength criterion is defined as the safety factor.
If this is ≤ 1 in a rock element then instability is indicated. Taking account of the individual element
sizes quantitative assessments of the intensity of failure in particular zones
around a wellbore/perforation can be made. With changes of the drilling
parameters it can quickly be determined which combination of these results in the lowest risk.

To enable the radial and tangential extent of the risks to be evaluated in each case the safety factor ranges
are plotted in colour for each element. This may be just for the rock or just for the weak zones, or in
combination showing the most critical values.
For some practical purposes the type of failure (e.g. breakout, frac etc.) can be of interest. In this case
the radial and tangential stresses, the normal stress along the wellbore and possible shear stress components
at the wellbore wall are analysed. The normal and shear stresses for the weak zones can also be considered.
This enables both drilling parameters and specific in situ structures to be identified as the origin of the
risk of instability.
For further information on BOREHOLE see:
Braun, R.; Tauber, F. & Stromeyer, D.: Methodik zur Einschätzung
der Bohrlochstandsicherheit. Erdöl Erdgas Kohle, Vol
108, No.9, S. 345-347, 1992
Braun, R.: A Commonly Neglected Factor in Rock Mass and Borehole Stability.
OIL GAS European Magazine, 2/2007, pp. OG79 - OG82
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