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Analyses

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

   
 

Stresses
   
 

Deformations
   
 

Stability
   

Analysis tools
   
 

RACOS®
   
 

BOREHOLE

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