Besides in situ stress field and rock strength there are other parameters which greatly affect borehole breakouts, particularly those which lead to local changes of the mechanical properties of the rock and alterations in the stress and strain conditions around the hole. Especially pre-peak nonlinearity, intensity of the drilling method and axial stress were the factors investigated through breakout experiments on thickwalled cylinders in a newly developed truly triaxial cell. The influence of nonlinearity on breakout initiation and geometry is more pronounced than that of the drilling technique.


A côte de contraintes in situ et la resistance des roches il y a autres paramètres qui determinent la formation des ruptures en parois. Ce sont specialement les quels qui conduisent à changements locals des proprietes mecaniques des roches et par consequence aussi à l'etat des contraintes et des deformations du parois. L'influence de non-linearite des deformations, de methode de forage et de contrainte axial sur la formation des rupture en parois ont ete investigues en un nouvel appareil vraiment triaxial à haute pression. L'influence de non-linearite sur Ie commençement et la geometrie de la rupture en parois est plus forte que les resultats causes par la technique de forage.


Neben in situ Spannungsfeld und Gesteinsfestigkeit bestimmen besonders auch solche Parameter die Bildung von Bohrlochausbruechen, die zu örtlichen Veranderungen der mechanischen Gesteinseigenschaften und damit zu Spannungs- und Verformungsumlagerungen fuehren. Experimentell Wurden in einer neuen echt dreiaxialen Druckzelle die Einfluesse von Verforrnungsnichtlinearitat, Bohrtechnik und Axialspannung auf die Ausbildung von Bohrlochausbrtuehen untersucht. Der Einfluß der Nichtlinearitat auf Ausbruchbeginn und -geometrie ist starker als der der Wandauflockerung durch Bohren.


Borehole breakouts are frequently observed in holes at large depth in zones of high in situ stresses and stress differences. Their orientation is always in line with that of the least in situ horizontal stress. Consequently, they can be useful as stress indicators; on the other hand, however, they can seriously affect the drilling progress or even stall it completely due to extreme deflecting or jamming of the drill bit and string in the damaged or collapsing hole section. The problem recently attracted much attention and considerable research efforts have been directed towards a better understanding of the effective failure mechanisms, an assessment of the relevant and critical rock properties, and the development of methods and models for the prediction, stress-related interpretation and possible prevention of borehole breakouts. The type of breakout process, either tensile spalling or shear fracturing, depends on the material properties; frequently, however, an apparent combination of both types can be observed with the latter more as a secondary effect. The mostly triangularly shaped breakouts occur where and when the high local compressive stress, respectively strain concentrations in the immediate vicinity of the wall exceed the critical value of the rock material. Factors affecting breakout initiation, depth, and width can therefore be reduced to those which either influence the stress or strain distribution around the hole, or to those which change the mechanical properties of the rock within the immediate vicinity of the hole boundary. A satisfactory agreement between the results of numerical simulations of the breakout process and observations in the field or the laboratory can only be expected, when the most relevant of these factors are included in the material model.

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