This study shows that the pore fluid pressure is an important parameter to characterize borehole instabilities in pelitic rock formations. Due to its dynamical fluctuations and its interactions with drilling mud the pore fluid pressure within the rock formation has a fundamental influence on the risk for unstable boreholes. The presented laboratory tests indicate, that an increasing pore fluid pressure progressively accelerates the softening process in mudstones as well as in claystones, shales and argillite formations. In such mudstone formations the maximum load capacity of about 20 - 40 MPa will be reduced to about 50%, if the pore pressure level increases due to drill mud pressure changes and at last reaches values near to the lithostatic pressure in this water-saturated formations. An additional effect of increasing pore fluid pressure is a stress fluctuation that will be induced in the microstructure, so that in some parts crack propagation (hydraulic fracturing) can be detected. Nevertheless, ductile behaviour and weakening are predominant in those rocks. Consequentely, crack growth result in borehole breakouts and the ductile deformation result in borehole convergency. Both types of borehole instabilities interact at increasing pore pressure conditions. Therefore, the risk of unstable drillholes mainly depends on the pore pressure level in the pelitic rock formations. Hydraulic fracturing as well as the struture-softening effects due to high pore pressure conditions of about 0.8 lithostatic pressure are also obtained in shales and argillites with stiffness of about 50 - 100 MPa. In this rocks the maximum load capacity and the Young`s modulus decreases to about 1/3, if pore pressure increases with 50 %. Accordingly, the deformation behaviour tends from brittle to ductile. These results represent an important reason to develop new drilling strategies in order to improve the stability of deep boreholes for example in oil exploration or in enhanced geothermal systems.
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