In this paper, we propose a method to estimate in-situ rock stress magnitudes from laboratory measurements on core material. Once the principal stress directions have been established by some other technique, our method allows the identification of the minor and major effective horizontal stress, and it moreover allows the determination of their magnitudes.

The problem in determining the principal in-situ stress magnitudes on core material is that we are dealing with a polyaxial stress state and therefore we have to estimate several independent principal stress magnitudes in one experiment. This is achieved by using a stress memory effect in Acoustic Emission (analogous to the Kaiser effect) in combination with the damage surface concept (cf. the yield surface in plasticity). From the shape of this surface in stress space, as determined through special testing procedures, we are able to derive the magnitudes of the previously applied stresses and, in the case of core material, the in-situ stress magnitudes.

The method has been studied in laboratory experiments in a polyaxial apparatus on sandstone outcrop samples. In these experiments, we have first imprinted a representative in-situ stress state on a rock sample, and then we have performed reloading experiments to determine the shape of the damage surface. The experiments show that the method may allow us to determine the in-situ stress magnitudes with an accuracy of 10-15%.

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