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Anisotropy of ‘Shales’: Implications for Geomechanical Investigations

Having a solid understanding of the mechanical properties of rock and rock masses is becoming increasingly important in the search for and production of hydrocarbons. One general assumption used in many geomechanical investigations is that the rock is isotropic, i.e., its physical properties do not vary with direction. It is generally recognized, however, that this is not reality, and that many rocks are anisotropic. One problem is that there are actually very few measurements of the anisotropy of rocks. Here we describe some examples of such measurements from the laboratory and the field. Laboratory measurements of the directional elastic properties of a variety of ‘shales’ from the Western Canada Sedimentary Basin that are important targets for unconventional exploitation (e.g Nordegg, Duvernay) have recently been published. Strains and ultrasonic wave speeds are measured in strategic directions on prisms carefully machined from core and allow for comparison between static and dynamic moduli. A complete set of the two Young’s moduli and the three Poisson’s ratios necessary to describe such rocks is obtained. The implications of these observations to our ability to infer stress states from sonic logs, among other observations, is illustrated by a direct comparison. At the rock mass scale, borehole seismic observations of anisotropy through a fractured rock mass are modelled and strongly suggest in this case that the distribution of microcracks more strongly affects the rock anisotropy than macroscopic fractures visible in ultrasonic imaging logging.

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