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The influence of effective stress history on reservoir stability and the response to hydraulic fracturing are simulated for two hypothetical shale reservoirs. It is numerically demonstrated that the effective stress history influences the present-day stability of faults and natural fractures. Any assumption of a homogeneous stress state is shown to be unrealistic in the majority of fractured shale reservoirs. The simulations demonstrate that, depending upon the effective stress history, shear displacements can be induced by stress changes associated with stimulation without a change of fluid pressure in the destabilized fractures. Elastoplastic shale and other fractured reservoirs are shown to have a memory of past geomechanical states. These numerical findings demonstrate the value of interpreting reservoir effective stress history when planning fluid injection. A previous case history is summarized to illustrate the overwhelming dependence of stress history analyses on a wide spectrum of earth science disciplines. It also illustrates the application of stress history analyses to activities other than hydraulic fracturing. The current inexperience of effective stress history description means that new approaches must be identified to optimize the necessary multidisciplinary investigations.

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Distinct element modelling of naturally fractured reservoirs is used to investigate changes of hydraulic conductivity induced by fluid injection. General conclusions are drawn relating to the effects of uniform changes of pore pressure on conductivity anisotropy in connected joint systems. The influence of cross-cutting discontinuous joints on the conductivity of the joints against which they terminate is emphasized. Results from a two dimensional model are related to the observed performance of a granitic geothermal reservoir. On emploie le modèle à éléments distincts des réservoirs fracturés de façon naturelle pour étudier les changements dans la conductivité induits par l'injection de fluide. Des conclusions générales sont tirées concernant les effets de changements uniformes de pression interstitielle dans des systèmes de joints reliés sur l'anisotropie de conductivité. On accentue l'influence exercée sur la conductivité des joints par des joints discontinus transversaux. Des résultats obtenus à partir d'un modèle bidimensionnel sont comparés avec lea performances observées d'un réservoir géotbermique granitique.