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Geothermal Energy

Science – Society – Technology

Fig. 7 | Geothermal Energy

Fig. 7

From: Stress sensitivity of porosity and permeability under varying hydrostatic stress conditions for different carbonate rock types of the geothermal Malm reservoir in Southern Germany

Fig. 7

Preconditioning of rock samples prior to hydrostatic permeability and porosity tests. a Confining and axial pressure were cycled simultaneously four times between 5 and 30 MPa to reduce inelastic behavior and to close microcracks. The bulk volumetric strain curves show irreversible deformation and relaxation hysteresis during unloading. b Each loading/unloading cycle during preconditioning increases the linearity of the stress–strain curves, indicating increasing elasticity. c Evolution of bulk volumetric strain during hydrostatic stress as a function of Terzaghi effective stress. The gap between loading and unloading indicates creep deformation. A nonlinear relaxation with a hysteresis is clearly visible with the onset of the unloading phase and an irreversible deformation at the end of the test cycle

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