Poromechanical and flow properties of fractured crystalline rock
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abstract
Crystalline rocks are the most plentiful igneous rock in Earth's crust and have been prominent for their low permeability and stiff mechanical response. These conditions allow crystalline rocks to be employed as a seal or a reservoir for various geomechanical projects, such as: geological carbon storage (GCS), enhanced geothermal systems (EGS), radioactive waste disposal, and a recent conceptual project where CO2 works as a working fluid of EGS. The present work aims to introduce laboratory experiments to characterize the response of Charcoal granite under three different conditions: pristine and thermally treated at 300C and 600C. Drained (dry), unjacketed, and undrained test were performed to measure the poroelastic parameters of rock and steady state flow tests allowed for the calculation of rock's permeability. Specific attention was put to achieving the full saturation of granite specimens and providing enough time (in order of a few hours) for pore fluid pressure equilibration. It was found that thermally induced damage decreased both unjacketed (solid) and drained moduli of the rock and significantly increased its permeability. On top of that, most of the material parameters (but the unjacketed modulus) were found to be strongly stress-dependent, which is attributed to the presence of cracks in the rock. Crack porosity and crack density were reported to be increasing by 50% when thermal damaged was introduced. The porosity-permeability relationship for the damaged material was established using the measured poroelastic properties and porosity sensitivity exponent was found to be equal to 14, indicating a strong influence of even small changes in porosity on the permeability of damaged granite.
