Abstract
The destruction of the rock that surrounds boreholes under thermo-hydro-mechanical coupling is an important factor for borehole stability in hot dry rock (HDR) geothermal energy extraction. Failure experiments for granite under triaxial stress ( σ 1 > σ 2 > σ 3 ) were conducted as 500°C superheated steam was transported through the borehole. High-temperature steam leads to large thermal cracks in the surrounding rock, which are randomly distributed around the borehole and gradually expand outwards. The randomly distributed thermally induced microcracks increase the complexity of the initial fracture morphology around the borehole and contribute to the appearance of multiple branch fractures. Fracture development is negligibly affected by ground stresses during the initial stages. However, fractures are deflected towards the maximum horizontal principal stress under ground stresses during later periods. During fracture propagation, high-temperature steam more easily penetrates the rock because its viscosity is lower than water. Towards the end of the crack expansion, the steam loses heat and liquefies, which increases the elongation resistance, and results in the arrest and intermittent expansion of the cracks.
Highlights
The geothermal energy of hot dry rock (HDR) is an undeveloped, safe, and renewable energy that is abundant all around the world [1, 2]
The destruction and stability of the granite that surrounds boreholes under thermo-hydro-mechanical coupling are essential factors that restrict the efficient extraction of HDR geothermal energy
The following conclusions are drawn from the experiments: (1) The effect of thermal stresses causes thermal cracking to be randomly distributed around the borehole, which gradually expands outwards with the borehole as the center
Summary
The geothermal energy of hot dry rock (HDR) is an undeveloped, safe, and renewable energy that is abundant all around the world [1, 2]. Most enhanced geothermal system (EGS) wells are vertical and located in a normal or strike-slip faulting regime. Under these conditions, hydraulic fractures would be expected to form axially along the wellbore. Geofluids wellbore observations from EGS projects indicate that flows have typically been localized at discrete zones along the wellbore and correlate with the locations of preexisting fractures This may be caused by mechanical instabilities in the rock matrix during stimulation [13]. The destruction and stability of the granite that surrounds boreholes under thermo-hydro-mechanical coupling are essential factors that restrict the efficient extraction of HDR geothermal energy
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