| Abstract |
Fractures are significantly important for the utilization of low-permeable geothermal reservoirs. The long-term sustainability of fracture aperture determines the lifespan and economic efficiency of a reservoir. In an aqueous fluid, chemical reactions between fluid and minerals are time-dependent under P and T conditions, yielding an irreversible impact on the fracture’s evolution. Pressure solution at the propping asperities results in a decrease of aperture, while mineral dissolution on the void walls enlarges the fracture. To investigate the effects of this coupled process, a long-term intermittent flow-through experiment was conducted with distilled water under a pore pressure of 1 MPa and confining pressure of 10 and 30 MPa at a temperature of 140 and 32 °C. A monomineralic sedimentary rock (Fontainebleau sandstone) with a composition of more than 99.5% quartz and an extremely low permeable matrix was used. A tensile fracture along the central long axis of the sample core was artificially produced and the two halves were assembled with a pre-offset of 0.75 mm. Permeability was intermittently measured after the flow was stopped for time intervals of 8 and 16 days, respectively. Before each permeability measurement, the effluent was continuously sampled from the outlet capillary with a sub-sample volume of 0.5 ml and a flowrate of 0.5 ml/min. The peak silica concentration of the subsamples represented the fluid’s composition within the fracture and indicated an ion diffusion process from the fracture towards the capillaries. Pre- and post-experiment µCT scans of the sample at atmospheric pressure illustrated the overall aperture and contact ratio changes before and after the experiment. The theoretically dissolved amount of quartz on the free fracture walls as accounted for by PHREEQC (1.73 mmol/l at 140 °C) was significantly smaller than the measured total silica concentration of the effluent (8~9 mmol/l), revealing that pressure solution mainly contributed to the dissolution process. However, it showed that confining pressure and the respective time intervals of stagnant flow had a negligible effect on the final peak effluent concentration, inferring that a chemical equilibrium has been reached rapidly at constant temperature consequently impeding pressure solution from proceeding. In addition, permeability showed to remain approximately constant throughout the 137 days of the experiment. In summary, it is concluded that the silica concentration of the pore fluid increased predominantly due to pressure solution at fracture asperities. The dissolution and diffusion of quartz (silica) within the water film between the stressed contacting asperities may be restricted by the ion concentration of the surrounding fluid, alleviating pressure solution creep. In contrast, continuous flow-through would disturb the chemical equilibrium within the pore fluid accelerating pressure-induced mineral dissolution. |