Long-term evolution of the transport proprieties of a fracture from the Coso Geothermal reservoir

Results are reported for a long-term circulation test on a calcite-filled fracture in diorite from the Coso Geothermal Field, California. Deionised water is circulated thought the fracture at 20°, then 60°, and then 90°C under a near constant effective stress of 13 MPa. Through the initial stages of the test, at 20°C, the fracture aperture drops from an initial mean hydraulic aperture of 30 μm to 0.6 μm in the first 100 hours, before reaching a steady magnitude. This corresponds to a net reduction in permeability of 7∼ 4 orders of magnitude through the initial duration of the experiment, and under constant stress. As temperature is increased, the average aperture further reduces, and a periodic change in hydraulic impedance is recorded under conditions of constant stress, temperature and pressure-controlled flow rate. The peak cyclic flow rate climbs rapidly to about 20 times the steady magnitude, within a period of 6000 minutes, and is interpreted as periodic clogging and flushing of mineral mass from the constricted and brecciated end of the sample. As the temperature is increased to 90°C the hydraulic impedance continues to decrease, ultimately reaching a final aperture of 1 μm and flow rate of 0.03 cc/min. This low magnitude of ultimate permeability persists, despite visible open voids within the calcite vein, implicating that much of this porosity is not well connected. Since effective stresses remain constant, this observed response is strongly conditioned by the evolving aqueous chemistry of the sample. Analysis of the data continues.