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Many reservoir rocks of productive geothermal energy resources display low porosity and matrix permeability. Therefore, to enhance fluid flow, fault zones and natural fracture networks are increasingly targeted for geothermal energy exploitation that are hydraulically connected to geothermal wells by stimulating the reservoir units. To this end, fluid is injected into the reservoir, which is generally believed to reduce effective stress and induce minor slip along stressed faults. Fluid injection can also lead to induced microseismicity and remotely-triggered earthquakes at great distances from the target reservoirs. The Blue Mountain geothermal field produces the largest seismic activity during maintenance shutdowns of injection and production requiring additional mechanisms such as poroelastic stress effects. In order to improve seismic hazard assessment and the understanding of induced seismicity around injection wells, we explore the coupling between matrix permeability, fault zone hydrology and mechanical behavior.One main goal of this work is to provide insight into the scale-dependence of permeability by comparing laboratory results with previous permeability measurements using tidal responses in three different idle wells in Blue Mountain. We present a series of laboratory experiments performed on rock samples collected from the DB2 well at the Blue Mountain geothermal site in Humboldt County, Nevada, USA. The geothermal field benefits from the intersection of two W- and NW-directed normal faults resulting in high permeability of the geothermal reservoir production zone controlled by a brittle damage zone. Samples were obtained from two different lithologies, both of Triassic age, that constitute the reservoir. The first set of samples are low-porosity, (0.4 vol%) quartz-dominated (~50 – 60 wt%) phyllites, which exhibit zones with pronounced fracturing and elevated porosity (3.8 vol%). The second set of samples are felsic intrusive rocks with moderate to high porosity (7 – 15 vol%) due to strong hydrothermal alteration and clay mineral formation. Samples from both lithologies were selected from different sections of the damage zone showing varying degrees of faulting, from intact to highly brecciated, containing mineralized veins. We determine flow and poroelastic properties of cylindrical samples with a length of 2 cm and a diameter of 5 cm, subjected to stepwise cyclic variation of pore ( |
