ABSTRACT: Hydraulic stimulation on the geothermal reservoir is the well-known operation for improving the transmissivity and fracture connectivity within the reservoir. In this operation, by increasing pore pressure, shear slip on pre-existing fractures are triggered or new fractures generate from the tip of pre-existing fractures. Due to these mechanisms, mechanical, hydraulic, and seismic properties of fracture/fracture network evolve during the stimulation, but it is unclear how these properties evolve and link in each other. We conduct the laboratory experiments to concurrently monitor the strength, permeability, and acoustic emissions during the hydraulic shearing of rough-walled fracture. Through the experiments, we find the shear slip is limited to between 0.023% and 0.33% of the representative length of pressurized zone, and the fracture permeability increases to from 4 to 12 times of the initial permeability (before slip). Interestingly, more than 50% of the permeability enhancement is achieved during the aseismic motion, which is commonly precedes the seismic/fast slip with acoustic emissions. These findings are well consistent with the mesoscale experiment at URL and will be useful in the actual operation of pressurization for geothermal reservoirs.
Hydraulic stimulation on the geothermal reservoir is the well-known operation for improving or maintaining the transmissivity and fracture connectivity within the reservoir (Evans et al., 2005; Haring et al., 2008). In this operation, by injecting pressurized water into the reservoirs, pre-existing fractures are reactivated in shearing mode with the opportunity for self-propping on asperities (Esaki et al., 1999). Due to these mechanisms, mechanical and hydraulic properties of fracture/fracture network evolve. During the shear slip on fractures, seismicity is possibly increased (Ellsworth, 2013; Majer et al., 2007), and it is necessary to be explored how the mechanical-hydraulic-seismic properties evolve and link in each other during the hydraulic shearing.
One of the most significant advances for understanding the linkages between mechanical, hydraulic, and seismic properties is achieved by Guglielmi et al. (2015a) through the in-situ reactivation experiment of mesoscale fault, which cuts thorough a carbonate formation. They concurrently monitor the mechanical, hydraulic, and seismic properties of the fault during the fluid injection and reveal that slow/aseismic slip primarily and initially occurs and then triggers the micro-earthquakes. Similar characteristics are also observed in in-situ reactivation experiment of shale fault (Guglielmi et al., 2015b). Thus, such a finding is possibly useful in assessing the seismic hazard associated with the pressurized water injection into the deep geothermal reservoir.