MR019-0010
The birth of a hydraulic fracture: Evolution of strain, displacement, seismicity, and temperature as measured by a borehole monitoring network during fracture propagation
Wednesday, 16 December 2020
Poster
Chet Hopp1, Verónica Rodríguez Tribaldos2, Martin Schoenball3, Yves Guglielmi1, Jonathan Blair Ajo-Franklin4, Aditya Chakravarty5 and EGS Collab Team6, (1)Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA, United States, (2)Lawrence Berkeley National Laboratory, Earth and Environmental Sciences Area, Berkeley, CA, United States, (3)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (4)Rice University, Earth, Environmental and Planetary Sciences Department, Houston, TX, United States, (5)Texas A&M University, College Station, TX, United States, (6)EGS Collab Project, Department of Energy, Geothermal Technologies Office, Berkeley, CA, United States
Abstract:
A series of hydraulic fracture stimulation experiments were performed at a depth of 1.5 km in the crystalline rock of the Sanford Underground Research Facility; part of the EGS Collab experiment. The stimulations were monitored by a dense array of instruments installed in six monitoring boreholes. These 60-meter boreholes contained loops of single-mode and multi-mode fiber optic cable as well as arrays of three-component accelerometers and hydrophones that were grouted in place. In addition, the injection and production boreholes each contained a micrometer-precision vector displacement sensor (SIMFIP), which measured the relative three-dimensional displacement between two inflatable packers. The fiber optic cables were interrogated for strain rate (DAS), strain/temperature (DSS), and temperature (DTS) by three separate recording systems, while the accelerometer array detected both seismicity and long-period (> 1 s) response to fracture propagation.
Over four days and four separate injection tests at pressures up to 27 MPa, the monitoring network showed a fracture initiating (and reactivating during subsequent tests) through normal opening followed by shear, with permanent increases in the mechanical aperture of ~40 and ~250 microns observed at the production and injection SIMFIP, respectively, during one particular injection scenario. Seismicity occurred ahead of the pressurized zone and reached the production borehole, roughly ten meters away, during the second injection test. DTS shows that the fracture also intersected one monitoring borehole halfway between injection and production, where both DSS and DAS recorded a permanent displacement of ~100 microns (extension along the borehole) throughout the four days of tests. Poroelastic response to fracture propagation was also measured by DAS in each of the other five monitoring holes. Finally, the monitoring network also reveals complex short-term fracture behavior during each test, with periods of apparent fracture closing observed at the monitoring holes during injection, followed by long-term reopening following shut-in. This behavior may reflect stress concentration ahead of the pressurized extent of the fracture.
