FAQ #7: Can the stimulated fractures grow in an uncontrolled way into undesirable places?
No. Friction, imposed by the weight of rock, will prevent the fractures from growing and extending in an uncontrolled way. Only where water pressure is increased by injection at the well will friction be reduced enough to cause fracture stimulation.
Starting at the well, a zone of increased water pressure will relieve the frictional forces on fractures and allow for small slip events, creating an EGS reservoir marked by a cloud of microseismicity (see FAQ #6). The increased water pressure zone and microseismic cloud will grow as the stimulation progresses until the EGS reservoir extends about 1500 feet from the injection well. Outside this zone of excess water pressure, the fractures will not slip. When the increased pressure is relieved at the end of the 30 day stimulation, the microseismicity will die off in the EGS reservoir.
Monitoring the locations of microseismicity with the seismic network will allow us to map where the fractures have been stimulated. If microseismicity occurs outside the volume approved for stimulation, we will take corrective measures. The upward growth limit is 6000 feet below the ground surface, which leaves more than 5000 feet of impermeable rock between the EGS reservoir and the local groundwater aquifer. You can read more about induced seismicity, the analysis of independent experts, and our mitigation plan here.