Fracture caging in a lab fault to prevent seismic rupture during fluid injection

Abstract

Seismic risk associated with deep fluid injection is a major holdback for geothermal energy development. Currently adopted mitigation measures include traffic light protocol and cyclic stimulation methods that retroactively limit injection pressures and flowrates based on observed seismic activity to hopefully reduce the likelihood of triggering large seismic events. Fracture caging presents an alternative proactive approach to seismicity mitigation that does not require flow rate or injection pressure limits. The caging concept is to pre-drill boundary wells around injection wells to contain all injected fluid. Prior experimental and numerical work demonstrated successful caging of tensile hydraulic fractures but did not investigate caging in shear faults. To fill this knowledge gap, this study focuses on caging of injection-induced shear fractures in a critically stressed lab-scale shear fault. Experiment variables include mechanical versus hydraulic shearing and cage size by varied well spacing. Acoustic activity was monitored using two calibrated acoustic emission systems, each having a different sensitivity bandwidth. This constitutes what we call Caged Geothermal Systems (CGS) as a modified version of Enhanced Geothermal Systems (EGS), but with CGS using more wells, an accelerated drilling timetable, much higher flow rate limits, and less proppant. We demonstrate successful caging in a lab-scale shear fault with a high recovery of the injected fluid and prevention of large critical rupture events.