Energy-based traffic light system (ETLS) to balance injection-induced seismicity and permeability enhancement: a laboratory scale modelling study

SUMMARY Hydraulic shear stimulation is a method to enhance permeability and heat extraction efficiency of geothermal systems. However, such reservoir treatments can have the risk of injection-induced seismicity. To address this issue, a novel traffic light system is proposed based on the change of the seismic injection efficiency rate which is the ratio between seismic energy and hydraulic energy. In a first step, we numerically investigate the behaviour of a naturally rough, slowly slipping, velocity-strengthening fracture in a granite at laboratory scale. The model is formulated on an evolution law for fractures within a rate-and-state friction framework, with fracture aperture varying as a function of both slip displacement and slip velocity. We compare the effects of the proposed energy-based traffic light system (ETLS) injection protocol against those of modelled monotonic and cyclic injection, focusing particularly on aperture evolution and slip velocity. We show how implementing the ETLS criteria can reduce slip velocity by 30 per cent compared to monotonic injection, while it was increased by 51 per cent for cyclic injection. Even with lower slip velocity, the ETLS injection scheme sustains a similar aperture per injected volume as the monotonic scheme once larger volumes are reached. Overall, our simulations suggest that an ETLS approach could provide a safer hydraulic shear stimulation strategy for enhanced geothermal systems by minimizing slip velocity while maximizing permeability, compared to monotonic or cyclic injection.