Multi-physics interactions in induced seismicity associated with hydraulic stimulation of unconventional geoenergy reservoirs

This study explores the complex interplay of multi-physics processes contributing to injection-induced seismicity, particularly in the context of hydraulic stimulation of deep geothermal and tight hydrocarbon reservoirs. Despite extensive empirical data collected from numerous deep geothermal sites since 1970s, and from over 250,000 hydraulically fractured wells drilled in shale reservoirs since the mid-2000s, a critical review comparing and evaluating the seismicity triggering mechanisms in these two typical unconventional geoenergy reservoirs is lacking. One aim of our study is to optimize the knowledge transfer about hydraulic stimulation between the disciplines of deep geothermal and shale reservoir studies. The use of microseismic data for reservoir characterization, and strategies for managing large-magnitude injection-induced earthquakes are highlighted. Furthermore, previous research has often prioritized single - or dominant-mechanism explanations without conducting comprehensive sensitivity analyses. Our study aims to bridge this gap by revisiting and expanding the fundamental physics for analyzing hydraulic stimulation and induced seismicity. Key contributing processes to induced seismicity in deep geothermal and tight shale reservoirs can be broadly classified into two perturbation categories: (1) Pore pressure and rock stress changes by direct fluid transmission, and (2) Similar changes in pressure and stress caused by elastic strain transmission. The interaction of the transmitted injection fluid in (1) and induced stress changes in (2) involve two key mechanisms, which may act either separately or in tandem: (1) Fluid diffusing rapidly into the rock, causing both pore pressure rise and associated elastic stress changes, and (2) stress changes transmitted from a single hydraulic fracture, which stores all injected fluid (and no or little injection fluid enters the host rock on the time scale of the fracturing treatment). In either case, pore pressure changes and stress variations may lead to the propagation of micro-fractures in the host rocks, seismic/aseismic slip and seismic wave emissions. Robust theoretical models are reviewed and used to quantify the relative significance of fluid and elastic transmission-induced pore pressure and rock stress changes under different conditions. This study underscores the need for unbiased, multidisciplinary approaches in seismic hazard assessment and hydraulic fracturing optimization. Our study may provide useful support for subsurface energy extraction, unconventional geo-reservoir management, and induced seismic risk mitigation, by highlighting improved predictive models as required for conducting engineering operations in a safer fashion.