Research Background: There is evidence that areas considered geologically stable have now experienced increased rates of seismicity (Ellsworth, 2013), particularly in the US. It is believed that this is due to fluid injection activities used in modern energy production (McGarr et al., 2015).
Amongst the main subsurface injection activities are carbon capture and storage (CCS), and enhanced geothermal systems (EGS) exploitation. Carbon capture and storage is a method of capturing CO2 emissions of electricity generation and industrial processes and injecting them into a geological reservoir. The injection of high pressure CO2 into a geological formation increases pore pressure and reduces effective stress within the formation. This can increase the chance of any pre-existing faults present to undergo reactivation and failure, and potentially generate seismic activity (Cappa and Rutqvist, 2012).
An enhanced geothermal system is comprised of: 1) a natural heat source, 2) a geothermal reservoir (granite, carbonates, etc.), 3) a wells loop system made by injection/production wells for cool water recharge and hot water discharge, 4) a fault system of open and connected fractures to close the injection and production wells loop. Coupling mechanisms between the source of fluids and the re-activation of the exploited fault system may potentially lead to induced seismicity.
Central Research Problem: During EGS activities, moderate to large events took place (e.g. 2006 ML 3.4 Basel; 2017 Mw 5.5 Pohang), which were larger than the forecasted maximum magnitude. Clearly, each of these subsurface injection practices represents a considerable hazard and potential risk in terms of induced seismicity. Fluid-induced seismicity can be investigated, for example, by monitoring active faults in a reservoir from in-situ borehole observations and measurements (e.g. strain, strain rate, pore pressure, fluid chemistry). However, these techniques are indirect in nature, and they have a high cost and a low resolution. As a consequence, our forecasting and mitigation strategies remain severely limited by our lack of specific knowledge about how rupture nucleation and propagation process are affected by injected fluids at depth (McGarr et al., 2015; Ellsworth, 2013).
Main Aim of the Project: The main aim of the proposed research is the systematic investigation of the role played by in situ heterogeneities of stress conditions and fault properties (e.g. lithology, roughness, rheology, structure) in controlling rupture propagation parameters (e.g. dynamic friction, slip weakening distance, dissipated fracture energy, slip and rupture velocity). In particular, the critical conditions leading to the transition between self-arrested ruptures, which are contained within the exploited reservoir volume, and large run-away ruptures, able to grow well beyond the exploited reservoir, will be investigated.
The expected results will allow gaining a deeper physical insight into the propagation and arrest process of natural and fluid injection induced earthquake rupture. They can be used by seismologists, geophysicists and geodesists to inform existing conceptual models used during risk and hazard assessment procedures.