Fluid Injection-Rate Controls on Seismic Moment
Authors: Roseboom M. , Eijsink A. , Elsworth D. , Yu J. , Marone C. , Rivière J. , Shokouhi P. , Wang J. 
Published: August 2024

ABSTRACT:
Fluid-induced shear reactivation of pre-stressed faults poses a project terminating risk during reservoir stimulation in geothermal, CO2 sequestration, wastewater disposal injection and hydraulic fracturing operations. Micro-earthquakes (MEQs) can be tolerated but fluid injection is capable of triggering large earthquakes that can cause major damage to surface infrastructure. Understanding the linkage between fluid-injection volume or injection rate and seismic moment is essential in safeguarding such infrastructure. We report shear experiments that specifically explore the link between fluid-injection rate and seismic moment resulting from shear reactivation of laboratory faults. Experiments are performed on granitoid cores from the UtahFORGE EGS demonstration site containing a single inclined fracture and with small-scale roughness added to the fracture surface. We inject fluids into the fracture and measure the resulting slip and drop in frictional shear stress. We record reactivation and pressure distributions over a range of fluid injection rates. Along-fault pressure distributions are progressively less-uniform as injection rates increase representing a switch from steady-state to transient conditions. Triggered shear displacement is used as a proxy for seismic moment and is indexed against cumulative injection volume and rate. Preliminary results suggest that seismic moment and injection volume increase with increased fluid-injection rates.

1. INTRODUCTION
Anthropogenic induced seismicity typically stems from industrial operations in energy developments such as engineered geothermal systems and shale gas recovery as well as in carbon sequestration and wastewater disposal. Fluid injection associated with these activities is known to alter the state of stress in subsurface formations due to the increase in pore pressure and reduction in fault shear strength, sometimes inducing earthquakes in critically-stressed faults (Nicholson and Wesson, 1990; McGarr et al., 2002; Evans et al., 2012; Ellsworth, 2013). In particular, triggered and induced seismic events have terminated geothermal projects in Basel, Switzerland in 2006 and Pohang, South Korea in 2017 (Giardini, 2009; Kim et al., 2018; Kwiatek, 2019; Ji and Wu, 2020). Additionally, fluid injection through wastewater disposal has triggered events, including that in Prague, Oklahoma in 2011, resulting in an M=5.7 earthquake (Sumy et al., 2014). It remains a challenge to define the seismic moment of potential seismic events triggered by fluid injection, as much of our understanding derives from analyzing unintended earthquakes and evaluating the mechanisms that control shear reactivation and fault slip (Keranen et al., 2014; Walsh and Zoback, 2015; Bao, 2016; Ji and Wu, 2020). Generally, induced seismicity may result from: 1) increase in pore pressure where the effective normal stress is reduced, therefore, destabilizing pre-existing faults in shear, or 2) regional tectonic stresses that are perturbed by poroelastic effects which increase shear stresses and trigger failure (Ellsworth, 2013; Bao, 2016; Guglielmi et al., 2016).