Effect of Fluid Viscosity on Earthquake Nucleation

Abstract

Injection of fluids in geo-reservoirs can reduce the effective stresses at depth, lubricating the nearby faults, promoting slip and, potentially, earthquakes. High-viscous fluids are often used during hydraulic fracturing and production phases in geo-reservoirs. Here, we performed dedicated experiments to study the influence of fluid viscosity on earthquake nucleation. We performed frictional sliding experiments at 30 and 50 effective normal stresses and fluids viscosity ranging from 1 to 1,226 mPa s and modeled them with a rate-and-state friction law. In the presence of fluid, the state variable is defined as the ability of the fluid to flow. Our results showed that static friction slightly decreases with increasing viscosity, the dynamic friction is governed by the dimensionless Sommerfeld number (S = 6ηVL/(σ' n H 2)). Moreover, we observed that the (a − b) parameters of the rate-and-state friction law decrease with increasing viscosity down to (a − b) < 0, possibly promoting unstable slip and earthquake nucleation. Plain Language Summary In the last 30 years, the exponential worldwide increase of human-induced seismicity has become an important issue in solid earth sciences. Most of the induced seismicity is due to engineering operations in deep geo-reservoirs for hydrocarbon production, CO 2 storage, wastewater disposal, and the exploitation of geothermal resources. While the reactivation of faults at the origin of this seismicity has been extensively studied, the influence of fluid properties including its viscosity has been overlooked, even if the viscosity of injected fluids spans from that of water to that of honey. In this study, we discuss the influence of fluid viscosity on the nucleation of earthquakes in fluid-permeated experimental faults and on induced earthquakes. Our experimental observations suggest that the viscosity of the fluid does not influence the fault strength. Instead, the viscosity of the fluid controls the behavior of the fault from stable to unstable sliding.

Influence of Viscous Lubricant on Nucleation and Propagation of Frictional Ruptures

“…Indeed, recent in situ experimental tests (Guglielmi et al., 2015) and numerical studies (Bhattacharya & Viesca, 2019) suggest that fluid overpressure initially promotes a slow slip. However, little is known about the influence of fluid viscosity on the nucleation length or nucleation process in general (Cornelio & Violay, 2020).…”

“…Another mitigation strategy can be related to the use of different fluid properties. Despite that, up to now, few studies focused on the influence of fluids, and their viscosity, on the reactivation and the propagation of the seismic rupture (Bayart et al., 2016; Cornelio et al., 2019; Cornelio & Violay, 2020). The influence of fluid viscosity on earthquake nucleation and propagation is moreover relevant knowing that the fluids naturally flowing into faults can present different viscosities, depending on the presence of gouge and its composition (Brodsky & Kanamori, 2001; Otsuki et al., 1999).…”

“…Depending on fault slip and the given characteristic lubrication lengths, the fault weakening can, in specific cases, be described by elastohydrodynamic lubrication. Recent experiments performed on rocks focused on understanding the influence of viscous fluids on faults mechanical behavior, spanning the three aforementioned lubrication conditions and showing that fluid viscosity influences fault stability (Cornelio et al., 2020) and weakening mechanism (Cornelio et al., 2019, 2020; Cornelio & Violay, 2020). In particular, they showed the dynamic friction to be strictly dependent on fluid viscosity and slip velocity, suggesting, under given conditions, elastohydrodynamics as a possible weakening mechanism (Cornelio et al., 2019).…”

The Permeability of Porous Volcanic Rock Through the Brittle‐Ductile Transition

“…Inset shows an image of the segmented 3D X‐ray CT volume used for the analysis. (f) Schematic of the sample assembly for the triaxial experiments (during the experiments, this sample assembly is within an oil‐filled pressure vessel, not shown; see also Cornelio & Violay, 2020 ).…”

“…Deformation experiments were performed using the Fluid Induced eaRthquake Simulator (FIRST; Cornelio & Violay, 2020 ; Noël et al., 2021 ) triaxial apparatus at the Laboratory of Experimental Rock Mechanics (Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland) (Figure 1f ). The permeability of cylindrical samples was measured during constant strain rate deformation under different effective pressures P e , assumed here to be P e = P c − P p , where P c is the confining pressure, and P p is the pore fluid pressure.…”