Effect of Fluid Viscosity on Earthquake Nucleation

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 = 6 eta VL/(sigma'H-n(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, CO2 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.

Fluid thermodynamics control fault heating and weakening under upper crustal stress conditions

Mateo Alejandro Acosta, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

Although fluids are pervasive among tectonic faults, thermo-hydro-mechanical couplings during earthquake slip remain unclear. In particular, fluid induced seismicity is today a subject of major interest for the geo-energy community. We report full dynamic records of stick-slip events, performed on saw cut Westerly Granite samples loaded under triaxial conditions (stresses σ1> σ2= σ3) at pressures representative of the upper continental crust (σ3~ 45-95 MPa) Three fluid pressure conditions were tested, dry, low, and high pressure (i.e. Pf=0, 1, and 25 MPa). Friction (μ) evolution recorded at 10 MHz sampling frequency showed that, for a single event, μ initially increased from its static pre-stress level, μ0 to a peak value μ p it then abruptly dropped to a minimum dynamic value μd before recovering to its residual value μr, where the fault reloaded elastically. Under dry and low fluid pressure conditions, dynamic friction (μd) was extremely low (~0.3 and ~0.2 respectively) and co-seismic slip (δ) was large (~250 and 200 μm respectively) due to Flash Heating (FH) and melting of asperities as supported by our microstructural analysis. Conversely, at high pressure (pf=25 MPa), μd was higher (~0.45), δ was smaller (~80 μm), and frictional melting was not observed on post-mortem surfaces. We calculated flash temperatures at asperity contacts including heat buffering by on–fault fluid. Considering the isobaric evolution of water’s thermodynamic properties with rising temperature showed that pressurized water controlled fault heating and weakening, through sharp variations of specific heat (cpw) and density (ρw) at water’s phase transitions. Injecting the computed flash temperatures into a slip-on-a-plane model for Thermal Pressurization (TP) showed that: (i) if pf was low enough so that frictional heating induced liquid/vapour phase transition, FH operated, allowing very low dynamic friction during earthquakes. (ii) Conversely, if pf was high enough that shear heating induced a sharp phase transition directly from liquid to supercritical state, an extraordinary rise in water’s specific heat acted as a major energy sink inhibiting FH and limiting TP, allowing higher dynamic fault strengths. Further extrapolation of this simplified model to geothermal reservoir depths shows that TP is the dominant weakening mechanism up to ~5 km depth. Increasing depth allows somewhat larger shear stress and reduced cpw rise, and so, substantial shear heating at low slip rates, favouring FH for fault weakening. Our results demonstrate that thermophysical properties of water should be taken into account in the formulation of the physically based model of earthquake forecasting.

2017

Mechanical behaviour of fluid lubricated faults during earthquake nucleation and propagation

Chiara Cornelio

The need for new forms of energy, and in particular the need for renewable and clean ones, led to develompment of Enanched Geothermal Systems, where the well designed stimulation of a geothermal reservoir is the key parameter for the safe development of this technology. Understanding the role that injected fluid properties, in particular viscosity, play on fault reactivation and induced seismicity is fondamental. During this Ph.D. we will apply experimental methods at laboratory scale to aim at recreate the conditions of a silicate bearing-fault under EGS conditions of stress and pore pressure. All the seismic cycle will be investigated, from the nucleation to the propagation of an earthquake. In particular, two type of laoding will be investigate in presence of different fluid: (1) imposing the propagation slip velocity, (2) loading the fault in critical conditions of stress and injecting the fluid directly on the fault. This study will shed light on influence of the fluid on fault stability and reactivation during fluid induced events but also in natural earthquake where fluid pressure is involved.

EPFL2020

Experimental studies of hydro-mechanical couplings in Enhanced Geothermal Reservoirs

Mateo Alejandro Acosta

Enhanced Geothermal Systems (EGS) allow for worldwide geothermal electricity production. They target deep (3-5 km), fractured rock reservoirs whose permeability is artificially increased through hydraulic stimulations (fluid injections). The injections modify the effective stresses, leading to new fracture creation and the reactivation of pre-existing ones, along with permeability changes. Today, the main issues with this technique are i) understanding and predicting the final fluid transport capacity of the reservoir and ii) avoiding the seismicity associated with the stimulation treatment. This thesis aims at understanding the two-way coupling between mechanical rock deformation and fluid transport in EGS reservoirs by reproducing them in the laboratory. Complex experimental protocols were developed and applied to various lithologies typically found in EGS. The experimental observations, coupled with analytical and numerical models, and with natural observations, allowed for the unravelling of some fundamental processes at play during EGS stimulations. A first part focused on how mechanical deformation influences fluid flow in i) anisotropic rocks and ii) fractures with customized roughness. In anisotropic rocks, the foliation orientation towards the stress field controls its mechanical and hydraulic transport properties. With ongoing deformation, anisotropic micromechanical models accurately predict the onset of damage, ultimate strength, porosity evolution, and fracture structure towards foliation orientation. Permeability is controlled by foliation orientation, fracture structure, and the applied stress. This shows that the full permeability tensor needs to be used with care on failed anisotropic rocks. In rock fractures with customized roughness, increasing the normal load decreases fracture transmissivity and thus fluid transport capacity. The transmissivity decrease is controlled by the contact geometry, as predicted by numerical models. Further, reversible shear loading and irreversible shear displacements (up to 1 mm offset) have little effect on transmissivity. Transmissivity evolution with shear displacement can be predicted by simple models only at low normal stress, where wear is not too prominent. These results question the concept of hydro-shear stimulations with small fault displacements. A second part targeted the effect of fluids across the earthquake cycle (nucleation and propagation) to approach induced seismicity. During nucleation, slight changes in fluid pressure drastically change the temporal evolution of earthquake precursors (slip and seismicity). Under all fluid pressure conditions, a semi empirical scaling relationship links the total moment (energy) released before the mainshock to its magnitude, in compatibility with several natural and anthropogenic earthquakes. This observation could help with real-time estimation of an impending earthquake's magnitude. During propagation, fluid pressure has a strong control on the thermal evolution of frictionally-heated asperity contacts. At low pressures, fluids vaporize and allow for contact melting, reducing shear resistance and leading to large magnitude quakes. At pressures close to the liquid-supercritical transition, water is an efficient thermal buffer, to the point that contact melting is impeded, leading to smaller quakes with little dynamic weakening. Consequently, injection strategies in EGS could be improved through careful extrapolation of the results.

EPFL2020

Fluid thermodynamics control fault heating and weakening under upper crustal stress conditions

Mateo Alejandro Acosta, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

2017

Experimental studies of hydro-mechanical couplings in Enhanced Geothermal Reservoirs

Mateo Alejandro Acosta

EPFL2020