Experimental studies of hydro-mechanical couplings in Enhanced Geothermal Reservoirs

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.

On the effects of fluid pressure variations on rock-mass and fault mechanical behaviour

Corentin Jean-Marie Rémi Noël

In the past 20 years, the growing interest in deep geo-reservoirs for purposes such as carbon storage, waste water disposal, or geothermal energy exploitation have led to large-volume fluid injections into the upper continental crust. These fluid injections have caused a massive increase in the seismicity rate in some normally âquietâ regions. Recent observations suggested that different injection strategies could produce different mechanical responses in these reservoirs. In this vein, cyclic fluid injections have been proposed as an alternative to conventional monotonic injections to mitigate induced earthquakes. Additionally, recent studies have suggested that injection into deeper geo-reservoirs, at the brittle-ductile transition, could reduce (or suppress) induced earthquakes. In this context, this research has aimed at bettering our understanding of reservoir deformation during different fluid injection strategies. Laboratory experiments were performed on porous reservoir rock under different reservoir conditions and fluid injection strategies. In the first section, it is demonstrated that water-saturated conditions, compared to dry conditions, cause a reduction in uniaxial compression strength in five sandstones. Additional experiments suggest that this water weakening of sandstones is due to the reduction of the fracture toughness and of the static friction of the materials. In the second section, it is shown that under drained conditions, pore fluid pressure oscillations affect the long-term mechanical behaviour of intact porous rocks. More than the amplitude of the oscillations, the period controls the time-to-failure, failure strength and dilatancy rate of the sample. Additionally, the pore fluid pressure oscillations control the AE events (a proxy for the seismicity): the AE events rate oscillates in-phase with the pore fluid pressure variations. Increasing the differential stress, the amplitude and the period of the oscillations accentuates this behaviour. In the third section, it is demonstrated that under drained conditions, pore fluid pressure oscillations strongly affect the mechanical behaviour of faults. The pore fluid pressure signal directly controls the instabilitiesâ (i.e., stick-slip and AE events) distribution, with an increase of events at elevated pore fluid pressure. For initially stable sliding faults, the pore fluid pressure oscillation signal controls the onset of unstable slip: the higher the pore fluid pressure oscillationâs amplitude, the lower the stress and displacement at the onset of a seismic event. In the fourth section, it is shown that under quasi-drained conditions, a pore fluid pressure increase within a rock undergoing ductile deformation causes instantaneous dilation of the system. Further increasing the pore fluid pressure leads to the development of localized deformations and the shear fracturing of the rock. The macroscopic shear fracturing is not instantaneous when the ductile to brittle transition is passed. Prior to this, a creeping phase is necessary. The time and strain length of this creeping phase are controlled by the rate of pore fluid pressure increase: the faster the injection, the longer the creeping phase. This work provides new insight into the mechanical behaviour of rock masses and faults under various crustal conditions during pore fluid pressure variations. It helps to constrain the mechanisms involved in deep geo-reservoirs submitted to different injection strategies.

EPFL2021

Stress management in the context of induced seismicity in subsurface reservoirs

Barnaby Padraig Fryer

The stress state of the subsurface has been shown to have an influence on a number of key processes. For example, the criticality of the stress state indicates how large stress changes need to be before a fault begins to slip, the mean effective stress controls compaction and permeability loss in hydrocarbon reservoirs, the differential stress has been seen in seismology to have a strong influence on the size distribution of earthquake catalogues, with stress state further influencing the affinity of a fault to slip either seismically or aseismically, and the stress state strongly influences the propagation of a hydraulic fracture, with stress jumps being capable of completely halting a fracture's propagation. For reasons such as these, it has long been recognized that the state of stress is an important parameter for subsurface industrial operations such as hydrocarbon production, Enhanced Geothermal System (EGS) stimulation, carbon storage, and hydraulic fracturing. Generally, however, the state of stress does not remain constant during many of these operations. Decreasing effective stresses have led to induced seismicity during injection, fluid production has led to total stress changes which have induced seismicity, even in areas where pore pressure has decreased, temperature-induced stress changes have led to shear stimulation, and changes in total stress have led to so-called "frac-hits" where hydraulic fractures propagate towards depleted zones. These examples demonstrate an operator's ability to alter the state of stress, yet deliberate attempts to alter the stress state for the benefit of future operations have only been suggested a handful of times. Here, this idea of altering the state of stress is extended through a series of investigations. It is shown that, because production-induced seismicity is caused by total stress changes associated with the gradient of the produced fluid pressure, a hydraulic fracture which reduces the gradient required for production will also reduce the resulting seismicity. Conversely, compaction which leads to permeability loss will result in higher pore pressure gradients and therefore more induced seismicity. Both of these studies, which also have implications for optimal horizontal wellbore orientation, are investigated through the use of a poroelastic reservoir simulator developed during the thesis. In a following study, it is suggested that high stress path reservoirs are attractive targets for fluid injection, as total stress changes will result in an increased stability despite increasing pore pressure. Further, the concept of stress preconditioning is introduced, such that operators alter the stress state of an EGS reservoir to promote more favorable earthquake distributions. These concepts, which make up the first three chapters of the thesis, all are forms of reservoir management that reduce the risk associated with induced seismicity. Following this, in a numerical study, the idea of stress preconditioning is extended to allow for directed stimulation treatments in EGS's. Finally, the idea of inducing stress jumps through production such that hydraulic fractures do not propagate vertically is evaluated by means of scaling analyses and numerical simulation, with implications for carbon storage. More than the individual propositions, this thesis promotes the mindset that, where appropriate, attempts should be made to alter the state of stress for the benefit of future operations.

EPFL2020

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

On the effects of fluid pressure variations on rock-mass and fault mechanical behaviour

Corentin Jean-Marie Rémi Noël

EPFL2021

Stress management in the context of induced seismicity in subsurface reservoirs

Barnaby Padraig Fryer

EPFL2020