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A thermo-hydro-mechanical analysis of pore pressure development due to mineral deposition in geothermal systems and subduction zones
Date de parution
2023
Nombre de page
127
Mots-clés
- Précipitation de silica
- Diminution de la porosité
- Réservoir géothermique
- Fracture hydraulique
- COMSOL Multiphysics
- Zone de subduction
- Tremblements et glissements épisodiques
- Modèles THC
- Quantification des incertitudes
- Silica deposition
- Porosity reduction
- Geothermal reservoir
- Hydraulic fracture
- subduction zones
- Episodic tremor and slip
- Uncertainty quantification
- THC models
Précipitation de sili...
Diminution de la poro...
Réservoir géothermiqu...
Fracture hydraulique
COMSOL Multiphysics
Zone de subduction
Tremblements et gliss...
Modèles THC
Quantification des in...
Silica deposition
Porosity reduction
Geothermal reservoir
Hydraulic fracture
subduction zones
Episodic tremor and s...
Uncertainty quantific...
THC models
Résumé
Une gestion optimale des réservoirs géothermiques requiert l’étude de la précipitation des
minéraux et de leurs effets sur le comportement du système. En effet, la précipitation des minéraux,
plus précisément de la silice, entraîne une diminution de la porosité de la roche et par conséquent
pourrait affecter la pression dans le système. Une recherche première confirme que la vitesse de
réduction de la porosité est le facteur déterminant une augmentation potentielle de la pression
dans le système. Quand la vitesse de réduction de la porosité est assez importante, le système
subit une augmentation de pression de sorte que l’écoulement de Darcy est inversé, transportant
ainsi la chaleur dans le sens inverse, expliquant ainsi une sous-performance de certains réservoirs
géothermiques.
En présence de fracture hydraulique, la diminution rapide de la porosité entraîne d’une part une
diminution de la largeur de la fracture et d’autre part l’absence de fuite de fluide de la fracture vers la
roche environnante. Cependant, une fois que le transfert de chaleur dans le sens inverse a lieu (dû à
l’augmentation de la pression en excès de celle hydrostatique), la largeur de la fracture recommence
à croître. Le développement de la surpression dans le système et l’introduction des contraintes de
chaleur (en excès) diminuent les contraintes effectives, affaiblissant ainsi la roche et provoquant sa
rupture.
Une étude finale des zones de subductions prouve que les tremblements et glissements
épisodiques sont liés à la diminution de la porosité de la roche en présence de la précipitation de la
silice (en forme de Quartz). En effet, la vitesse de diminution de la porosité est le facteur contrôlant
l’augmentation de la pression et par conséquent une diminution des contraintes effectives et la
rupture éventuelle de la roche. Une fois que le glissement a lieu, la pression diminue et le processus
de précipitation de silica recommence. Ceci est un processus répétitif.
Abstract
One fundamental aspect of geothermal reservoir management involves the study of mineral deposition
and its controlling factors. Silica, in its various forms, is one of the most studied minerals
and its deposition has been linked to porosity reduction and fluid flow impairment. In geothermal
systems, heat is exchanged between the porous rock and the fluid leading to shifts in the mechanical
behaviour of the rock. The mechanical behaviour of the reservoir rock is further unsettled by the
presence of silica (or other mineral) deposition and its resulting pore pressure buildup. In fact, pore
pressure may become in excess of hydrostatic thus decreasing the effective stresses and rendering
the reservoir rock unstable. This concerning issue is a source of disagreement within the scientific
community, where researchers differ in approaches to incorporate porosity reduction in the suite of
governing equations describing the geothermal system, and in some cases suggesting simplifications
by neglecting the porosity reduction problem. While the simplification may be true in some scenarios,
an increasing number of literature agrees on the importance of porosity reduction, its effects on
fracture instability, and its link to slow earthquakes or episodic tremors and slip in subduction zones.
Accordingly, the main purpose of this thesis is to reconcile the equations governing the behaviour
of the geothermal system with the porosity reduction and evaluate its influence. We introduce a key
concept of a time-dependent porosity reduction rate based on the variation of the concentration of
deposited silica in the system. That is, the evolution of pore pressure in the geothermal reservoir
becomes dependent on this introduced porosity reduction rate, thus affecting the advection term
and eventually the effective stresses. Furthermore, geothermal systems are constituted of solid and
fluid phases, and include inherent discontinuities, i.e. fractures, and the superposition of several
continua, each with its unique properties and constraints but interacting and interchanging fluid,
heat and minerals. This thesis extends the porosity reduction study to target fractured geothermal
reservoirs and explores its effects on fracture aperture evolution and their stability.
Silica deposition, a primarily temperature-dependent process, is also encountered in subduction
zones due to dehydration processes and fluid transport by the subducting slab and the corner flow of the mantle wedge. The study of tremor data in the Cascadia subduction zone shows that slip
events vary from large and infrequent to small and frequent with increasing depth. Measured ratios
of compressional (P)-wave to shear (S)-wave velocities are in the range of 1.6 and 2.0, decreasing with
increasing depth and are proportional to the episodic recurrence intervals. This observation indicates
the presence of quartz at greater depth. All evidence shows that porosity reduction via progressive
silica enrichment near the base of the forearc crust and upward mineralization of quartz veins enables
slow earthquakes at subduction zone forearcs, otherwise called episodic tremor and slip (ETS).
Episodic healing and permeability reduction of the silica-rich fault gauge elicit a reduction in tremor
recurrence time. At higher temperature, faster silica deposition occurs, leading to faster porosity reduction
rates, and consequently faster fluid overpressure. Accordingly, the fault is subjected to lower
effective normal stress and hence shorter tremor recurrence times.
In this study, we present numerical simulations of fluid pressure, heat transfer and reactive
transport in a geometrically constrained fractured hydrothermal system undergoing time-dependent
porosity reduction. We use the finite element based commercial software COMSOL Multiphysics.
The simulations explore the effects of porosity reduction which occurs at the vicinity of the injection
well, where temperatures are low, on injectivity and fracture stability. The simulation also identifies
the controlling factors, such as the porosity reduction rate and the fracture initial aperture, the injection
pressure and concentration of silica (as quartz) in excess of the equilibrium concentration. The
simulations further highlight the consequences of silica enrichment (porosity reduction) in subduction
zones and the resulting heat and fluid flow dynamics. Although fluid in these high enthalpy
systems is saline, we opt for water as the modeling fluid.
Simulations results show that porosity reduction rate is the principal controlling factor of the
behavior and stability of the hydrothermal system undergoing mineral deposition. In fact, pore pressure
can become in excess of hydrostatic and lead to a reverse Darcy flow (in reverse of its presumable
direction) at the vicinity of the injection well, overtime decreasing the injectivity rate and producing
underperforming wells. Furthermore, excess pore pressure at the fracture boundary brings a decrease
in the effective stresses and instability for a range of fracture inclination angles. Finally, fault
reactivation and ETS are not large scale events, rather events caused by local variations in porosity
and pore pressure. Furthermore, only a time-dependent porosity reduction rate at the subduction
zone controls the decrease in the effective stress and causes ETS. Nevertheless, the cycle of fault reactivation
then healing is incessant, and faster pore pressure development leads to lower changes in
effective stress and hence shorter recurrence times of episodic tremors and slip (ETS).
minéraux et de leurs effets sur le comportement du système. En effet, la précipitation des minéraux,
plus précisément de la silice, entraîne une diminution de la porosité de la roche et par conséquent
pourrait affecter la pression dans le système. Une recherche première confirme que la vitesse de
réduction de la porosité est le facteur déterminant une augmentation potentielle de la pression
dans le système. Quand la vitesse de réduction de la porosité est assez importante, le système
subit une augmentation de pression de sorte que l’écoulement de Darcy est inversé, transportant
ainsi la chaleur dans le sens inverse, expliquant ainsi une sous-performance de certains réservoirs
géothermiques.
En présence de fracture hydraulique, la diminution rapide de la porosité entraîne d’une part une
diminution de la largeur de la fracture et d’autre part l’absence de fuite de fluide de la fracture vers la
roche environnante. Cependant, une fois que le transfert de chaleur dans le sens inverse a lieu (dû à
l’augmentation de la pression en excès de celle hydrostatique), la largeur de la fracture recommence
à croître. Le développement de la surpression dans le système et l’introduction des contraintes de
chaleur (en excès) diminuent les contraintes effectives, affaiblissant ainsi la roche et provoquant sa
rupture.
Une étude finale des zones de subductions prouve que les tremblements et glissements
épisodiques sont liés à la diminution de la porosité de la roche en présence de la précipitation de la
silice (en forme de Quartz). En effet, la vitesse de diminution de la porosité est le facteur contrôlant
l’augmentation de la pression et par conséquent une diminution des contraintes effectives et la
rupture éventuelle de la roche. Une fois que le glissement a lieu, la pression diminue et le processus
de précipitation de silica recommence. Ceci est un processus répétitif.
Abstract
One fundamental aspect of geothermal reservoir management involves the study of mineral deposition
and its controlling factors. Silica, in its various forms, is one of the most studied minerals
and its deposition has been linked to porosity reduction and fluid flow impairment. In geothermal
systems, heat is exchanged between the porous rock and the fluid leading to shifts in the mechanical
behaviour of the rock. The mechanical behaviour of the reservoir rock is further unsettled by the
presence of silica (or other mineral) deposition and its resulting pore pressure buildup. In fact, pore
pressure may become in excess of hydrostatic thus decreasing the effective stresses and rendering
the reservoir rock unstable. This concerning issue is a source of disagreement within the scientific
community, where researchers differ in approaches to incorporate porosity reduction in the suite of
governing equations describing the geothermal system, and in some cases suggesting simplifications
by neglecting the porosity reduction problem. While the simplification may be true in some scenarios,
an increasing number of literature agrees on the importance of porosity reduction, its effects on
fracture instability, and its link to slow earthquakes or episodic tremors and slip in subduction zones.
Accordingly, the main purpose of this thesis is to reconcile the equations governing the behaviour
of the geothermal system with the porosity reduction and evaluate its influence. We introduce a key
concept of a time-dependent porosity reduction rate based on the variation of the concentration of
deposited silica in the system. That is, the evolution of pore pressure in the geothermal reservoir
becomes dependent on this introduced porosity reduction rate, thus affecting the advection term
and eventually the effective stresses. Furthermore, geothermal systems are constituted of solid and
fluid phases, and include inherent discontinuities, i.e. fractures, and the superposition of several
continua, each with its unique properties and constraints but interacting and interchanging fluid,
heat and minerals. This thesis extends the porosity reduction study to target fractured geothermal
reservoirs and explores its effects on fracture aperture evolution and their stability.
Silica deposition, a primarily temperature-dependent process, is also encountered in subduction
zones due to dehydration processes and fluid transport by the subducting slab and the corner flow of the mantle wedge. The study of tremor data in the Cascadia subduction zone shows that slip
events vary from large and infrequent to small and frequent with increasing depth. Measured ratios
of compressional (P)-wave to shear (S)-wave velocities are in the range of 1.6 and 2.0, decreasing with
increasing depth and are proportional to the episodic recurrence intervals. This observation indicates
the presence of quartz at greater depth. All evidence shows that porosity reduction via progressive
silica enrichment near the base of the forearc crust and upward mineralization of quartz veins enables
slow earthquakes at subduction zone forearcs, otherwise called episodic tremor and slip (ETS).
Episodic healing and permeability reduction of the silica-rich fault gauge elicit a reduction in tremor
recurrence time. At higher temperature, faster silica deposition occurs, leading to faster porosity reduction
rates, and consequently faster fluid overpressure. Accordingly, the fault is subjected to lower
effective normal stress and hence shorter tremor recurrence times.
In this study, we present numerical simulations of fluid pressure, heat transfer and reactive
transport in a geometrically constrained fractured hydrothermal system undergoing time-dependent
porosity reduction. We use the finite element based commercial software COMSOL Multiphysics.
The simulations explore the effects of porosity reduction which occurs at the vicinity of the injection
well, where temperatures are low, on injectivity and fracture stability. The simulation also identifies
the controlling factors, such as the porosity reduction rate and the fracture initial aperture, the injection
pressure and concentration of silica (as quartz) in excess of the equilibrium concentration. The
simulations further highlight the consequences of silica enrichment (porosity reduction) in subduction
zones and the resulting heat and fluid flow dynamics. Although fluid in these high enthalpy
systems is saline, we opt for water as the modeling fluid.
Simulations results show that porosity reduction rate is the principal controlling factor of the
behavior and stability of the hydrothermal system undergoing mineral deposition. In fact, pore pressure
can become in excess of hydrostatic and lead to a reverse Darcy flow (in reverse of its presumable
direction) at the vicinity of the injection well, overtime decreasing the injectivity rate and producing
underperforming wells. Furthermore, excess pore pressure at the fracture boundary brings a decrease
in the effective stresses and instability for a range of fracture inclination angles. Finally, fault
reactivation and ETS are not large scale events, rather events caused by local variations in porosity
and pore pressure. Furthermore, only a time-dependent porosity reduction rate at the subduction
zone controls the decrease in the effective stress and causes ETS. Nevertheless, the cycle of fault reactivation
then healing is incessant, and faster pore pressure development leads to lower changes in
effective stress and hence shorter recurrence times of episodic tremors and slip (ETS).
Notes
Accepted on the recommendation of:
Prof. Stephen A. Miller, University of Neuchâtel, Switzerland
Prof. Benoît Valley, University of Neuchâtel, Switzerland
Prof. Stefan Schmalholz, University of Lausanne, Switzerland
Defended on December 16th 2022
No de thèse : 3029
Prof. Stephen A. Miller, University of Neuchâtel, Switzerland
Prof. Benoît Valley, University of Neuchâtel, Switzerland
Prof. Stefan Schmalholz, University of Lausanne, Switzerland
Defended on December 16th 2022
No de thèse : 3029
Identifiants
Type de publication
doctoral thesis
