Miller, Stephen Andrew

2023, Gisler, Batoul, Miller, Stephen Andrew

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).

2016-11-26, Schmitt, Nicole, Verbeken, B., Grassi, R., Miller, Stephen Andrew, Perrochet, Léa, Valley, Benoît, Mosar, Jon

2014, Fuchs, Florian, Lupi, Matteo, Miller, Stephen Andrew

2014, Lupi, Matteo, Saenger, Erik H., Fuchs, Florian, Miller, Stephen Andrew

2021-4-5, Valley, Benoît, Miller, Stephen Andrew

Switzerland initiated an energy transition plan for a massive development of renewable energy sources in response to climate change challenges and the decision to shut down nuclear power plants. Geothermal energy represents one component of this transition, with energy scenarios planning for more than 5% of the Swiss electricity demand produced from geothermal energy by 2050. Geothermal energy potentially provides base-load electricity supply, while also contributing to direct and indirect heat supply for replacing fossil fuels, and thus reducing greenhouse gas emissions. In response to this initiative, the Swiss Geological Survey (swisstopo) compiles information of the subsurface relevant for deep geothermal energy, including well data, seismic data interpretation of major stratigraphic horizons and faults, heat flux maps, thermal models of the underground, and geothermal potential studies. Access to this database provides opportunities for reviewing the geothermal potential of Switzerland using a quantitative play-fairway approach. In this contribution, we first review the available data sets and propose conceptual classifications of geological and structural settings favorable for deep-seated fluid circulation in Switzerland. We use the available data to determine best-estimate stress models, which are then used to compute slip and dilation tendency on the main faults identified in the database. We also combine all available information to provide quantitative mapping of the fairway score (favorability maps) for geothermal exploration. Model resolution does not yet capture local effects relevant for specific project development, but does identify general trends at the scale of Switzerland. Currently, this approach provides best estimate models for the currently available data, and will be refined and better-resolved with the acquisition and implementation of future data.

2015-10-26, Schmitt, Nicole, Grassi, R., Miller, Stephen Andrew, Perrochet, L., Valley, Benoît, Mosar, Jon

2014, Lupi, Matteo, Miller, Stephen Andrew

2018, Jansen, Gunnar, Valley, Benoît, Miller, Stephen Andrew

Understanding the dynamics of naturally fractured systems and fractured reservoirs in terms of flow, heat transport and fracture stability (e.g. induced seismicity) is important for a range of applications associated with waste water injection, renewable energy (e.g. geothermal systems), and greenhouse gas mitigation (e.g. geological sequestration of CO2). Here we present the implementation and validation of an open source MATLAB package for efficient numerical simulations of the coupled processes in fractured systems. We take advantage of the embedded discrete fracture model that efficiently accounts for discrete fractures. We perform a series of numerical benchmark experiments to validate the implemented approach against analytical solutions and established numerical methods. Finally, we use a simplified geomechanical model and an integrated fracture stability analysis that allows estimating the potential for shear stimulation, and thus a mechanistic assessment of induced seismic risk during stimulation. The open source distribution of the source code and results can be used as a blue print for the re-implementation of the method in a high performance computing (HPC) framework or as a standalone simulation package for investigating TH(m) problems in geothermal reservoirs.

2014, Farias, Cristian, Lupi, Matteo, Fuchs, Florian, Miller, Stephen Andrew

2014, Lupi, Matteo, Kenkel, Johannes, Ricci, Tullio, Suski, Barbara, Fuchs, Florian, Miller, Stephen Andrew