A thermo-hydro-mechanical analysis of pore pressure development due to mineral deposition in geothermal systems and subduction zones
Authors:   , Gisler Batoul 
Published: November 2023

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).