A thermo-hydro-chemical model with porosity reduction and enthalpy production: Application to silica precipitation in geothermal reservoirs

Abstract

This work investigates the influence of porosity reduction by silica precipitation on pore pressure development, heat transfer and reactive flow in a geothermal system. Porosity reduction due to silica precipitation can generate excess pore pressure, thus affecting the stability of the rock mass. In addition, porosity reduction increases the thermal diffusivity of the rock, affecting the thermal front, while silica precipitation generates enthalpy. Consequently, the effective stresses associated with solute deposition must be considered for accurate studies of rock failure. We use COMSOL multiphysics to solve the governing equations and explore these complex and interacting processes. We investigate a single-phase, reactive flow model coupled to hydrodynamic flow and heat transfer. Model results show that porosity reduction rate depends on the initial porosity and can lead to overpressure in the matrix, thus reversing the Darcy velocity. This would result in silica deposits on the walls of the well and heat transfer from the reservoir to the well, thus accelerating the cooling front. Consequently, the system observes a drastic decrease in the effective stresses and an increased likelihood of tensile failure of the rock. These results have important implications regarding the long-term thermo-hydraulic properties of geothermal reservoirs.