Effects of injection fluid type on pressure-dependent permeability evolution of fractured rocks in geothermal reservoirs: An experimental chemo-mechanical study

Abstract

In geothermal reservoirs, coupled thermal-hydrological-mechanical-chemical processes lead to gradual closure of fractures and consequent production decline. The objective of this study was to investigate the effects of injected fluid type on the evolution of fracture aperture/permeability at various stress levels through a series of flow-through experiments on a fractured phyllite specimen, retrieved from a production well in the Blue Mountain geothermal field. The injected fluids included deionized water, silica super- and under-saturated fluids, and the geothermal fluid extracted from the Blue Mountain geothermal field. It was found that injection of geothermal and silica super-saturated fluids led to lowest and highest permeability reduction, respectively. In addition, the degree of permeability recovery was lower in the experiment using geothermal fluid compared to the experiment using silica super-saturated fluid. On the other hand, chemical analysis of the effluent samples revealed that injection of deionized water resulted in dissolution of feldspar and quartz, while precipitation of silica occurred in the experiment using geothermal fluid. Post-test observation by scanning electron microscopy of fracture surface area in the test using silica super-saturated fluid indicated some degree of mineral precipitation. A comparison between pre- and post-test computer tomography scan images for the experiment with injected silica super-saturated showed that mostly decrease in fracture aperture occurred, with increase in fracture aperture in a few regions along the fracture surface. Finally, the Three-Element rheological model successfully predicted the fracture permeability decay. The results of this study suggested that precipitation of silica, a major problem in permeability loss of geothermal systems, can be potentially minimized by using an injection fluid that is closer to chemical equilibrium state with the host rock.