An Experimental Approach to Assist in Quantifying Fracture Sealing Mechanisms in Geothermal Systems

Abstract

Fluid flow in geothermal reservoirs with low primary permeability is largely controlled by faults and fracture networks. Under certain conditions, hydrothermal veins can precipitate within fractures to form barriers to fluid flow, decreasing permeability and the efficiency of a potential resource. The microstructure and geochemistry of veins can be used to determine the stress, strain, pressure and temperature that a rock mass has experienced, as well as fluid pathways and composition. Fracture sealing in rocks containing pore fluid has been shown experimentally to be a function of time, temperature and fracture dimensions. Sealing is also dependent on changes in fluid pressure and composition; however, this is less well understood. Using a combination of published and new experimental datasets, the evolution of quartz and calcite vein precipitation rates, microstructure and permeability during changes in fluid flux can be constrained. To help understand the processes involved in vein evolution, a new high-pressure, high-temperature triaxial deformation apparatus has been designed to simulate a range of upper crustal geothermal gradients whilst under confining pressure. Two types of experiment using fractured core samples have been designed to investigate vein growth mechanisms and resultant effects: 1. Static, where the sample is overpressured with supersaturated fluid and fluid pressure is reduced at varying rates; 2: Evolution, where supersaturated fluids are flowed through the sample at different rates. Data emerging from this new experimental setup, when considered alongside natural microstructures and other theoretical and experimental data, can be used to predict the conditions under which precipitation is most likely to increase scaling in wells or reduce reservoir permeability, thereby improving subsurface models of geothermal reservoir production or stimulation.