Fracture sealing in geothermal systems: A combined EBSD and chemical approach

Abstract:

Development of natural and enhanced geothermal resources hosted in crystalline, volcanic and plutonic reservoir rocks, or in indurated, metamorphic basement reservoirs has increased over recent years. In these reservoir rocks, permeability is dominated by faults and fractures, with small contributions made by primary permeability. As such the study of how these structures are generated, their properties (e.g. orientation, spatial distribution, aperture, orientation with respect to the stress field), and how they become filled with precipitated minerals is vital to understanding the evolution of these geothermal systems, and is key to their successful development. In particular, fracture sealing is known to decrease the overall permeability of, or create permeability barriers in a geothermal reservoir, limiting its effectiveness as a resource. As such study of this sealing process is vital to discerning the evolution of fractured geothermal systems.

We use electron backscatter diffraction combined with cathodoluminescence and energy dispersive X-ray data from calcite and quartz filled veins from high temperature geothermal fields in New Zealand to investigate chemical patterns and microstructures in sealed reservoir fractures. Results indicate that while chemical zonation patterns may appear diverse or complicated, accompanying physical mineral growth and microstructure can either be simple or tell a more convoluted story. Calcite veins explored show little to no deformation and chemical suggesting postkinematic vein growth into free space with no subsequent deformation, while chemical zonation suggests fluid chemistry variation as sealing progressed. Quartz filled veins show crystal orientation of depositing vein crystals is controlled by that of the fracture wall minerals, and that varying chemistry has little to no impact on quartz microstructure.