Fracture Wall Crystallography Control on Mineral Scaling in Geothermal Reservoirs: Preliminary Results

Abstract

Efficient and sustainable production of geothermal resources are dependent on several factors, most critically, the maintenance of open structural pathways for fluid flow. An interconnected fracture system is essential for heat and fluid migration within a reservoir, particularly in crystalline host rocks where intrinsic permeability is low. Over time, numerous fluid-rock interactions within these reservoirs lead to mineral nucleation and growth, eventually resulting in partial or complete fracture sealing i.e. reservoir scaling. This work presents initial findings of microstructural characterisation of calcite scaling in a geothermal system in Kibiro, Uganda. Electron Backscatter Diffraction (EBSD) reveals that calcite can display a special orientation relationship with quartz involving the calcite m and quartz a directions. However, when adularia is present at the fracture wall interface alongside quartz, calcite will nucleate preferentially on adularia via a special orientation relationship involving the c and a directions of both minerals. This indicates that adularia requires less energy for nucleation and attachment of calcite molecules than quartz. Preliminary observations suggest that it is not simply host rock lithology determining nucleation and subsequent growth of these fracture sealing minerals, but the crystal lattice arrangement of specific minerals in the fracture wall assemblage exerting a preferential control.