Microanalysis of Calcite Scaling in a Fractured Geothermal System

Abstract

The microscopic crystallographic and chemical make-up of geothermal minerals precipitated in fractures within active geothermal reservoirs can record significant information concerning reservoir scaling. It is vitally important to characterize these microscale signatures as they can potentially provide insight into a range of factors that control the onset of fracture scaling and scaling rates, including temperature, pressure, and geothermal fluid composition. By investigating examples of sealed fractures from geothermal systems with a range of microanalytical techniques a deeper understanding of the processes operating during fracture scaling can be obtained. Information on nucleation and growth of scaling minerals in geothermal fractures is crucial in order to incorporate reservoir scaling accurately into reservoir models, and defining fully the influence of secondary permeability in geothermal systems. This study presents results of a multi-staged analysis of calcite scaling in geothermal fracture flow pathways in the Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand. The application of EBSD, EDS, cathodoluminesce, and LA-ICPMS of calcite sealed fracture samples helps establish the action of ‘non-classical’ bladed calcite nucleation in the Kawerau Geothermal Field, provides insights into crystal growth processes and growth rates and their controlling conditions such as fluid chemistry and temperature, and provides insights on the effects these processes have on fracture permeability over time.