Thermal cycling of saturated Ghibli granite and Rotokawa andesite using distilled water and geothermal brine.

Thermal stimulation is a technique commonly employed by geothermal operators to enhance permeability of both production and reinjection reservoirs. Through the cyclic injection of cold fluid, induced thermal stresses lead to significant development of micro-fractures, porosity and permeability. How thermal-cycling affects the physical and mechanical properties of a rock-mass is not well understood. Previous experiments by Siratovich et al. (2013, 2015a) consider thermal-cycling using only distilled water. In this study thermal-cycling methodology has been developed to also consider the effects of thermal-cycling using saline low-pressure brine retrieved from Nga Awa Purua (NAP) geothermal power station. Results obtained in this project clearly indicate that the chemistry of H₂O and NAP brine play significant roles in the alteration of Ghibli granite. Methods were then translated to samples of Rotokawa andesite, the main reservoir lithology at the Rotokawa geothermal field, with differences in how the two lithologies responde to the thermal-cycling being observed. Variability in the physical disturbance and chemical alteration of samples was observed in thin-section and by the measurement of porosity, permeability, density, ultrasonic-wave velocity and limited XRD and SEM. In geologically short periods of time, saturated thermal-cycling using both H₂O and NAP brine has resulted in measurable amounts of mineral alteration to samples, with alteration products observable in thin-section. Ghibli granite samples that were thermally-cycled in H₂O developed more permeability than those cycled in NAP brine, despite developing less porosity. This is associated with the alteration of biotite to chlorite, which was more prevalent in H₂O treated experiments. It is suggested that the new chlorite assemblage has allowed for the linking of flow-paths, reducing tortuosity and enhancing permeability. Samples thermally-cycled in NAP brine were observed through XRD to have greater amounts of albitization than those cycled in H₂O . Albitization and associated reordering process are thought to be responsible for the measured differences in porosity development, with NAP brine treated samples on average developing more porosity than H₂O treated samples. When these experiments were applied to the Rotokawa andesite, both porosity and permeability were observed to initially decline and then recover, although not to their initial level. The specific mechanisms responsible for these changes could not be identified. To comprehensively understand reservoir processes during thermal-cycling, and to better understand reservoir operational management, more experimentation and numerical modelling is required. In the Further Research section, key areas have been indicated where additional studies could be advantageous