Laboratory Simulation of Granite-Fluid Interactions Up to Supercritical Conditions

Abstract

As a viable resource, any developed Hot Dry Rock (HDR) system must be long-lived, and the chemistry of its fluid-rock interaction processes understood; both for maintaining fluid pathways and field management. We used a recently developed tube-type, flow-reactor system (Bignall et al., 2000) to simulate reaction processes in a hypothetical HDR geothermal system, operating at sub-critical to supercritical conditions. Our goal is to elucidate dissolution-deposition processes and the character of induced and natural fractures in the HDR hydrothermal system, both for energy extraction and in-situ production of hydrothermal synthetic materials. Two preheated (hydrothermal) fluids, (i) deionised (unmineralised) water and (ii) a moderately saline (~1200 mg/kg Cl, ~60 mg/kg Si), mildly acidic fluid, have been reacted with unaltered granite in the flow-type reactor system, at a range of temperatures (up to 425oC), and typical operating pressures of 15-25MPa. Output solutions were analysed, after interaction with the ëreservoirí (granite) rocks, by ICP-AES and AAS techniques and show, for a 400oC/20MPa/H2O experiment, that an ëapparent equilibriumí fluid is produced, with a major element chemistry of 140ppm Si; 7.1ppm Al; 0.09ppm Fe; 0.03ppm Mg; 0.07ppm Ca; 7.0ppm Na; 5.2ppm K. Zones of granite weight gain/loss, as a result of fluid-rock interactions reflect shifts in Kw, dielectric constant and fluid density, in response to changes in fluid composition and the thermal regime. Dissolution of quartz, feldspar and ferromagnesian mineral phases occur in the zone where temperatures are <340oC, where SEM examination reveal primary minerals have a pitted appearance, although in this zone there is also minor deposition of sericite. In the zone where the thermal regime is between 360oC and about 380oC, there is a strong weight gain, characterised by deposition of quartz and subordinate secondary albite. In zones where the temperature is higher, at >3900C, there is little net weight change, with remobilisation of quartz producing minor dissolution of primary crystals, but also occurrence of secondary quartz, together with some sericite and chlorite(?). Our laboratory studies demonstrate that host rock characteristics, fluid chemistry, flow rate, temperature and the temperature gradient are all important factors in dictating mineral dissolution and/or precipitation processes, as well as the condition of the fracture-pathways.