| Abstract |
High levels of CO2 are found in the Broadlands-Ohaaki geothermal system in the Taupo Volcanic Zone of New Zealand. The system has been studied for over forty years, and over that time a significant body of information on rock-water-gas interaction in the system has been obtained. CO2-rich water flows up from fractures in the low permeability basement greywacke into the overlying reservoir, which is dominated by rhyolites, dacite, pyroclastics, and some lake sediment. It is an excellent natural analog for rock-water-gas interactions that would occur in an EGS system using CO2 as a heat extraction fluid. In terms of rock chemistry and mineralogy, this is a more “felsic” system (e.g., rhyolite, granite) that might be expected to yield less reaction with CO2 than a more “mafic” system (e.g., basalt, gabbro). Nevertheless, a significant amount of reaction does occur. We are conducting new geochemical modeling studies, laboratory experiments, and a field experiment to develop an improved understanding of the waterrock- gas interactions in this system as it relates to being a natural analog of an EGS-CO2 system. Here we review the most pertinent known aspects of what is known about the Broadlands-Ohaaki system and describe initial geochemical modeling studies of the deep fluid in the natural system using more recent and extensive thermodynamic data than used in earlier, published studies. Attention is focused on the role of sheet silicates (micas, chlorites, and clays) in stable mineral assemblages. |