Experimental Thermal Stimulation of the Rotokawa Andesite
Thermal cycling of rock by heating and rapid quenching in water significantly affects its physical, mechanical and elastic properties. In this study we present a novel technique where specially designed equipment simulates the cyclic thermal stimulation processes employed by the conventional geothermal industry. To enhance productivity and injectivity of geothermal wells, geothermal operators commonly inject fluids cooler than reservoirs into wells at pressures less than natural fracture gradients which can result in enhanced fluid handling capacities. In an attempt to better understand this process, the investigation of thermal stimulation at a laboratory scale has been conceived and implemented. We have designed and built an apparatus that allows the heating and quenching of representative samples by thermal stimulation in a pressure vessel capable of attaining 350°C and 24 MPa and sustaining pressure during quenching cycles. Core sourced from production wells in an active commercial geothermal field has been tested in the apparatus. Our studies have characterized specimens prior to and subsequent to thermal stimulation for density, porosity, permeability, micro-structural texture, mineralogical fabrics, acoustic velocities, dynamic and static elastic moduli. Our results indicate that our stimulation apparatus is capable of enhancing both microscopic and macroscopic permeability, increasing porosity, reducing bulk density and attenuating seismic velocities. We have enhanced porosity in our specimens by up to 1.0 (vol% ) over original values, attenuated compressional wave velocities by up to 15% and enhanced permeabilities by nearly an order of magnitude over initially observed values. We utilize scanning electron microscopy to evaluate the microstructural change to samples, supplementing physical property investigations. The results imply that thermal stimulation can be successfully replicated in the laboratory and is coupled with both thermal and chemical components. The implications of this study for future laboratory and field scale stimulation testing are then considered.