Creating granitic geothermal reservoirs by carbon dioxide injection

Abstract

This article summarizes a PhD dissertation titled Creating granitic geothermal reservoirs by carbon dioxide injection, submitted to the Graduate School of Environmental Studies, Tohoku University. Carbon dioxide (CO₂) has been proposed as an alternative fracturing fluid to create geothermal reservoirs, because it is less reactive to rock-forming minerals, capable to reduce water footprint, and easier to handle compared to the competing alternative gasses. CO₂ has also low viscosity (‹ 100 μPa⋅s) across wide range of conditions, potentially allowing its injection to induce complex, cloud-fracture network (CFN) at conventional (c.a. 150–300 °C) and superhot (› c.a. 400 °C) geothermal conditions. Experiments on intact granite samples clarified that CO₂ injection achieves CFN at conventional and superhot geothermal conditions, through the stimulation of pre-existing microfractures by the low-viscosity CO₂. The aperture of fractures in the CFN increases with temperature and differential stress, and the fracturing pressure can be predicted using the Griffith failure criterion. Then, experiments on cylindrical granite samples with sawcut, serving as an analogue of a natural fracture, elucidated that CO₂ injection achieves CFN in naturally-fractured granite, along with the shearing of the natural fractures. Finally, experiments into granite samples with CFN revealed that chelating agent solution injections improves the permeability of the CFN without inducing excessive rock deformation and acoustic emission, both at slightly acidic and alkaline conditions, and under varying stress state. At large scale, and in radial flow condition, chelating agent solution injection under slightly acidic condition induces higher degree of mineral dissolution around injection borehole; thus, the injection should utilize a lower chelating agent concentration to allow for a lower solution-viscosity, and more uniform degree of mineral dissolution over a greater distance from borehole.