Characteristics and effectiveness of water-assisted CO2 fracturing for creating geothermal reservoirs in volcanic rocks

Water-assisted CO 2 fracturing is a potential permeability enhancement method for creating or recreating geothermal reservoirs in volcanic rocks. Thus, clarifying the influence of variations in rock texture on the fracturing characteristics is essential. Moreover, in geothermal environments, volcanic rocks are most likely saturated with water; therefore, elucidating the influence of pre-existing pore water on the fracturing process and induced fracture characteristics in water-assisted CO 2 fracturing is necessary. Hence, this study experimentally elucidated the effectiveness of water-assisted CO 2 fracturing in various volcanic rocks under geothermal conditions. We conducted a set of experiments under triaxial stress conditions at 250 °C on cylindrical samples, each containing a borehole, prepared from various volcanic rocks. The analysis revealed that fracturing was inhibited (attained short and narrow aperture fractures) in the initially water-saturated sample fractured using only CO 2 . However, the fracturing inhibition effect was diminished in experiments where CO 2 within the sample borehole was compressed by injecting water, and the injected water further widened and propagated the fractures induced earlier by CO 2 (water-assisted CO 2 fracturing). The induced fractures were generally wider in porous volcanic rocks, such as pyroclastic rocks, than in low-porosity volcanic rocks, such as lava. Moreover, the fracture apertures tended to increase with increasing clast size in the pyroclastic rocks. Thus, our findings show that pyroclastic rocks containing large clasts, such as agglomerate, lapillistone, and lapilli tuffs, are preferable for creating geothermal reservoirs. • Pre-existing pore water inhibits fracture propagations in CO 2 fracturing. • Water-assisted CO 2 fracturing diminishes the inhibition effect. • Pyroclastic rocks attained wider fractures than lava. • Fracture aperture tended to increase with increasing pyroclastic clast size.