Carbon Dioxide (CO₂) Fracturing of Volcanic Rocks Under Geothermal Conditions

An enhanced geothermal system that uses carbon dioxide (CO₂) for both reservoir creation and thermal energy extraction has been attracting attention in Japan. For this system, CO₂ fracturing is conducted to create highly permeable fractures in low-permeability reservoirs, for instance, consisting of volcanic rocks. However, there is no previous study on CO₂ fracturing of volcanic rocks under geothermal conditions, and possibility and characteristics of such fracturing are therefore unknown. Here we present results of CO₂ fracturing experiments on andesite and basalt at 250 °C and a confining pressure of 30 MPa. It is demonstrated that CO₂ fracturing occurs at a lower pressure than water fracturing and produces a more complex fracture pattern with a substantial permeability improvement. Additionally, it is shown that CO₂ fracturing with water, in which CO₂ is chased and pressurized by water, can produce larger-aperture fractures (i.e., larger permeability improvement) with keeping the advantage in CO₂ fracturing.

An enhanced geothermal system that uses carbon dioxide (CO₂) for both reservoir creation and thermal energy extraction has been attracting attention in Japan. For this system, CO₂ fracturing is conducted to create highly permeable fractures in low-permeability reservoirs, for instance, consisting of volcanic rocks.

However, to the best of the authors’ knowledge, few laboratory CO₂ fracturing experiments have been conducted using CO₂ and geothermal conditions to date (Isaka et al., 2019; Pramudyo et al., 2021; Pramudyo et al., 2023). Additionally, there is no previous study on CO₂ fracturing of volcanic rocks under geothermal conditions, and possibility and characteristics of such fracturing are therefore unknown. In this context, the present study aimed at clarifying possibility and characteristics of CO₂ fracturing on volcanic rocks under geothermal conditions. This paper presents results of CO₂ fracturing experiments on andesite and basalt at 250 °C and a confining pressure of 30 MPa.

Cylindrical rock samples (diameter, 30 mm; length, 25 mm) with a single borehole (diameter, 1.5 mm; length, 10 mm) were prepared using Genbudo basalt from Hyogo prefecture, Japan, and Honkomatsu andesite from Kanagawa prefecture, Japan. X-ray computed tomography (CT) and permeability measurements were conducted on each sample before and after the experiment. The X-ray CT was carried out under dry, room temperature, and atmospheric pressure conditions at an X-ray tube voltage of 120 kV, a tube current of 150 μA, and a voxel size of 25 μm × 25 μm × 25 μm, to confirm formation of fractures. The permeability of the sample was measured in a radial flow geometry at room temperature and atmospheric pressure, by injecting water into the borehole with graphite gaskets.