10:00 AM - 10:15 AM
[HSC07-05] Evoliution of basalt permeability induced by CO2-water-mineral reactions under geothermal conditions
Keywords:Permeability, Basalt, CO2-enhanced geothermal system, CO2-water-rock interaction
The injection of CO2 into an Enhanced Geothermal System (CO2-EGS) for geothermal energy production is proposed as a means of reducing CO2 emissions into the atmosphere. As part of the CO2-EGS project, the authors have investigated changes in hydraulic properties resulting from geochemical reactions. In the CO2-EGS, hydrothermal water is acidified by the dissolution of injected CO2, which promotes the mineral dissolution. Dissolution increases the concentrations of divalent ions, leading to the precipitation of carbonate and clay minerals. Such geochemical reactions are likely to change the permeability, but the relationship between geochemical reactions and permeability is not well understood. Here, we conducted a flow-through experiment using a hot CO2-charged water to examine how the geochemical reactions change the reservoir permeability.
We used a porous basalt with porosity of 33 % because basalt formation is considered one of potential reservoirs in a CO2-EGS project in Japan. The basalt is mainly composed of plagioclase, pyroxene, glass, and olivine. For the flow-through experiment, we developed a hot supercritical CO2 flow-through apparatus designed to flow water, supercritical CO2, and their mixture through a long rock core with 200 mm in length. The maximum temperature that can be used is 300 °C. By applying a constant differential pressure to the core, CO2-charged water was flowed at 200 °C and backpressure of 10 MPa. Different differential pressures of 0.08, 0.6, 0.7, and 4.2 MPa were applied to investigate the effect of flow rate on the reaction-induced change in the permeability.
At all differential pressures, the infiltration of CO2-charged water caused a continuous decrease in permeability, up to about two orders of magnitude. The temporal permeability decrease was initially fast and then became slower. After the experiments, submicron- to micron-sized pores formed due to the dissolution of plagioclase, glass, and peridotite, which were especially prominent at the upstream of core. Needle-like secondary minerals precipitated on the pore surfaces 20 mm downstream from the upstream. The change in porosity before and after distribution was less than 1 %. The permeability of volcanic rocks can be described by the equation k = 0.00078φr2 (k: permeability, φ: porosity, r: characteristic pore-throat radius; Yokoyama and Takeuchi, 2009, J. Geophys. Res.). Based on this relationship, permeability is primarily controlled by porosity and pore radius. Because the porosity does not change significantly, the decrease in permeability of basalt likely resulted from the reduction in pore radius caused by secondary mineral precipitation associated with CO2-water-basalt interaction.
We used a porous basalt with porosity of 33 % because basalt formation is considered one of potential reservoirs in a CO2-EGS project in Japan. The basalt is mainly composed of plagioclase, pyroxene, glass, and olivine. For the flow-through experiment, we developed a hot supercritical CO2 flow-through apparatus designed to flow water, supercritical CO2, and their mixture through a long rock core with 200 mm in length. The maximum temperature that can be used is 300 °C. By applying a constant differential pressure to the core, CO2-charged water was flowed at 200 °C and backpressure of 10 MPa. Different differential pressures of 0.08, 0.6, 0.7, and 4.2 MPa were applied to investigate the effect of flow rate on the reaction-induced change in the permeability.
At all differential pressures, the infiltration of CO2-charged water caused a continuous decrease in permeability, up to about two orders of magnitude. The temporal permeability decrease was initially fast and then became slower. After the experiments, submicron- to micron-sized pores formed due to the dissolution of plagioclase, glass, and peridotite, which were especially prominent at the upstream of core. Needle-like secondary minerals precipitated on the pore surfaces 20 mm downstream from the upstream. The change in porosity before and after distribution was less than 1 %. The permeability of volcanic rocks can be described by the equation k = 0.00078φr2 (k: permeability, φ: porosity, r: characteristic pore-throat radius; Yokoyama and Takeuchi, 2009, J. Geophys. Res.). Based on this relationship, permeability is primarily controlled by porosity and pore radius. Because the porosity does not change significantly, the decrease in permeability of basalt likely resulted from the reduction in pore radius caused by secondary mineral precipitation associated with CO2-water-basalt interaction.