10:45 〜 12:15
[SCG58-P06] Flow-Through Experiments on the Formation of the Silica Sealing Layer Above Supercritical Geothermal Reservoirs
キーワード：supercritical fluid, silica sealing, quartz precipitation, flow through, geothermal reservoir, hydrothermal fractures
In active volcanic areas as temperature increases with depth, a transition of hydrological properties from permeable to impermeable occurs at several km within the crust, controlling the formation and maintenance of supercritical geothermal reservoirs. As temperatures reach ~350°C (depth of ~2,300 m for the case of Kakkonda geothermal field), quartz solubility in water increases, but the solubility of quartz drops sharply at around ~380°C, then it increases again. At this depth where the silica solubility is at a local minimum, fractures and open pore spaces are thought to be sealed rapidly by silica precipitation [1]. However, it is still uncertain how the transition from dissolution to precipitation of minerals occurs in these narrow zones, and how the porosity and flow path variation are produced at the permeable-impermeable boundary.
We conducted the hydrothermal flow-through experiments with Iidate granite samples to understand the dissolution and precipitation behaviors of silica minerals in response to a temperature gradient. Twenty granite blocks (2.5 mm x 2.5 mm x 10 mm) were placed along the vertical flow path. A Si-rich aqueous solution (Si = 283 mg/kg (H2O), Al = 5 mg/kg (H2O)) was used to promote quartz precipitation and flowed across the granite surface for 48 hours. The reactions were performed at a constant pressure of 25 MPa with a temperature gradient from 350°C (sub-critical) to 430°C (supercritical conditions). Samples were analyzed by X-Ray Computed Tomography (X-Ray CT), X-Ray Fluorescence (XRF) and Scanning Electron Microscope Energy Dispersive Spectrometry (SEM-EDS). Input and output solution chemistry was analyzed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).
Systematic changes of mineral dissolution and precipitation were found along the flow path. At 1-8 cm from the inlet, preferential dissolution of quartz occurred until 9 cm when amorphous silica began to precipitate. However, from 6-9 cm, small quantities of Na- and K-rich accessory minerals precipitated, often atop dissolved quartz. Amorphous silica precipitated from 9-12 cm uniformly on the granite surface regardless of underlying minerals, along with cristobalite forming at 11 cm. From 12-20 cm, euhedral isolated quartz crystals grew which peaked in mass at 12-13 cm. Such nucleation and growth of quartz resulted in flow path blockage in the final hours of one experiment. After the massive precipitation, 14-20 cm continued to see nucleated quartz but its mass steadily declined. At the same locations, syntaxial quartz overgrowth became the prominent feature until the last cm of flow path. The X-Ray CT volume change data supported what we observed in the SEM-EDS with a loss of mass among early samples (dissolution) and a sharp transition to a gain of mass in the latter half samples (precipitation). Using the input and output solutions, we found that the dissolution from the first 8 cm slightly increased Si concentration levels higher than the starting concentration. Si concentration started decreasing after 9 cm with a significant drop at 12 cm. These results are in line with what has been found in previous studies [2]. Our new experimental results indicate that (1) quartz particle formation via nucleation plays essential roles on the formation of sealing layers at the top of supercritical geothermal reservoirs, (2) transition from dissolution to precipitation occurs at a very narrow region, and (3) assuming the downward fluid flow in the crust, the high porosity zone is expected just above the sealing layers. Future experiments will investigate aperture growth and sealing in interior granite pathways and in fractured granite core samples, and the effects of seal-breaking will be also be explored. References: [1] Saishu, H., Okamoto, A., & Tsuchiya, N. (2014). Terra Nova, 26(4), 253-259. [2] Okamoto, A., Saishu, H., Hirano, N., & Tsuchiya, N. (2010). Geochimica et Cosmochimica Acta, 74(13), 3692-3706.
We conducted the hydrothermal flow-through experiments with Iidate granite samples to understand the dissolution and precipitation behaviors of silica minerals in response to a temperature gradient. Twenty granite blocks (2.5 mm x 2.5 mm x 10 mm) were placed along the vertical flow path. A Si-rich aqueous solution (Si = 283 mg/kg (H2O), Al = 5 mg/kg (H2O)) was used to promote quartz precipitation and flowed across the granite surface for 48 hours. The reactions were performed at a constant pressure of 25 MPa with a temperature gradient from 350°C (sub-critical) to 430°C (supercritical conditions). Samples were analyzed by X-Ray Computed Tomography (X-Ray CT), X-Ray Fluorescence (XRF) and Scanning Electron Microscope Energy Dispersive Spectrometry (SEM-EDS). Input and output solution chemistry was analyzed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).
Systematic changes of mineral dissolution and precipitation were found along the flow path. At 1-8 cm from the inlet, preferential dissolution of quartz occurred until 9 cm when amorphous silica began to precipitate. However, from 6-9 cm, small quantities of Na- and K-rich accessory minerals precipitated, often atop dissolved quartz. Amorphous silica precipitated from 9-12 cm uniformly on the granite surface regardless of underlying minerals, along with cristobalite forming at 11 cm. From 12-20 cm, euhedral isolated quartz crystals grew which peaked in mass at 12-13 cm. Such nucleation and growth of quartz resulted in flow path blockage in the final hours of one experiment. After the massive precipitation, 14-20 cm continued to see nucleated quartz but its mass steadily declined. At the same locations, syntaxial quartz overgrowth became the prominent feature until the last cm of flow path. The X-Ray CT volume change data supported what we observed in the SEM-EDS with a loss of mass among early samples (dissolution) and a sharp transition to a gain of mass in the latter half samples (precipitation). Using the input and output solutions, we found that the dissolution from the first 8 cm slightly increased Si concentration levels higher than the starting concentration. Si concentration started decreasing after 9 cm with a significant drop at 12 cm. These results are in line with what has been found in previous studies [2]. Our new experimental results indicate that (1) quartz particle formation via nucleation plays essential roles on the formation of sealing layers at the top of supercritical geothermal reservoirs, (2) transition from dissolution to precipitation occurs at a very narrow region, and (3) assuming the downward fluid flow in the crust, the high porosity zone is expected just above the sealing layers. Future experiments will investigate aperture growth and sealing in interior granite pathways and in fractured granite core samples, and the effects of seal-breaking will be also be explored. References: [1] Saishu, H., Okamoto, A., & Tsuchiya, N. (2014). Terra Nova, 26(4), 253-259. [2] Okamoto, A., Saishu, H., Hirano, N., & Tsuchiya, N. (2010). Geochimica et Cosmochimica Acta, 74(13), 3692-3706.