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[HSC07-04] Reaction-induced fracturing on serpentinite promoted by selective dissolution of brucite
Keywords:Carbon mineralization, Reaction-induced fracturing
Direct carbon mineralization of mafic and ultramafic rocks underground (i.e., in-situ carbon mineralization) is considered as a stable and effective ways of carbon fixation (Carbfix 2020). Serpentinite has more divalent cation than basalt, and in situ carbonation of serpentinite has been expected as a method to store a large amount of carbon dioxide, however, limited reactive surface area of serpentinite has been a challenge. Field observation suggests that reaction-induced fracturing due to volume-increasing reactions enhances the reactions inside the rocks (Plumper et al. 2012, Kelemen&Hirth 2012). However, such reaction-induced fracturing has not been clearly reproduced in experiments using natural rocks for carbonation reactions.
This study presents the results 6 different conditions of batch experiments. Core samples (6 mm in diameter and 5 mm in height) of brucite-rich serpentinite were heated to 90, 150, and 200℃ in CO2-saturated water (CO2-pressure: 10 MPa) and 1M NaHCO3 solution for one week. Then textures of the products were compared to geochemical modeling and their reaction process was studied. The greatest amount of magnesite precipitated at 200℃ in 1M NaHCO3 solution. Reaction-induced fractures were clearly observed in CO2-saturated water 150℃, NaHCO3 solution 150 and 200℃. Further observation of NaHCO3 solution 200℃ confirmed 2 types of fractures: diagonal fractures inside the sample and vertical fractures on the surface. In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentine mixture was formed in a mesh texture outside of it.Then textures of the products were compared to geochemical modeling and their reaction process was studied. The greatest amount of magnesite precipitated at 200℃ in 1M NaHCO3 solution. Reaction-induced fractures were clearly observed in CO2-saturated water 150℃, NaHCO3 solution 150℃ and 200℃. Further observation of NaHCO3 solution 200℃ confirmed 2 types of fractures: diagonal fractures inside the sample and vertical fractures on the surface. In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentine mixture was formed in a mesh texture outside of it.
Geochemical modeling shows a high solubility of Mg in CO2-saturatd water during brucite dissolution (i.e., ~10-2 mol/L), suggesting intense outward diffusion of Mg ions, which would cause outside precipitation. On the other hand, it shows a low solubility of Mg (~10-10 mol/L) and a high solubility of HCO3- (~1 mol/L) in NaHCO3 solution, which may cause HCO3- to diffuse preferentially into the rock and lead precipitation of magnesite to occur in situ.
Based on the above, the reaction process was discussed as follows. Selective dissolution of brucite from the serpentine-brucite mixture supplies Mg ions, locally raises the pH, and leaves behind porous serpentine. (2) Magnesite precipitates in the veins and gaps in the porous serpentine causing the volume expansion. (3) Diagonal cracks are generated due to the expansion of the reacted surface. (4) Fluid permeates through the cracks, and the same reaction and expansion as in (1)-(2) occur on the crack surfaces. (5) Vertical cracks are generated on the surface due to the expansion of the inside. Therefore, local reaction and expansion by selective reaction of brucite may play an important role in the continuous reaction-induced fracture process in serpentinite.
This study presents the results 6 different conditions of batch experiments. Core samples (6 mm in diameter and 5 mm in height) of brucite-rich serpentinite were heated to 90, 150, and 200℃ in CO2-saturated water (CO2-pressure: 10 MPa) and 1M NaHCO3 solution for one week. Then textures of the products were compared to geochemical modeling and their reaction process was studied. The greatest amount of magnesite precipitated at 200℃ in 1M NaHCO3 solution. Reaction-induced fractures were clearly observed in CO2-saturated water 150℃, NaHCO3 solution 150 and 200℃. Further observation of NaHCO3 solution 200℃ confirmed 2 types of fractures: diagonal fractures inside the sample and vertical fractures on the surface. In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentine mixture was formed in a mesh texture outside of it.Then textures of the products were compared to geochemical modeling and their reaction process was studied. The greatest amount of magnesite precipitated at 200℃ in 1M NaHCO3 solution. Reaction-induced fractures were clearly observed in CO2-saturated water 150℃, NaHCO3 solution 150℃ and 200℃. Further observation of NaHCO3 solution 200℃ confirmed 2 types of fractures: diagonal fractures inside the sample and vertical fractures on the surface. In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentine mixture was formed in a mesh texture outside of it.
Geochemical modeling shows a high solubility of Mg in CO2-saturatd water during brucite dissolution (i.e., ~10-2 mol/L), suggesting intense outward diffusion of Mg ions, which would cause outside precipitation. On the other hand, it shows a low solubility of Mg (~10-10 mol/L) and a high solubility of HCO3- (~1 mol/L) in NaHCO3 solution, which may cause HCO3- to diffuse preferentially into the rock and lead precipitation of magnesite to occur in situ.
Based on the above, the reaction process was discussed as follows. Selective dissolution of brucite from the serpentine-brucite mixture supplies Mg ions, locally raises the pH, and leaves behind porous serpentine. (2) Magnesite precipitates in the veins and gaps in the porous serpentine causing the volume expansion. (3) Diagonal cracks are generated due to the expansion of the reacted surface. (4) Fluid permeates through the cracks, and the same reaction and expansion as in (1)-(2) occur on the crack surfaces. (5) Vertical cracks are generated on the surface due to the expansion of the inside. Therefore, local reaction and expansion by selective reaction of brucite may play an important role in the continuous reaction-induced fracture process in serpentinite.