17:15 〜 18:45
[SCG46-P05] MgOの水和膨張反応における変形・応力・浸透率の同時計測
キーワード：水和反応、浸透率、同時計測
Serpentinization and carbonation reactions of ultramafic rocks cause a significant volume increase. This volume change in rocks reduces porosity and permeability, preventing further reaction progress. In contrast, the theoretical consideration and numerical simulations that fracturing induced by volume increasing reactions can occur in the cases that the surface reaction is faster than fluid flow[1]. This has been confirmed by the laboratory experiments using analog materials[2]. However, to date, no experiments have been performed wherein the deformation and fracture behavior is measured in real-time while simultaneously observing the reaction, fluid flow, and deformation. Therefore, the feedback system of fluid flow-reaction-fracturing is still unclear. In this study, using a newly developed experimental apparatus, we investigate the temporal evolution of permeability, reaction progress and fracturing for the progress of volume-increasing reaction.
We conducted experiments on the hydration reaction of sintered aggregate of periclase to form brucite (MgO + H2O → Mg(OH)2 which is +119% solid volume expansion. Flow-through experiments were carried out using an experimental setup where external stress conditions (confining and axial pressure) were controlled, and changes in the overall reaction process (stress/strain, volume change, acoustic emission (AE), and permeability) were investigated in real-time. Two types of MgO sample (height: 20 mm and diameter: 10 mm) was used two different connected porosities; high porosity: 9-11% and low porosity: 0.01%. We conducted three runs with varying axial pressure and the porosity of the sample: Exp 1 (high porosity sample and axial pressure of 20 MPa), Exp 2 (high porosity sample and axial pressure of 40 MPa), and Exp 3 (low porosity sample and axial pressure: 20 MPa).
In Exp1, the axial pressure gradually increased and reached the maximum of 35 MPa, then gradually decreased. The axial strain also showed an increase by volume expansion with increasing axial pressure, followed by contraction with decreasing axial pressure. During the decrease of axial pressure or strain, flow rate was decreased. After 3 hours run, the sample was 80% reacted and expanded homogeneously in the circumferential direction. These results suggest that the porosity decrease and clogging of cracks caused by the compression of the entire sample, resulting in a decrease in permeability. In contrast, in Exp 2, homogeneous expansion was observed as similar to Exp 1, but with more shortening in the axial direction, and the permeability increased by about one order of magnitude. This is thought to be due to the formation of microcracks in the direction parallel to the axis.
While the experiment with the high porosity sample began to expand within 10-20 minutes after the start of the experiment, Exp 3 with the low porosity sample took 27 hours to the observe of the initiation of reaction that is monitored by increase in axial pressure. Sample expansion was observed after an increase in axial pressure and deformation in the direction of expansion. An increase in flow rate was accompanied with activation of AE signals, and the permeability increased by two orders of magnitude from the initial state. After the runs, many polygonal fractures were formed, suggesting that the reaction-induced fracturing drastically changed the hydrological properties of the sample. In both experiments, the maximum stress generation was observed in the initial phase of the reaction, followed by volumetric expansion.
This study showed a novel features on the volume-expanding hydration of MgO that (1) permeability increase can occur even in the porous rocks if differential stress is applied, by deformation of weak Mg(OH)2 or microcracks, and (2) in the low-porosity rocks, the reaction-induced fracturing drastically enhanced the permeability, that initiated after the long-induction time.
[1]Shimizu and Okamoto 2016, Contrib Mineral, 171, 73
[2]Uno et al. 2022 PNAS
We conducted experiments on the hydration reaction of sintered aggregate of periclase to form brucite (MgO + H2O → Mg(OH)2 which is +119% solid volume expansion. Flow-through experiments were carried out using an experimental setup where external stress conditions (confining and axial pressure) were controlled, and changes in the overall reaction process (stress/strain, volume change, acoustic emission (AE), and permeability) were investigated in real-time. Two types of MgO sample (height: 20 mm and diameter: 10 mm) was used two different connected porosities; high porosity: 9-11% and low porosity: 0.01%. We conducted three runs with varying axial pressure and the porosity of the sample: Exp 1 (high porosity sample and axial pressure of 20 MPa), Exp 2 (high porosity sample and axial pressure of 40 MPa), and Exp 3 (low porosity sample and axial pressure: 20 MPa).
In Exp1, the axial pressure gradually increased and reached the maximum of 35 MPa, then gradually decreased. The axial strain also showed an increase by volume expansion with increasing axial pressure, followed by contraction with decreasing axial pressure. During the decrease of axial pressure or strain, flow rate was decreased. After 3 hours run, the sample was 80% reacted and expanded homogeneously in the circumferential direction. These results suggest that the porosity decrease and clogging of cracks caused by the compression of the entire sample, resulting in a decrease in permeability. In contrast, in Exp 2, homogeneous expansion was observed as similar to Exp 1, but with more shortening in the axial direction, and the permeability increased by about one order of magnitude. This is thought to be due to the formation of microcracks in the direction parallel to the axis.
While the experiment with the high porosity sample began to expand within 10-20 minutes after the start of the experiment, Exp 3 with the low porosity sample took 27 hours to the observe of the initiation of reaction that is monitored by increase in axial pressure. Sample expansion was observed after an increase in axial pressure and deformation in the direction of expansion. An increase in flow rate was accompanied with activation of AE signals, and the permeability increased by two orders of magnitude from the initial state. After the runs, many polygonal fractures were formed, suggesting that the reaction-induced fracturing drastically changed the hydrological properties of the sample. In both experiments, the maximum stress generation was observed in the initial phase of the reaction, followed by volumetric expansion.
This study showed a novel features on the volume-expanding hydration of MgO that (1) permeability increase can occur even in the porous rocks if differential stress is applied, by deformation of weak Mg(OH)2 or microcracks, and (2) in the low-porosity rocks, the reaction-induced fracturing drastically enhanced the permeability, that initiated after the long-induction time.
[1]Shimizu and Okamoto 2016, Contrib Mineral, 171, 73
[2]Uno et al. 2022 PNAS