5:15 PM - 7:15 PM
[SCG54-P01] Self-induced Differential Stress and Reaction-induced Fracturing in Hydration of Periclase: Insights from Real-time measuring Experiments
Keywords:Reaction-induced stress, hydration, Reaction-induced Fracturing
Serpentinization and carbonation of ultramafic rocks cause significant solid volume expansion, reducing porosity and permeability, which inhibits reaction progress. However, natural rocks exhibit extensive serpentinization and carbonation, and the mechanisms and controlling factors of these reactions remain poorly understood. Recent numerical simulations and experiments have suggested that when the reaction rate exceeds the fluid flow rate, reaction-induced fracturing occurs, which may enhance reaction progress[1][2]. However, in previous experiments, reaction, fluid flow, and deformation/fracturing behaviors have not been simultaneously measured, and the feedback system between fluid flow, reaction, and fracturing remains unclear. In this study, we conducted batch and flow-through reaction experiments on the hydration of periclase (MgO + H2O → Mg(OH)2, volumetric expansion: +119%) using a newly developed experimental apparatus capable of measuring deformation, fluid transport, and fracturing behaviors. We investigated the temporal evolution of permeability, reaction progress, and fracturing.
Two types of sintered periclase aggregates (height: 20mm, diameter: 10mm) were used: high-porosity samples (connected porosity: 9–11%) and low-porosity samples (connected porosity: 0–0.01%). Flow-through experiments at 180–210°C under 20 MPa confining pressure controlled external stress conditions, with real-time measurements of stress-strain, volume, acoustic emissions, and permeability. To compare reaction behavior and reaction rates, batch reaction experiments were also conducted under hydrostatic conditions.
In batch experiments, high-porosity samples reacted uniformly, while low-porosity samples formed reaction fronts progressing inward. In the flow-through experiments, the temporal evolution of fracturing behavior and permeability differed significantly depending on the initial porosity. High-porosity samples reacted uniformly, initially lowering permeability slightly but without major expansion. Over time, axial stress increased, leading to yielding. Samples expanded circumferentially with no macroscopic fracturing, and permeability remained stable or increased. These results suggest initial pore closure without the external shape change, followed by self-induced stress causing yielding. Expansion in high-porosity experiments began within 10–20 minutes. In contrast, in low-porosity samples, reaction onset was significantly delayed, requiring 1500–2900 minutes to start the reaction. During the flow-through experiments with low-porosity samples, an extended induction period was observed, characterized by slow reaction due to surface layer spallation. This was followed by a temporary increase in permeability, a rise in stress, and volume expansion. Eventually, volume stabilized, while permeability increased by two orders of magnitude. This suggests that local stress concentration caused by surface layer spallation and heterogeneous reaction led to sudden large-scale fracturing. This fracturing triggered cascading reactions, generating reaction-induced stress, yielding, and further fracturing, thereby enhancing permeability. Although the flow-through experiments required a long induction time, once reaction initiation occurred, the reaction rate was approximately 18 times higher than in the batch experiments. This suggests large-scale fracturing and cascading reactions accelerate reactions even in low-porosity materials.
This study has revealed the effects of self-induced differential stress on expansion reactions and has provided detailed insights into reaction progress mechanisms: (1) Even under isotropic initial stress, volume expansion generates differential stress that promotes fracturing, and (2) In low-porosity samples, heterogeneous reactions after long induction periods cause fracturing, enhancing permeability and reaction acceleration.
[1]Shimizu and Okamoto 2016, Contrib Mineral, 171, 1-18
[2]Uno et al., 2002 PNAS, 119.3
Two types of sintered periclase aggregates (height: 20mm, diameter: 10mm) were used: high-porosity samples (connected porosity: 9–11%) and low-porosity samples (connected porosity: 0–0.01%). Flow-through experiments at 180–210°C under 20 MPa confining pressure controlled external stress conditions, with real-time measurements of stress-strain, volume, acoustic emissions, and permeability. To compare reaction behavior and reaction rates, batch reaction experiments were also conducted under hydrostatic conditions.
In batch experiments, high-porosity samples reacted uniformly, while low-porosity samples formed reaction fronts progressing inward. In the flow-through experiments, the temporal evolution of fracturing behavior and permeability differed significantly depending on the initial porosity. High-porosity samples reacted uniformly, initially lowering permeability slightly but without major expansion. Over time, axial stress increased, leading to yielding. Samples expanded circumferentially with no macroscopic fracturing, and permeability remained stable or increased. These results suggest initial pore closure without the external shape change, followed by self-induced stress causing yielding. Expansion in high-porosity experiments began within 10–20 minutes. In contrast, in low-porosity samples, reaction onset was significantly delayed, requiring 1500–2900 minutes to start the reaction. During the flow-through experiments with low-porosity samples, an extended induction period was observed, characterized by slow reaction due to surface layer spallation. This was followed by a temporary increase in permeability, a rise in stress, and volume expansion. Eventually, volume stabilized, while permeability increased by two orders of magnitude. This suggests that local stress concentration caused by surface layer spallation and heterogeneous reaction led to sudden large-scale fracturing. This fracturing triggered cascading reactions, generating reaction-induced stress, yielding, and further fracturing, thereby enhancing permeability. Although the flow-through experiments required a long induction time, once reaction initiation occurred, the reaction rate was approximately 18 times higher than in the batch experiments. This suggests large-scale fracturing and cascading reactions accelerate reactions even in low-porosity materials.
This study has revealed the effects of self-induced differential stress on expansion reactions and has provided detailed insights into reaction progress mechanisms: (1) Even under isotropic initial stress, volume expansion generates differential stress that promotes fracturing, and (2) In low-porosity samples, heterogeneous reactions after long induction periods cause fracturing, enhancing permeability and reaction acceleration.
[1]Shimizu and Okamoto 2016, Contrib Mineral, 171, 1-18
[2]Uno et al., 2002 PNAS, 119.3