Reaction-Induced Fracturing Under Subsurface Conditions
a Warren Distinguished Lecture with Wen-lu Zhu, Geology, University of Maryland
The rate and extent of serpentinization and carbonation of ultramafic rocks are a subject of debate. On the one hand, the products from these volume expansion reactions may fill existing pore space and passivate reactive surfaces, preventing the reactions from completing. On the other hand, the increase of solid volume may generate enough stress to fracture the host rock, which provides positive chemical-mechanical feedback to maintain reaction by adding new fluid pathways and exposing unreacted surface. Realistic assessments of the efficiency of geological carbon sequestration require better understanding of the mechanics of reaction-induced fracturing. Zhu and team conducted dynamic microtomography experiments to investigate the effects of confinement and pore fluid pressure on hydration of periclase MgO to brucite Mg(OH)2 at subsurface conditions. In the first suite of experiments, they quantified the control of reaction-induced fracturing over the rate of periclase to brucite transition. At temperatures of 180–200°C, they observed that reaction-induced fracturing took place only when effective confinements were less than 30MPa. Below this threshold, the hydration of periclase led to reaction-induced fracturing and the brucite-for-periclase replacement was able to proceed to completion within a few hours. Above 30MPa, the periclase to brucite transition caused a porosity reduction and the reaction became self-limited. The dependence of reaction-induced fracturing on effective confining pressure is intriguing. Several mechanisms for the observed threshold, including disjoining pressure and ductile creep, are discussed. In the second suite of experiments, they studied how the fracking serpentine rocks via hydration reactions. A periclase rod was inserted into a central hole of a serpentinite sample. They found that the expansion of the periclase/brucite composite against the surrounding serpentinite exerted sufficient normal stress to nucleate fractures at the inner wall of the serpentinite. Using time-resolved three-dimensional imaging, they quantified the spatial and temporal distribution of the reaction-induced fractures. Their results show that the reaction-induced fractures developed a hierarchical fracturing pattern, similar to that observed in serpentinized mantle rocks. With increasing pore fluid pressure, the fracture spacing decreases and the number of fractures increases. These experimental results have important implications in understanding microseismicity in the serpentinization of oceanic crust.
Wen-lu Zhu, Professor, Department of Geology, University of Maryland. Wen-lu received her Ph.D. at Stony Brook University. She worked at the Woods Hole Oceanographic Institution as a research scientist before moving to the University of Maryland. Wen-lu’s research focuses on the coupling between rock deformation and fluid flow in Earth’s crust and mantle. She measures the evolution of mechanical and transport properties and quantifies the change of the 3-D rock structure at various scales. She received the 2020 Louis Néel Medal from the European Geosciences Union for her contributions to understanding coupling between fluids and rock deformation.