Dissolution of randomly distributed soluble grains: post-dissolution k 0 -loading and shear

2019

JGR Solid Earth

Gas migration mechanisms control the release of gas from seafloor sediments. We study underlying phenomena using transparent sediments subjected to controlled effective stress; this experimental approach allows high‐resolution real‐time monitoring of gas migration through cohesionless granular materials under 3‐D boundary conditions. Observed migration patterns depend on the effective stress at the time of injection and the stress history. Gas migration transitions from pore invasive to grain displacive when the capillary pressure for air entry ΔPAE is greater than the effective stress σ′. This study focuses on grain‐displacive gas migration. The morphology of grain‐displacive gas bodies changes with depth as the sediment stiffness G increases and the effect of surface tension γ vanishes: spheroidal gas bubbles form in the near‐surface, faceted cavities further down, and eventually open‐mode fractures develop at depth. The gas injection pressure is proportional to the effective stress in grain‐displacive migration. Preloading and overconsolidation cause the rotation of principal stresses and gas‐driven openings align with the new minimum principal stress direction. Cyclic loading promotes the upward migration of gas‐filled openings, and there is mechanical memory of previous gas pathways in sediments.

Section: Resultssupporting

confidence: 58%

Grain‐Displacive Gas Migration in Fine‐Grained Sediments

Sun

2019

JGR Solid Earth

“…Rather than wormhole formation (limited by arching instability), grain dissolution readily affects interparticle coordination and forces, triggers fabric rearrangement, and the void ratio evolves towards a stress-dependent terminal value closer to e max . Consequently, both hydraulic and mechanical properties are affected; in particular, there is an increased contractive tendency (Cha and Santamarina, 2014). Furthermore, experimental and discrete element DEM simulation results show that 1) the horizontal stress ratio k o decreases during dissolution and it can reach the active value k a (i.e., internal shear failure), often followed (Shin and Santamarina, 2011b).…”

Section: Reactive Fluid Flowmentioning

confidence: 99%

Fluid-Driven Instabilities in Granular Media: From Viscous Fingering and Dissolution Wormholes to Desiccation Cracks and Ice Lenses

Liu

2022

Front. Mech. Eng.

Single and multi-phase fluids fill the pore space in sediments; phases may include gases (air, CH4, CO2, H2, and NH3), liquids (aqueous solutions or organic compounds), and even ice and hydrates. Fluids can experience instabilities within the pore space or trigger instabilities in the granular skeleton. Then, we divided fluid-driven instabilities in granular media into two categories. Fluid instabilities at constant fabric take place within the pore space without affecting the granular skeleton; these can result from hysteresis in contact angle and interfacial tension (aggravated in particle-laden flow), fluid compressibility, changes in pore geometry along the flow direction, and contrasting viscosity among immiscible fluids. More intricate fluid instabilities with fabric changes take place when fluids affect the granular skeleton, thus the evolving local effective stress field. We considered several cases: 1) open-mode discontinuities driven by drag forces, i.e., hydraulic fracture; 2) grain-displacive invasion of immiscible fluids, such as desiccation cracks, ice and hydrate lenses, gas and oil-driven openings, and capillary collapse; 3) hydro-chemo-mechanically coupled instabilities triggered by mineral dissolution during the injection of reactive fluids, from wormholes to shear band formation; and 4) instabilities associated with particle transport (backward piping erosion), thermal changes (thermo-hydraulic fractures), and changes in electrical interparticle interaction (osmotic-hydraulic fractures and contractive openings). In all cases, we seek to identify the pore and particle-scale positive feedback mechanisms that amplify initial perturbations and to identify the governing dimensionless ratios that define the stable and unstable domains. A [N/m] Contact line adhesion

“…The gradual dissolution was controlled by progressively lowering the salt concentration in the permeated fluid (duration: 3 days). These slow and progressive changes in salt concentration over the long period ensured all soluble particles decreased in size at the same time (a “homogeneous dissolution” as in Cha and Santamarina 47 ). The low reaction rate (low Damkohler number) guaranteed uniform dissolution and avoided preferential dissolution modes such as localized dissolution and face dissolution initiated from the inlet boundary.…”

Section: Experimental Studymentioning

confidence: 99%

Effect of grain dissolution on sloping ground

Cha

2022

Sci Rep

The static and dynamic stability of natural or constructed slopes can be affected by dissolution or dissolution-like phenomena. Their underlying mechanisms, however, remain unclear. New experimental results and discrete element simulations provide particle-level and macroscale information on the consequences of mineral dissolution on slope behavior. At the microscale, load-carrying grain arches develop around dissolving particles, the porosity increases, and contact force chains evolve to form a honeycomb topology. At the macroscale, while vertical settlements are the prevailing deformation pattern, lateral granular movements that create mass wasting are prominent in sloping ground, even under the quasi-static granular loss. Horizontal grain displacement is maximum at the surface and decreases linearly with the distance from the slope surface to become zero at the bottom boundaries, much like vertical granular displacement along the depth. Sediments with smaller friction angles and steeper slopes experience greater displacement, both vertically and horizontally. Slopes become flatter after dissolution, with the reduction in slope angle directly related to the loss in ground elevation, ΔH/Ho. Yet, because of the porous fabric that results from dissolution, vertical shortening is less than the upper bound, estimated from the loss in the solid mass fraction, ΔH/Ho≈SF. Under water-saturated conditions, the post-dissolution fabric may lead to sudden undrained shear and slope slide.