Abstract
Soft sediments exhibit complex and varied deformation behavior during in situ bubble growth; however, the sediment microstructure is often neglected when predicting bubble networking or fracture propagation dynamics. This study considers three chemically similar Mg(OH)2-rich sediments, which differ slightly in their particle size distributions and morphologies but exhibit significant differences in their porosity, stiffness, and pore throat dimensions at equivalent yield strengths. At low yield strengths, microstructure greatly influenced the size distribution and connectivity of spherical bubble populations, with narrow sedimentary pore throats promoting coarser bubbles with diminished connectivity. Increased connectivity of the bubble population appeared highly significant in limiting bed expansion, either by establishing pathways for gas release or by dissipating excess internal bubble pressure, thereby diminishing further growth. During in situ gas generation, each sediment demonstrated a critical fracture strength, which demarcated the populations with high void fractions (0.27 < ν < 0.4) of near-spherical bubbles from a fracturing regime supporting reduced void fractions (ν ≈ 0.15) of high aspect ratio cracks. However, critical fracture strengths varied significantly (in the 60-1000 Pa range) between sediments, with coarser-grained and higher porosity sediments promoting fracture at lower strengths. Fracture propagation greatly enhanced the connectivity and diminished the tortuosity of the void networks, thereby augmenting the continuous gas release flux.
Highlights
In situ gas generation and retention have significant environmental implications for numerous natural and industrial waste sediments
The solid concentrations of corroded Magnox sludge (CMS) and corroded magnesium sludge (CMgS) samples are determined from the loss in mass following 24 h drying at 80 °C
The magnesium hydroxide powder sludge (MHPS) and CMgS sediments were imaged using an Inveon dual positron emission tomography (PET)/computed tomography (CT) scanner (Siemens, Germany) in accordance with the method outlined in Johnson et al.[7]
Summary
In situ gas generation and retention have significant environmental implications for numerous natural and industrial waste sediments. An ebullient flux of methane is promoted by elevated winds,[4] tides, and atmospheric pressure fluctuations,[5] which effectively reduce the critical diameter necessary for a bubble to achieve buoyancy.[6] under quiescent conditions, both theoretical models[6] and X-ray computed tomography (CT) imaging[3] have shown the critical bubble diameters required for bubble ebullition to be prohibitively large, even under low yield strength conditions of
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