Initial rise of bubbles in cohesive sediments by a process of viscoelastic fracture

Abstract

[1] An understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates. Past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid (e.g., magmas and hydrofractures) appear not to be applicable to bubbles in soft sediments, and this paper presents a new model for gas bubble rise in soft, fine-grained, cohesive sediments. Bubbles in such sediments are essentially "dry" (little if any free water) and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics, which assume the existence of a spectrum of flaws within the sediment fabric. By extending this theory, we predict that bubbles initially rise by preferential propagation of a fracture in a (sub) vertical direction. We present a criterion for initial bubble rise. Once rise is initiated, the speed of rise is controlled by the viscoelastic response of the sediments to stress. Using this new bubble rise model, we estimate rise velocities to be of the order of centimeters per second. We again show that capillary pressure plays no substantive role in controlling bubble growth or rise.

Laboratory Modelling of Volcano Plumbing Systems: A Review

“…(ii) The propagation direction is governed by the balance between the buoyancy forces and the strength of the gel: if buoyancy is small, the dyke propagates in all directions, whereas if the buoyancy is large, the dyke propagates dominantly upward. (iii) A buoyant crack of constant volume propagates itself only if it is larger than a critical length (see also Algar et al 2011). …”

Methane bubble growth in fine-grained muddy aquatic sediment: Insight from modeling

“…However, fracture event results in sudden increase in bubble volume and drop in internal gas pressure (Johnson et al, 2002), implying that transient reaction-diffusion equation must be solved in combination with LEFM, as implemented by Boudreau (2009, 2010). When buoyancy was added to the model, an initial rise (propagation) of a large (mature) bubble was simulated by Algar et al (2011b), neglecting mass transfer between the bubble and sediment. Fracturing in the model was permitted to occur at the bubble head only.…”

“…The sediment cell is presented as a block cut by a symmetry plane (only half of the block is modeled). The initial bubble seed is presented as a small pennyshaped crack with one of its free surfaces located on the symmetry plane (Algar et al, 2011b).…”

A new instrument for high-resolution in situ assessment of Young’s modulus in shallow cohesive sediments

“…The ability of infauna to move in finegrained sediment is dependent on the energy expended by animals in elastic deformation until fracture occurs. Linear elastic fracture mechanics (LEFM) modelling of gas bubble growth (Algar and Boudreau 2009) and rise (Algar et al 2011) also require evaluation of Young's modulus, E, in near-surface sediments as model input parameters.…”