Enhanced gas migration through permeable bubble networks within consolidated soft sediments

Many consolidated sediments experience in situ gas generation from methanogenesis, corrosion or radiolysis reactions and can retain bubbles for long periods. Particular interest is motivated by the retention and acute release of flammable hydrogen from nuclear legacy waste sludge. X-ray computed tomography was employed to observe 0:07-10mm bubble populations within 30-1112Pa yield strength Mg(OH)2 sediments. High rates of partial coalescence were observed amongst sub-millimetre microvoids, forming extensive bubble networks which spanned the 32mm field of view. Lattice Boltzmann and Monte Carlo modelling demonstrated these networks to be highly pervious to gas, with effective diffusivities for hydrogen of 3:7-12:5 x 10^-5 m2 s^-1. Continuous vessel-spanning bubble networks, dynamic conductivity between ganglia of coalesced bubbles, Haines jumps and composite diffusion through the gas and aqueous phase can account for enhanced gas migration over length-scales of several metres, thus enabling chronic gas release from low-intermediate strength sediments (10 Pa <τ< 1 kPa) too strong for buoyant bubble ebullition and too weak for vertical channel formation.