Competitive methane bubble growth in aquatic muds

Abstract

Methane (CH4) bubbles residing in shallow aquatic muds pose a significant threat to the environment. Impeded by the muddy sediment opacity and insufficient resolution for their characterization, past studies overlooked bubble interactions as they grow. Here, the competitive fracture-driven growth of CH4 bubble pair with different initial sizes in the aquatic muds is simulated, using a mechanical/reaction-transport numerical model. Mechanical and solute transport interactions were found to dominate at the different stages of the bubble growth, both retarding the smaller competitive bubble growth. The stress from large competitive bubble affects the inner pressure and diffusive CH4 flux to the smaller bubble, causing its slower initial growth (at t < 40 s). The large competitive bubble also diverts CH4 from the smaller one later on, thus inhibiting its growth even more. Bubble stress interactions may produce more laterally oriented smaller bubble and significant deformations of the larger one, both amplified under the decreasing distance between the growing bubble pair. Competitive bubble growth may shape a bubble size distribution pattern observed in the lab experiments and in situ, which may promote muddy sediment CH4 gas retention and produce gas domes. The latter acts as pockmark precursors whose formation induces a violent gas release to the water column and potentially to the atmosphere. Our study presents a basis for a proper upscaling to various effective gassy muddy sediment characteristics and gas retention evolution transient models, while preserving the single bubble growth descriptors. It contributes to evaluation and even reduction of a long-persisting uncertainty related to CH4 flux from the shallow aquatic sediments.