Influence of anisotropy in sediment mechanical properties on CH4 bubble growth topology and migration pattern in muddy aquatic sediments

Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane, an important greenhouse gas. Past accounts of gas bubbles developed in shallow aquatic sediments (and in their surrogates) have reported a controversial occurrence of vertical as well as horizontal bubbles topologies. Within the framework of tensile fracturing of muddy sediment produced by the growing bubbles, the vertical orientation of bubbles is well understood, however factors controlling horizontal bubble growth are largely unclear. This study is conducted by employing a mechanical/reaction–transport numerical model, which couples diffusion-led expansion of gas bubble and elastic-fracture mechanical response of sediment to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (a property describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Our results demonstrate that bubbles growing in isotropic sediment develop a vertically oriented topology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow horizontally at the initial stages, however at later stages they start evolving in vertical direction as well, under influence of gravity, and eventually initiate their vertical ascent as well. Our results suggest an explanation of apparent conundrum about preferred orientations of bubbles in muddy sediments. Laterally growing bubbles produced in anisotropic sediment are able to coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results reveal that anisotropy-led initial lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale seeps and outlets, at continental margins and inland water bodies sediments. Additionally, horizontal bubbles tend to be stationary (in contrast to the vertical bubbles) thus being responsible for high gas storage (or retention) capability of aquatic sediments.