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

Abstract

Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane (CH4), an important greenhouse gas. They are a source of a permanent concern for their contribution to destabilization of coastal and aquatic infrastructure. Past accounts of gas bubbles developed in shallow muddy aquatic sediments and in their surrogates have reported a controversial occurrence of vertical as well as horizontal bubbles morphologies. Our study suggests an explanation of the apparent conundrum about preferred orientations of bubbles in muddy sediments. It is conducted by employing mechanical reaction–transport numerical model, which couples CH4 diffusion-led expansion of gas bubble and elastic-fracturing mechanical sediment response to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Bubbles growing in isotropic sediment develop a vertically oriented morphology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow preferentially horizontally. These bubbles can coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results suggest that anisotropy-led lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale outlets in aquatic sediments. Moreover, in contrast to the vertical bubbles, horizontal bubbles tend to be stationary, thus being responsible for high gas storage capacity of anisotropic sediments.