| Recently it has been suggested that methane bubbles in fine-grained sediments grow by a process of elastic expansion and fracture, according to the principles of linear elastic fracture mechanics (LEFM). To test this theory, bubbles were formed in sediments by gas injection and imaged using a CT-scanner. From this it is found that the sizes and shapes of these bubbles are consistent with the predictions of LEFM.;Finally a rise mechanism, based upon the principles of LEFM, is suggested and it is hypothesized that the speed of rise is controlled by the viscoelastic response of the sediments. Based upon this a finite element model of bubble rise is developed and used to estimate rise velocities.;A finite element model (FEM) of bubble growth, based upon the principles of LEFM, is presented and contrasted with a previous LEFM growth model that relies on an assumption of quasi-steady state. The FEM model consists of a reaction-diffusion (RD) module describing the production and transport of methane to the growing bubble and coupled to a sediment mechanics module describing the stress response of the sediments. When compared with the previous growth model it is found that the quasi-steady state assumption is flawed due to violation of mass conservation during fracture events. As a result bubbles grow faster then previously thought. The sensitivity of the model to its parameters is studied to gain an understanding of how bubbles may behave under different sediment conditions. The effect of forcing by pressure and temperature variations is also studied. It is found that temperature can have a significant influence on growth. Pressure fluctuations brought on by tides does not, but low tides can induce bubble release due to the reopening of previously formed fractures. In addition it is shown that a balance between gas supply and sediment mechanics can, under certain conditions, result in the cessation of growth, and can provide an explanation for the bubble morphology observed in natural sediments. |
