Sedimentation-driven cyclic rebuilding of gas hydrates

Abstract

<p>Gas hydrate recycling is an important process in natural hydrate systems worldwide. The recycling of hydrates often leads to high hydrate saturation close to the base of the gas hydrate stability zone (GHSZ). However, to date it remains enigmatic how free gas is recycled back into the GHSZ and what the controlling factors are. Here we use a 1D compositional multi-phase flow model to investigate the dominant mechanisms that control natural gas hydrate recycling. As case study, we apply the numerical model to study hydrate recycling at the Green Canyon Site 955 in the Gulf of Mexico, where high sedimentation rates are thought to drive vigorous hydrate dissociation and re-invasion of free gas into the stability zone. Our novel results suggest that hydrate recycling is a highly dynamic process in which hydrates form and dissociate at surprisingly rapid rates with an inherent cyclicity. These cycles can be divided into three phases of 1) gas accumulation phase, 2) gas breakthrough phase and 3) uninhibited hydrate build-up phase. During the first phase hydrates are dissociating and free gas accumulates below. After the free gas saturation reaches a threshold value (given by the mutual effects of entry pressure, bulk permeability, and relative permeability function), gas breaks through the barrier of the remaining hydrate layer. Controlled by permeability and kinetic rate a new hydrate layer forms.  In the absence of external perturbations to the GHSZ, gas migration leads to a distinct hydrate layer with a convex distribution of hydrate saturation. Such a hydrate layer acts like a converging-diverging `nozzle' for the gas flow, when gas enters the hydrate layer, it decelerates until it reaches the peak hydrate saturation (i.e. the nozzle throat), and then accelerates until it exits the hydrate layer on the other side. This nozzling effect, together with the hydrate dissociation kinetics, leads to the cyclic behavior of hydrate recycling. We suggest that the evident cyclicity of burial-driven gas hydrate build-up process provides a new advanced understanding of natural gas hydrate recycling process, and free gas invasion mechanisms into the GHSZ.</p>