Gas Saturation Resulting from Methane Hydrate Dissociation in a Porous Medium: Comparison between Analytical and Pore-Network Results

We develop predictive tools for methane gas saturation that results from hydrate dissociation in porous media at different length scales, ranging from the single-pore scale up to the pore-network scale. Initially, we examine the case of a single spherical hydrate grain dissociating inside a bulk continuum (i.e., without the constraints from the solid surfaces that are present inside a porous medium). The growth of the resulting gas bubble is limited only by the liquid pressure of the domain, the capillary pressure of the growing bubble, and the gas solubility in the liquid phase. This case corresponds to an upper limit for the gas saturation. Next, we consider the case of a hydrate grain located inside a single pore body as well as the case of multiple hydrate grains that are distributed randomly inside the pore network. To this purpose we consider a simple porous domain that is represented by a pore network with all pores/throats being of the same size. When the hydrate phase is confined inside a porous domain, the growth of the resulting gas bubble is controlled, mostly, by the capillary thresholds of the interconnections (i.e., the pore throats) between the different pore bodies. For all cases we develop analytical solutions for the ratio of the gas to the hydrate saturation, S_g/S_H, and compare the solutions with results obtained from pore network simulations [Phys. Rev. E 2006, 74, 056303] where the pore bodies/throats follow appropriate size distributions. Very good agreement is observed between the different approaches considered in this study.