Pore-scale investigation of methane hydrate phase transitions and growth rates in synthetic porous media

Abstract

A growing demand for energy and natural gas in particular, makes methane gas hydrates a potential target to supplement natural gas production from conventional resources. Several pilot projects have proven the feasibility of gas production from hydrates both onshore and offshore, but the proposed models of gas hydrate behavior in sediments lack experimental support. In particular direct pore-scale observations of gas hydrate phase transitions can assist in a better understanding of the fundamentals of gas hydrate phase transitions in sedimentary systems. In this thesis, methane gas hydrate formed from methane gas and distilled water was visually investigated in high-pressure micromodel based on a thin section of Berea sandstone. The main objective of this work was to supplement previous research at the Reservoir Physics groups to determine hydrate formation and dissociation mechanisms on pore scale; and to estimate the hydrate growth rates. Fifteen primary and twenty-two secondary hydrate formation experiments were performed in this work. The first seven primary formations were carried out with initial water saturation ranging from 0.30 to 0.60 and nearly constant pressure (83-84 bar) and temperature (1.2-1.4 °C), with the main intention to study the hydrate formation mechanisms and the effect of initial water saturation and pore sizes on the hydrate growth rates. The remaining experiments were conducted at various pressure-temperature conditions to further investigate formation mechanisms and provide growth rate measurements as function of driving force (degree of subcooling). The hydrate formation was first observed in the continuous water-gas interface, followed by hydrate growth into trapped gas. For both gas configurations, the hydrate growth initiated at the water-gas interface along the pore walls and then progressed towards the pore center. The hydrate growth resulted in two different hydrate configurations: crystalline hydrate with total gas consumption and hydrate films/shell with enclosed gas. Hydrate was also observed to grow in water at the water-hydrate interface, but this was rare and restricted to locations where the free gas was available within a relatively short distance. Massive growth in water was only achieved due to forced dissolution of methane in water. The hydrate formation was quantified by estimating the hydrate film growth rates. The hydrate growth at the water-gas interface along the pore walls was approximately two orders of magnitude faster compared to the hydrate growth towards pore center. The hydrate film growth rates were studied with respect to initial water saturation, pore sizes and driving force (degree of subcooling, ΔT). No clear effect of initial water saturation and pore sizes on the growth rate was observed. The hydrate growth rate towards the pore center seemed to increase with driving force but the relationship was weak. The growth rate along the pore walls was proportional to ΔT^2 and showed a good agreement with the measurements of hydrate film growth reported in literature for bulk systems. Thirty-four hydrate dissociation experiments were conducted by reducing the pressure several bars below the equilibrium pressure. The hydrate dissociation was studied with respect to hydrate configurations (crystalline and hydrate films/shell). The dissociation of crystalline hydrate was slower and resulted in a gas front that flowed through the pore space, inducing further hydrate dissociation. The dissociation of hydrate films/shell with enclosed gas initiated in the pore center and then continued towards the pore walls, leading to the mobilization of enclosed gas. The direct contact with mobile gas rather than water favored hydrate dissociation. The overall dissociation rate of crystalline hydrate seemed to increase with decreasing hydrate saturation.