Destabilization of gas hydrate-bearing sediments induced by thermal changes

Abstract

Hydrate-bearing sediments may destabilize spontaneously as part of geological processes, unavoidably during petroleum drilling/production operations, or intentionally as part of gas extraction from the hydrate itself. In all cases, high pore fluid pressure generation is anticipated during hydrate dissociation. This dissertation centered on the destabilization of gas hydrate-bearing sediments in relation to behavioral characteristics and implications, which is induced by thermal changes. Partial dissociation during thermal stimulation is characterized by a pressure-temperature evolution along the phase boundary until all hydrate has dissociated. Pore fluid pressure generation is proportional to the initial hydrate fraction and the sediment bulk stiffness; is inversely proportional to the initial gas fraction and gas solubility. When the sediment stiffness is high, the generated pore pressure reflects thermal and pressure changes in water, hydrate, and mineral densities. Eventually, excess fluid pressure generation is limited by failure conditions. Shallower sediments require lower amounts of hydrate dissociation to reach failure than do deeper sediments, and hence they need a smaller increase in temperature. However, in the case of $CO_2$ hydrate, the pore fluid pressure evolution is limited by a $CO_2$ vapor-liquid phase equilibrium boundary due to the liquefaction of $CO_2$. This also applies to particular gases which have a liquefaction pressure above zero degree Celsius, such as ethane, propane and iso-butane. Hydrate dissociation in small pores experiences melting point depression induced by depressed water activity the capillarity between the hydrate-water interface yields; and lower fluid pressure generation due to the additional confinement the water-gas interface exerts on small gas bubbles. Therefore, lower excess pore water pressure develops in finer sediments with disseminated hydrates. Capillary effects vanish when pores exceed ~1 $\mu$ m (s...