Dissociation Behavior of CO2 Hydrate in Sediments during Isochoric Heating

Abstract

As CO2 is sequestered into sediments in the oceanic environment, CO2 hydrate can form as a byproduct. This study explored the dissociation behavior of CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size. We synthesized CO2 hydrate in three types of particulate sediments: glass beads, fine sand, and crushed silt. We then dissociated them through isochoric heating. We observed the excess pore fluid pressure build-up and self-preservation behavior, in which the pressure-temperature state evolves along the hydrate phase boundary until either it reaches the second quadruple point or all hydrates dissociate. The pore fluid pressure evolution is limited, however, by the CO2 vapor-liquid phase equilibrium boundary due to the liquefaction of CO2. The presence of CO2 liquid in sediments forces the pressure-temperature evolution to follow the CO2 vapor-liquid phase equilibrium boundary, regardless of hydrate formation and dissociation processes. CO2 hydrate in fine-grained sediments experiences capillary pressure-induced melting point depression, but this effect vanishes when the pores exceed approximately 1 microm, such as in coarse-grained sediments. In particular, any fracture generation in sediments which involves the local release of confinement eliminates the melting point depression induced by the capillary effect.