Numerical simulation of dissociation of methane hydrate in response to depressurization by periodic uplift

In marine sedimentary environment, dissociation of methane hydrate (MH) in sediment occurs when pressure and temperature condition of the sediment changes from MH stability to gas-water by either increasing temperature or decreasing pressure, or both. Up to now, characteristics of dissociation of MH in marine sedimentary environment by increase of bottom-water have been investigated in several studies. In the present study, we present a numerical model of dissociation of pore-space MH in sandy sediment in response to periodic events of rapid decrease of pressure. This model is based on one-dimensional heat conduction equation of uniform physical properties and takes into account the latent heat of formation and dissociation of MH. The pressure in sediment is assumed as hydrostatic. The upper boundary as seafloor is set to a constant temperature. A constant heat flow is supplied in the lower boundary. This model also assumes for the sediment with pore-space MH that when pressure changes from MH stability to gas-water, temperature of the sediment decreases to the temperature of hydrate stability boundary at that pressure by endothermic dissociation of MH. We apply the numerical model to investigate characteristics of dissociation of MH and the effects of the dissociation of MH on the subbottom thermal regime in response to depressurization by periodic uplift that occurs every 100 years. For the numerical computation, different values of initial water depth, saturation of MH, displacement of uplift and thickness of pore-space MH layer in sediment are assigned to investigate how these parameters contribute to the dissociation of MH in response to the periodic uplift. In sediment that MH changes to gas-water by uplift, endothermic dissociation of MH makes temperature of the sediment decrease to the temperature of hydrate stability boundary at that depth. After that, dissociation of MH progresses at the base of gas hydrate stability (BGHS) at the depth until the next occurrence of uplift. Because of the endothermic dissociation of MH, temperature of sediment around BGHS decreases. This acts as self-cooling of the sediment and works to slow dissociation of MH. Dissociation of MH becomes faster for the model of shallower initial water depth, larger displacement, or low MH salutation. On the other hand, thickness of pore-space MH layer only controls completion time of dissociation of MH. This study is supported by MH21, Research consortium for methane hydrate resource in Japan.