Stability Condition of Methane Hydrate in Fine-Grained Sediment

Abstract

Stability condition is of critical importance for methane hydrate exploitation, transportation, and reserves. This study measured the stability conditions of methane hydrate in fine-grained sediment with different dry densities (ρd = 1.40, 1.50 and 1.60 g/cm3) and various initial water saturations by the multi-step heating method. The experimental result showed that the methane hydrate formation in fine-grained sediment required lower temperature and/or higher pressure compared to that in bulk state. At the same time, it is found that the deviation degree of P–T conditions of methane hydrate in fine-grained sediment with different dry density and initial water saturation are completely different from that in pure water. In addition, according to the nuclear magnetic resonance technique (NMR), the changes in NMR signal intensity during the formation and decomposition of methane hydrate in silt were analyzed. Regardless of formation and dissociation stages, liquid water always distributes in the small sediment pores. An empirical formula is developed to address the capillary suction of water and hydrate with respect to the unhydrated water within sediment. Furthermore, a phase equilibrium model is proposed to predict the stability conditions of hydrate-bearing fine-grained sediment.