Importance of Pore Size Distribution of Fine-grained Sediments on Gas Hydrate Equilibrium

Abstract:

Gas hydrates have been considered as a new source of natural gases. For the gas hydrate production, the gas hydrate reservoir should be depressurized below the equilibrium pressure of gas hydrates. Therefore, it is important to predict the equilibrium of gas hydrates in the reservoir conditions because it can be affected by the pore size of the host sediments due to the capillary effect. In this study, gas hydrates were synthesized in fine-grained sediment samples including a pure silt sample and a natural clayey silt sample cored from a hydrate occurrence region in Ulleung Basin, East Sea, offshore Korea. Pore size distributions of the samples were obtained by the nitrogen adsorption and desorption test and the mercury intrusion porosimetry. The equilibrium curve of gas hydrates in the fine-grained sediments were found to be significantly influenced by the clay fraction and the corresponding small pores (>50 nm in diameter). For the clayey silt sample, the equilibrium pressure was higher by ~1.4 MPa than the bulk equilibrium pressure. In most cases of oceanic gas hydrate reservoirs, sandy layers are found interbedded with fine-grained sediment layers while gas hydrates are intensively accumulated in the sandy layers. Our experiment results reveal the inhibition effect of fine-grained sediments against gas hydrate formation, in which greater driving forces (e.g., higher pressure or lower temperature) are required during natural gas migration. Therefore, gas hydrate distribution in interbedded layers of sandy and fine-grained sediments can be explained by such capillary effect induced by the pore size distribution of host sediments.