Experimental study of methane hydrate formation and decomposition in the porous medium with different thermal conductivities and grain sizes

Gas hydrate found in nature is mainly existed in deposit, such as marine deposit and permafrost. The thermophysical properties of the deposit are largely influenced by the formation features of gas hydrate, thereby affecting the hydrate production. In this manuscript, the experiments of formation and decomposition behavior of gas hydrate in the deposit with different grain sizes (40-60 mesh, 80-120 mesh, 325-400 mesh) and different thermal conductivities (0.926 W/m.K, 28.8 W/m.K, 41.9 W/m.K) were conducted in the small cubic hydrate simulator, and the coupling effect of the heat and mass transport on the hydrate formation and dissociation were researched. It was concluded that the mass transport rate in the deposit dominated the hydrate formation. The formation of gas hydrate was initial at the contact surfacing of gas-water and grew gradually in the gas-rich region, and the hydrate formation amount in the water-rich region was little. In the deposit with the grain size of 80-120 mesh and 325-400 mesh, there was no obvious induction time for the hydrate formation that occurred during gas injection. Be different from the hydrate formation, the heat transfer rate of the deposit restricted chiefly methane hydrate dissociation. With the raise of grain size and thermal conductivity of deposit, methane hydrate decomposition rate enhanced. It's also found that the formed hydrate acted as cementation in porous medium with the grain size of 80-120 mesh and 325-400 mesh. The heat transfer rate of the deposit would significantly decrease when most of the cementation between the porous medium was disappeared as a result of hydrate dissociation, and the hydrate saturation in sediments at this time was defined as the critical hydrate saturation (50-80% of the initial hydrate saturation). The critical hydrate saturation is meaningful for gas hydrate resource prospecting and the risk assessment of gas hydrate production in actual fields.