Experimental Study on the Difference of Fluid Flow between Methane Hydrate-Bearing Sand and Clay Sediments

The gas-water seepage of natural gas hydrate sediments in the South China Sea significantly influences the efficiency of reservoir production of water/gas. However, there are significant differences between gas and water flow in methane hydrate-bearing sand and clay sediments. In this study, a series of permeability experiments were carried out in quartz and montmorillonite sediments in the presence of methane hydrate. The experiment results indicated that gas permeability decreases with increasing effective stress in montmorillonite and quartz in the absence of methane hydrate. The decrease in porosity has a higher impact on the permeability of quartz sand than that of montmorillonite. Due to the different effects of methane hydrate formation on the pore structure of sand and clay, with the increase in methane hydrate saturation in quartz sand and montmorillonite, the effect of effective stress on the permeability reduction of quartz sand is gradually weakened but the effect of effective stress on the permeability reduction of montmorillonite is gradually enhanced. Under the condition of constant total stress, the gas permeability decreases with the decomposition of methane hydrate. The permeability damage rate of montmorillonite caused by methane hydrate decomposition is much higher than that of quartz sand due to the swelling effect of montmorillonite particles. In addition, the water-phase seepage of quartz in the presence of methane hydrate is in line with the Darcy seepage characteristics. However, the water-phase seepage of montmorillonite in the presence of methane hydrate has non-Darcy seepage characteristics and the calculated water permeability is far less than the gas permeability.