Experimental study on the gas phase permeability of montmorillonite sediments in the presence of hydrates

Permeability is a key parameter influencing fluids flow capacity and the gas production rate of the marine hydrate-bearing sediments. In this study, a series of experiments were carried out to find out the dependence of the gas phase permeability of montmorillonite on different hydrate saturations at effective axial stresses of 1 MPa, 3 MPa. And an experimental study of the decomposition of methane hydrate (MH) by depressurization was carried out. It is found that with the increase of gas hydrate saturation, the gas phase permeability of montmorillonite shows a trend of decreasing first and then increasing, due to the effect of blockage of hydrate particles and the increase of pores in inter-aggregate zones during methane hydrate formation. Furthermore, the gas phase permeability of clay clearly decreases gradually with the increasing decomposition time under different hydrate saturations, due to the formation of the bound water and the swelling of clay. The magnitude of CH4 coefficient of permeability Ka and that of transmission D∗ reache 10−7and 10−8, respectively. Ka is also known as hydraulic conductivity in the Darcy law. It is defined as the unit flow rate under the unit hydraulic gradient, indicating the difficulty of the fluid passing through the pore skeleton. And D∗is the coefficient of transmission based on a modified form of Fick's law, which is a function of the volume-mass properties of the soil and the gas density. After the complete decomposition of the hydrate, the permeability coefficient Ka decreases with the increase of initial hydrate saturation when 4.02%≤Sh≤15.15%. However, 15.15%≤Sh≤27.26%, the values of Ka increase with the increase of initial hydrate saturation. These experiments helped us understand some important aspects of MH behavior such as saturation of hydrate formation, effective stress, and permeability change during decomposition. This work could be useful to research on the production efficiency of marine gas hydrate reservoir.