Gas transport model in pore heterogeneous bedded salt rock: Implications for tightness evaluation of salt cavern gas storage

To address the gas flow behaviors in bedded salt rock, an apparent permeability model considering Knudsen diffusion, pore surface diffusion, and pore heterogeneity was presented. The evolutions of gas permeability and gas flow behaviors with pressure were hinted by a gas seepage experiment carried out for model verification. The results show that the new apparent permeability model could accurately invert the seepage behaviors in the cores of salt rock and interlayer, with an average error of 2.39% and 1.22% for helium and nitrogen, respectively. When the pore pressure is 0.5 MPa, the permeability contribution of Knudsen diffusion accounts for 82.35% of the apparent permeability. When the pore pressure is higher than 1.24 MPa, the permeability of Knudsen diffusion is lower than that of viscous flow. Once the gas pressure is higher than 2 MPa, the permeability contribution of Knudsen effect is basically equal to 0. A special focus was given to the different gases in salt cavern gas storage, which indicated that the leakage ratio of hydrogen is the highest, followed by methane, and helium is the lowest, with 7.27%, 5.76%, and 4.58%, respectively during the 30 years operation period. This work could add further insights into the gas flow behaviors in the pore heterogeneous salt rock and be employed for tightness evaluation of salt cavern gas storage and pillar width design of adjacent salt caverns.