Effect of marine clay minerals on the thermodynamics of CH4 hydrate: Evidence for the inhibition effect with implications

Clay minerals are widely distributed in hydrate-bearing sediments worldwide, especially in the clayey-silty sediments in the South China Sea. The debate on the influence of clay on the thermodynamics of natural gas hydrate (NGH) remains and impedes the fundamental understanding of hydrate-clay interactions and the design of effective production strategies for energy recovery. Specifically, the phase equilibria of MH in the presence of clay and how different types of water associated with clay particles affect NGH thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the phase equilibria of methane hydrate (MH) were measured in the presence of typical clays from the South China Sea, i.e., montmorillonite, kaolinite, and illite. The shift of MH phase equilibria curve to a more stringent pressure was identified with Na-montmorillonite showing the most significant inhibition. The leftward shift (inhibition) of phase equilibrium temperature (Teq) is up to 3.9 K at a pressure of 10.40 MPa for Na-montmorillonite compared with pure water. Furthermore, we develop a thermodynamic model based on the classical Chen-Guo model first accounting for the change in water activity due to cation exchange of Na-montmorillonite. The developed model exhibits good predictability with measured thermodynamic data aligns favorably with our model prediction. The inhibition on the phase equilibria of MH is linked to the reduction of water activity due to clay-bound water for clays with a swelling nature. The findings of this study offer direct evidence and explanations on how clay minerals thermodynamically inhibit CH4 hydrate. It provides valuable insights into the resource estimation of clay-silty natural gas hydrates (NGH) in nature and facilities the design of suitable production strategies targeting clay-rich hydrate-bearing sediments.