Methane hydrate stability in pore water: A simple theoretical approach for geophysical applications

Geophysicists have recently expressed an interest in understanding how pore water composition affects CH4 hydrate stability conditions in the marine environment. It has previously been shown in the chemical engineering literature that CH4 hydrate stability conditions in electrolyte solutions are related to the activity of water (a(w)). Here we present additional experimental data in support of this relationship and then use the relationship to address issues relevant to geophysicists. Pressure and temperature conditions of CH4 hydrate dissociation were determined for 10 solutions containing variable concentrations of Cl-, SO42-, Br-, Na+, K+, Mg2+, NH4+, and Cu2+. The reciprocal temperature offset of CH4 hydrate dissociation between the CH4-pure water system and each of these solutions (and for other electrolyte solutions in literature) is directly related to the logarithm of the activity of water (lna(w)). Stability conditions for CH4 hydrate in any pore water system therefore can be predicted simply and accurately by calculating lna(w). The effect of salinity variation and chemical diagenesis on CH4 hydrate stability conditions in the marine environment can be evaluated by determining how these processes affect lna(w) of pore water.