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

Abstract

Geophysicists have recently expressed an interest in understanding how pore water composition affects CH 4 hydrate stability conditions in the marine environment. It has previously been shown in the chemical engineering literature that CH 4 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 CH 4 hydrate dissociation were determined for 10 solutions containing variable concentrations of Cl − , SO 4 2− Br − , Na + , K + , Mg 2+ , NH 4 + , and Cu 2+ . The reciprocal temperature offset of CH 4 hydrate dissociation between the CH 4 ‐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 (ln a w ). Stability conditions for CH 4 hydrate in any pore water system therefore can be predicted simply and accurately by calculating ln a w . The effect of salinity variation and chemical diagenesis on CH 4 hydrate stability conditions in the marine environment can be evaluated by determining how these processes affect ln a w of pore water.