Predictions of gas hydrate phase equilibria and amounts in natural sediment porous media Jeffery B. Klauda, Stanley I. Sandler * Department of Chemical Engineering, Center for Molecular and Engineering Thermodynamics, University of Delaware, Newark, DE 19716, USA Received 12 August 2002; received in revised form 10 May 2003; accepted 15 May 2003 Abstract Hydrates in marine sediments are estimated to contain more methane than conventional reserves. Previous estimates of the amounts of methane hydrate ignore pore size effects and variations in hydrate saturation of the pore space. The model presented here uses sediment type, geothermal gradient, and seafloor depths as inputs, and leads to predictions for the maximum depth of hydrate stability for data collected in the Ocean Drilling Program with an average error of 5%. Reaction-mass transfer partial differential equations are solved to estimate the amount of hydrate filling of the pore space, rather than using previous ad hoc choices for hydrate saturation. Predictions for the amounts of methane hydrate in the Mediterranean Sea, the Black Sea, the Gulf of Mexico, and the Northern Indian Ocean are made. q 2003 Elsevier Ltd. All rights reserved. Keywords: Gas hydrates; Clathrates; Porous media 1. Introduction Gas hydrates or clathrates are crystalline compounds that form from water with at least one other compound, and are stable at conditions above the normal freezing point of water. Methane is one of many guests that can form hydrates and will be considered here because of its abundance in the ocean seafloor regions where large masses of hydrates exist. There has been interest in developing methods to harvest the huge amounts of methane present in natural gas hydrates. A recent estimate of the amount of methane trapped in hydrates is as much as 300 times that in conventional US reserves (Sloan, 1998). Also, the melting and dissociation of gas hydrates in the ocean floor and in permafrost regions might increase global warming (Hatzikiriakos & Engle- zos, 1993; Hatzikiriakos & Englezos, 1994) resulting in further hydrate dissociation. Conversely, sequestering of carbon dioxide in hydrates has been proposed to reduce the amount of this greenhouse gas in the environment. Brewer, Friederich, Peltzer, and Orr (1999) and Brewer, Orr, Friederich, Kvenvolden, and Orange (1998) have experimentally shown the formation of CO 2 hydrates in the ocean. Most measurements and models have been for bulk gas hydrates that are important in industrial pipelines, but for which capillary effects are unimportant. Methane hydrates in nature occur in porous media, e.g. clay, silt, and sand, where capillary forces (surface tension) can be important, and result in a hydrate equilibrium pressure that increases as the pore size decreases. Handa and Stupin (1992) first examined the effect of pore size on the equilibrium pressures of methane hydrates with laboratory prepared porous silica gel, and Uchida, Ebinuma, and Ishizaki (1999) measured the equilibrium pressures of methane hydrates in pores of different sizes. However, measurements of hydrate equilibrium pressures in natural porous media (clay, silt, or sand) in the laboratory are limited. Seafloor hydrates are thermodynamically stable when at a given temperature the hydrostatic pressure is higher than the equilibrium pressure. However, the hydrate equilibrium pressure increases with depth due to increasing temperature as a result of the geothermal gradient. Therefore, there is a maximum depth of hydrate stability in the seafloor below which the hydrostatic pressure is less than the hydrate equilibrium pressure. Clennell, Hovland, Booth, Henry, and Winters (1999) and Henry, Thomas, and Clennell (1999) 0264-8172/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-8172(03)00064-3 Marine and Petroleum Geology 20 (2003) 459–470 www.elsevier.com/locate/marpetgeo * Corresponding author. Tel.: þ 1-302-831-2945; fax: þ1-302-831-3226. E-mail address: sandler@udel.edu (S.I. Sandler).