We have initiated a program of research into the ocean geochemistry of gas hydrates, or clathrate hydrates of natural gases (Brewer et al., 1997). Gas hydrates are a solid ice-like phase formed at low temperature and high pressure by van der Waals forces between gas and water molecules, with the "host" water molecules forming a molecular cage which confines the "guest" gas molecules through their mutual electrostatic interaction. Most low molecular weight gases (O2, N2, CO2, CH4, H2S, Ar. Kr, Xe etc.) will form a hydrate under some pressure-temperature conditions. In the ocean, gas hydrates composed dominantly of methane are common constituents of the shallow marine geosphere (Kvenvolden, 1993), and they occur both in deep sedimentary structures (Dickens et al., 1997), and as outcrops on the ocean floor (MacDonald et al., 1994). The laboratory synthesis of gas hydrates, for study of their thermodynamic properties, typically involves rocking, or vigorously shaking the reactants in a pressure vessel (Sloan, 1990). The reaction has been found by many workers to be slow, and ice crystals are commonly added to initiate the reaction. Gas hydrate formation in the ocean does not involve shaking, and ice crystals are not part of the natural deep sea environment, and therefore laboratory experiments carried out to date have not been able to simulate natural processes. Gas hydrates are believed to form by migration of gas from depth along faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water.In our experiments we used MBARI's remotely operated vehicle Ventana to simulate nature, and to carry out a series of experiments injecting gas: methane, carbon dioxide, and a methane+ethane+propane gas mixture, into sea water, and sediments of varying grain sizes, in the deep waters of Monterey Bay. The local oceanographic conditions are such that the PT boundary for methane hydrate formation at our site is close to 525 meters depth. We dove Ventana to about 910m so as to provide conditions favoring hydrate formation.
Figure 1. (Larger image [55Kb]).
The apparatus used to control gas flow is shown in Figure 1. It consists of a gas tank, flow regulator, and a set of quarter turn valves which are actuated hydraulically by the vehicle. Release of gas into acrylic reaction cylinders (60 cm x 4.5 cm) produced almost instant crystallization of a hydrate mass. A movie clip of this is shown here ( Figure 2). The rapidity of the reaction in this natural system is both remarkable, for both experimental and theoretical studies in the laboratory have shown very slow reaction rates, and is important for understanding the formation of hydrates in nature.
Figure 2. MPEG-1 movie [1.2 Mb]
Injection of gas into a column of coarse sand quickly flooded the pore space with hydrate, and turned it into a solid block of clathrate "ice". Gas flow then fractured and lifted this solid mass, the movie clip in Figure 3 shows:- on the far left a large hydrate mass formed in water, in the center a block of solidified sand that has been fractured and is in the process of being lifted, and on the far right veins of gas hydrate formed in a fine grained black mud.
Figure 3.MPEG-1 movie [1.6Mb]
- Brewer, P.G., F.M. Orr, Jr., G. Friederich, K.A. Kvenvolden and D.L. Orange. Gas Hydrate Formation in the Deep Sea: In Situ Experiments with Controlled Release of Methane, Natural Gas and Carbon Dioxide. Energy & Fuels 12: 183-188 (1998).
- Brewer, P.G., F.M. Orr, Jr., G. Friederich, K.A. Kvenvolden, D.L. Orange, J. McFarlane and W. Kirkwood. Deep-ocean field test of methane hydrate formation from a remotely operated vehicle. Geology 25: 407-410 (1997).
- Dickens, G.R., C.K. Paull, P. Wallace & the ODP Leg 164 Scientific Party. Direct measurement of in situ methane quantities in a large gas-hydrate reservoir. Nature 385: 426-428 (1997).
- Kvenvolden, K.A., Gas hydrates - Geological perspective and global change. Reviews of Geophysics 31: 173-187 (1993).
- MacDonald, I.R., N.L. Guinasso Jr., R. Sassen, J.M. Brooks, L. Lee and K.T. Scott. Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico. Geology 22: 699-702 (1994).
- Sloan, E.D., Clathrate hydrates of natural gases. Marcel Dekker, NY, 641 pp. (1990).