Innovative instrumentation for exploring the deep biosphere of the Mediterranean Sea
Gas hydrates are ice-like crystalline structures of water molecules, within the cavities of which hydrocarbon gases of low molecular weight are trapped, especially methane. Natural methane hydrates have been located in vast quantities around the world, restricted below arctic permafrost soils and beneath the sea in sediments of the outer continental margins. They represent an enormous potential energy resource if a technology can be developed for their economic recovery. A practical method to extract methane gas from its reservoir requires fundamental information on the mechanisms underlying the formation and dissociation of gas hydrates and their properties, including kinetics and crystal growth. Within the ANAXIMANDER project, financed by the European Commission, an integrated approach was attempted for the exploration and evaluation of the gas hydrates in the eastern Mediterranean Sea. With the use of advanced sampling instruments, cores of sediments containing gas hydrates were brought on board from the Anaximander sea-mountains to carry out subsampling with minimum distortion of their properties. For the optical study of gas hydrates crystals and their macroscopic properties, a novel cell was designed that enabled pressure and temperature control in defined spaces, achieving the conditions found in the sediments. The high-pressure optical cell was constructed with a narrow glass capillary, and installed in a slotted brass container which generated a defined constant temperature gradient. Fast formation of gas hydrates can be initiated at the gas-water interface placed at a predetermined level by withdrawing liquid from inside the capillary. Depending on the pressure, the thermal stability of gas hydrates is typically in the 270-295 K range. Cubic sII hydrate was grown from a gas composed of approximately 0.5 % ethane and 99.5 % propane at low temperatures. Competing sI crystals grew when gas hydrates were transferred from a colder zone to a warmer one. Further identification of specific hydrate phases under conditions similar to their natural environment has the potential to complement the usual characterisation method of evolved gas analysis from samples returned from the laboratory.
