Glossary of hydrate terms

Hydrate science is still in its early stages. Different terminology is often found in the literature for the same gas hydrate related objects and mechanisms. Soon, common use and concensus will determine the accepted nomenclature. Hydrate descriptive terms are strongly science based at present and the use of potential economic hydrate terms in an economic sense (Max and Chandra, 1998) may differ somewhat from scientific usage. 'Hydrate' vs 'hydrates', for instance often appear to be used indescriminately, as does 'hydrate' or 'gas hydrate'.

BGR: Base of Gas Reflector. This is sometimes a strong positive impedance contrast at the base of a gas-rich zone below hydrate, where gas has largely separated from pore fluids, or at the base of a gas zone within the HSZ.

Blanking: Blanking refers to a weaker pattern of reflection traces in reflection seismic records that is regarded by some as being caused by the presence of hydrate. The seismic model of the effect is: hydrate fills the pore spaces and reducing the impedance contrasts of the original bedded sequence. Some geophysicists are not convinced that apparent blanking is evidence of the presence of hydrate.

BOH Base of hydrate. Also: BGHS, Base of Gas Hydrate Stability. This refers to the actual base of naturally occurring hydrate, which may be either higher or lower than present calculations may indicate. The gas hydrate physical system is imperfectly understood in nature. The base may be difficult to identify when there is no BSR, especially where the bedding surfaces of marine sediments are parallel bedded with the seafloor.

BSR: Bottom-Simulating Reflector. Seismic term. This is a strong negative impedance contrast at the contact between higher Vp and density hydrate-rich zone and sediments having lower Vp and density below. BSR is usually recognized as a reversed impedance contrast gas zone. The BSR is commonly taken as is a surface marking the phase boundary of and the actual base of the hydrate stability zone.

Dissociation. The 'melting' of gas hydrate owing to rising temperature or diminishing pressure. Pioneered as a methane recovery method from hydrate in Russia (see chapter 6).

Dissociation Feedback: Gas hydrate contains within it a self-moderating or feedback system which act as a rate-controlling mechanism during dissociation. Whatever the cause of dissociation, heat is absorbed. This cools the system and may cause dissociation to slow or cease. Run-away dissociation feedback may take place when hydrate is brought to near-

atmospheric pressures which usually will initiate a complete dissociation; the system may absorb enough heat under this condition to freeze the local water.

Depressurization (recovery): Term for artificial dissociation process where pressure in a gas section in contact with hydrate is pumped down and the pressure lowered. Hydrate in contact becomes unstable and will dissociate, absorbing heat.

Inhibitor (recovery): Term for artificial dissociation process where fluids having the effect of an antifreeze with respect to gas hydrate is introduced. The presence of the fluid (e.g. brines, methanol, etc.) changes the position of the phase boundary in pressure-temperature space and causes hydrate lying between the original and the inhibitor-determined position of the phase boundary to dissociate. Water-ice analog is seen where icy streets are melted by the application of salt.

Thermal (recovery): Term for artificial dissociation process where warm fluids are introduced in a hydrate section and the rise in temperature and the availability of heat allows the hydrate to 'melt'.

GAIL. Gas Authority of India, Ltd. Original coordinators of the Indian Gas Hydrate Research Program.

HEZ: Hydrate economic zone. This includes all of the HSZ and sediments below solid hydrate that include both gas-charged and sediment which does not display a significant level of compaction for its depth. This is the zone in which the geotechnical parameters must be extremely well known to allow for safe extraction from gas reservoirs and production of gas from the hydrate. It is presently estimated that this economic zone may be up to 1.5 to 2 times the thickness of the HSZ.

HGZ: Hydrate Gas Zone. Zones of free gas that occur mainly trapped below the HSZ where the formation of hydrate has lowered permeability enough to trap gas. Gas zones may also occur within the hydrate stability zone.

HSZ: Hydrate Stability Zone. This is the zone in which hydrates are expected to be stable based on calculations of pressure and temperature with depth in the seafloor. Also known as Gas Hydrate Stability Zone (GHSZ). It extends from the seafloor downward in the marine sediments to some depth determined by rising temperature. Hydrate in the HSZ is most stable near the surface and progressively less stable downward.

Hydrate, gas hydrate, methane hydrate: Oceanic gas hydrate, is dominantly methane plus water. Hydrate is a diagenetic crystalline solid that forms as a diagenetic material in sediment pore spaces within the local HSZ. Hydrate (singular) is commonly used where one gas species is dominant such as in methane hydrate. Where more than one gas is present in the hydrate lattice and where more than one hydrate structure is present, hydrates (plural) is correct usage.

HZ: Hydrate Zone. This is the zone within the HSZ in which hydrate is actually stable. The upper surface of the HZ may extend to the surface or it may be depressed below a sulfate reduction, or other chemically-active zone. The base of the HZ may occur either higher or lower, or be imbricated and complex, depending on local chemical, structural, and thermal deviations from a simple theoretical model.

JNOC. Japanese National Oil Company. Coordinated the first 5-year program of basic research and development of the Japanese gas hydrate research program.

OBC: Ocean Bottom Cable. Also Seafloor Array or OBA (Ocean Bottom Array). Linear array of geophones resting on the seafloor used primarily for sensing shear and interface waves.

OBH: Ocean Bottom Hydrophone. Used for sensing acoustic energy in the sea, usually near the seafloor in association with OBS, OBH or OBC(A), to allow separation of pressure wave from shear wave energy.

OBS: Ocean Bottom Seismometer. This device records acoustic energy propagating through the bottom; usually on 3-axes so that a full 3-dimensional solution can be found.

Oceanic hydrate system. Economic term referring to all natural gas, which is concentrated as both solid, crystalline hydrate and associated free gas. Useful for describing entire potential economic methane section in which hydrate or gas is concentrated. Where either hydrate or free gas is the economic target, use of the term 'hydrate' alone is unintentionally misleading.

Ryukyu Trough: Also spelled Ryokyo. Subduction zone collisional margin of the eastern Eurasian plate that can be traced from north of Taiwan to offshore SE Japan. Synonymous in its northern end with the Nankai Trough.

TGR: Top of Gas Reflector. This is a strong negative impedance contrast that marks the top of a gas-rich zone. Where this occurs immediately below the HSZ, it may be coincident with the BSR. It may occur within an HSZ where gas has expelled pore fluids and has precluded further formation of hydrate. Both the top and the base of gas reflectors may not necessarily mark a thermodynamic passage (phase boundary) similar to that defined by the BSR.

Vertical Array: A vertical array of hydrophones used in conjunction with stepped-out sources to acquire an acoustic dataset used primarily for determining physical properties of the upper seafloor for acoustic propagation modeling.

VSP: Vertical Seismic Profile. This is the standard method for carrying out reflection seismic profiles from ships at sea. A horizontal array consisting of a number of hydrophones are towed behind the ship. A source of energy, such as an airgun, watergun, transducer (boomer), or a sparker, is used to provide for regular, short bursts of acoustic energy, which is recorded by the hydrophones after being reflected from the seabottom and from reflectors within the bottom.

Was this article helpful?

0 0

Post a comment