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18-10-2010, 03:50 PM
Gas hydrate.doc (Size: 3.24 MB / Downloads: 104)
Gas hydrate, also known as clatherate, are ice like crystalline molecular complexes composed of water and natural gas that form when gases, (mainly methane produced by microbial break down of organic matter) combine with water at low temperature and high pressure. Although Gas hydrate looks very much like ordinary ice but it burns with a soft orange flame like the pilot light on a gas stove.
Gas hydrate was first produced in 1810 by Sir Humphrey Davy by bubbling chlorine gas through water under elevated pressure. In 1930s presence of gas hydrate was observed in oil & gas pipe lines. In 1970s Russian scientist postulated the occurrence of natural deposits of gas hydrate. This theory was confirmed in 1980s by recovery of sea sample in black sea. In 1990s it was estimated that amount of organic carbon recoverable from hydrate are more that all other known fossil full sources combined. The world wide amount of methane gas hydrate is estimated to be at least 1x 104 giga tons of carbon from 80 possible fields. This is twice the amount of carbon held in all fossil fuels on earth. By the end of 20th century many countries such Japan, Canada, USA, Germany and India has started research programs for investigation of gas hydrate.
There are three primary conditions that must be satisfied in order that methane hydrates are naturally able to form and be preserved. These are:
1. Sediment porewaters (or rarely water column) is saturated with CH4 (free gas)
2. Sufficient pressure is available (hydrostatic pressure, P)
3. The temperature (T) of the water and sediment is suitably cold.
The first condition, i.e., that the waters are saturated with respect to methane is frequently met in shallow coastal waters. There are numerous sources for this methane, but most commonly in this setting the gas is of bacterial or thermogenic origin. The combination of temperature and pressure (water depth) necessary for methane hydrate formation and stability are shown in Figure 1. In the unshaded region, the pressure is either too low (too shallow) or the temperature too high for hydrate to exist. The shaded region in Figure 1 shows the depth-temperature (P, T) region in which gas hydrate is stable.
The gas hydrate stability zone (HSZ) in sediments can be delineated on a temperature versus depth profile with respect to the hydrothermal gradient (for subsea gas hydrates), geothermal gradient and clathrate phase boundary, as shown in Figure (for subsea sediments). The position of the hydrate phase boundary is primarily a function of gas composition, but may also be controlled by pore fluid composition (e.g. presence of salts), pore size, and possibly sediment mineralogy. Hydrothermal and geothermal gradients are locality dependent, and can differ markedly with geographical location and tectonic setting. The predominant hydrate-forming gas is methane, with lesser CO2 and hydrogen sulfide (H2S), all of which are generally produced in-situ by microbial breakdown of sedimentary organic matter. In hydrocarbon-rich provinces, clathrates may contain a more deep-seated thermogenic gas component, generally in the form of ethane and propane, which, due to increased thermodynamic stability, can shift the HSZ to considerably shallower depths.
Methane hydrates have been found in the subsurface in permafrost regions, but most occur in oceanic sediments hundreds of meters below the sea floor where water depths are greater than about 500 meters. Most occurrences of gas hydrate are inferred from geophysical logs in the permafrost regions and from anomalous seismic reflectors within ocean sediments called bottom-simulating reflectors, or BSR’s, which are thought to indicate the base of the gas hydrate stability zone. Examination of 90 gas hydrate samples recovered from 15 different geologic regions indicates that most samples consist of individual hydrate grains or particles in pores of sedimentary rocks. In general, the gas-hydrate-bearing zones in sedimentary sections range from tens of centimeters to tens of meters in thickness. Gas hydrate also occurs as nodules, laminae, and veins within sediment and, in one case, as a pure gas hydrate layer as much as 4 meters thick
EVIDENCE FOR GAS HYDRATES
Although gas hydrate has been recognized in drilled cores, its presence over large areas can be detected much more efficiently by acoustical methods, using seismic-reflection profiles.
Hydrate has a very strong effect on acoustic reflections because it has a high acoustic velocity (approximately 3.3 km/s - about twice that of sea-floor sediments), and thus grains cemented with hydrate produce a high-velocity deposit due to the mixing of hydrate with the sediment. The existence of free gas below the BSR causes a drastic decrease of seismic velocity, sometimes to levels lower than the seismic velocity of sea water (1500 m/sec). If there is no free gas below a deposit of gas hydrate, there will not be a BSR and thus it will be impossible to detect the deposit by conventional seismic methods. The fact these geophysical methods sometimes fail to detect gas hydrate may indicate that the total volume of gas hydrate worldwide is still underestimated.
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