The solubility of methane in seawater depends on the depth of seawater and the size of methane bubbles (Italiano et al., 200 1). The research shows (Swinnerton et al., 1997) that the solubility of methane in seawater is different at different depths. In the offshore surface, the equilibrium concentration of methane dissolved in seawater relative to atmospheric methane is supersaturated, and its main source is hydrocarbon fermentation, which is caused by plankton and fish. When the depth of seawater exceeds 100m, the dissolved methane is in an unsaturated state, which is caused by methane oxidation by methane-oxidizing bacteria. When there are hydrothermal and volcanic activities on the seabed, a large amount of methane will be produced, and the solubility of methane is very high. However, when it escapes from a hole or crater, its solubility drops rapidly (Welhen et al., 1979). It can be seen that only when there is a considerable leakage on the seabed can methane be continuously input into seawater through submarine leakage or injection. At the high-intensity leak, methane even enters the seawater in a large amount in the form of free gas, in which undissolved and consumed methane rises to the sea surface in the form of bubble column or bubble string, escapes and is released into the atmosphere (Etiope et al., 2002a).
Italiano et al. (200 1) simulated and calculated the conditions under which methane bubbles rose to the sea surface through seawater in the form of bubble columns or bubble strings and escaped into the atmosphere. This process is the result of the balance between the dissolution rate of methane bubbles in seawater, the rate of bubbles entering the atmosphere from the sea surface and the increase of bubble velocity due to the decrease of hydrostatic pressure when bubbles rise in seawater. The factors affecting this process are seawater depth, bubble size, methane solubility around bubble column, seawater temperature and fluid movement (Etiope et al., 2002). When the radius of methane bubbles is 0.05 ~ 3 cm and the rising speed is 10 ~ 40 cm/s, if the water depth in the leakage area is less than 20m, almost all the leaked methane can enter the atmosphere. When the water depth in the leakage area is 50m, only 50% of bubbles with radius greater than 5mm can escape from seawater and be released into the atmosphere (Italiano et al., 200 1).
According to some studies on methane flux released by seabed leakage (Table 3.3)(Judd et al., 1997), the methane released by marine geology in some parts of Britain was investigated and estimated. It is considered that the methane emission flux of marine leakage sources is generally in the order of 1 ~ 104 t/a, and most of them are less than100 t/a. In the marine leakage area with the investigated area of 105km2, the methane emission is103 ~/kloc-. It is estimated that the methane emission flux of the whole British ocean leakage is 0. 12 ~ 3.5 TG/a (Judd et al., 1997). Simpson et al. (1999) estimated that the methane flux released by the ocean leakage in Nordic activities was10000t/a. According to the current investigation and statistics, assuming that the methane flux released into the atmosphere by ocean leakage was 5-40g/m2/a, the methane released by the global ocean leakage was 8-65tg/a (hovland et al., 1993) or 18 ~ 48tg/a (Hornafius et al., 18). The uncertainty of these data is mainly caused by the uncertainty of global ocean leakage area (Judd et al., 1997).
Table 3.3 Methane Flux Released by Submarine Leakage