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A gaseous phase occurs within the liquid when the sum of partial pressures of all the gases dissolved in the water reaches hydrostatic pressure. During a recent field-trip in November 2003, financed by UN-OCHA, we were able to measure precisely the concentrations in CO2 and CH4 according to depth (cf. Depth curves below). We have shown the partial pressures of CO2 and CH4, their sum and hydrostatic pressure. Note that on this figure the sum of partial pressures reaches 57% of hydrostatic pressure at a depth of 270 m, whereas on previous evaluations the level of saturation showed only 40% saturation. One also notes that an upsurge of gas-filled water from 120 m provokes the dangerous ex-solution phenomenon, whereas previous measurings established this point at 180 metres. The present level of risk is therefore much higher than we believed it to be before the results from this study. | Moreover, note that the partial pressure of CH4 accounts for around 85% of the total pressure of dissolved gas, the CO2 for only 15%. The presence of dissolved methane gas thus constitutes by far the greatest menace of a gas leakage. The carbon dioxide alone would, practically speaking, be insufficient to provoke an explosion of gas. To resume the situation concerning risk evaluation, one can say that without the presence of methane the risk of a gas explosion would be negligible. The methane, due to its low solubility, is the factor which would set off the process of 'avalanche' ex-solution. Because of its large volume, the carbon dioxide would have a catastrophic impact. Thus the methane can be seen as the detonator of the delayed-action bomb which the huge quantity of dissolved CO2 constitutes. The extraction of methane thus represents - when we are talking of risk evaluation - disarming the detonator and thus making the lake safe. | |||||
| Concentration in dissolved gas (CO2 and CH4) according to depth (November 2003 field trip) | |||||
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