gaseous phase occurs within the liquid when the sum of partial pressures of all
the gases dissolved in the water reaches hydrostatic pressure.
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
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.
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.
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.