Review of the degassing project
1986 - 1990 period
The degassing of lakes
Nyos and Monoun was proposed independently by several
scientists in the aftermath of the Nyos disaster. Klaus
Tietze in Germany, and a French team, led by Adelin
Villevieille (then Director of the French Delegation aux
Risques Majeurs) were inspired by the gas lift technique
used at the time for extracting methane from the stratified
lake Kivu at Gisenyi (Ruanda). In a document issued in 1987,
the Delegation aux Risques Majeurs developed a proposal
extrapolating the method for off-shore oil production (see
fig. 1, fig. 2,
fig. 3). The
scheme, including several rigs and metal pipes, was then too
costly to obtain financing. The breakthrough came some years
later with the choice of high density polyethylene as the
material for pipes, the density of which is very close to
the density of water: setting of the pipes could thus be
done by hand, at a much lower cost. Feasibility of the
concept was demonstrated in 1992 at Monoun, in a mission
backed by UATI, and in 1995 at Nyos. The success of those
trials paved the way for a fully fledged experiment at lake
Nyos, to begin as of January 2001.
A modest self-siphon
experiment was carried out at lake Nyos and Monoun in March
1990 by means of a 9 mm inner diameter PVC pipe.
A little soda geyser (1 m high) was
primed for the first time on both lakes indicating the
feasibility of the controlled degassing plan.
The experiment was also designed to
provide Cameroonian scientists with a simple means for
determining, in the field, the CO2 concentration profiles of
The self-siphon brings
the gas-water mixture to the surface where the flow rate of
each component is readily measured volumetrically by means
of a graduated cylinder and a stopwatch (cf. figure).
The results are in perfect
accordance with the CO2 concentration profiles measured by
other authors with less simple methods (e.g., Niskin-type
profile in lakes Nyos and Monoun,
March 1990. Those results were gained using the
above-mentionned method: comparison of the liquid and gas
flow rates at the outlet of a small pipe
Shown are the concentration of
ex-soluble carbon dioxide; add 0.8 l STP.l-1 to obtain the
total CO2 concentration.
1992 experiment at Monoun
self-siphon experiment was conducted in March April 1992 at
lake Monoun, after determining an up-dated profile (fig. 1
b). Two polyethylene pipes (50 mm and 140 mm inner-diameter,
respectively) were fixed between moorings on the lake bed
and a floating raft, at a distance of 100 m or so from the
meteorological monitoring platform set up by the US team
(fig. 1 a). The main results of the experiments are
presented in table. Two to three 140 mm diameter pipes (fig.
2) could readily remove in one year the carbon dioxide
dissolved in Lake Monoun.
The steady-jet flushing out the 140
mm inner-diameter and 65 m long pipe moored vertically in
the middle of lake Monoun, April 1992 (Photo: J.
A bathymetric profile
of Lake Monoun.
The width of the crater is 450 m at 30 m depth
(vertical exaggeration: 3)
Table: experimental and
calculated results corresponding to the flow in two pipes of
different diameter and length. More than 90% of the energy available
through gas expansion is lost by friction. The temperature drop is
measured between the two openings of the pipe. The mass of the gas
and the liquid contraction upon ex-solution are neglected, as is the
presence of methane in dissolved gas (methane constitutes 2% of the
total gas; G. W. Kling, personal communication).