distribution of methane in economically exploitable concentrations in the main
basin of Lake Kivu. Only the 280 m and 340 m isobaths are shown.
OF METHANE GAS DEPOSIT
Using a bathymetric chart
established by Lahmeyer and Osae in 1998 we have added the depth contours 280
m (in red) and 340 m (in yellow).
This diagram facilitates the estimation
of the productive horizontal surface of the lake, depending on the depth at which
extraction takes place. It also allows for the rapid targetting of favourable
sites for setting up methane extraction plants after examining the distance between
productive layers and the shore.
have measured the respective volumes of water, CO2 and CH4 in layers 10 m thick.
Notice on this diagram the clear distinction between
the lake water above 260 m in depth (weak in dissolved gas) and that below 260
m which contains a high proportion of dissolved gas. Only the methane-rich layers
below this depth contour can be considered economically viable.
presentation of carbon dioxide and methane according to depth.
of all, look at some quantative measurements extrapolated from this diagram :
total volume of lake 560 km3
total volume (TPN° of dissolved
CO2 : 255 km3
total volume of dissolved CH4 : 65km3.
distribution of volume of methane in 10 m layers : economically interesting exploitable
zones are shown in transluscent form. This diagram allows for an estimation of
the quantity of economically exploitable gas, using the criteria of richness in
gas and distance from shore.
We make a distinction
between the quantity of methane dissolved and the quantity of methane extractible
(net result at outlet of exploitation plant). We estimate in fact, following our
calculations, that about 20% of the methane will be discharged (lost) to the deep
waters of the lake during the separation and washing stages. Note that all these
figures are given in volumes of pure methane TPN (1 bar pressure, temperature
Schematically we can divide the lake into four layers, of
which three contain methane which can be considered economically exploitable.
These three layers should be considered in terms of difficulty, because of depth
(distance from the shore) in setting up extraction columns.
at a depth of 300 m
(260 - 300m layer : zone 1)
Volume of dissolved
methane : 13.3 km3,
volume of extractible methane : 10.6 km3..
at a depth of 350 m
(300 - 350m layer : zone 2)
Volume of dissolved
methane : 15 km3,
volume of extractible methane : 12.3 km3.
at the deepest part of the lake
(350 - 485m layer : zone 3)
of dissolved methane : 14.9 km3,
volume of extractible methane : 11.9 km3.
To conclude, of the 65km3 of gas dissolved in the lake
28.3 km3 are easy to exploit and would give 22.6 km3 of pure methane
TPN, 15 km3 are accessible with more difficulty and would provide 12 m3 of pure
methane TPN. 17 km3 are to eb considered as not economically viable.
volume of extractible methane : 34.5 km3.
It is important to optimise the
exploitation system so as to minimise the quantity of methane discharged in deep
water during the separation and washing stages. We think we can limit this loss
of methane to between 20% and 25%. One should also note that these calculations
do not take into account the recharging of the lake in gas over a period of time.
This recharging is very difficult to estimate and figures put forward at the present
time vary between 125 Mm3 and 250 Mm3 per year.