EVALUATION 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.

We 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.

Cumulative presentation of carbon dioxide and methane according to depth.

First 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.

Vertical 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 0°C°.

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.

Extraction 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..

Extraction at a depth of 350 m
(300 - 350m layer : zone 2)

Volume of dissolved methane : 15 km3,
volume of extractible methane : 12.3 km3.

Extraction at the deepest part of the lake
(350 - 485m layer : zone 3)

Volume 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.

Total 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.