Pourquoi l'exploration du pétrole sous le lac Kivu comporte des risques énormes !
Why exploring for oil under Lake Kivu carries unique and huge risks
Robert Hecky, Conversation May 28, 2017
Rwanda and the Democratic Republic of the Congo have signed a five-year joint exploration agreement for oil under Lake Kivu. But Professor Robert Hecky warns that there are extraordinary risks. The lake has a huge amount of gas in its deep waters and could explode with devastating consequences.
What’s so special about Lake Kivu?
With a surface area of 2,370km² – almost the size of Mauritius – and a maximum depth of 480metres, Lake Kivu is one of the Great Lakes of Africa. It is the 3rd deepest in Africa after Lakes Malawi and Tanganyika. All three are located in East Africa’s Western Rift valley. Kivu lies on the border between Rwanda and the Democratic Republic of the Congo (DRC).
Other lakes in the western Rift Valley are rich in fish species. But not Lake Kivu. There are only 28 compared with over 400 in downstream Lake Tanganyika. Nevertheless, the lake supports an important fisheries industry – based on an introduced sardine native to Lake Tanganyika – yielding over 20,000 tonnes per year.
The Kivu basin is home to two million people, with more than 400 inhabitants/km². This is one of the highest population densities in the area surrounding the African Great Lakes. This includes several large refugee camps near the shores of the lake.
Kivu is an important transport corridor for the informal economy between Rwanda and the DRC, with steady traffic in produce, fish and domestic goods. It is also a major reservoir providing water for power production downstream on the Ruzizi river.
Although it has a mean depth of 240 metres, oxygen does not penetrate below 60 metres. This means that only bacteria grow through most of the volume of the lake while fish and other large organisms are restricted to the surface layer.
In common with all deep lakes, Kivu has density stratification– the densest water is found at the bottom and the least dense water at the top. Density is a function of temperature (cooler waters are more dense) and salinity (more saline waters are more dense). Unlike most other lakes, Kivu gets warm, saline inflows into the deep water which enforces a very strong density gradation from bottom to top which results in layers of different density, and results in an uncommon temperature structure with the warmest waters occurring at the deepest depths.
Temperatures are affected by inflowing deep springs heated by the active volcanoes which form its northern watershed. But these warm springs are also rich in volcanogenic carbon dioxide and other salts. Because of the salt inputs there is a a very strong and permanent density gradient, with the deep water in the lake trapping dissolved gases such as carbon dioxide and methane.
There are over 60km³ of methane dissolved in the deep water of the lake, and as a result they are supersaturated in methane and would degas spontaneously if the deep water was significantly displaced upward. A simple example is the vigorous bubbling that occurs when a shaken bottle of soda has its cap removed.
The cap on Lake Kivu is the density gradient which prevents water from moving upward rapidly and degassing to the atmosphere.
The potential for rapid degassing is real, especially in a geologically active region such as the Kivu basin. Volcanoes and earthquakes are common in the rift valley. An example of the risk is illustrated by the 1986 tragedy at Lake Nyos in Cameroon which released a gas cloud as a result of a landslide. Over 1,700 people died.
Because of its much greater size and gas content, Kivu has the potential for a cataclysmic gas release.
Its peculiar characteristics have led to the creation of a commercial scale methane gas reserve which is now beginning to be extracted to produce electricity for the Rwanda national grid. This gas extraction has the dual benefit of producing electricity but also reduces the risk of spontaneous degassing as it will release the gas from the deep water in a controlled way and gradually reduce the gas pressure over time. But currently, on-lake exploration drilling or commercial extraction of oil would add an additional risk of an uncontrolled gas release from the deep gas-rich deep water of Kivu.
What potential energy resources does the lake hold?
The current lake was predated by ancient lakes that left sediments below the modern lake that may hold oil.
There has been seismic exploration in Kivu – using shock waves to explore potential subsurface resources such as oil or gas. But exploratory drilling hasn’t happened. Yet.
Because the Kivu basin is the highest in altitude of the lakes in the Western Rift Valley, the sediment thicknesses may not be as deep and oil rich as in the Albert basin or the Lake Tanganyika basin. Nevertheless, there’s still interest as is evident in the recent Rwanda and DRC oil exploration agreement. If seismic exploration identifies positive signs of oil in the sediments under the lake, then exploratory drilling is likely to follow.
What are the main concerns about oil exploration under the lake?
Drilling on the lake surface would be very hazardous because it could lead to rapid upwelling of deep water. If a blowout were to occur, the upwelling of oil or gas would displace the currently stable gas rich layers upward until the gas pressure exceeded the hydrostatic pressure at shallower depths allowing spontaneous degassing to occur. This could release a powerful explosion, due to gas expansion, such as happened on Nyos – but potentially much larger.
As elsewhere, oil extraction runs the risk of contamination. This would affect people who depend on the lake for their livelihoods. Concerns like this were highlighted at the recent African Great Lakes Conference in Entebbe.
But concerns extend beyond contamination because Kivu carries much bigger risks than other lakes. These include potential loss of human life in the surrounding catchment due to the release of carbon dioxide gases and the destruction of infrastructure by large waves due to the gas expansion.
Of course, oil could bring positive benefits if it is carefully done, well regulated and the economic benefits equitably distributed.
What advice would you give any exploration teams?
I would advise them not to do exploratory drilling on the lake itself. Shore/land based drilling would eliminate the risk of disturbing the lakes density structure. But even then care must be taken to avoid possible contamination.
I would encourage companies and governments to ensure that there’s good baseline data before drilling starts. But eliminating all risks won’t be possible in such a geologically active area. Insurance should be sought or bonds posted to guarantee that funds would be available if anything goes badly wrong.
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