The Lake’s Gas Resource Formation
Lake Kivu contains an important methane gas resource. It is one of the Great Lakes of Africa in the Western (Albertine) Rift Valley. It lies on the border between the Democratic Republic of the Congo and Rwanda. The lake has a length along the dividing border of 98km and at its widest it’s 48km. The lake is some 10 000 years old. This is because it was dammed by the lava fields of two active volcanoes in the north. With its ancient outlet to the Nile River blocked by the lava fields and volcanoes, it now spills to the south.
The Ruzizi River flows south from Lake Kivu through a deep canyon. The river flows south in a cascade which drops over 700m in 20km to Lake Tanganyika. However, the river has a series of hydro-electric run-of-river power sites which are exploited to only 5% of their 560 MW potential. In contrast, Lake Tanganyika has no methane resources.
Gas Resource Estimating
Lake Kivu’s methane gas resource can now be measured very quickly, cheaply and accurately. But it must be measured every few years. This is because gas is continuously forming, making it a biogenic and renewable resource. The methodology for updating resource measurement is simple. It is described below.
However, the greatest uncertainty associated with Lake Kivu’s resource is the recoverability of gas in situ. This varies greatly by the gas recovery method used. This in turn is hugely variable. Indeed it is mainly dependent on the technology and plant design deployed.
The Kivu gas resource certainty is in stark contrast to typical gas in-situ measurement for conventional oil and gas. In fact, such data and their interpretation are slow to acquire. They are also expensive and yet most often lack great certainty.
Along with the inaccuracy of predicting the contents of the resource, there is the uncertainty multiple of % recovery. One calculates reserve estimates by multiplying the resources in place by their recovery. This percentage is highly variable, depending on the characteristics of the oil and gas resources present. It is also impacted by the “tightness” of the reservoir rock. Producers can choose or vary their production techniques to increase recovery potential.
Conventional O&G: Reserve Estimating
For conventional oil and gas, the field discovery and delineation drilling are just the beginning of the process. Typically, discovery requires several methods used in sequence, in a process that may take years. For instance, these will include geological mapping for initial targeting. For deeper interpretation, geophysicists will select the respective methods and tools for onshore and offshore survey.
The survey data produce 2-D and 3-D seismic mapping. These maps are used by experts to delineate a potential reservoir. Experienced technicians identify their drilling targets for reserve delineation and quantification to gather the field data. Essentially, they use drill data logged in the database such as depth, metres of intersect, pressure, flow-testing and assays. More recently, newer geophysical methods use sophisticated down-hole probes to establish more extensive data. New tools such as ML and AI are thus becoming the tools of choice for quicker interpretation of vast “data lakes”. The risk of “dry” wells and uneconomic reservoirs remains high, despite expensive seismic modelling.
Indeed, such oil or gas reservoirs can have estimates of P50 and P90 certainty reserves that are wide apart. The certainty and cost both hinge on the number of holes drilled, their depths, the grid spacing etc. The costs of completing a conventional field’s reserve estimate can add up to hundreds of millions of dollars. The process can easily take ten years to complete.
Lake Kivu: Gas Resource Estimating
For Lake Kivu, the reservoir capacity is known quite precisely by depth in a 3-D mapped lake (see maps above & below). The second factor in the calculation, the concentration profile of gases by depth, is measured roughly every five years. The concentration of gases by depth is highly consistent across the lake. Therefore, for any given year, taking a few profile measurements allows for a quick reserve recalculation.
EAWAG has estimated dissolved gas resources in Lake Kivu several times. Reserve estimates were made recently in 2004 and 2010. The prior estimate was back in 1975. In these cases, they calculate the lake’s volume from the lake bathymetry maps (shown below). They assessed detailed gas concentrations from sampling across the lake, at all depths. Multiplying gas concentrations, for each metre of depth by the gas concentration, they can quantify the resource.
These re-estimates can be very accurate, much more so than typical for conventional gas reservoirs. This accuracy is good because gas concentrations at any depth are very consistent across the whole lake. Therefore a few measurements across the lake can confirm the lake-wide dissolved gas concentration profile by depth. 2010 assessments of lake methane resource were 65 km3 (>2 tcf) and carbon dioxide 260 km3 (10 tcf).
Understanding Lake Kivu’s Unique Nature
Lake Kivu is arguably the world’s largest natural freshwater bio-digester. The lake contains vast amounts of algae, providing feedstock for producing recoverable biogas. The lake may also be one of the world’s largest carbon sinks, after the oceans, due to it unique capability to store vast volumes of these greenhouse gases. Gas resources in-place can provide inexpensive power for the region’s population for generations. But a program to draw-down gas inventories is also essential to avert a massive limnic eruption.
Unique Layering of the Lake
Distinct layers within the lake form five unique, stratified gas containments. In fact, the lake’s salinity-based density gradients (chemoclines) clearly separate and define these strata. Therefore, left undisturbed, these containments retain the continuously generated, biogenic CH4 and CO₂. The gases are trapped in solution, as strata of varying concentrations. The richest biogenic gas inventory is stored in the lower two strata of the lake. Significant volumes of CO₂ of magmatic origin also accumulate within this total gas inventory.
Gases are therefore accumulating ever closer to saturation levels. As such, they threaten to cause a future limnic eruption. For example, an eruption occurred at Lake Nyos in Cameroon in 1986 with 1746 reported casualties. In this case, an eruption is inevitable if not actively prevented. However Lake Kivu has the size, potential, location and gas inventory for even more dire consequences. An eruption may be of the order of 1000 times larger.
Outline Hypothesis on Changes to the Lake
In a detailed study, Philip Morkel has co-authored a paper with Dr Finn Hirslund of COWI. The paper outlines a novel hypothesis to interpret an improved understanding of the lake’s history. We used a complex, multidisciplinary approach in this work to analyse the lake’s unique situation. Our hypothesis identifies the impact of diffusion mechanisms on vertical transport of gas and nutrients in the lake. Through this body of work, one can foresee the probable outcomes and recognize the effects of system constraints.
The paper outlines how one should initiate gas extraction projects on a basis that enables three vital outcomes. These are; a) for developers to pursue economic projects, while b) society enforces developers to follow extraction methods that ensure safety, through lifetime prevention of disasters, while c) gas extraction maximizes useful energy output.
The key to safety is proper management of the chemoclines. This must be achieved while drawing down the gas inventory. The instrument to achieve both, safely and productively, is derived from defining and making the right processing choices. The choices correctly specify all critical requirements for plant design of gas production facilities. Indeed, these are derived from thorough analysis of what may happen and what must be done technically.
Fulfilling the MP’s Principles
We had to first deal with the primary concern for continued public safety. Thereafter we could examine, through further analysis, ways of minimizing environmental impact on the mixolimnion. Slow, natural upwelling of meteoric water out of surrounding lava strata causes impactful changes to the stratification. These meteoric waters flow into the mixolimnion of one of the world’s most nutrient-rich water bodies. In addition, we need to consider the impact of washing the extracted raw gas with some of this water. This scrubbing of unwanted gas is essential to upgrade gas quality for use in power-generating engines. The wash water source is from the shallow stratum of the mixolimnion.
Keeping Lake Kivu Safe Long-term
Finally, we considered how to continue holding the chemoclines at their current levels. To achieve this need, we discussed how to evacuate a fraction of the degassed water from deeper resource strata. Counter-intuitively, the best solution may be to re-inject degassed water into the mixolimnion, in the top 60m of the lake. Displacing degassed water can maintain chemoclines in place. However, it introduces an environmental challenge. The challenge lies in defining how to minimize any safety-driven or environmental impacts on the mixolimnion. Potential toxic effects come from H₂S, from CO₂-induced acidity, and from oxygen depletion caused by CH4 and H₂S.
Defining the role and methods of sustainable commercial gas extraction as the tool that for the foreseeable future, serves to avert a disastrous limnic eruption caused by nature’s continued accumulation of gases in Lake Kivu.
Other Studies of Lake Kivu
EAWAG, at Kastanienbaum on Lake Lucerne in Switzerland, is a faculty specialising in studies of lakes and their behaviours. EAWAG Studies of Lake Kivu demonstrate how it is one of the most-studied lakes on the planet because of its unique characteristics.
The Lake Kivu methane is partly magmatic in origin. It is noteworthy that microbial reduction of the volcanic CO2 produces some of the methane. The role of archaea methanogens is being studied. But the gas inventory is mostly biogenic. This gas forms through the anaerobic bio-digestion of dead algae and fish on the lake floor. The gas resource is renewable, with a rate of renewal at present about 0.5-1.0% per annum. The rate of renewal has increased over recent decades, possibly due to changes in nutrients present in the Biozone.
Kivu is one of three known turn-over lakes. But Kivu shares this distinction with Cameroonian volcanic lakes, Lake Nyos and Lake Monoun. With turnovers recurring less often than 1000-year intervals, history fails to provide a detailed record any one incident.
Historic Lake Eruption Impacts
Analysis of Lake Kivu’s geo-morphology history indicates multiple major disturbances from the examination of drill-cores. The cores show evidence of one or more massive biological extinctions on millennial timescales. These extinction events relate to gaseous eruptions that occur every few millennia. These eruption events can release hundreds of billions of cubic metres of gas in short catastrophic events.
But the gaseous chemical composition of releases from exploding lakes can be unique to each lake. Lake Kivu has a signature gas content of 20% methane and 80% carbon dioxide. It has some nitrogen and sulphur dioxide (2000 ppm) as lesser constituents.
Potential eruptions release emissions of gases that pose a serious risk to all oxygen-dependent life, anywhere in the vicinity of the lake. But the inventory of methane resource gas in situ also provides an accessible, exploitable, renewable energy resource. If we properly exploit these, the value of production from the methane resource can provide tens of billions of dollars for the Kivu region.
Lake Basin Geography
The lake covers a total surface area of some 2,400 km2 (1,040 sq mi). Its surface elevation lies at 1,463 metres (4,790 ft) above sea level. But the lake also lies in a rift valley that is slowly being pulled apart. This rifting results in volcanic activity in the area. Rifting also makes the lake particularly deep. Its maximum depth of 486 m (1,575 ft) ranks fifteenth in the world.
Renewable Methane Resource
EAWAG studies assess Lake Kivu to contain approximately 65 billion cubic metres dissolved methane gas. A density gradient at 260 m (850 ft) traps most of the gas below it. This density gradient blocks any upward diffusion of gases and nutrients into shallower water.
Until 2004, extraction of the gas was done on a small scale by a 1965 era extraction plant. The piped extracted gas was being used to run boilers at the Bralirwa (Heineken-owned) brewery in Gisenyi, Rwanda, until 2003.
Basins of the Lake
Productive Methane Resource Basins
- The main basin or Northern Basin (North, Gisenyi, East on the Map) has a maximum depth of approximately 485 m. As a result this basin holds by far the most important volume of the water in Lake Kivu. Gas harvesting is feasible from this zone as it contains the bulk of methane resources. Therefore, gas extraction facilities will primarily be sited in this zone for practical and economic reasons.
- The South-eastern Basin (Kibuye) is the most southerly resource-bearing basin. Noteworthy, KivuWatt has located its gas concession and gas extraction facilities here. Due to the lack of depth and distance to shore, this location limits its productivity. So it also lowers the effectiveness of the plant investment.
- There is an isolated bay to the north-west, Kabuno Bay. It only has a maximum depth of 150 m. First of all, this bay has accumulated high concentrations of carbon dioxide, but little methane. The carbon dioxide should perhaps support an elevated risk of localised eruption. But recent finding show that this bay is too shallow to sustain a large columnar eruption. As a result, unlike the main lake basin, we may not require to vent the carbon dioxide build-up. Because the bay is shallow, it averts any large, localised gas eruption risk.
- The south-western part of the lake has three separate basins. However none of these, even the Kalehe (West) Basin, is resource-bearing. It has a maximum depth of 230 m, shallower than the main gradient. The Ishungu Basin is further south, with a maximum depth of 200 m. The Bukavu Basin is furthest to the south, with a maximum depth of only 100 m.
- Part of the Northern basin, plus part of the Ishungu and Bukavu basins lie in Rwandese waters. To the west, the rest of the lake lies in Congolese waters.
Lake Kivu’s Density Structure
Lake Kivu has an extraordinary feature in the stratification and resultant stability of the deep waters of the lake. The vertical density profile is a key feature defining the lake’s behaviour. However, this phenomenon is not evident in any other lakes to this degree.
Oxygenated water persists only the uppermost layer, the Biozone. This Biozone supports aquatic life in the form of small, sardine-like fish called “Sambaza”. Indeed, seasonal turnovers oxygenate the Biozone. Hence this occurs more strongly in the dry, windy seasons. Oxygenation penetrates to a depth of up to 60m. However, oxygen-rich water only penetrates to as shallow as 40m during the calmer, wet seasons. These wet seasons run from January to April and from October to November due to the lake’s equatorial latitude.
Impacts of Density Structure
The deeper layers are progressively more dense with depth. Increasing concentrations of salts dictate the density. These consist mainly of magnesium carbonates and sulphates. For that reason, density layers separate the lake into definable strata. Figure 5 show these density gradients, illustrating the defined zones. Gradient layers act as virtual traps, preventing significant vertical migration of gases.
Gas formation happens on the lake bottom. The bulk of the gas resource accumulates in the Resource Zones, as shown above. Gas reserves remain in the same lake strata as as they are generated. This is because the strength of the gradient layers prevents gas diffusion through a sharp change in density.
This is a sectional view of Lake Kivu, looking to the north from the deeper northern basin. We show this view schematically. The vertical transitions characterise the lake as it is at present. The lake bottom composition consists of many layers of sediments. We know that some of the sediment layers date to the older and shallower lake’s existence, prior to 10,000 years ago. The belief is that the anoxic bio-degradation of sediments occurs mainly on top of the sediments.