Geo-energy | GESTCO (Geological Storage of CO2)

Keywords

Global warming, greenhouse gases, CO₂, sequestration, reservoir geology, geo-energy.

Objectives

In order to reduce the impact of greenhouse gases on global climate change, Belgium has agreed to ratify the Kyoto protocol and reduce its industrial emissions of CO₂ by 7,5% compared to the 1990 level towards 2008-2012.
In practice, this means a reduction of 25 million tonnes of CO₂ whereas combined emissions of the industrial sectors targeted for reduction measures attain 55 million tonnes yearly. Current policies aim at rational use of energy (improving energy efficiency), renewable energies (current share less than 1% in Belgium, the lowest in Europe), reducing carbon intensity (switch from coal to gas which produces less CO₂ during combustion).
The effect of these measures is largely insufficient to reach the target set for Belgium, not counting the additional effect of phasing out of nuclear power. Therefore, capture and sequestration of CO₂ will soon become a necessary option. The Geological Survey of Belgium is studying the potential for geological sequestration of CO₂ in the Belgian subsurface.

Realisation

top

Budget

Personnel

Start of the project

Related activities

top

Figures

top

Modeling vertical reservoir properties with CO₂-VR

Abstract

Geological reservoirs are usually horizontal structures with large horizontal dimensions and only a limited height. The general approach for modeling such reservoirs is to assume a constant density of the fluid that is injected or produced. This assumption is no longer valid when calculations are performed for reservoirs with a large vertical extent, up to several hundred meters, or for fluids that are near critical or near saturated. A typical coal mine has a large vertical dimension that well exceeds the 100 m range. When it would be used for CO₂-sequestration, the pressure and temperature conditions would be close to the saturation conditions of CO₂ or its super-critical extension. In order to calculate the reservoir conditions in detail, the one-dimensional vertical reservoir simulator CO₂-VR was developed.

The core of the simulator calculates the reservoir pressures and densities for CO₂ at each depth. Unlike for hydrostatic conditions, where density and pressure are almost static functions of depth, a system involving CO₂ is much more sensitive. Calculation therefore starts from a point for which the reservoir pressure is known. Usually this the top of the reservoir where the maximal pressure determined by the seal, or a depth at which the reservoir pressure is chosen to be in equilibrium with the hydrostatic pressure prevailing in the hosting formations. Since the temperature inside the reservoir is considered to be equal to the temperature outside the reservoir, the density of CO₂ can be calculated at this starting point. Using this data, the pressure conditions of the cells above and below the starting cell are estimated in an upward-downward calculation scheme. This approach produces pressure and density profiles that are in equilibrium with a given geothermal gradient.

These data are used to calculate the storage capacity of the reservoir in the non-flooded parts (density of CO₂), the flooded parts (solubility calculations based on temperature, pressure and salinity), and adsorbed to coal (adsorption data based on pressure, temperature and coal type).

Extended features allow calculating the effect of flooding of underpressured parts of the reservoir and of the amount of heat that can be produced by CO₂-geothermics.

Emphasis was on developing a basic calculation scheme. In order to test this outline, it has been fed into an MS Excel spreadsheet. In this preliminary configuration normal calculation time for a vertical grid of 100 cells is about 15 minutes, whereas the extended features that require iterative calculations are performed overnight. The central database holds pressure-temperature-density data for CO₂ for temperatures between -50 and +1000 °C and for densities between 10 and 1000 kg/m³. It calculates static conditions only, but may be developed in future as an extension for other simulators. The main challenge for further development is the prediction of the behavior of gas mixtures, because currently the only equations that are accurate close to critical conditions are those for pure systems.

In spite of the rudimentary nature of CO₂-VR, it is in its current configuration able to produce accurate and reliable results. One striking conclusion is that the rule of thumb that CO₂ will be supercritical below ~800 m and gas-like at shallower depths, is not applicable to coal mine reservoirs, but that on the contrary CO₂ at shallow depths will be denser than at deeper levels.

CO₂-sequestration in abandoned coal mines

Kris Piessens & Michiel Dusar

Abstract

Growing evidence for the influence of human induced emissions of green house gasses on the climate has led to a line of climate conventions. The best known is the Kyoto-protocol that has been ratified by several countries. Reducing the emission of CO₂, the most important green house gas, into the atmosphere requires a major change towards renewable energy, increased nuclear energy production, or capturing CO₂ that is produced when burning fossil fuels. The last option requires that CO₂ is stored after capturing. The only reservoirs that are sufficiently large for this purpose are geological reservoirs.

CO₂-sequestration in Europe is the topic of the EC-sponsored GESTCO-project. One option, inspired by storage of natural gas in coal mines, is to use abandoned coal mines as reservoirs. However, unlike the natural gas storage projects, sequestration of CO₂ requires that gas would be secured for thousands of years. A specific hazard is that formation water would infiltrate in a CO₂-filled mine. The gas would then be compressed at the top of the reservoir, until possibly the pressure would be high enough to allow the gas to escape. CO₂ is not a toxic gas and such releases normally would not pose a direct threat to humans and other living creatures, but the stored green house gasses would be released into the atmosphere.

To prevent such a scenario, the mine should be developed as an overpressured reservoir. This means that the reservoir pressure would be higher than the hydrostatic pressure of the surrounding formation water, thereby preventing the influx of formation water. A critical issue is if the top seal of the reservoir is able to withstand sufficient amounts of overpressure, typically around 130 % of the hydrostatic pressure. In case of the Belgian Campine coal mines, the seal is formed by Cretaceous chalks and marls.

The limited tests and the documented behaviour of natural faults indicate a very low permeability, comparable to clay, but the seal is currently insufficiently explored to allow a definite conclusion.

CO₂ in a coal mine may occupy 'free-space', may be in solution in the formation water, or adsorbed on coal. Each of these amounts can be calculated, but not all parameters are sufficiently known to produce useable results. Therefore they are split into an ascertained capacity, that can be calculated with a known accuracy or that is sufficiently conservative, and an additional potential capacity, an estimated surplus that can not be guaranteed or calculated accurately.

A case study for the Belgian Beringen-Zolder-Houthalen collieries shows that sequestration in these coal mines is a viable option, even for the ascertained capacity alone. At an injection rate of 300 000 ton/y, sequestration can be guaranteed for about 25 years. This is a conservative estimate, and it may prove possible to inject at 500 000 ton/y during 25 years. This is a small, but still considerable contribution, approximately 3 to 6 % of the mitigation required to reach the Kyoto-target of Belgium, relative to the 1990 emission level.

CO₂-geothermics in abandoned coal mines

Abstract

Geothermics is a way to extract heat from the subsurface. It has recently gained new interest, as it produces renewable and emission free power. The most common way is to extract warm formation water or steam and use this for heating or power generation. A second technique, Hot-Dry-Rock (HDR), is under development. It aims at exploiting heat by injecting cold water in a fractured reservoir and recovering it after it has been heated. A third method is CO₂-geothermics. This concept has been developed in combination with assessment studies for the sequestration of CO₂ in abandoned coal mines.

Suited conditions for CO₂-geothermics are only met in CO₂-filled reservoirs that are located at a correct depth and have a sufficient vertical extent. A typical coal mine may range from 500 m to 1000 m depth. When such a mine would be used for CO₂-sequestration, the reservoir pressure at 1000 m would be set equal to hydrostatic pressure. In this way, the whole reservoir will become overpressured and the influx of formation water would be prevented.

The reservoir temperature is determined by the geothermal gradient of the host rock. The pressure gradient, however, depends on the density profile of CO₂, and not on the hydrostatic gradient, which prevails outside the reservoir. Since the density of CO₂ is, also in its liquid state, lower than the density of water, the pressure gradient (increase of pressure with depth) in the reservoir will be lower than the hydrostatic gradient. This low pressure gradient, the normal thermal gradient and the physical properties of CO₂, result in a reversed density profile with the most dense fluid occurring at the top of the reservoir. This is not a stable situation. The dense, cold CO₂-fluid will move down, whereas the lighter and warmer fluid from the deeper parts of the reservoir will rise. This convection will transport warmth from deeper to more shallow parts of the reservoir, and will continue as long as the temperature difference between the bottom and top of the reservoir is sufficiently large to sustain the reversed density profile.

CO₂-geothermics uses the heat transporting capacity of a CO₂-filled reservoir. In a conceptual system, a heat exchanger is placed at the top of the reservoir to extract warmth from the reservoir. This will cool the top of the reservoir, causing the CO₂ to become denser and thus enlarging the reversed density gradient. The most effective system can be developed in a low-density reservoir, this is a reservoir which is filled with gas or gas-like CO₂. When the top of such a reservoir is cooled sufficiently, part of the CO₂ will condense. The liquid CO₂ will rain or run down the reservoir, until it warms up and vaporises. The cooling and downward mass transport will result in a pressure drop. In response to the pressure and density gradients, warm CO₂ will rise to the top of the reservoir. This can in turn be condensed by the heat exchanger to extract thermal energy.

This convection has the potential of transporting heat in a very efficient way because of the large amounts of energy that are released and absorbed during the condensation (top of reservoir) and vaporisation (lower parts of reservoir). The heat itself is mainly extracted from the rocks, which are present as gob in the panels. The required energy input and surface facilities may be minimal, because it is not necessary to pump up large amounts of fluid and, given a suited mine design, heat from a large portion of the subsurface is transported to a limited number of heat exchangers.

Greenhouse gas mitigation in Belgium. Status and potential contribution of geological sequestration.

compilation Michiel Dusar, April 2003

Paper

STATUS

As a densely populated and highly industrialised country with growing economy, Belgium has a balanced diversification of energy sources. CO₂ emissions grew in the 1990s and reached 127 Mt in 2000, about 7,7% higher than in 1990 and 12,7% above the target level (EEA, 2002). Contrary to other greenhouse gases, CO₂ emissions are still growing 1% annually (Quarterly Newsletter of the Federal Planning Bureau); for the Flanders region, CO₂ emissions rose 13% over the period 1990-2001 (Van Steertegem, 2002). Belgium's Kyoto target is a 7,5% reduction of emissions by 2008-2012 compared to 1990 of which CO₂ accounts for 84%.

The federal government ratified the Kyoto protocol in 2001 but effective reduction will depend on actions by the regions, which have a high degree of autonomy in energy policy and are not committed to the same energy efficiency and environmental target. The first Federal Plan for Sustainable Development was published in 2000 and provides a framework for federal schemes, including a National Climate Plan that is still in preparation (Task Force Sustainable Development, 2002). No quantitative policy goals have been set, however. Complexity of the government structure and occasional disparities between federal and regional policy objectives and targets are the main causes for the slow progress of decision-making in key policy issues on greenhouse gas mitigation (Priddle, 2001).

source: http://mineco.fgov.be/homepull_en.htm (this site will be opened in a new window.)

Advances in energy efficiency, with the introduction of benchmarking, and partial substitution of coal by gas, have contributed to restraining growth in CO₂ emissions since 1990. Great efforts in increasing energy efficiency have already been made by the key industrial sectors (electric power, metallurgy, petrochemicals, cement), which reduces possibilities for further reduction without threatening the international market position of the industrial sector. The 'dash for gas' has rapidly increased the share of gas at the detriment of coal in the energy supply. Rising gas prices and an uncertain future in a liberalising market has brought this switch to a halt, which may be beneficial in the long term, in view of declining European reserves and the great dependency of the domestic sector in Belgium on gas. Neither renewable energy nor combined heat and power generation, which could limit carbon emissions, can be easily introduced in Belgium, because of higher cost and local resistance, and will not contribute significantly in attaining the Kyoto target. Renewable electricity generation is less then 1% in Belgium (lowest in the European Union), is not growing much faster than the total electricity production and will not reach the 3% active policy goal and thus fall way beyond the 6% indicative target set for 2010 (ECOFYS, 2002). The regions have introduced "green certificate schemes" in an attempt to address this problem. Geological sequestration is not part of the National Climate Plan, nor included in the green certificate schemes.

It is generally accepted, and conceded by the regional authorities, that the Belgian Kyoto target will not be attained, though opinions on causes and effects strongly differ. Actually, more than 16 Mt of CO₂ have to be avoided annually in Belgium. Greenhouse gas mitigation in a growing Belgian economy can only be achieved by combined action on the four variables: energy efficiency, reduction of carbon intensity, green energy, capture and sequestration.

Kyoto is a first modest step towards stabilisation of CO₂ concentration and is as such not leading to a change of global warming trends. In order to stabilise the atmospheric CO₂ concentration at a level preventing more pronounced human interference with the climate system - as recommended by the UN Framework for Climate Change at 550 ppm - emissions have to be cut by at least 50%. CO₂ sequestration (partial or total capture and storage) is necessary to achieve deep reductions.

Belgium is also committed to phase out nuclear power from 2014. Although this will not affect the already existing difficulties for reaching the national Kyoto target it will be an additional challenge for achieving deep reductions in the future. Nuclear power accounts for almost 60% of electricity generation and for 22% of total energy supply in Belgium. Finding a cost-effective and low-emission alternative for nuclear power is not yet considered, which means that fossil fuels will continue to supply energy for electricity generation for decades to come (AMPERE, 2000 and International Peer Review group, 2001), stressing even more the need for capture and storage of CO₂.

Prevent fossil carbon to be emitted in the atmosphere, capture it and send it back from where it comes in a long-lasting storage is the message for the 21st century (Mathieu, 2002; IEA Working Party on Fossil Fuels, 2002).

STORAGE POTENTIAL

The small size of Belgium, about 39.000 km2 including the offshore area, and its complex geology do not offer unlimited choices for geological sequestration. At a depth of 800 m, generally considered as the minimal technical limit for sequestering CO₂, 95% of the territory is occupied by tight Paleozoic rocks. There are no extensive deep saline aquifers, no known oil and gas fields, no salt stocks. The potential for geological storage is restricted to the Dinantian aquifer in the Campine basin, to unmined coal beds and abandoned coal mines in the Hainaut and Campine coal basins.

Aquifer

The Dinantian aquifer in the province of Antwerp, which is already utilised for seasonal storage of natural gas (the Loenhout storage site holds over 1 billion Nm³ of gas) has an estimated CO₂ storage potential of 125 Mt in closed structures and under an angular structural trap. Other aquifers in the Campine basin are either insufficiently known or too risky for leakage; the latter problem also affects the Dinantian aquifer in the southern parts of Belgium. Notwithstanding its rather small capacity (compared to some offshore reservoirs) and need for additional seismic reconnaissance, the Dinantian aquifer is interesting because of good injectivity and high completion factor, in relation to nearby sources of CO₂. The best scenario for an early opportunity project is the linking with a pure CO₂ source from an ammonia plant. Other, poorly known potential reservoirs are in Devonian carbonates and Triassic sandstones.

Coal

Storage of CO₂ in unmineable coal seams has a much higher potential, but presents the disadvantage of slow injection rates, requiring many boreholes. The Campine coalfield is able to hold 950 Mt of CO₂. Coal already contains methane adsorbed to the internal pore surfaces. Generally, CO₂ is preferentially absorbed on coal compared to CH4, and the exchange ratio approximates 2:1. In practice, CO₂ thus will enhance recovery of coal bed methane, still producing green energy as there will be a net carbon reduction. Six anomalous zones in the Campine coal basin could produce 132 billion m³ of methane and store 432 Mt of CO₂. Unmined coal zones, e.g. east of Mons in the Hainaut coal field have a potential for ECBM allowing CH4 production of 1 Mm3/day (Mostade, 1999) and storing all surplus process CO₂ from an ammonia plant nearby (cf. Lysen, 2002). For this reason, the government of the Walloon region decided early 2003 to join the RECOPOL project (Reduction of CO₂ emission by means of CO₂ storage in coal seams in the Silesian Coal Basin of Poland) as an end-user.

Coal mines

Abandoned coal mines in the Campine and Hainaut coal fields are quite tight and gassy. The Anderlues and Péronnes coal mines in Hainaut have been temporarily but safely developed as a reservoir for seasonal gas storage. However, long-term storage of CO₂ requires resistance to flooding or else dissolution in deep groundwater, practically limiting the capacity to be stored safely. Because of shallow depth of closure, mines in the Hainaut coal field are not interesting for CO₂ storage unless deep seals are established. The Campine collieries are possibly among the world best for developing new energy systems, starting with methane drainage, continuing with CO₂ sequestration and ending with geothermal exploitation. Total storage capacity is small, ca 35 Mt but high injection rates and possibilities to earn revenues from CH4 recovery, which otherwise would be spilled, make them a good target for early opportunities (possibly in combination with pure CO₂ sources from across the national boundary as part of a transnational pilot project). Re-utilisation of abandoned mines could also provide the learning-time how to develop the larger, yet uneconomical coal reserves in the longer term.

CO₂ emission sources in Belgium, used for scenario development (Laenen, 2003; Mostade, 1999). Data source: ECOFYS datasheet for GESTCO.

CONCLUSION

Geological sequestration must be seen in conjunction with other options of greenhouse gas mitigation and should complement but not replace other policy measures to encourage energy efficiency and reduction of carbon intensity. However, in view of the apparent difficulties of Belgium to reach even the Kyoto target, underground storage might be considered as unavoidable. The potential for geological sequestration of CO₂ in Belgium is restricted and does not offer solutions for continuous and unlimited dependency on fossil fuels, but possesses nevertheless opportunities for early application in the Campine basin. The vast coal deposits, both in north and south Belgium offer hopes for future utilisation as a reservoir for energy carriers and repository for derived gases. Ideally, CO₂ sequestration could be integrated in the optimised exchange of energy products between the surface and the subsurface.

REFERENCES

• AMPERE (Pauwels, J.P. & Streydio, J.-M., eds.), 2000 - Report of the AMPERE ('Analysis of the Means of Production of Electricity and the Restructuring of the Electricity Sector') Commission to the State Secretary for Energy and Sustainable development. October 2000.
• ECOFYS, 2002 - Pretir. Implementation of Renewable Energy in the European Union until 2010. Project executed within the framework of the ALTENER Programme of the European Commission, DG Transport and Energy.
• EEA European Topical Centre on Air and Climate Change, 2002 - Greenhouse gas emission trends in Europe, 1990-2000. European Environmental Agency, Copenhagen. 168 p.
• International Energy Agency Working Party on Fossil Fuels, 2002 - Solutions for the 21st Century. Zero Emissions technologies for Fossil Fuels. Technology Status Report, May 2002.
• International Peer Review Group (Bourdeau, Ph.; Laponche, B.; Morrison, R.; Mortensen, J.B. & Savelli, P.), 2001 - Assessment of the AMPERE commission report by an International Peer Review Group to the State Secretary for Energy and Sustainable development. April 2001.
• Laenen, B., 2003 - DSS scenarios Belgium. VITO 2003/ETE/R/0R0. 38 p.
• Lysen, E.H., ed., 2002 - PEACS Project: Opportunities for early application of CO₂ sequestration technology. IEA GHG Report Number PH4/10, September 2002; 100 p.
• Mathieu, Ph., 2002 - Towards the hydrogen era using near zero CO₂ emission energy systems. ECOS 2002 (Efficiency, Costs, Optimization, Simulation and Environmental Impact of Energy Systems), Berlin, July 2002.
• Mostade, M., 1999 - Coalbed Methane Potential of the Southern Coal Basin of Belgium. International Coalbed Methane Symposium, May 3-7, 1999, Tuscaloosa, USA.
• Piessens, K. & Dusar, M., 2002 - Feasibility of CO₂-sequestration in coal mines. Geological Survey of Belgium, August 2002. 50 p.
• Priddle, R., 2001 - IEA commends introduction of new energy efficiency measures but stresses need for more competition in Belgium's energy sector. IEA/Press 3 October 2001.
• Task Force Sustainable Development, 2002 - A Step towards Sustainable Development. Federal Report on Sustainable Development 2002 (in NL and FR). http://www.icdo.fgov.be/pub/rapports.stm (this site will be opened in a new window.) and Vlaams Klimaatsbeleidsplan. Periode 2002-2005.
• Van Steertegem, M., red., 2002 - Milieu- en Natuurrapport Vlaanderen: thema's 2002. MIRA-T 2002. VMM. 388 p.
• van Tongeren, P.C.H., 2001 - CO₂-sequestration possibilities in the deep aquifers of the Campine Basin (northern Belgium). VITO 2001/ETE/R/030. 20 p.
• van Tongeren, P.C.H. & Laenen, B., 2001 - Coalbed methane potential of the Campine Basin (N. Belgium) and related CO₂-sequestration possibilities. VITO 2001/ETE/R/042. 39 p.
• van Tongeren, P.C.H. & Laenen, B., 2001 - Residual space volumes in abandoned subsurface coalmines of the Campine Basin (northern Belgium). VITO 2001/ETE/R/054. 38 p.
• van Tongeren, P.; Dreesen, R.; Laenen, B. & Dusar, M., 2002 - Influence of geologic and economic parameters on the (E)CBM-development in the Campine basin (Belgium). Polish Geological Institute Special Papers, 7: 271-280.

INSTITUTIONS WHICH CAN OFFER FURTHER INFORMATION AND ASSISTANCE

• Geological Survey of Belgium
  Website: http://www.naturalsciences.be/geology/
  Contact: michiel.dusar@naturalsciences.be - kris.piessens@naturalsciences.be
• VITO (Flemish Institute for Technological Research)
  Website: http://www.vito.be/english/energytechnology4.htm (this site will be opened in a new window.)
  Contact: roland.dreesen@vito.be - peter.vantongeren@vito.be - ben.laenen@vito.be - david.lagrou@vito.be
• University of Liège, Department of Power Generation
  Website: http://www.ulg.ac.be/genienuc/ (this site will be opened in a new window.)
  Contact: pmathieu@ulg.ac.be
• Polytechnical Faculty of Mons, Mining-Geology Section
  Website: http://gefa.fpms.ac.be (this site will be opened in a new window.)
  Contact: marc.mostade@swing.be - christian.dupuis@fpms.ac.be