The reduction of anthropogenic CO2 in the atmosphere is of paramount importance for the future world climate. Various available options for the sequestration of CO2 in the subsurface have been proposed and discussed. Both on the international and national level significant progress has been accomplished on this issue. Yet there is a continued need and also still unexploited potential for a judicious combination of carefully designed laboratory experiments and numerical simulations of the physical and chemical processes. The aim of this CO2Trap project, funded by the German Federal Ministry of Education and Research (BMBF under grant 03G0614A, GEOTECHNOLOGIEN special program - Erkundung, Nutzung und Schutz des unterirdischen Raumes -), is to develop, study, and evaluate two alternative approaches for the subsurface deposition of CO2: Technology I: Precipitation of aqueous CO2 as calcium carbonate (CaCO3) in formations containing calcium sulphates (CaSO4) and feldspars. Technology II: Sorption of CO2 on (i) residual coal and (ii) waste coal dust or sludge from coal processing plants in abandoned coal mines. As an additional, overriding issue relevant for all options of underground storage of CO2 we propose to study the long-term effects of supercritical CO2 gas and dissolved aqueous CO2 on the sealing properties of typical cap rocks above potential CO2 storage formations.

Feasibility Study

The FeasibilityStudy emphasizes that both Technology I - mineral trapping of CO2 in geothermal reservoirs or using fly ashes- as well as Technology II - physical trapping of CO2 sorbed on residual coal and mining waste material in abandoned coal mines - provide alternative approaches for long-term and safe subsurface immobilisation of greenhouse gases. Significant storage capacities are available in geological formations for millions of tonnes of carbon dioxide.

Project Partner

RWTH Aachen University

• Applied Geophysics
• Geology and Geochemistry of Petroleum and Coal
• Engineering Geology and Hydrogeology
• Clay and Interface Mineralogy

University of Bayreuth - Hydrology

University of Stuttgart - Hydraulic Engineering

RWE Dea AG

RWE Power AG

DMT GmbH

Deutsche Steinkohle AG

STEAG Saar Energie AG

Technology I - mineral trapping: Precipitation of aqueous CO2 as calcium carbonate in formations containing calcium sulphates and feldspars

Objectives

The novel approach is to sequester CO2 not only by physical trapping within a reservoir, but to convert dissolved CO2 into the geochemically more stable form of calcite in a reaction with calcium obtained from dissolution of sulphates and feldspars. Influence factors will be studied in the laboratory, reactive transport is going to be modelled in numerical reservoir simulations, and optimum conditions explored for this process to take place.

Scientific and technical goals

Mineral trapping of CO2

For modelling and thus testing the potential of a CO2 sequestration by transforming anhydrite into calcite, the dynamics of anhydrite redistribution within the reservoir as well as the trans-formation mechanisms and rates need to be known. In detail, three topics will be studied: (i) dissolution rates of anhydrite at near-equilibrium conditions and (ii) at expected variations in solution chemistry. The understanding of the mechanism and rate of the transformation of anhydrite into calcite requires further data from experiments performed as close as possible to reservoir conditions in order to estimate the efficiency of various kinds of anhydrite precursors (anhydrite as cementing agent, as nodules, as cloudy nests). New insight gained into dissolution, growth, and transformation kinetics of anhydrite will form a solid base for detailed simulations of a technical scenario, which deals with the input of alkalinity in combination with heat extraction. Reactive transport will be simulated for injection of a cool fluid, conditioned to contain an optimum amount of CO2. The simple rate laws currently implemented in the SHEMAT software (Clauser 2003) will be refined and replaced by improved relations derived from the specific laboratory experiments performed in this project. The result will be a realistic simulation which accounts for all of the important physical and chemical processes in this highly non-linear, coupled reactive transport problem. This allows exploring different scenarios in order to provide a ranking with regard to CO2 storage volume, logistic, or economic feasibility, and to propose an optimum mix between these different criteria.

Geothermal Energy Use

The cooling of large volumes of reservoir fluid can be transformed from a cost into a benefit factor if the geothermal heat is used and not spilled as waste heat. The produced heat can be used and marketed for space heating like in commercial geothermal heating plants or transformed into electricity using organic Rankine cycle or Kalina cycle technology. The cooled water is then loaded with dissolved CO2, and after reinjection into the reservoir this cold water becomes enriched in calcium and subsequently precipitates CO2 as calcium carbonate (CaCO3). This way the cost reduction for cooling the injected water is achieved by producing ecologically desirable geothermal energy. Injecting aqueous CO2 at a hydro-geothermal installation has the advantage that the fluid pressure in the aquifer will remain more or less unchanged as the fluid volumes produced from the aquifer roughly equal those injected. Ultimately, it is one of the prime goals of this project to develop a scientifically and technically feasible new technology in which CO2 sequestration and geothermal energy use are combined to achieve a safe and economically attractive long-term storage of CO2 trapped in minerals.

_Geothermal heating plant consisting of a well doublet (production and injection well)_

Technology II - physical trapping: Sorptive storage of CO2 on residual coal and coal dust in abandoned mines

Objectives

Due to their high gas sorption capacity, coal and dispersed organic matter are considered as promising targets for subsurface storage of CO2. The planned studies will explore whether a combination of this storage process with the production of coal bed methane or coal mine methane can be expected to provide synergetic effects. Further, and in contrast to earlier studies, focus lies on the injection of flue gas instead of pure CO2 in order to reduce costs associated with separating CO2 from flue gas. Additionally, joint deposition will be investigated of waste coal dust or sludge and CO2 in abandoned mines adsorbed to these substances. This procedure may provide an innovative option for both the deposition of waste coal dust or sludge and a productive future use of abandoned mines.

Scientific and technical goals

Utilization of gob zones of abandoned coal mines for CO2 storage

The use of gob areas and formation damage zones in and around abandoned coal mines as an alternative to storing CO2 in coal seams will be studied. These damage zones offer various advantages: (i) they have high permeability; (ii) they provide access to large volumes of residual coal, dispersed sedimentary organic matter and mineral surfaces with potentially high CO2 sorption capacities; (iii) the geological situation is usually well known; (iv) parts of the existing mining infrastructure (pumps, pipelines, ventilation shafts) may be used for gas injection; (v) long-term gas (methane) monitoring and water management plans are operational in these areas for public safety reasons (little additional investment required for CO2 monitoring). The work will involve initial laboratory sorption experiments on coals, dispersed organic matter and bedrocks to assess fundamental data on CO2 storage capacities and kinetics. The main challenge will then consist in the integration of physico-chemical data with engineering and mining information to arrive at a reliable feasibility assessment.

Flue gas injection

As a specific feature and timely topic, this project will consider the injection of flue gas rather than pure CO2. This may have several advantages: (i) it avoids costs for separating CO2; (ii) it reduces safety risks due to relatively low CO2 contents and low injection pressures. The concept involves using large volumes of residual organic matter in subsurface mining dam-age zones as - geologic filters - to remove CO2 from flue gas. Research will focus on the com-position and sorption properties of flue gas, such as: preferential sorption of individual flue gas components; rates (kinetics) of sorption and desorption on coals, dispersed organic mat-ter and mineral components under different temperature and pressure conditions. Further, the thermodynamic properties (equations of state) and the aqueous solubility of flue gas will be studied in detail.

Combination of coal dust or sludge and CO2 deposition

The third innovative aspect to be studied in this project is the combination of the underground deposition of waste coal dust or sludge with CO2 disposal. Large amounts of coal sludge are being produced every year as a waste product during the processing of mined hard coal and lignite (brown coal). This material contains significant amounts of organic carbon (>60 % of dry mass), and therefore can be expected to have a high sorption capacity for CO2. Its small particle sizes correspond to higher sorption rates compared to that in compact coal seams. On the one hand, deposition of this coal dust or sludge in abandoned coal mines appears as an attractive way to dispose this waste material while on the other to create additional gas sorption capacity in the underground mines. A combination of these two aspects - disposal of waste coal dust or sludge and underground deposition of CO2 - generates positive economic and ecologic synergy. It is proposed to use coal dust or sludge to extract CO2 on site by adsorption from flue gas. This process might require a high partial pressure to avoid de-sorption during deposition. However, an appropriate method should be developed to increase the economical and ecological benefits of the process. In case of success, this new and low-cost CO2 extraction method could replace existing traditional separation techniques which are uneconomical and deteriorate power production efficiency by about 15 %.

Numerical simulations

One of our major goals is to be able to predict the processes of gas transport and sorption/desorption in the workings (shafts and passages), gob areas, and damage zones of abandoned coal mines. Numerical modelling of gas transport processes in underground mines will be used as an integrative tool to: (a) assess the usefulness of conventional mine ventilation software for modelling flue gas injection; (b) to simulate flue gas injection using the MUFTE_UG simulation software (Helmig 1997, Helmig et al. 1998).

Sealing efficiency of reservoir cap rocks over geological time with respect to gaseous and supercritical CO2

Objectives

The long-term sealing efficiency of cap rocks above potential CO2 storage reservoirs is a major concern in the selection and design of subsurface storage facilities for this reactive gas. In this project sealing efficiency will be characterized of geological barriers (low-permeability cap rocks) exposed to CO2 gas and dissolved CO2 at reservoir pressure and temperature.

Scientific and technical goals

Fine-grained clastic rocks form the seals above most natural gas and petroleum reservoirs. Laboratory tests will be perform on selected types of cap rocks (shales, marls, siltstones, carbonates) to assess their sealing efficiency with respect to CO2 and study the influence of reactive CO2 gas or dissolved CO2 on their geochemical and petrophysical properties. Our primary objective is to develop an improved quantitative understanding of CO2 transport processes in sedimentary rocks by combining well-constrained laboratory experiments, rock characterization, and numerical modelling. Short-term, long-term, and corresponding leakage processes will be addressed. The laboratory experiments comprise capillary gas-breakthrough tests, permeability measurements with water, determination of effective gas permeabilities after breakthrough, and diffusion experiments. All experiments will be conducted under controlled effective stress conditions at reservoir temperatures and fluid pressures. Detailed petrophysical, mineralogical and geochemical analysis will be performed for sample characterization and correlation. The experiments will be accompanied by numerical modelling studies from laboratory to field scale involving multi-component and multiphase flow and reactive transport to provide a quantitative basis for a qualitative and quantitative appraisal of long-term cap rock sealing efficiency.

Project links

• RWTH Aachen - press release
• Kioto Protokoll

WDR nano tv report about CO2Trap project

• Real Media Stream (good quality)
• Real Media Stream (high quality)

Media players

• Realplayer (Windows) - Download Link
• MPlayer (Windows/Linux/Unix/MacOS X) - Download Link

References

Clauser C (2003) Numerical Simulation of Reactive Flow in hot Aquifers using SHEMAT/Processing SHEMAT. Springer Verlag, Heidelberg-Berlin.

Helmig R (1997) Multiphase Flow and Transport Processes in the Subsurface. A Contribution to the Modeling of Hydrosystems (Environmental Engineering), Springer Verlag, Heidelberg-Berlin.

Helmig R, Class H, Huber R, Sheta H, Ewing J, Hinkelmann R, Jakobs H, Bastian P (1998) Architecture of the Modular Program System MUFTE_UG for Simulating Multiphase Flow and Transport Processes in Heterogeneous Porous Media, Mathematische Geologie, 2, 123-131.

Contact

For further information please contact:
Robert Meyer, RWTH Aachen, Applied Geophysics:
Lochnerstr. 4-20, 52056 Aachen, Tel.: +49 241 80-99772, Fax: +49 241 80-92132:
e-mail: r.meyer@geophysik.rwth-aachen.de

Prof. Dr. Christoph Clauser, RWTH Aachen, Applied Geophysics:
Lochnerstr. 4-20, 52056 Aachen, Tel.: +49 241 80-94825 Fax: +49 241 80-92132:
e-mail: c.clauser@geophysik.rwth-aachen.de

Funding