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

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.

The CO2TRAP project was successfully completed in May 2008 and initiated these follow-up projects:

CO2 Seals

Integrity of sealing rock formations for CO2 storage
http://www.co2seals.de

CO2SINUS

CO2 Storage in in situ Converted Coal Seams
http://www.co2sinus.org

ALCATRAP

Optimisation of CO2 binding by reaction with alkaline residual materials through the ALCATRAP process
http://www.hydro.uni-bayreuth.de/pros/detail.php?lang=de&id=32