| Title | Mineral Trapping of CO2 in Operated Hydrogeothermal Reservoirs |
|---|---|
| Authors | Michael Kühn, Martin Back, Christoph Clauser, Helge Stanjek and Stefan Peiffer |
| Year | 2007 |
| Conference | European Geothermal Conference |
| Keywords | CO2 storage, mineral trapping, geothermal energy, anhydrite |
| Abstract | Storage of carbon dioxide (CO2) by precipitation of carbonbearing minerals in geological formations is, on the long run, more stable and therefore much safer than direct storage or solution trapping. Furthermore, options for CO2 sequestration which offer additional economic benefits besides the positive effect for the atmosphere are attractive. Both arguments motivate us to study the novel approach of storing dissolved CO2 as calcite in geothermally used aquifers. Geothermal energy in Germany is mainly provided from deep sandstone aquifers by a so called ?doublet? installation consisting of one well for hot water production and one well for cooled water re-injection. After re-injection of CO2 enriched, cold brine into the reservoir, anhydrite abundant as matrix mineral dissolves. As a consequence, the water becomes enriched in calcium ions. Numerical simulations demonstrate that alkaline buffering capacity provided by plagioclase in the reservoir rock or through surface water treatment with fly ashes subsequently result in the reaction of dissolved Ca and CO2 to form and precipitate calcium carbonate. We show that anhydrite dissolution with concurrent pore space increase is important to balance pore space reduction by precipitation of calcite and secondary silicates. A core flooding experiment under increased pressure and temperature conditions showed that the average permeability increases continuously. Laboratory experiments prove the feasibility of literally transforming anhydrite into calcite and provide necessary kinetic input data for the modelling. Suitable geothermal reservoirs exist with anhydrite as matrix mineral and plagioclase supplying alkalinity. Their CO2 storage capacities depend on their volume and porosity as well as on the chemical and mineralogical composition of the formation brine and reservoir rock, respectively. Mass balance calculations yield that the storage capacity can be estimated from the abundance of anhydrite in the reservoir. Based on an operation time of 30 years this theoretical, quite significant storage capacity amounts to million of tons of CO2 around geothermal heating plants. |