Synergy through CO_2 Storage by Mineral Trapping in Combination with Geothermal Heat Production

摘要

Storage of carbon dioxide (CO_2) by precipitation of carbon-bearing minerals in geological formations is, on the long run, more stable and therefore much safer than direct storage or solution trapping. Furthermore, options for CO_2 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 CO_2 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 CO_2 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 CO_2 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. 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 CO_2 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 0.5 million tons to 25 million tons of CO_2 around geothermal heating plants.