CO2 storage through mineral trapping in geothermal reservoirs

Abstract

Costs for carbon dioxide sequestration into deep saline aquifers can be transformed into a benefit when combined with ecologically desirable geothermal heat or power production. The produced energy can be used and marketed. Aim is a scientifically and technically feasible new technology to achieve a safe and economically attractive long-term storage of CO2 trapped in minerals. We develop, study, and evaluate a novel approach not only to sequester CO2 by physical trapping within a reservoir, but to convert dissolved CO2 into the geochemically more stable form of calcite. Due to the geological situation exploitation of geothermal energy in Germany is mainly provided from deep aquifers. The common arrangement of bore holes is the well doublet, consisting of one well for hot water production and one well for cooled water re-injection. The cooled water is loaded with dissolved CO2, and after re-injection into the reservoir this cold water becomes enriched in calcium e.g. due to dissolution of anhydrite (CaSO4). Subsequently CO2 precipitates as calcium carbonate (CaCO3), provided that alkalinity is present either by the dissolution of feldspars in the aquifer or by surface water treatment with fly ashes. Processes are studied both in laboratory and by numerical simulations. The latter are essential to quantify the entire process of CO2 storage and to deepen the understanding of the detailed chemical processes. Reaction modelling and reactive transport simulations are done on multiple scales since the combination of all scales is not feasible in numerical models up to now. The relevant scales studying CO2 storage in combination with geothermal energy production reach down from the reservoir scale (ca. 10 km) to the micro scale (ca. 1 cm). Results from larger scale models provide constraints for smaller scale scenarios. For processes which cannot be resolved on the larger scale, due to restrictions of discretization of the applied numerical mesh, functionalities are derived from the smaller scale. To be predictive and capable of quantifying amounts of storable CO2 numerical investigations on the reservoir scale are vital. Simulations on the borehole scale are necessary, because the near vicinity of wells is vulnerable to permeability decrease as a result of mineral reactions. Laboratory experiments are used to calibrate the numerical tools and simulations on the micro scale allow further investigation of the overall process of mineral dissolution and precipitation. Simulation results as well as laboratory experiments prove that anhydrite can be successfully transferred into calcite and thus are evidence for the feasibility of the new technology.