Importance of Co-captured Gases in the Underground Storage of CO_2: Quantification of Mineral Alterations in Chemical Experiments

摘要

Carbon dioxide capture and geological storage (CCS) is being developed to reduce the carbon dioxide (CO_2) emissions from anthropogenic point sources, e.g. fossil-fuel power plants, to the atmosphere. To establish CCS technology, it is indispensable to develop a reliable database and geochemical models concerning the geological storage of CO_2, e.g. in saline aquifers, which are to be filled with "overwhelmingly CO_2" (Directive 2009/31/EC). To establish reliable models it is essential to have applicable thermodynamic properties, kinetic data, and a good understanding of the occurring chemical reactions. So far most experiments and existing data apply to pure CO_2 gas instead of the captured CO_2 waste gas that will contain minor amounts of co-captured gases, e.g. O_2, N_2, NO_x, SO_x, CO, H_2, H_2S. Quantitative measures of the chemical alterations due to these accessory gases are scarce. In the national COORAL project "CO_2 Purity for Separation and Storage", a number of institutions work towards a better understanding of environmentally and economically feasible concentrations of the accessory gases during capture, transport, injection and storage. The sub-project at BGR focuses on high-pressure and high-temperature (HPHT) experiments to elucidate mineral and fluid alterations and quantify kinetic rates for the mineral-fluid-CO_2-co-injected gas system. An unstirred batch-reactor system allows for four contemporaneous experiments at precisely defined p-T conditions of up to p 590 bar T 350°C. Runs are conducted using three components: 1) natural mono-minerals, 2) salt solutions representing brines of deep saline aquifers in Northern Germany and 3) binary gas mixtures of CO_2 plus one accessory gas. All experiments take place in an inert environment, using gold reaction cells with volumes of up to 130 ml, which allow the addition or removal of fluids throughout the experiment without altering the experimental conditions. Further experiments comprise experiments using (1) multi-mineral set-ups in a batch experiment and (2) up to 45-cm-long sedimentary rock cores in flow-through reactors. The latter system is currently under construction, while batch - and capsular - experiments run successfully. To further optimize the experimental design and to evaluate the experiments the project combines laboratory experiments and numerical simulations, applying the geochemical simulators PHREEQC and ChemApp which will be coupled to OpenGeoSys (OGS) for thermo-hydro-mechanical-chemical (THMC) process simulations.