Effects of carbon dioxide leakages on shallow aquifer properties

Abstract

Abstract The mineralogy of well-sorted silica sand is modified by mixing it with an aqueous solution of metal (Fe, Mn, Cu, Co, Ni, Cr, Zn, Cd) salts and increasing very slowly the pH so that insoluble metal oxides/hydroxides co-precipitate on sand grains. The modified sand is packed in two fixed-bed columns saturated with water which is injected at a constant and low influx rate. In the one column, gas CO2 is co-injected at a flow rate that is one order of magnitude higher than that of water, while the flow test in the other column is used as control experiment. The pressures drop across the aqueous and gas phases are recorded continuously to track any potential changes of the (column averaged) relative permeabilities. Water effluent samples are collected to measure physicochemical parameters (e.g. pH, conductivity, etc) and metal cation concentration by atomic absorption spectroscopy. The mineralogy of sand is identified with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and quantified with atomic absorption spectroscopy. The kinetics of CO2 dissolution and aqueous phase acidification along the porous medium is simulated under conditions of constant and uniform distribution of water saturation. The highest rates of metal cation release, due to geochemical solid/aqueous phase reactions, are observed during the initial and transient phase of the experiment as the CO2 dissolution and water acidification have not yet been completed, whereas the fluid saturation and relative permeability change respectably. The CO2 dissolution rate is enhanced as the water saturation decsreases due to the higher specific interfacial area.