Evaluation of the CO2-sequestration capacity of sandstone aquifers in the Campine Basin (NE-Belgium) based on autoclave experiments and numerical modelling

摘要

The Campine Basin in NE-Belgium houses importantCO2-emitting industries. Injection of CO2 in sandstoneaquifers in this sedimentary basin could significantly reduce emissions towards the atmosphere.An integrated study was set up to evaluate the effects ofCO2-water-rock interactions on the reservoir properties of 3sandstone aquifers in the basin, i.e. the fluvial sandstones of the Westphalian C, Westphalian D and the Lower Triassic (Buntsandstein). A number of representative samples, from boreholes, of each reservoir were characterised by means of a broad spectrum of petrographical, geochemical and petrophysical methods. Five samples of each reservoir were exposed for a period of 6 months to the conditions prevailing in the reservoirs during and after CO2 injection, in high temperature - high pressure autoclaves. CO2-water-rock interactions were inferred from the evolution of the chemical composition of the brine in the autoclaves and comparison of the treated and untreated samples after the experiments. Data from the detailed characterisation of the experimentally treated samples was used to construct a reaction model in PHREEQC. Reaction kinetics of 17 rockforming minerals are based on user-defined rate laws and parameters. CO2-water-rock interactions inferred from the experiments were used to adjust reaction progress. Numerical modelling confirms that the sequestration capacity of the studied reservoirs will be greatly enhanced by CO2-water-rock interactions. During injection carbonate dissolution can enhance permeability of the reservoirs. In the first 15 years after injection alteration of Al-silicates (feldspars and clays) to kaolinite and illite buffers the pH-drop caused byCO2-injection. Higher pH and release of K and Na promoteionic trapping, i.e. sequestration as dissolved bicarbonatespecies. Slow release of Fe and Mg from altering Al-silicates offers some potential for mineral trapping, i.e. sequestration of CO2 as carbonate minerals. Dissolution of hematite and pyrite causes reduction of Fe3+ and precipitation of siderite. Substantial siderite precipitation occurs after more than 25 years and is initiated when a certain pH is reached due to Al-silicatereactions. These numerical simulations illustrate that thesequestration capacity offered by CO2-water-rock interactions is highly variable, depending on the reservoir mineralogy. The sequestration capacity of the studied reservoirs ranges from approximately 100 to 500 gCO2/kgw.