Mineralogical and microstructural investigations of two natural CO2-bearing sandstone reservoirs

Abstract

Geological storage of CO2 in sandstones is expected to have a large influence on the characteristics of reservoir rocks. Two natural CO2-bearing sandstone reservoir rocks are studied to investigate the influence of the long term storage of CO2 on the geochemical, mineralogical and microstructural processes operating in natural reservoirs. Optical- and electron microscopy and micro-XRF techniques are used to investigate microstructural and geochemical processes like mineral trapping, mineral growth, diagenesis through metamorphic reactions and porosity-permeability changes. By using CO2-free samples as a reference for CO2 natural analogue sites, diagenetic features could have been distinguished from CO2-related reactions. Mineral reactions that can be linked to the presence of CO2 in the Werkendam reservoir are the partial dissolution of anhydrite and the precipitation of magnesium-rich siderite. The formation of Mg-rich siderite out of ferroan dolomite reduces the porosity since it grows in the pores. Consequences of CO2-rich fluids penetrating through the Entrada sandstones are the removal of hematite grain coatings and the dissolution of considerable amount of clays. Both reactions increase the porosity. In addition, illite is altered to kaolinite, which has significant implications for the permeability of a reservoir rock chosen for future subsurface CO2 storage. Results of this study can be applied for the large-scale investigations of the influence of CO2 on reservoir rocks. The Entrada sandstone acts as an open system: a useful reference for the behaviour of reservoirs in the case of CO2 leakage and faulting. The Werkendam reservoir rocks are a valuable reference for the long-term implications of CO2 accumulation.