A CHEMICAL MODEL FOR THE SEAWATER-CO2- CARBONATE SYSTEM – AQUEOUS AND SURFACE CHEMISTRY

摘要

We present a chemical model of the aqueous and surface chemistry of a calcium carbonate rock in equilibrium with a brine and CO2(g). The predicted surface chemistry match zeta potential measurements reported in the literature. We calculate the surface charge and potential of a rock in equilibrium with high salinity brines, such as seawater and Ekofisk formation water in the temperature range 25-130°C. We show that a calcium carbonate (chalk) core exposed to seawater at high temperature will experience dissolution and mineralogical change. Experiments performed on Stevns Klint chalk (which is a very clean chalk with a high content of calcium carbonate) show a significant water weakening effect when exposed to seawater at high temperatures. These cores also behave more water wet at high temperatures. From our calculations, we find no clear evidence that the explanation for these experiments could be due to changes in surface potential or charge, simply because the temperature dependence in the surface potential is too weak. Instead we find that dissolution of calcite have the right temperature dependence. We therefore suggest that a dissolution process inside the core could explain the experimental results. We present calculations that show a nice correlation between the oil produced in six imbibition experiments, and the calculated amount of dissolved calcium inside the core.