An investigation of CO2-brine interactions using North Sea oil well data with respect to geological storage of carbon dioxide and enhanced oil recovery

Abstract

Geochemical trapping, comprising mineral and solubility trapping, is the most secure and permanent trapping mechanism for CO2 geological storage. CO2 dissolution in brine is also an important mechanism for CO2 enhanced oil recovery, as it improves sweep efficiency and increases oil displacement. This study aims to address current knowledge gaps that relate to how brine composition affects CO2 geochemical trapping mechanisms and the effect that CO2-saturated brine has on the permeability and porosity of host rock. Several different geochemical models that can be used to predict geochemical trapping potentials were reviewed and the importance of selecting the correct equation of state (EoS) was addressed. The geochemical modelling software, HydraFLASH was selected to calculate CO2 solubility in brine and PHREEQC was chosen to predict mineral trapping potentials. Hydrothermal experiments were performed to investigate the importance of well selection within a field, concluding that geochemical trapping potentials can vary within the same field as a consequence of brine compositional changes, in particular changes in SO42- concentrations due to seawater flooding, at individual wells. Further experiments were performed to assess the potential for calcite, which overlays many potential aquifer stores, to buffer brine and promote mineral trapping, as well as to investigate the potential effect that CO2-saturated brine has on the permeability and porosity of local host rock. The addition of calcite resulted in a significant increase in brine pH, but not sufficient enough to promote mineral trapping. In addition, CO2-saturated brine reacting with host rock resulted in the dissolution of Ca, Mg and S bearing minerals within the rock. However, an overall decrease of rock porosity and permeability was observed, due to the formation of clays and Na-micas and the mobilisation of fines.