Two-phase flow effects on the CO₂ injection pressure evolution and implications for the caprock geomechanical stability

¹ École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
² GHS, Institute of Environmental Assessment and Water Research (IDAEA), CSIC, 08034 Barcelona, Spain
³ Dept Geotechnical Engineering and Geosciences, Technical University of Catalonia (UPC-BarcelonaTech), 08034 Barcelona, Spain

^a Corresponding author: victor.vilarrasariano@epfl.ch

Abstract

Geologic carbon storage is considered to be one of the main solutions to significantly reduce CO₂ emissions to the atmosphere to mitigate climate change. CO₂ injection in deep geological formations entails a two-phase flow, being CO₂ the non-wetting phase. One of the main concerns of geologic carbon storage is whether the overpressure induced by CO₂ injection may compromise the caprock integrity and faults stability. We numerically investigate the two-phase flow effects that govern the overpressure evolution generated by CO₂ injection and how this overpressure affects the caprock geomechanical stability. We find that fluid pressure increases sharply at the beginning of injection because CO₂ has to displace the brine that fills the pores around the injection well, which reduces the relative permeability. However, overpressure decreases subsequently because once CO₂ fills the pores around the injection well, CO₂ can flow easily due to its low viscosity and because the relative permeability to CO₂ increases. Furthermore, the pressure drop that occurs in the capillary fringe due to two-phase flow interference decreases as the CO₂ plume becomes larger. This overpressure evolution induced by CO₂ injection, which remains practically constant with time after the initial peak, is very beneficial for maintaining caprock stability. Thus, the sealing capacity of the caprock will be maintained, preventing CO₂ leakage to occur across the caprock.