Short- and long-term regional pressure build-up during CO2 injection and its applicability for site monitoring

Abstract

Large-scale pressure build-up due to a CO2 injection is investigated in the vertical and horizontal direction to determine locations and strategies suitable for pressure monitoring of the injection operation, cap rock integrity and large-scale geological setting. A realistic site-scale multi-layered model within the North German Basin is used, which explicitly accounts for a heterogeneous vertical structure including semi-permeable layers above the storage formation and below the cap rock. Results show that characteristic trends of the pressure signal are found for specific monitoring locations, depending on the horizontal and vertical distance to the injection well. Pressure signals and thus maximum pressures may be strongly delayed in the vertical direction, with a time shift larger than the injection period. This demonstrates that the maximum leakage risk in the cap rock may occur significantly after the injection period, not at the end of the injection. Vertical permeability contrasts between the cap rock and the storage formation as well as spatial heterogeneities cause variations in the shape and time evolution of the pressure signals and could thus be used to evaluate the vertical connectivity and the leakage risk. Boundary conditions and formation compressibilities control the pressure signal far from the injection well. It is shown that buoyant rise of CO2 into overlying formations cannot be detected by pressure monitoring.