FACTORS AFFECTING CAPROCK INTEGRITY DURING AQUIFER PRESSURIZATION

The importance of caprock behaving as an ideal seal to potential percolating flow paths during aquifer pressurization associated with CO2 injection cannot be overstated. Most models developed to simulate pressure buildup in aquifers are aimed at investigating stochastic aquifer properties. However, in our investigation we create a modified stochastic shale model characterized by evaluating the occurrence of a low proportion of sand bodies (< 5% by volume) in which the shale facies in the caprock is assigned a stochastic permeability distribution and stress-dependent permeability is considered. We discovered that even unconnected sand bodies can create percolating flow paths if certain conditions are met. These conditions include a relatively high pore pressure gradient across the caprock and a favorable geometric configuration of sand bodies, which play different roles in forming a percolating flow path. Sand bars/dykes can lead a pore pressure front that extends through a considerable vertical thickness quickly, while thin sand sheets tend to transfer the pore pressure front across a large horizontal range and thus increase the possibility of involving other local sand bodies in the percolating flow paths. The presence of percolating flow paths in caprock can also limit the pore pressure increase in the aquifer formation during fluid injection. The more active the flow paths, the less increase in the pore pressure that occurs in the aquifer. A relatively large pore pressure gradient is the prerequisite for causing a percolating flow path to occur. The pore pressure differential required to sustain a percolating flow path can be much lower once the path has formed. Stress-dependent permeability appears to facilitate the formation of percolating flow paths.