Two-phase fluid flow properties of cataclastic fault rocks: Implications for CO2 storage in saline aquifers

Abstract

Fault rocks can function as barriers to subsurface fluid flow and affect the storage of CO 2 in geological structures. Even though flow across faults often involves more than one fluid phase, it is typically modeled using only single-phase functions due to a lack of fault rock relative permeability data and complexities in incorporating two-phase flow properties into flow simulations. Here we present two-phase fluid flow data for cataclastic fault rocks in porous sandstone from the 90-Fathom fault (northeast England). The study area represents a field analogue for North Sea saline aquifers of Permian–Triassic age that are currently being considered for CO 2 storage. We use the experimental data to populate a synthetic model of a faulted saline aquifer to assess the impact of these fault rocks on CO 2 injection. We show that even fault rocks with low clay contents and very limited quartz cementation can act as major baffles to the flow of a non-wetting phase if realistic two-phase properties are taken into account. Consequently, pressure may increase far more rapidly in the storage compartment during CO 2 injection than anticipated based on models that only incorporate absolute fault rock permeabilities. To avoid high pressures, which may lead to hydrofracturing and CO 2 leakage, either more complex injection strategies need to be adopted or seismic data acquired to ensure the absence of faults in aquifers selected for CO 2 storage.