Modelling of long-term along-fault flow of CO2 from a natural reservoir

Geological sequestration of CO2 requires knowledge of the flow properties of fault-related fracture networks in the low-permeability shale caprocks that overly most of the considered storage sites. A safe, sustainable and economical storage operation requires a profound understanding of these risks, recognising that quantification is challenging due to the many length and timescales involved and the very limited availability of data. The Green River site in Utah is a rare case of leakage from a natural CO2 reservoir, where CO2 (dissolved or gaseous) migrates along two fault zones to the surface. This provides a unique opportunity to understand CO2 leakage mechanisms and volumes along faults. A successful modelling of measured leakage rates will provide confidence in modelling approaches and will help select safe storage sites, de-risk storage operations and guide containment monitoring. Here, we present an integrated workflow to model the measured leakage rates and locations at this site. We combine laboratory experiments to obtain single-fracture stress-sensitive permeabilities; single-fracture modelling for stress-sensitive relative permeabilities and capillary pressures; fracture network characterisation and modelling for the primary and secondary caprocks; upscaling of properties and constitutive functions in fracture networks; and full compositional flow modelling at field scale modelling. Our results predict locations accurately and, within an order of magnitude, leakage rates correctly without extensive history matching. Subsequent history matching achieves accurate leak rate matches within a-priori uncertainty ranges for model input parameters.