Modeling potential CO2 leakage rate along a fault in Mahogany Field

•Field data yield two independent estimates of the fault permeability, and the estimates are consistent.•The worst-case CO2 flux along the fault is above the background range, but below high fluxes in nature.•The attenuation of CO2 flux by permeable layers is temporary if layers are sealed at other end.

In geological CO2 storage projects, faults and the damage zones surrounding them may provide a leaky conduit for CO2 to escape the storage formation and enter shallower sensitive zones. Upward migration of CO2 whether within the damage zone or along the fault itself can be attenuated by permeable layers that intersect the fault. Quantifying CO2 flux along leakage paths requires values of transport coefficients along the pathways, which are rarely available. This work provides a data point for one type of escape path permeability, namely a conductive fault. The key feature is that the field data yield two independent estimates of the permeability, and the estimates are consistent.We describe two methods to estimate plausible range of fault properties from field data in the Mahogany Field, Trinidad. The Mahogany field is well characterized and this study uses the geology of this hydrocarbon-bearing structure to represent the geology of analogous structures that do not contain hydrocarbon. Fault permeability and capillary entry pressure are estimated from a shale gouge ratio (SGR) correlation. An independent estimate of fault permeability is obtained from a simple model of cross flow between the adjacent fault blocks and the measured pressure response during the production period. The two estimates agree, suggesting that the SGR method could be useful in assessing risk associated with along-fault leakage of stored CO2.We use a simple leakage model to estimate the worst-case CO2 flux along the fault. Results show that for the inferred fault permeability and a plausible range of pressure elevation at the base of the fault, the predicted CO2 fluxes at top of the fault are in tenths mg/m2/s. That is above the range of background CO2 fluxes measured at surface, but below high fluxes in nature. The CO2 flux along fault could be attenuated to zero by permeable layers that intersect the fault. However the attenuation is temporary if layers are sealed at other end. The flow rate at top of fault increases asymptotically after the attenuation starts decreasing.