Fault-seal analysis for CO₂ storage: an example from the Troll area, Norwegian Continental Shelf

Fault seal plays a critical part in hydrocarbon traps, and the same will be true for CO₂ storage. The standard workflow for prediction of capillary seal of hydrocarbons can be readily adapted to prediction of CO₂ seal since the fluid properties of CO₂ at reservoir temperatures and pressures are within the range shown by hydrocarbons. The workflow is applied in a feasibility study into the proposed CO₂ storage in the Johansen Formation of the Troll Field.

Computation of Shale Gouge Ratio (SGR) over the fault surfaces, in combination with juxtaposition diagrams, was used to estimate the sealing potential of faults cutting the Johansen Formation. SGR values were converted to potential CO₂ column heights that might be trapped at each fault. Column heights are generally less than 100 m at each fault, allowing a cross-fault migration route from the Johansen Sand via the Statfjord Formation, Cook Formation and Brent Group.

Analysis of in-situ stresses suggests that the faults in the Troll Field are not close to failure and therefore up-dip leakage of CO₂ is unlikely. Extremely large CO₂ columns (>300 m) would be required to change this stress stability, and across-fault capillary leakage would occur first.