Dynamic modelling for feasibility study of the shallow CO2 injection experiment at the CO2CRC Otway site, Victoria, Australia

A dynamic modelling study was undertaken to assess the feasibility of a planned CO2 injection experiment into a shallow fault at the CO2CRC’s Otway Research Facility. The aim was to identify key physical properties that strongly influence migration behaviour but are presently unmeasured. Two different simulators (CMG-GEM and TOUGH2) were used to model this experiment. Both simulation efforts indicate that the proposed experiment is feasible, but show the need for better data on the maximum injection pressure and the permeability distribution in the near-surface region (including the continuity of the clay layer). During the simulation with high injection rate, there could be a rapid accumulation of CO2 at the early injection stage due to the constraints of maximum injection pressure. The modelling results suggest that the dominant trapping mechanisms are likely to be free CO2 gas trapped by the upper clay layer and residual trapping. The total amount of CO2 that could be injected increased with greater injection pressure, injection rate and maximum residual gas saturation. The results suggest that dissolution of CO2 is likely to continue to increase during the injection and post-injection stages. After the CO2 injection phase, the gas was found to spread laterally within the reservoir and moved upward along the permeable grid cells at the modelled fault. A comparison between the modelling approaches suggests that if there is a desire to have CO2 migrate up the fault and reach the upper clay layer, it will be important to conduct the injection experiment at the most permeable sections of the fault and inject CO2 into a shallow high permeability layer. It is necessary to clarify whether there is an unsaturated zone beneath the clay layer as this is speculated to exist but is unknown.