Monitoring of CO2 storage in a depleted natural gas reservoir: Gas geochemistry from the CO2CRC Otway Project, Australia

The CO2CRC Otway Project in southwestern Victoria, Australia has injected over 17 months 65,445 tonnes of a mixed CO2–CH4 fluid into the water leg of a depleted natural gas reservoir at a depth of ∼2 km. Pressurized sub-surface fluids were collected from the Naylor-1 observation well using a tri-level Utube sampling system located near the crest of the fault-bounded anticlinal trap, 300 m up-dip of the CRC-1 gas injection well. Relative to the pre-injection gas–water contact (GWC), only the shallowest Utube initially accessed the residual methane gas cap. The pre-injection gas cap at Naylor-1 contains CO2 at 1.5 mol% compared to 75.4 mol% for the injected gas from the Buttress-1 supply well and its CO2 is depleted in 13C by 4.5‰ VPDB compared to the injected supercritical CO2. Additional assurance of the arrival of injected gas at the observation well is provided by the use of the added tracer compounds, CD4, Kr and SF6 in the injected gas stream. The initial breakthrough of the migrating dissolved CO2 front occurs between 100 and 121 days after CO2 injection began, as evidenced by positive responses of both the natural and artificial tracers at the middle U-tube, located an average 2.3 m below the pre-injection GWC. The major CO2 increase to ∼60 mol% and transition from sampling formation water with dissolved gas to sampling free gas occurred several weeks after the initial breakthrough. After another ∼3 months the CO2 content in the lowest U-tube, a further average 4.5 m deeper, increased to ∼60 mol%, similarly accompanied by a transition to sampling predominantly gases. Around this time, the CO2 content of the upper U-tube, located in the gas cap and an average 10.4 m above the pre-injection GWC, increased to ∼20 mol%. Subsequently, the CO2 content in the upper U-tube approaches 30 mol% while the lower two U-tubes show a gradual decrease in CO2 to ∼48 mol%, resulting from mixing of injected and indigenous fluids and partitioning between dissolved and free gas phases.