The CO2CRC Otway Project in southwestern Victoria, Australia has injected over 17 months 65,445 tonnes of a mixed CO 2 –CH 4 fluid into the water leg of a depleted natural gas reservoir at a depth of ∼2km. Pressurized sub-surface fluids were collected from the Naylor-1 observation well using a tri-level U-tube sampling system located near the crest of the fault-bounded anticlinal trap, 300m up-dip of the CRC-1 gas injection well. Relative to the pre-injection gas–water contact (GWC), only the shallowest U-tube initially accessed the residual methane gas cap. The pre-injection gas cap at Naylor-1 contains CO 2 at 1.5mol% compared to 75.4mol% for the injected gas from the Buttress-1 supply well and its CO 2 is depleted in 13 C by 4.5‰ VPDB compared to the injected supercritical CO 2 . Additional assurance of the arrival of injected gas at the observation well is provided by the use of the added tracer compounds, CD 4 , Kr and SF 6 in the injected gas stream. The initial breakthrough of the migrating dissolved CO 2 front occurs between 100 and 121 days after CO 2 injection began, as evidenced by positive responses of both the natural and artificial tracers at the middle U-tube, located an average 2.3m below the pre-injection GWC. The major CO 2 increase to ∼60mol% 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 CO 2 content in the lowest U-tube, a further average 4.5m deeper, increased to ∼60mol%, similarly accompanied by a transition to sampling predominantly gases. Around this time, the CO 2 content of the upper U-tube, located in the gas cap and an average 10.4m above the pre-injection GWC, increased to ∼20mol%. Subsequently, the CO 2 content in the upper U-tube approaches 30mol% while the lower two U-tubes show a gradual decrease in CO 2 to ∼48mol%, resulting from mixing of injected and indigenous fluids and partitioning between dissolved and free gas phases. Lessons learnt from the CO2CRC Otway Project have enabled us to better anticipate the challenges for rapid deployment of carbon storage in a commercial environment at much larger scales.