The Otway shallow fault experiment: Insights from surface monitoring

A controlled release experiment was conducted at the CO2CRC Otway International Test Centre to quantify vertical CO 2 migration through a shallow fault. Over eight-days, 16.2 tonnes of CO 2 was injected at approximately 70 m depth adjacent to the Brumbys Fault. The fault, a highly permeable near-vertical, strike-slip structure, was extensively characterised prior to the experiment and the injection conditions were optimised for leakage. CO 2 detection at the ground surface was monitored via soil gas sampling, fixed and portable flux chambers, and atmospheric CO 2 transects using a mobile sensor. CO 2 breached the surface rapidly, first detected by a fixed soil flux chamber 27 h after injection began. Measurement of soil gas samples shortly thereafter revealed a nearby soil gas well was almost entirely comprised of CO 2 with a carbon-13 signature similar to the injected CO 2 . The CO 2 leakage rate was estimated by integrating portable soil flux measurements and using Bayesian analysis of atmospheric CO 2 transects. The rates estimated from atmospheric measurements were higher than estimates using soil flux measurements, potentially influenced by additional CO₂ leakage observed from soil cracks and insect burrows. Towards the end of the experiment, the leakage rate was estimated to be approximately 40% of the CO 2 injection rate, suggesting that permeable shallow strike-slip faults have the potential to be highly transmissive for CO 2 . The CO 2 leakage rate quickly decreased after injection stopped. The study provides critical insights into the timing, location and CO 2 migration behaviour within a highly permeable fault. The data can be used to constrain conceptual and numerical models of CO 2 migration within faults, whether shallow or deep.