Using earth tides to monitor CO2 storage: Field trials at the Otway International Test Centre, Victoria

Tidal forces from the Sun, Moon and other astronomical bodies cause daily variable strains in the solid earth. These strains cause variations in the pressure in fluid-saturated rocks. Their amplitude and phase are affected by the in-situ fluids, so that CO${}_2$  may be detectable when injected into an aquifer for a CO${}_2$  storage site. We report comprehensive monitoring of earth tides from six deep observation wells surrounding a 15 kt CO${}_2$  plume injected into the Paaratte aquifer in the Otway Basin, Australia. This plume was repeatedly monitored with time-lapse seismic. The observation wells were up to 500 meters from the edge of the CO${}_2$  plume, but the observed pressure amplitudes and phases of the earth tides responded to its creation and propagation. This response is at distances from the plume comparable to the pressure diffusion length in a day, which in this setting is about 350 meters. Earth tides may be a useful passive, supplementary method of monitoring CO${}_2$  plumes created by geological storage.