Using time-lapse seismic to monitor injection of CO2 into a depleted gas reservoir—Otway pilot project

The CO2CRC Otway Pilot project involves the injection of over 65,000 tonnes of CO2/CH4 gas mixture (80% of carbon dioxide and 20% of methane) into a deep depleted gas reservoir. The test site is located onshore, some 300 km west of Melbourne. CO2 injection into a depleted gas reservoir will provide important experience for monitoring and verification under these conditions. In this study we address the application of time-lapse seismic for assurance monitoring, which aims to verify that the injected gas remains confined to the target formation. Moreover, we also attempt to image CO2 plume within the reservoir. This is particularly challenging due to the small size of the reservoir and the associated CO2 plume, the presence of residual gas saturation, as well as the significant depth and complexity of the reservoir. In order to meet these challenges we developed a comprehensive seismic monitoring program, which included both time-lapse surface and borehole seismic methods. Prior to the base line survey, a series of 2D test surveys was conducted that aimed to investigate seismic repeatability at the site. It was crucial to keep the repeatability at the highest level considering that the time-lapse seismic signal was expected to be quite small. In fact, it is widely adopted that injection of CO2 into a depleted gas reservoir or equivalently enhanced methane recovery is not traceable by repeated seismic measurements. Careful planning of 3D surveys allowed us to achieve excellent repeatability, which is at the reservoir level represented by normalised RMS difference values of about 20%. Single, coherent time-lapse anomaly at the reservoir level was observed; its location and amplitude agree with the position of CO2 plume predicted by reservoir simulations. Measured amplitude differences agreed with the one predicted by seismic modelling.