Borehole seismic monitoring of CO2 storage using fiber-optic sensors: Otway Project example

Abstract

Continuous time-lapse multi-well vertical seismic profiling (VSP) is a cost-effective option for monitoring CO2 underground storage. Such an approach was tested at the CO2CRC Otway site by monitoring of 15,000 t of supercritical CO2 injected into an aquifer 1,500 m deep with nine permanent seismic sources (surface orbital vibrators or SOVs) and five downhole fibre-optic receivers. This continuous monitoring is complemented by multi-well 4D VSP using a conventional vibroseis source and the same DAS receivers, which included baseline and two monitor surveys after injection of 4,000 and 12,000 t of CO2. The continuous DAS-SOV monitoring detected and tracked the growth of the areal CO2 plume by mapping changes of reflection amplitudes. The plume is also detected by time-lapse changes of reflection amplitudes in multi-well 4D VSPs. The plume images obtained from continuous offset VSP and 4D VSP are broadly consistent with each other, but with some differences, which serve as a measure of uncertainty of 4D VSP images.