The estimation of changes in the elastic properties of rocks during CO₂ injection is critical for the monitoring and management of CCS projects. Such estimates can be used to optimize injection strategies, assess the long-term storage capacity of reservoirs, and minimize the risk of induced seismicity.

 

To date, seismic techniques remain the most accurate methods for determining the volume of the CO₂ plume; however, estimating the residual CO₂ saturation within the plume is still challenging.

 

To address this challenge, we implemented an innovative laboratory method for rock sample characterization using Distributed Acoustic Sensing (DAS). Using samples from the CRC-8 well (OITC, Victoria, Australia), we simulated subsurface reservoir conditions. After characterizing the samples with advanced ultrasonic techniques, we injected supercritical CO₂ (scCO₂) and observed the dependence of ultrasonic wave response on scCO₂ saturation. Subsequently, using a novel force-oscillation technique, we measured the response at seismic frequencies with DAS. We found that the relationship between DAS-measured strain amplitude and scCO₂ saturation is non-linear and influenced by several factors, including reservoir properties, injection rate, and pressure. Nevertheless, with careful calibration and modelling, DAS measurements can provide reliable estimates of CO₂ saturation.

 

DAS technology shows strong potential to transform the monitoring and management of subsurface reservoirs in CCS projects. Real-time monitoring of strain and other parameters with DAS can deliver valuable insights into reservoir performance and safety, support optimized injection strategies, and help reduce the risk of induced seismicity.