Technology innovation that can enhance the monitoring of CO2 in the subsurface explained at the US DOE Carbon Storage Meeting

The US DOE annual Carbon Storage Review meeting, held in Pittsburgh 1-3 August, included presentations on the latest technology innovations currently under development in the country. One of the challenges of secure subsurface storage is the ability to track CO₂ in deep reservoirs. Fibre-optic distributed acoustic sensors (DAS) are attracting increasing interest as a reliable medium for detecting subtle changes in reservoirs. The technology has been deployed and tested at CO₂ storage sites and there is evidence that it could become a promising means of monitoring.

DAS works by responding to external pressure from a seismic source acting on the fibre that then induces a back-scattering effect. This response can be translated into a strain measurement that can be correlated with wave forms detected by conventional geophones. DAS generated signals have now been successfully correlated with vertical seismic profiles (VSPs) using arrays of geophones suspended in a wellbore. One key advantage of fibre optic systems that has become evident from field deployment is their durability. These systems are robust and have remained intact even after 5 years in the ground. The technology has now reached a stage where it is being commercialised. Helical configurations of fibres will improve the sensitivity of system and therefore signal quality and seismic images.

DAS, and other monitoring techniques, are run prior to CO₂ injection into reservoirs to generate a base-line image. CO₂ is then injected into the subsurface and tracked by running repeat surveys. Time-lapse and cross-well comparisons between base-line conditions and later surveys can provide evidence of CO₂ distribution within reservoirs. Future research is planned to analyse different forms of shear-wave and heat pulse monitoring. These advances should lead to enhanced seismic images especially in deep reservoirs where conventional seismic techniques can be less effective.