Fiber Optic Monitoring of CO2 Injection in a Carbonate Saline Aquifer Onshore Abu Dhabi

A carbon sequestration site has been developed onshore Abu Dhabi where, for the first time, 100% of the injected CO2 is to be captured and sequestered in a carbonate saline aquifer (Xu et al., 2023). This project is part of ADNOC's $23 billion multi-year action plan to decarbonize its operations and supports its goal to reduce carbon intensity by 25% by 2030 and deliver on its Net Zero by 2045 ambition. The well was spudded in April 2023 and CO2 injection started in November of the same year. The site currently has one well, equipped with fiber optic sensors on both the casing and the tubing string, which can monitor passive and active seismic activity, distributed temperature, acoustics and strain over the entire length of the fiber cables. The intention is to monitor and verify CO2 injection and containment as part of the MMV (Monitoring, Measuring and Verification) specifications for site certification. Unfortunately, time-lapse seismic is notoriously difficult in Abu Dhabi carbonate reservoirs due to their stiffness and the low sensitivity of their elastic properties to reservoir changes. As a result, detecting fluid changes with surface seismic and 4D time lapse seismic has not been very successful in the region. While there are many published examples of successful CO2 plume monitoring using time-lapse seismic (Bacci et al., 2017, Pevzner et al., 2021), they overwhelmingly concern clastic formations. Rock physics modeling of supercritical CO2 replacing hypersaline water (23% concentration) showed increased seismic detectability compared to injections in oil reservoirs for Enhanced Oil Recovery (EOR) purposes. While this gave us hope that seismic monitoring could work in this carbonate environment, we nonetheless took every precaution to maximize repeatability and signal-to-noise ratio in our design. We first tested a seismic monitor survey using only freshwater injection, which has much lower detectability than CO2, and obtained very encouraging results. We then implemented a thorough monitoring plan, which this paper will detail.