Fiber Optics Monitor CO2 Injection in Carbonate Saline Aquifer

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 223005, “Fiber-Optic Monitoring of CO2 Injection in a Carbonate Saline Aquifer Onshore Abu Dhabi” by Jane E. Mason, SPE, Guillaume M. Cambois, and James Cowell, ADNOC, et al. The paper has not been peer reviewed. _ A carbon sequestration site has been developed onshore Abu Dhabi in which, for the first time to the authors’ knowledge, 100% of the injected CO2 is to be captured and sequestered in a carbonate saline aquifer. Rock-physics modeling of supercritical CO2 replacing hypersaline water (23% concentration) showed increased seismic detectability compared with injections in oil reservoirs for enhanced oil recovery purposes. A thorough monitoring plan, detailed in the complete paper, was implemented. Minute changes in rock properties caused by CO2 injection in an onshore carbonate saline aquifer can be detected by seismic monitoring using a highly repeatable fiber-optic-based system. Method Changes in elastic rock properties as the result of production in carbonate reservoirs in the Middle East are typically approximately 5%, while the normalized root mean square for repeated land-based surface seismic is typically approximately 30%. Although case-dependent, the large gap between these two numbers explains why 4D seismic largely has been unsuccessful in the Middle East for reservoir monitoring. The caprock formation is approximately 150 m thick (950–1100 m) and the carbonate saline aquifer is approximately 700 m thick (1100–1800 m). The well was completed with fiber-optic cable cemented in place behind the casing, which terminates 200 m into the injection interval at 1309 m and approximately 50 m above the perforations. Because the fiber-optic cable is cemented in place, the receiver positions and coupling do not change over time. The permanently installed fiber-optic cable also allows cost-effective vertical seismic profile (VSP) surveys to be performed. One important early decision was the termination depth for the casing cable. A perforated cable could have entirely compromised VSP acquisition; as a result, it was decided to terminate the cable above the perforations. The casing-deployed cable was specifically selected to carry out multiple simultaneous distributed acoustic sensing (DAS), distributed temperature sensing (DTS), and distributed strain sensing (DSS). DTS data also is required to deconvolve temperature from strain obtained from the DSS measurement. A hybrid fiber-optic cable also was run clamped to the injection tubing, terminating at 1260 m at the permanent downhole gauge above the production packer, allowing for direct comparison of measured results from the casing while complementing the data-acquisition program. A previous study had indicated that DAS VSP measurements on the tubing would suffer from strong tube wave and injection noise. The fiber on the tubing, however, provides useful information with regard to compositional thermal transient analysis and well integrity.