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Monitoring CO2 storage using well-casing source electromagnetics

Successful geologic storage of CO2, whether in a carbon capture, utilization, and storage process or as part of enhanced oil recovery (EOR) operations, requires careful monitoring of the concentration of CO2 within the reservoir. Because injected CO2 is more electrically resistive than the in-situ brines and pore fluids in most storage sites, electromagnetic (EM) methods are well suited to map the distribution of injected CO2 over time. In particular, utilizing existing metallic well casings as electrodes to channel current to formation depth and measuring the resulting electrical and magnetic fields at the surface is an effective technique to image the deep and thin reservoirs of interest. This paper describes the theory behind the well-casing source EM method and discusses a series of field surveys carried out over a three-year period using the method to measure CO2 concentration at an EOR site in West Hastings, Texas. The surveys identify resistive zones that agree with the injection locations and geologic structure of the reservoir and show an increase in reservoir resistivity related to CO2 concentration between the initial field status and the middle survey. However, they also show an unexpected decrease in the resistivity within the reservoir between the last two surveys, which may be due to changes in survey parameters or CO2 injection patterns during that interval.