CSEM sensitivity study of CO₂ layers with uniform versus patchy saturation distributions

A comprehensive 3D finite difference time domain (FDTD) forward modeling based on idealized subsurface CO₂ resistivity distribution is carried out. The CO₂ is conceived in layers bounded above by impermeable shale layers. Additionally a 2.5D inversion of the modeled data is included in order to demonstrate the possibility of marine controlled source electromagnetic (CSEM) survey to distinguish between the uniform and patchy saturation in CO₂ sequestration projects. Modeled data show that a typical CO₂ plume, with diameter and thickness of 2000 and 40 m, respectively, at 750 m below seafloor with patchy saturation gives 50% electric (E) field magnitude increase for 1 Hz source frequency at 2500 m source-receiver offset. In comparison, a similar plume with uniform CO₂ saturation gives an E-field magnitude gain by 25% for the same frequency-offset combination. CSEM inversions reconstruct the modeled plume with good lateral resolution. We find the 3D forward modeling and inversion as effective tools for CSEM sensitivity analysis and mapping of the injected CO₂ layers.