Controlled source electromagnetic modeling and sensitivity analysis for monitoring subsurface CO₂ storage

The electrical resistivity of reservoir rocks, according to Archie’s law, is very sensitive to changes in fluid saturation. Consequently, time-lapse resistivity anomalies due to CO₂ storage within subsurface reservoirs can be detected by controlled source electromagnetic (CSEM) repeat surveying. Three-dimensional (3D) finite difference time domain (FDTD) forward modeling [1], based on subsurface resistivity structures, demonstrates the possibility of CSEM surveying to monitor movement of subsurface CO₂ storage. Preliminary studies in this direction (see [2] and [3]) confirm the feasibility of CSEM as a monitoring tool. In [2] a study based on 2D finite element method is carried out for general case of reservoir monitoring. The focus in [3] has been the detectability of CO₂ by CSEM surveying for the specific case of Utsira formation. CSEM sensitivity study assuming realistic CO₂ storage within a subsurface reservoir in a shallow water (100 m) geological setting is presented in this work. Similar studies assuming idealized CO₂ storage at deep water (700 m) setting is under review [4] The present study focuses on:
• Monitoring time-lapse CSEM anomalies with respect to variations in geometry and/or saturation of CO₂ storage
• Evaluation of the effects of reservoir depths and lithology on time lapse anomalies
• Detection of potential CO₂ leakage from the main storage
Air-wave strongly influences the subsurface CSEM responses in shallow water settings [5] Airwave effects in the modeled data are investigated for a realistic 4D CSEM analysis. The