Controlled-Source Electromagnetics Provide Promise for CCS Applications

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225486, “Experience With Using Controlled-Source Electromagnetics for CCUS Monitoring and Future Applications,” by Panagiotis Kirmizakis and Pantelis Soupios, King Fahd University of Petroleum and Minerals, and Christophoros Benetatos, Politecnico di Torino, et al. The paper has not been peer reviewed. _ An electromagnetic (EM) geophysical survey using controlled-source electromagnetic (CSEM) methods was conducted in North Dakota to evaluate its feasibility for monitoring subsurface CO2 fluid plumes. This study assesses the advantages, constraints, and necessary enhancements of both passive and active EM techniques in the context of carbon capture and storage (CCS). Surface log-scale resolution was successfully achieved, demonstrating the method’s capability to delineate fluid-plume boundaries and estimate fluid volumes with high accuracy. CSEM Background CSEM methods are increasingly recognized for their ability to detect subtle electrical resistivity contrasts associated with subsurface fluid movements. Accurate and efficient monitoring of CO2 injection plumes is particularly crucial for ensuring operational integrity, safety, and environmental compliance in CCS projects. Inherent resolution limitations of surface-based EM methods, however, necessitate integrating supplemental borehole log data for accurate interpretation. Recent advancements in EM instrumentation and data-processing methodologies have enhanced the effectiveness of CSEM techniques for subsurface fluid monitoring. Innovations such as high-resolution inversion algorithms, real-time data-acquisition systems, and adaptive noise-filtering techniques have improved the detection of resistivity contrasts associated with fluid saturation changes and plume migration. CSEM methods inject low-frequency EM energy into the subsurface through artificial dipole transmitters. As the EM field propagates through geological layers, its behavior is governed by the conductivity distribution of the surrounding materials. Variations in resistivity, caused by changes in fluid saturation or lithology, influence the amplitude and phase of the received electric and magnetic field responses. Receivers placed at the surface or in shallow boreholes detect these fields, and the resulting data are analyzed using inversion techniques to reconstruct the subsurface resistivity structure. CSEM is particularly sensitive to conductive features embedded in more-resistive matrices, such as CO2 replacing brine in porous rock formations. Its depth of investigation and lateral resolution are primarily controlled by source-receiver geometry, signal frequency, and resistivity contrasts. Moreover, incorporating anisotropic inversion algorithms allows for the resolution of directional conductivity features. CSEM can offer detailed, noninvasive imaging of fluid fronts and reservoir heterogeneity when properly calibrated with petrophysical and seismic data.