On the Feasibility of Using Marine CSEM for Reservoir Monitoring

Marine CSEM exploration for hydrocarbons exploits that thin resistive layers in the subsurface transport EM energy with higher velocity and less attenuation than the conductive surroundings. The successful application of marine CSEM for exploration suggests that the method can be used for reservoir monitoring. Conventional survey design for exploration uses low-frequency signals from an electric dipole source towed close to the seabed and an array of receivers situated on the seabed for measuring the electric and magnetic fields. Low-frequency signals lead to low resolution and the attenuation of field response with offset implies high sensitivity to source and receiver positioning. Thus, when evaluating if monitoring of a reservoir is feasible, it is important to investigate the dependency of the EM response to parameters such as source frequency, overburden conductivity, and reservoir thickness and conductivity. We have considered stratified media and used analytical expressions for the EM fields when studying how the reservoir response depends on these parameters. Moreover, we have derived simple expressions for estimating the reservoir conductivity or thickness from two or more offset measurements when one knows the overburden conductivity as well as expressions for estimating conductivity or thickness changes from time-lapse data.