How Repeatable 4D Csem Surveys Need To Be?

Repeatability of acquisition parameters for the base and monitor surveys is an important consideration for time-lapse studies of hydrocarbon reservoirs using controlled-source electromagnetics (CSEM). Variations in parameters such as source and receivers positions, conductivity and depth of seawater, etc lead to differences in the recorded EM fields that are often comparable to or exceed EM response due to production-induced changes in the reservoir resistivity. In that case, 4D CSEM is not feasible as long as 4D effects are analysed in the data domain. In the present study, we demonstrate the feasibility of 4D CSEM even for large differences in the acquisition parameters if the analysis is performed in the model domain. Using the “canonical” model considered by Orange et al. [Geophysics, 2009], we show that the repeatability requirements for water conductivity and receiver positions are relaxed approximately by an order of magnitude if the conventional sensitivity analysis is replaced by examination of inverted resistivity volumes.