Theoretical analysis of long offset time‐lapse frequency domain controlled source electromagnetic signals using the method of moments: Application to the monitoring of a land oil reservoir

[1] We present a sensitivity study applied to water front monitoring of an onshore oil reservoir, using a remote controlled source electromagnetic method (CSEM) with electric dipoles and a borehole‐to‐surface configuration. We have developed an optimized and parallelized code based on the method of moments, in order to study the influence of several static or time‐varying background uncertainties on the time‐lapse CSEM signal (also called 4‐D CSEM). Analysis of the relative and absolute variations in phase or quadrature of the time‐lapse signal induced by the fluid substitution process, inside the reservoir, has shown that the vertical electric dipole allows the shape of the water front to be monitored, while remaining less sensitive (compared to a horizontal electric source dipole) to the total volume of substituted fluid. We have examined the influence of missed anomalies (1‐D/3‐D), with more or less conductive properties, near to the ground surface or the reservoir, and with or without time‐varying properties. In most cases, the 4‐D signal behaves like a reliable filter, canceling almost all response anomalies. However, it can also lead to strong, local perturbations of the time‐lapse signal. We have also shown that in the presence of steel cased boreholes at the source location, or with outlying steel cased boreholes, the recording of exploitable data does not present insurmountable difficulties at low frequencies (∼1 Hz), and for a dense array of surface receivers. These positive results with CSEM monitoring suggest that minimal, coarse‐time 3‐D explorations should be used to ensure reliable interpretation of the monitored data.