Recent technological advances have made time-lapse EM a real possibility. In particular, EM methods appear to be a promising tool to identify the process of salt water breakthrough between an injection and a production well. We will examine the CSEM method that uses vertical, stationary transmitters and receivers, as has been developed by Petromarker. A simple reservoir model is studied where small parts of a larger box-sized reservoir have reduced resistivity. The repeatability is affected by changes in weather conditions and ocean currents, which may cause slight inaccuracies in positioning and transmitter tilt. The seawater conductivity is measured routinely and may be compensated for. These uncertainties along with source waveform changes may accumulate to significant systematic errors, but they are controllable and their sizes easily estimated. An example of a repeated measurement where both the transmitter and receiver were moved is shown. The average deviation at late times of four such measurements is 16 nV. In our model, this corresponds to the detection of 10% reduced resistivity. Our conclusion is that time-lapse EM monitoring is indeed feasible for large reservoirs at the time scale of months, provided the saturation changes are sufficiently localized.


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