A first attempt at monitoring underground gas storage by means of time‐lapse multichannel transient electromagnetics

Abstract

Two successive transient electromagnetic surveys were carried out over an underground gas storage site in France. The idea was to monitor changes in the gas bubble from the differences in the data. If successful, the new methodology could help to reduce the number of monitoring wells and finally reduce costs. Preliminary 3D modelling indicated that the resistivity changes caused by movements of the gas/water contact should be detectable in the electric field transients provided that the signal‐to‐noise ratio is at least 100:1. The surveys were performed with the TEAMEX multichannel acquisition system, adapted from a seismics system. The highly redundant data were analysed by calculating the relative differences in the electric field transients. The differences were common‐midpoint‐sorted and spatially stacked. Another approach was the calculation of electric field time derivatives in a log–log domain, to eliminate static shift effects which are present in the data. Even though the data quality is excellent from a classical point of view, neither of the two approaches reveals changes in the data which might be caused by changes in the gas reservoir. In future applications to monitoring, transmitters and receivers should be installed permanently, and the transmitter input waveform should be monitored continuously, to avoid some of the problems encountered here. Moreover, the signal‐to‐noise ratio will have to be further increased by at least one order of magnitude.