Magnetic signatures produced by fluid flow in porous sediments

Abstract

Laboratory experiments have demonstrated that a magnetic field is created during the displacement of water by oil in a porous medium. This magnetic field is interpreted to be generated by the electrokinetic effect. The results of the experiments are up-scaled to permit modelling of the electrokinetic-magnetic field generated by a producing oil field. Predicted maximum magnetic anomalies (measured at the Earth's surface) near oil wells, where the oil-water contact moves vertically at rates between 1 to 5 m/yr (conventional production rates), are in the vicinity of 1 nT or less. However, in the situation where producing wells have unstable pressure regimes, vertical flow rates can exceed 100 m/yr for short periods; maximum magnetic anomalies in the vicinity of 150 nT are predicted under such conditions. Accepting the interpretation that the electrokinetic-magnetic field is produced by randomly changing current loops in the partially oil-saturated zone (transition-zone) of the reservoir, the observed magnetic field is expected to exhibit time-transient behaviour. It is proposed that such a technique could be used to monitor changes to the pressure regime in producing petroleum reservoirs.