Multi‐transient electromagnetic repeatability experiment over the North Sea Harding field ‡

We present results of synthetic time-lapse and real repeatability multi-transient electromagnetic surveys over the North Sea Harding field. Using Archie’s law to convert porosity and fluid saturation to resistivity we created 3D isotropic models of the reservoir resistivity at different stages of production from the initial state in 1996 through to complete hydrocarbon production by 2016 and, for each stage, we simulated an east-west transient electromagnetic survey line across Harding. Unconstrained 1D full-waveform Occam inversions of these synthetic data show that Harding should be detectable and its lateral extent reasonably well-defined. Resistivity changes caused by hydrocarbon production from initial pre-production state to production of the oil rim in 2011 are discernible as are significant changes from 2011–2016 during the modelled gas blowdown phase. The 2D repeatability surveys of 2007 and 2008 tied two wells: one on and the other off the structure. Between the two surveys the segment of the field under investigation produced 3.9 million barrels of oil – not enough to generate an observable time-lapse electromagnetic anomaly with a signal-to-noise ratio of 40 dB. Processing of the 2007 and 2008 data included deconvolution for the measured source current and removal of spatially-correlated noise, which increased the signal-to-noise ratio of the recovered impulse responses by about 20 dB and resulted in a normalized root-meansquare difference of 3.9% between the data sets. 1D full-waveform Occam inversions of the real data showed that Harding was detectable and its lateral extent was also reasonably well-defined. The results indicate that the multi-transient electromagnetic method is suitable for exploration, appraisal and monitoring hydrocarbon production.