A B S T R A C TWe 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.

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Following experimental success in the application of 4D technology in the Magnus and Foinaven fields and the delivery of commercial value from the application of time-lapse seismic (4D) studies in the Forties and West of Shetland fields and in the Pompano Field in the Gulf of Mexico, BP has embarked on the systematic use of 4D monitoring in its NW Europe operations to enhance value in the light of an increasing production challenge. Within the past two years, 12 datasets have been acquired over 19 fields of varied reservoir age, type and property. Although specific questions differ from field to field, in all cases a major objective is to influence the location and design of infill wells. Useful time-lapse information has been gained from post-stack matched ‘legacy’ surveys of varying data quality. However, it is clear that more value is added by custom-designed 4D surveys: seismic quality and repeatability are critical to the successful delivery of reliable time-lapse seismic monitoring. Integration of seismic data with dynamic data is still relatively ‘low-tech’ and further benefits will be gained by improvement in this area. The paradigm is shifting to a position where 4D monitoring in the North Sea is the rule rather than the exception. Now that there is clear evidence of added value in the most obvious target reservoirs, surveys are being carried out in areas that are technically more challenging – and the early results indicate that these too will be of real commercial benefit. Use of 4D data is also being progressed in other regions, including the Gulf of Mexico, and application in other basins around the world is being planned. On the technology development front, BP is seeking continuous improvements in acquisition and processing, both in quality and turnaround time, and richer integration with production and conventional well-based surveillance data. The ultimate goal is for effective reservoir management based on a range of surveillance methods.