Reservoir characterization using crosswell electromagnetic inversion: A feasibility study for the Snorre field, North Sea

The spatial resolution of a commercially available crosswell electromagneti c (EM) system is demonstrated using models derived from three time steps fr om a reservoir simulation of the Snorre field in the North Sea. The numeric al simulation of the Snorre field waterflood shows that crosswell EM field measurements provide high sensitivity to changes in the reservoir over time . This sensitivity is achieved by combining the reservoir geometry derived from surface 3-D seismic interpretation, reservoir conductivities at well l ocations, and constrained EM inversion of the reservoir's electrical conduc tivity. Inversions of 2-D and 3-D numerical models show that the changes in electri cal conductivity attributable to changes in water saturation can be quantit atively mapped as a function of time. The inversions provide smooth estimat es of the spatial variation of reservoir electrical conductivity that can d iscriminate between the level of water saturation at different stages of th e waterflood. Inversions performed on 2-D data show that for the Snorre exa mple, 3%-5% Gaussian random noise (depending on the model) can be added wit hout a significant degradation in the inverse models. Two-dimensional inver sions of the full 3-D data in the Snorre example can map the vertical avera ge electrical conductivity within the reservoir in the interwell region alm ost as well as when the model is two dimensional (constant in strike direct ion). The effect of 3-D structure does not seriously degrade 2-D inversion in the Snorre example-even between wells that lie in a line parallel to str ucture. A series of 2-D inversions where various constraints and starting models ar e used demonstrates the importance of incorporating a priori information in the form of starting models and restricting the inversion domain to the re servoir zone. These tests show that totally unconstrained, smooth inversion s of the interwell volume provide very limited quantitative information. Ho wever, when the reservoir geometry is constrained by seismic data and start ing models are provided by linear interpolation of conductivities at well l ocations, the reservoir's vertical average electrical conductivity can be p redicted to within a few percent by 2-D inversion. The Snorre field consists of a full-scale reservoir with interwell spacings that exceed 1 km where previous work has demonstrated the applicability of crosswell EM in shallow reservoirs with well separations on the order of 1 00 m. The simulations show that, given current transmitter and receiver tec hnology, the magnetic fields could be measured in the Snorre field in steel -eased wells separated from the transmitter by up to 725 m.