Advances in Electromagnetic Survey Instrumentation and the Use of a Cased Borehole for Imaging a Deep Formations

Electromagnetic (EM) resistivity imaging methods have the potential to map subsurface fluid distribution in applications ranging from derisking potential prospects to quantifying the propagation of hydrofractures in unconventional reservoirs. However, to date EM surveys have been limited to reservoirs too shallow or too large to be of general applicability in the oil industry. The critical technology issues in extending EM methods are: a) generating an adequate subsurface electrical current, and b) the sensitivity and stability of the EM resistivity measurement apparatus. A borehole provides a physical means to contact to deep hydrocarbon formations. To date boreholes have been exploited by lowering a current injection electrode to the depth of interest and completing the electrical circuit with a counter electrode at the surface. This borehole to surface EM method has been successfully used to image oil-to-water contact in an oil reservoir but has the disadvantage that the well must be opened, and, for a producing reservoir, taken off-line. We have now demonstrated a new approach that uses the borehole casing as a current injection electrode. In addition, we have developed the first capacitive sensors for geophysical use and have adapted them for use with commercial seismic data recorders.