Crosswell electromagnetic tomography: System design considerations and field results

Abstract

Electrical conductivity is an important petroleum reservoir parameter because of its sensitivity to porosity, pore fluid type, and saturation. Although induction logs are widely used to obtain the conductivity near boreholes, the poor resolution offered by surface‐based electrical and electromagnetic (EM) field systems has thus far limited obtaining this information in the region between boreholes. Low‐frequency crosswell EM offers the promise of providing subsurface conductivity information at a much higher resolution than was previously possible. Researchers at Lawrence Livermore National Lab (LLNL) and Lawrence Berkeley Laboratories (LBL), together with an industrial consortium, recently began a program to conduct low‐frequency crosswell EM surveys and develop suitable inversion techniques for interpreting the data. In developing the field instrumentation we used off‐the‐shelf components whenever possible, but custom‐designed induction coil transmitters and receivers were built for the field experiments. The assembled field system has adequate power for moderate to high‐resolution imaging, using boreholes spaced up to 500 m apart. The initial field experiment was undertaken in flat lying terrain at the British Petroleum test site in Devine, Texas. Using wells spaced 100 m apart, we collected a complete crosswell EM data set encompassing a 30 m thick, 10 ohm‐m limestone layer at a depth of 600 m. The resulting profiles were repeatable to within 1% and showed an excellent sensitivity to the layered structure, closely matching the borehole induction resisitivity log. At the UC Richmond field station, crosswell EM measurements were made to track an injected slug of salt water. Conductivity images of data collected before and after injection showed a clear anomaly as a result of the salt water plume and indicated that the plume had migrated in a northerly direction from the injection borehole.