The crosshole EM technique provides an image analogous to a smoothed two-dimensional induction resistivity log. Inductive EM sources are placed in one well and magnetic field receivers in a second well, up to a km away. Sources and receivers are positioned above within and below the depth interval of interest, and we image the resistivity of the interwell volume to fit the collected data. Data may be collected in open and/or steel-cased boreholes, although steel casing reduces this range. The resolution of images is roughly 5 percent of the well spacing.
The technique has been used for more than 5 years in imaging thermal oil recovery operations, but more recently for reservoir characterization and water flood imaging. The resistivity contrast between salt water saturated zones and oil and gas pay zones usually provides an excellent signal for the EM data and makes the imaging of the water flood fairly straightforward.
In this paper we provide a technology overview and then show a case history where the EM resistivity images have defined the initial water saturation distribution, and have tracked changes over a two year period due to water flooding in a fracture dominated southern California reservoir. We further describe simulations for crosshole EM imaging in carbonates for a WAG (water alternating gas) process where the technique is used for initial site characterization and to monitor saturation changes during the injection.