- Copyright © 2004 Society of Exploration Geophysicists
The geophysics we describe here will sound different from what you have heard before, because it is different. It uses electric fields instead of seismic waves. It is not designed for exploration. It is a distant cousin of cross borehole seismic tomography, although it is quite different in terms of its application and results. Probably the most curious aspect is that it doesn't chase the holy grail of geophysics—high resolution.
What good could possibly come from a low-resolution, electrical geophysical tool? The answer is found in the paradigm gradually finding favor by oil executives and reservoir engineers: while finding and exploiting large new fields is important for long-term growth, improving recovery efficiency for existing resources also makes sense because developing a new field is increasingly expensive. As a result, secondary recovery methods are receiving increasing attention.
This shift in view requires a parallel shift in geophysics. Now geophysics needs to deliver useful information about field production using low-cost, long-term monitoring techniques that have minimum impact on production operations. Electrical resistance tomography (ERT) is ideally suited to these new goals. ERT is a method for tomographically reconstructing the electrical resistivity distribution in the subsurface using an array of electrodes. Typically, current is driven between two of the electrodes and the resulting voltage distribution is measured on the remaining electrodes. Repeating this with other pairs of current electrodes in the array makes it possible to sample the subsurface electrical properties in a manner that allows calculation of the spatial distribution of those properties. The electrical properties depend strongly on pore fluid content. Consequently, ERT is especially good at giving information on the movements of oil, water, and gas—the principal fluids of interest in a petroleum reservoir.
An important aspect of this method is that the oil well casings themselves are used as …