Abstract 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.
Introduction With the advent of crosshole seismic technology in the 1980's a new generation of high resolution geophysical tools has become available for reservoir characterization and process monitoring. The chief improvement over other methods is simply that the tools are deployed in boreholes which enables the measurements to take place much closer to the region of interest.
Beginning in 1990 researchers began development of a low frequency crosshole EM technology (3). The crosshole EM induction system can be thought of as an extension of the borehole induction logs into the region between wells. The system, in fact, operates very similar to single well logging tools but with the transmitter and receiver tools deployed in separate boreholes.
Figure 1 provides an excellent illustration of why field developers should consider collecting EM in addition to seismic data. Here we plot seismic velocity and electrical resistivity as a function of porosity, water saturation and temperature in water flooded sandstone cores. The plots indicate a high sensitivity of the electrical resistivity to variations in reservoir conditions, and a smaller sensitivity of seismic velocity to the same reservoir variations. Typically, the resistivity varies up to an order of magnitude over the range of typical reservoir conditions whereas the seismic velocities vary by no more than 10 to 20 percent. The contrast is most pronounced in Figure 1b where the seismic data indicate a very small change due to fluid saturation whereas the electrical data are greatly affected.
This above plot is not surprising considering the physics. Seismic waves are predominantly supported by the rock matrix and variations due to pore fluids are secondary effects. EM induced currents primarily flow in the pore fluids, electrical property variations of these fluids therefore have a large effect on EM data. The combination of the two techniques thereby allows high definition of both the rock matrix and pore fluids.