Chapter 12 - Principle of Dielectric Logging Tools

Abstract

In the previous chapters, we discussed electromagnetic (EM)-based logging tools working at relatively lower frequencies. In Chapter 2, Fundamentals of Electromagnetic Fields Induction Logging Tools and Chapter 3, Electrical Properties of Sediment Rocks: Mixing Laws and Measurement Methods, we noticed that in EM wave propagation, the conductivity is dominant (conduction current) at lower frequencies. However, when the frequency is getting higher, e.g., in megahertz like in logging-while-drilling (LWD) resistivity frequencies, displacement current will have to be considered, which is determined by the dielectric characteristics of the formation. When the frequency further increases, the displacement current’s role becomes more important than the conduction current in the EM propagation process. Therefore, from measurement point of view, higher frequency is preferred when we want to measure the dielectric constant of the formation. As discussed in Chapter 3, Electrical Properties of Sediment Rocks: Mixing Laws and Measurement Methods, the dielectric constant of oil and gas are relatively small (1–3) whereas the water has very high dielectric constant (70–81). It is very natural to use dielectric measurement to identify water versus oil and gas. On the other hand, the conductivities of the fresh water and oil or gas are very similar. Unfortunately, the conventional logging tools such as induction, LWD, and laterolog tools are not sensitive to the dielectric constant change due to the low frequency operation. It is seen from Chapter 3, Electrical Properties of Sediment Rocks: Mixing Laws and Measurement Methods, that the dielectric constant is not really a constant, it changes with the frequency. The dispersion characteristics of the dielectric constant may also be used to identify the formations. Due to the high-frequency nature of the dielectric tools, the investigation depth is rather limited.