![]() ![]() The dielectric antenna pad and the antenna equivalent dipole. If detailed and accurate analysis or design is necessary, three-dimensional (3D) analysis must be used.įigure 12.1. All the analysis method discussed in the previous chapters using magnetic dipoles can be applied. Therefore the analysis of the dielectric tool becomes very simple. Due to the small size of the slot, we can approximate the H field in the slot by a uniform magnetic dipole and the effect of the pad can be replaced by an image dipole. It is easy to see that the E field in the slot has vertical component and the H field is in the direction of the slot. The radiation of EM field is from the slot opening of the cavity ( Fig. The cavity is filled with ceramic materials with high dielectric constant (as high as 100) to reduce the antenna dimensions since the dimensions of the antenna is determined by the operating frequency and is inversely proportional to the square root of the relative dielectric constant of the material filled inside the cavity. When used, the tool pad is pushed against the borehole wall by the pusher mechanism.ĭue to the high frequency, dielectric tools usually use cavity-backed slot antennas. Actually the dielectric tool can not only measure the dielectric constant of the formation, but also measure formation resistivity simultaneously, and further derive out the water-saturated formation porosity based on the measured dielectric constant and resistivity. ![]() Therefore the dielectric tools are made as wireline tools. To measure the dielectric properties of the formation, the EM signals generated by the tool must be able to penetrate through the mud layer before reaching the formation, which makes the tool to be designed as a pad-type tool so that it can be mechanically pushed against the borehole wall and reduce the influence from the mud to the tool performance. Due to the high-frequency nature of the dielectric tools, the investigation depth is rather limited (a few inches, see Table 1.2).Īs we know, the borehole mud has high content of water for water-based mud (WBM) and is very lossy to the EM signals, and has high dielectric constant. The dispersion characteristics of the dielectric constant may also be used to identify the formations. 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. 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. On the other hand, the conductivities of the fresh water and oil or gas are very similar. It is very natural to use dielectric measurement to identify water versus oil and gas. 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). Therefore, from measurement point of view, higher frequency is preferred when we want to measure the dielectric constant 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. ![]() 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. ![]() 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. In the previous chapters, we discussed electromagnet (EM)-based logging tools working at relatively lower frequencies. Richard Liu, in Theory of Electromagnetic Well Logging, 2017 12.1 Introduction ![]()
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