A New Electromagnetic Propagation Tool for Well Logging

Abstract

ABSTRACT The use of electromagnetic (EM) energy to evaluate rock formations and, in particular, to determine the water saturation, as well as to infer the oil and gas saturations within pores, has recently received much attention. The dielectric properties of rocks measured by these tools at microwave frequencies are strongly dependent on the water saturation and are relatively insensitive to salinity changes. Therefore, dielectric properties measurements, when used with the data obtained from other well-logging tools (e.g., those involving neutron and gamma ray densities measurements), provide valuable information that helps distinguish between hydrocarbon-bearing and water-bearing zones. The presently available EM tools are generally single frequency devices. In addition, it is difficult to design low-frequency tools of reasonable sizes and of controlled radiation patterns. In this paper, we present a new EM tool which is based on the design of an open transmission line coupler. The main features of this tool include its broadband performance and its compact size, even down to frequencies as low as in the kHz range. It thus provides the possibilities of: Logging at various depths of investigation by varying the frequency and the separation between the couplers.The combination of low- and high-frequency measurements to separate specific dielectric parameters, such as the dc conductivity.The broadband coupling characteristics of the new coupler makes it possible, for the first time, to make swept frequency and time-domain well logging measurements.The borehole effects are minimized, since the tool is a patch-type device. Unlike the other low-frequency EM tools, the new coupler has very well-controlled coupling characteristics, with the major part of the energy transmitted to the rock formation. Results of our experimental evaluation of the designed tool, including measurement of the depth of investigation and the attenuation and phase shift between the transmitting and receiving couplers as a function of frequency, will be presented. It is generally shown that lower frequencies provide less attenuation and larger depth of investigation. The tool performance is also evaluated by utilizing a computer program where the rock formation is simulated by two-dimensional cross sections with a borehole and a flushed zone of variable thickness. Results of these numerical simulations are presented. At lower frequencies, it is shown that the mud cake effects are minimized and the sensitivity of the measurements to the formation beyond the flushed zone is improved. Limitations on using planar models to evaluate the EPT performance are also discussed. The new EM tool is capable of realizing the full potential of dielectric logging. It has the advantages of both presently available high- and low-frequency EM tools combined together. It also facilitates the unique capabilities of swept-frequency and time-domain well logging.