By far the most important use of resistivity logs is the determination of hydrocarbon-bearing versus water-bearing zones. Because the rock's matrix or grains are nonconductive and any hydrocarbons in the pores are also nonconductive, the ability of the rock to transmit a current is almost entirely a function of water in the pores. As the hydrocarbon saturation of the pores increases (as the water saturation decreases), the formation's resistivity increases. As the salinity of the water in the pores decreases (as Rw increases), the rock's resistivity also increases. A geologist, by knowing (or determining) several parameters (a, m, n, and Rw), and by determining from logs the porosity (ϕ) and formation bulk, or true, resistivity (Rt), can determine the formation's water saturation (Sw) from the Archie equation:
Resistivity logs produce a current in the adjacent formation and measure the response of the formation to that current. The current can be produced and measured by either of two methods. Electrode tools (also called galvanic devices or, for presently available versions, laterologs) have electrodes on the surface of the tool to emit current and measure the resistivity of the formation. Induction tools use coils to induce a current and measure the formation's conductivity. These two types of tools have many variations, which are summarized in Table 5.1. In many cases, it is desirable to use both electrode and induction tools to produce a single resistivity log. For example, an electrode device might be used to measure
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Basic well log analysis
This publication is a general introduction to common openhole logging measurements, both wire line and MWD/LWD, and the interpretation of those measurements to determine the traditional analytical goals of porosity, fluid saturation, and lithology/mineralogy. It is arranged by the interpretation goals of the data, rather than by the underlying physics of the measurements. The appendix files contain digital versions of the data from the case studies, a summary guide to the measurements and their interpretation, and a simple spreadsheet containing some of the more common interpretation algorithms. This Second Edition of Basic Well Log Analysis delivers a great impact on training and self-training along with superior workbook exercises, newer measurements, borehole imaging, and nuclear magnetic resonance in separate chapters, all directed to provide a guide through the lengthy and sometimes ambiguous terminology of well logging and petrophysics. It provides readers with interpretation examples (and solutions) so that the techniques described here can be practiced.