Chapter 12 - Basic Well-Log Interpretation

Abstract

This chapter provides an introduction to basic well-log interpretation. There are several attributes of hydrocarbon zones that are very important to well-log interpretation—the free water level (FWL) occurs at a capillary pressure, equal to zero, the oilwater contact occurs above the FWL in a water-wet reservoir, and in an oil-wet reservoir, the FWL and the water–oil contact occur at the same location. The self-potential (SP) log records the electrical potentials that develop in the borehole because of the presence of a conductive drilling mud and the contrasting formation water conductivity. The voltage between a borehole and a surface reference electrode is recorded as the SP log. The gamma ray (GR) log basically distinguishes between shales and other types of formation. The GR log is recorded so that the curve deflects to the right as the radioactivity increases, and thus the curve is very similar to the SP log. The GR log can be recorded in open holes with any type of fluid in the hole, or in cased wells. Used alone with a casing collar locator, the GR log permits direct correlation of depth measurements with open-hole logs. This is used to ensure accurate depth control for cased-hole wireline services, such as perforating, formation testing, fracturing, location of leaks, channeling behind casing, and many other applications where a small amount of radioactive element can be added to a fluid for subsequent detection downhole.