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Introduction to Well Bore Logs

by Barry Wilmot

In oil-field terminology, a "log" is a record made during or after drilling a well. It indirectly gives a report on the geological formations penetrated.

Logs can be classified into two basic types:

  • Resistivity logs, and
  • Porosity logs

The standard practice is to make an well bore log of every well drilled. The resistivity log is presented in columns with the S. P. (spontaneous potential) curve displayed in the left most column, a depth column in the center, and several resistivity curves in the right hand column(s).

Well Log illustrated

Resistivity logs are very effective geologic and engineering tools and are used chiefly for correlation purposes, identifying and measuring types of rock and also reservoir fluids.  The resistivity of the formations are important clues to their lithology and fluid content. Formations conduct electric current only by means of the mineralized water they contain. The solid parts of the rock are insulators. When the rock is dry, it will not conduct electricity. In a similar manner, oil, gas and fresh water are highly resistive and if they are present in the formation they are electrically nonconductive.   Salt water is highly conductive.

Porosity logs are also presented in columns. The gamma ray log is displayed to the left of the depth column, and measures the natural radioactivity of the formations.  The main purpose of the gamma ray curve is for correlations, as well as the identification of sand and shale or non-reservoir rocks.

Porosity is displayed to the right side of the depth column and can consist of neutron curves, density curves and sonic curves.  Sonic curves were popular before the 1970's but are now rarely used in heavy oil logging.

The neutron log measures the effect of artificially bombarding the formation with neutrons. How far the neutrons will travel before capture depends on the type of atoms they encounter. The larger the nuclei of the atoms encountered, the less effective they are in slowing down the neutrons. However, hydrogen has a small nuclei, and is most effective in slowing down neutrons. Hydrogen concentration means water or hydrocarbons are present, which in turn means porosity. Therefore, for this log, the lower the neutron count, the higher the porosity.

The density log has a radioactive source which emits gamma rays. These can be thought of as high velocity particles which collide with the minerals which make up the rock. The gamma ray continues with diminished energy. The more electrons the gamma rays encounter, the more quickly they are slowed down. In other words, the denser the rock, the higher the electron density. Therefore, scattered gamma rays reaching the detector is an indication of formation density. The electron density is related to the bulk density of a rock type and is a function of its porosity. The higher the gamma ray count, the less dense (or more porous) the rock is.