Well logging chronicles the depths, subsurface formations and events encountered while drilling. Well logs can include visual observations or be made by instruments lowered into the well during drilling.
Tradition holds that the term “well logs” is borrowed from ship nomenclature. Similar to a ship’s log that tracks the events aboard the vessel, a well log tracks the events of drilling, but instead of being plotted in a timeline, a well log is recorded by depth drilled. In the early 1800s, well loggers scaled the oilfield derricks and simply wrote down what happened at certain depths, including problems, types of formations encountered, speed of drilling and, of course, oil or gas flows.
In the early 1900s, Conrad Schlumberger envisioned the concept of using electrical measurements to map subsurface formations; and in 1927, he and his brother Marcel performed the world’s first electrical resistivity well log in France. (Resistivity is the measurement of the level of difficulty an electric current has passing through a formation.)
Well logging today means anything recorded having to do with the drilling versus the depth of the well at that moment, many times represented by a graph and corresponding notes.
Logging tools are inserted into the well to measure the electrical, acoustic, radioactive and electromagnetic properties of the subsurface formations. Sometimes the logging tools are incorporated into the drilling tool, and sometimes the drilling tools are lowered into the well at regular intervals to collect data.
Engineers and drillers use well logs to measure depths of formation tops, thickness of formations, porosity, water saturation, temperature, types of formations encountered, presence of oil and/or gas, estimated permeability, reservoir pressures and formation dip — ultimately determining whether a well is commercially viable or not and whether casing, cementing and completion should be run on a well. It’s not only a journal of what is perforated below the surface, but also a predictor of success.