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.
Reading a Well Log
The well log includes the header, which provides specific information about the well, such as the operating company, well information and type of log run; as well as the main log section, or the graph. The graph charts vertically the depth reached, and the horizontal scale is the measurement scale, which can be represented linearly and/or by logarithms.
Inserts are found throughout the graph at each major section of the log, identifying each curve. Curves on the log, also called traces, readings or measurements, can be represented by solid, long-dashed, short-dashed or dotted lines to decipher between the different measurements represented on the log.
The final part of the log includes the tool calibrations for before and after the log was conducted, ensuring that the log is accurate.
Types of Well Logs
As the technology of well logging has improved over the decades, myriad types of well logs have emerged. From Gamma Ray (GR) Logs that measure radioactivity of the rocks to determine the amount of shale in a formation to Sonic (or Borehole Compensated) Logs that measure porosity by measuring how fast sound waves travel through rocks, different tools are used to determine different subsurface characteristics.
As previously mentioned, Resistivity Logs measure how electricity travels through rocks and sediments. This determines what types of fluids are present because oil and fresh water are poor conductors of electricity, while formation waters are salty and easily conduct electricity.
Induction Logs are used in wells that do not use mud or water, but oil-based drilling fluids or air, which are nonconductive and, therefore, cannot use electric logs. Induction uses the interaction of magnetism and electricity to determine Resistivity.
Spontaneous Potential (SP) Logs show the permeability of the rocks in the well by calculating the electrical currents generated between the drilling fluids and formation water held in the pore spaces. SP is used many times to determine between shale and sandstone.
Methods of Well Logging
Mud Logs refers to the drilling mud, or drilling fluid, used to provide buoyancy to the drill, as well as remove cuttings from the well. Information from a mud logger supplements the driller's log, cuttings log and evaluation log, and is used along with logs of nearby wells to determine the commerciality of a well. Additionally, mud logs monitor the wellbore to help prevent blowouts.
For many years, well logging tools were lowered into the well at regular intervals while drilling to retrieve data. With the advent of directional drilling, well logging had to develop to be able to log a well that was no longer vertical. Logging While Drilling and Measurement-While-Drilling (or MWD) place the logging tools on the end of the drilling column. That way, drillers can use the information immediately to determine the direction and future of the well.
Well logs of today use Computer-Generated Logs to immediately interpret information gathered while drilling. In addition to keeping measurements, these sophisticated logs can notify drillers of a potential hazard and transmit data via satellite to computers offsite.