WIRELINE LOGGING BASICS
To perform a logging operation on wireline, the measuring instrument, often called a probe, sonde or logging tool, is lowered into the borehole on the end of an insulated electrical cable. The cable provides power to the downhole equipment. Additional wires in the cable carry the recorded measurement back to the surface. The cable itself is used as the depth measuring device, so that properties measured by the tools can be related to particular depths in the borehole.

Wireline logs descend into the wellbore under the force of gravity. In deviated wells, the tools may not travel all the way to bottom. Roller and powered tractor subassemblies can be used to assist. These are not especially useful in rough or horizontal wells, so coiled tubing or pipe conveyed tools are used.

The wireline logging operation showing logging truck (right), logging cable strung into the rig, then lowered into the borehole (left), with logging tools at the end of the cable (bottom). Logs are usually recorded  while being pulled upward by the winch in the logging truck. Most logs can also be run as an integral part of the drill string (logging while drilling or LWD) or attached to coiled tubing. These methods are useful in deviated, horizontal, or other hostile well environments.

Cutaway view of a modern logging truck showing winch with logging cable (left) and computerized operator's station (center)

A logging tool is made up of a sonde and a cartridge. The sonde is the portion of the tool which gives off energy, receives energy, or both. The cartridge contains the electrical circuitry or computer components needed to control the downhole equipment, and to transmit data to and from the surface.

Combination logging tools consist of more than one sonde and cartridge, so that more than one log can be recorded on a single trip into the wellbore.

Surface equipment is mounted in a logging truck, van, or skid unit from which all logging operations are controlled. The logging unit contains hoisting equipment for lowering and raising the tools in the hole, and electronic or computer equipment for controlling and recording the downhole measurements.

Recording the well log involves a number of steps, beginning with sensing and pre-processing the measurement in the logging tool itself, transmission of this information to the surface over several miles of wireline, further processing in the logging truck computer, data storage on disc or magnetic tape, and finally display of the data on film or paper.

Measurements are recorded in two forms, analog and digital. The analog data may be recorded on photographic film, electronic plotter, or chart recorder. The same data are captured in digital form on magnetic tape or disc for later use in computer aided petrophysical analysis. Many instrument control and calibration functions are now handled by the same computer used to record the digital data, with some human control. The result is a log, as seen below.

 
Example of a Well Log, with a standard 3-track presentation on the left and an image log on
 the right. Curve names and scales in the scale heading help identify which curve is which.

All logging tools and surface equipment must be properly calibrated. Service companies have calibration procedures for most tools, some of which are based on standards established by the American Petroleum Institute (API). Each tool must be calibrated at the surface before placing it in the hole to make measurements, and must pass certain calibrations after the measurements are complete to verify that measurement accuracy has not drifted. Some tools also have downhole calibration checks.

After reaching total depth, or some other location of interest in the borehole, measurements are made while pulling the tool upward over several hundred feet of the borehole. This is called the repeat run, and is used to determine the repeatability of the measurements when compared to the main logging pass. After the repeat run is complete, the tool is lowered to the bottom of the hole, and the main logging pass is commenced. During the early portion of these measurements, the responses are compared to those of the repeat run to determine that no instrument drift has occurred. Results of all field calibrations and repeats are attached to the bottom of the well log record.

In addition to the actual measurements, the well log itself contains information about the logging process which supports use and interpretation of the data. The well name, location, date, surface measurements on the mud system, drill bit size, casing information, and logging equipment data are found on the log heading, Any pertinent information or comments regarding the logging job may be recorded in the remarks section.

The logging equipment is carried to the wellsite on a truck (for land based operations near roads), or transported by helicopter on skids (for remote land operations) or are permanently mounted on offshore rigs. Some typical logging units are shown below. 

 
Logging Trucks and Skid Units

Computerized surface equipment is now the rule rather than the exception. Such units, on a truck and with logging tools on board, can cost over $1,000,000.