1. vb. [Reservoir Characterization, Formation Evaluation, Drilling]

To continuously measure formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others. For wireline measurements, the logging tool (or sonde) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline cable. Once the tool string (link to ID 2964) has reached the bottom of the interval of interest, measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the downhole temperatures for long enough to allow the tool to be lowered to the bottom of the hole and measurements to be recorded while pulling the tool up the hole. In this case, "down log" measurements might be conducted on the way into the well, and repeated on the way out if possible.) Most wireline measurements are recorded continuously while the sonde is moving. Certain fluid sampling and pressure-measuring tools require that the sonde be stopped, increasing the chance that the sonde or the cable might become stuck. Logging while drilling (LWD) tools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole.

See related terms: hostile environment,  wireline log

2. n. [Formation Evaluation]

The measurement versus depth or time, or both, of one or more physical quantities in or around a well. The term comes from the word "log" used in the sense of a record or a note. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging while drilling (LWD) logs are also taken downhole. They are either transmitted to surface by mud pulses, or else recorded downhole and retrieved later when the instrument is brought to surface. Mud logs that describe samples of drilled cuttings are taken and recorded on surface.

For more details, see The Defining Series: Introduction to Wireline Logging.

Synonyms: well log

See related terms: real-time data,  recorded data

3. n. [Formation Evaluation]

The display of one or more log measurements on a strip of paper or film (a hard copy) with depth in one axis. In this sense, the term refers to the display not only of the measurement but of other relevant information. A typical log is presented on folded paper of indeterminate length, but about 8.5-in. [21.5-cm] wide. It consists of a heading, well sketch, logging tool sketch, insert, main log, repeat section and tail. When the term is used in this sense, each log measurement is usually referred to as a curve.

4. vb. [Formation Evaluation]

To record a measurement versus depth or time, or both, of one or more physical quantities in or around a well.

Synonyms: survey

5. adj. [Formation Evaluation]

Associated with the information from a log. For example, a log print is a paper print on which log data have been recorded.