5.1 Overview of Logs

In the oil and gas industry, the term “log” can be used as either an action (verb) or a thing (noun). In very general terms, the action refers to measuring something related to drilling operations. A wide variety of surface and downhole sensors, hardware, and software technology are used to take these measurements.

The measurements occur over a measured time period or at different depths in the well (or sometimes both). The “things” that are measured include operational parameters, drilling progress and parameters, and formation properties. Sometime the measured properties or data are direct measures used in operations (e.g., rate of penetration (ROP) or hole size) and sometimes they are indicators (e.g. electrical resistivity as an indicator of hydrocarbon presence) or as inputs to additional analysis.

The output of the logging process—traditionally a very long piece of paper that records values at each time or depth index—is also referred to as a log. Today, this information is captured digitally, but still consists of a time or depth index and an associated measurement at that index. Logs are often informally referred to as “curves” (because when the data is plotted—an index and its associated value—it produces a curve). A Log may consist of 1 curve or a collection of many curves.

The volume of log data is huge (both historical and current) and has many vital uses. So log data is transferred frequently—as it is collected and used for analysis in realtime drilling operations, for overall field evaluation, and for future use when evaluating similar prospects and/or operational conditions.