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Along with the influx of sailors came many of their nautical expressions. That's why the drilling derrick and its equipment are called a "rig," the derrick is a "mast," the changing room is called a "doghouse." The term "log" is another of these merchant marine expressions.
Nearly everyone is familiar with the ship's log kept by the captain. It's simply a chronological record of what happens aboard a ship. The record of what occurs on a drilling rig is the driller's log. Oil companies are interested in what happens as a bit drills deeper, so the driller's log is usually recorded by depth rather than by time.
In the early days of the industry, the driller's log was about the only information available on subsurface formations. On it were recorded the types of rock brought up from the borehole, how many feet per hour the bit was drilling, oil or gas flows, equipment breakdowns, accidents such as stuck drillpipe, and any other occurrence that might have a bearing on evaluating the well. Today, "log" has stretched to mean any data recorded vs. depth (or time), in graph form or with accompanying written notes.
When someone mentions a log, he/she is usually referring to records run on an uncased wellbore using an electric wireline logging truck and tools. Logs can also refer to the driller's log, mud logs, computer-generated logs, and MWD (measurement while drilling) logs.
WHY DO WE RUN LOGS?
What are we trying to accomplish with a log? What does it tell us that is so important? One of the advertising slogans of Schlumberger (pronounced slumber-jay) during the '60s was ". . . the eyes of the oil industry." This slogan aptly describes the importance of logging. Geologists and engineers literally work blindly when they try to imagine what is happening at the bottom of a well. Layers upon layers of sediments have amassed over the years and have been deformed and altered so much that we can't guess exactly what lies beneath our feet.
Before logging, drillers had only the information from their driller's logs and the behavior of nearby, or offset, wells. This information was and is important and useful, but it still left a lot to "by guess and by golly." Electric wireline logs have turned on the light for the petroleum geologist and engineer. In particular, they provide information in areas such as these:
* depths of formation tops
* thickness of formations
* porosity
* temperature
* types of formations encountered (shale, sandstone, limestone, dolomite)
* presence of oil or gas
* estimate of permeability
* reservoir pressures
* formation dip (the angle the formation makes to the horizontal and its direction)
* mineral identification
* bonding of cement to the casing
* amount and kind of flow from different intervals in a producing well
The list goes on and on, and new logs, as well as new uses for old logs, are being developed continually.
But very simply, the real reason for running logs is to determine whether a well is good or bad. A good well is commercially productive - it produces enough oil or gas to pay back its investors for the cost of drilling and leaves a profit. A bad well is not commercially productive. Logs help us make this determination.
By the time a log can be run, thousands of dollars have been spent for leases, possibly for seismic studies, and for drilling. However, thousands of dollars more are still to be spent to complete the well - running the casing, cementing, perforating, testing, setting production tubing and packers, and installing wellhead equipment and surface production facilities. If a company can determine that a well won't be productive before it spends thousands of dollars on the completion costs, it will minimize its loss. As in poker, there's no sense in throwing good money after bad.
Logs help us determine whether the formation we are penetrating contains commercial reserves of oil or gas, thus minimizing costs on bad wells. On good wells, the logs also show us where the oil or gas may lie, how much there is (reserves), and whether more than one zone is productive.
Practically everyone in the oil industry uses logs at one time or another. And logs are certainly used by everyone involved in the decision-making processes necessary in drilling and completing a well.
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