because initial hydrocarbon (oil and gas) recovery fractions may have been quite low (20–30 percent of the resources in the formation), and improved logging techniques may enable drillers to double that recovery fraction. It is much less expensive to reexamine existing wells, of which the oil companies have tens of thousands, than to drill new exploratory wells, 1,000 or 2,000 of which may be drilled each year in the southern United States and Gulf of Mexico. Small, independent well logging companies find a substantial market in relogging old wells, while the major oil field services companies (Baker-Hughes, Halliburton, Schlumberger, and Weatherford) win most of the contracts for logging while drilling.
All well logging was done by wireline until the 1980s when measurement-while-drilling and logging-while-drilling tools first became available. Measurement while drilling provides data on the location and direction of the drill head and logging while drilling gathers information about the features of a formation to the surface while the drill head is still in the formation. Crude signals can be used to carry some minimal information from the logs to the surface as the borehole is being drilled. When coupled with the ability to direct the drill head (geosteer) toward promising targets, these techniques offer several advantages over wireline logging.
With geosteering, the pitch of a borehole can be increased to a high angle relative to a vertical line to create a so-called deviated well. Such wells can follow one of the long dimensions of an oil or gas deposit, enabling much higher recovery fractions relative to a well that just traverses the short dimension of the deposit. Because of difficulties in data transmission, only critical data are transmitted, at a very low data rate (about 10 bits per second) from logging while drilling tools to the surface by mechanically vibrating the drilling fluid, which is called mud pulsing. More detailed data are stored in electronic data chips which are extracted from the logging collar when it is removed from the borehole. Another advantage of logging while drilling is in offshore drilling where the well is cased as the hole is drilled to prevent fluid intrusion into the well. In both deviated wells, where strings of instruments would get stuck, and offshore wells, where wireline logging is only possible through the borehole casing, logging while drilling is the most attractive option.
In both wireline and logging while drilling, time is a critical factor. The cost of running operations on an offshore drilling rig is very high: drilling a well might cost $1–2 million per day of operations. In such operations, downtime and logging-equipment failures are expensive. Well logging equipment costs are only a small part of the cost of drilling operations and generally a very small fraction of the hydrocarbon production costs. Modifications that improve the accuracy of logging without compromising reliability of the data are welcome in the industry even if they raise the cost. As a result, many techniques have been used for well logging. Several techniques are discussed below.
Well loggers use combinations of both radiation-based and non-radiation-based tools (called nuclear and nonnuclear tools in this field) to examine the earth formations surrounding the well and sensors to detect the media’s response to interrogation tools. An analyst examines detector logs to look for some or all of the following parameters of the formation: formation water saturation, porosity, rock characteristics, carbon/oxygen ratio, and permeability.
Because of the complexity of earth formations, only a combination of all the logs allows the log analyst to draw accurate conclusions for the formation parameters. For example, combining resistivity and nuclear logs, the log analyst can determine porosity, water content, and density. Figure 9-1 illustrates a typical set of well logs. Even with multiple logs, well log analysis is an interpretive science in that it relies on data that do not uniquely determine the solution. Different well log analysts may, and often do, interpret the same logs differently. As a result, there are differences of opinion on which tools are the most important ones and which ones are valuable in what media and in combination with what other tools (see, e.g., differing views in Jacobsen et al., 2006; Badruzzaman, 2002; Ayan et al., 1999; Chang et al., 1993; Ellis, 1987).