John Kowalski, Dresser Atlas
Forty years ago there was little problem with the analysis of a well log. The only type of log that was available was an Electrolog. This survey was used to determine the depth of the pay zone, the thickness of the bed, and the location of the oil-water contact. As time went on, more and more different types of logs were developed and the analysis of each was fairly simple and straightforward. However, a problem was developing in the interpretation of wells. What could be done on a well which was logged with two, or three, or four different tools? Each tool required a different method of analysis because of the different theories and types of data that were being measured. Log analysis was rapidly developing into a highly specialized discipline. At the present time, one of the most important people in any oil company is the log analyst. The exploration people depend on him for information about the rocks in a wildcat and the exploitation group depends on him to tell them about the reservoir characteristics. Now, instead of one or two types of logs, he has to contend with about forty of them. Each of these logs responds differently in a sequence of different lithologies. Because of this distinctive response, certain combinations of logs may be used to determine characteristics of the rock through which they are recorded. In most cases, these logs are recorded as a line trace on film in a camera (Fig. 1). There are, however, certain specialized logs which are recorded on special equipment to facilitate their interpretation (Fig. 2). The modern log analyst uses all methods of interpretation that are available to him, from simple charts to computers. In today's oil field most wells are high cost, high risk ventures. A single zone can make the difference between putting a well on production and abandoning it. The responsibility of deciding on the location and quality of pay zones places such a burden on the log analyst that he cannot afford to leave anything to chance. The advent of computers has given him a tool which he can depend on giving a total uniform analysis of any section. At the present time there are many different types of programs, a great many of which are highly specialized. However, there are six programs which are in widespread usage - Shaly-Sand, Complex (carbonates), Elastic Properties, Coal, Diplog and Cased Hole Analyses. Each of these computer programs requires certain input data to calculate all of the output information. After the data has been converted to magnetic tape it is plotted on a graph-like chart. Because of the characteristics of the logs, distinctive patterns, which depend on the rock, are formed in these cross plots (Fig. 3). The log analyst then uses these cross plots to determine parameters for the computed log.