New logs and greater flexibility in electrical well logging applications are now provided by truck-mounted computer systems. These special computers digitize and record on tape the data from well surveys. Simultaneously with the recording of a subsequent log, the computers merge taped data and perform continuous log analysis computation. The optical recordings of these computations provide a rapid means of formation evaluation in the field. Two computer systems have been developed. In one the data are recorded digitally on punched paper tape. The second system records the data digitally on magnetic tape using a format compatible with most digital computers. A major application of the truck-mounted computers systems has been the merging of data from an induction-electrical survey with that from either a sonic or a formation density log. The resulting formation analysis log consists of continuous curves of Rwa, Fr/Fs (or Fr/Fd), and apparent porosity. The location of potential pay zones is more easily and quickly accomplished by scanning these computed logs than by manually performing the many computations required for a similar study. Since computations are automatically performed on all zones, it is unlikely that productive zones will be overlooked. Other combinations of log data are also possible.
Several years ago a method was proposed by Tixier, et al., in which simple computations of data from resistivity and sonic logs provide more reliable "quick look" interpretations. These techniques, which involve the manual computation of an apparent water resistivity Rwa and of an Fr/Fs ratio, have been widely used by the industry. The methods use the sonic data to normalize formation resistivities for variations in porosity and shaliness. In the application of the techniques, Rwa and Fr/Fs values are computed and tabulated for many levels. Computations are made not only for zones of apparent interest on the resistivity curves, but also for obvious water-saturated intervals and shaly sections. The tabulated results or, even better, semi-continuous plots, as shown in Fig. 1, focus attention on the zones of interest. Oil- or gas-bearing intervals are indicated by Rwa and Fr/Fs values significantly higher than obtained for water-saturated zones. The log analyst may then concentrate his attention on the formations that offer production possibilities. He can selectively use his more complete and more precise methods of interpretation and, thus, perform the necessary evaluations in the shortest time. These quick look methods are particularly beneficial at the wellsite.