A new electrical logging method called Laterolog is described which providesfor better recording of formation resistivity. In this method a current, preferably of constant intensity, is forced into the formation perpendicularlyto the wall of the hole as a sheet of predetermined thickness by means of aspecial electrode arrangement and of an automatic control system.
The essential advantages of the Laterolog over the regular resistivity logsare explained. With the Laterolog, the mud column has much less influence thanwith conventional methods. The Laterolog is, therefore, particularly useful inwells drilled with highly conductive mud. Moreover, when the electrode systemis located opposite a bed of moderate or small thickness, the disturbing effectof the adjacent formations becomes practically eliminated, provided the bed isthicker than the sheet of current, which itself is usually a few inches to afew feet thick. The sequences of beds are, therefore, much more sharplydifferentiated, and, in many cases, it is possible to read directly from thelogs values close to the true resistivities of the formations without furthercorrections.
Field examples are shown to illustrate the results of the method in varioustypes of formations.
As soon as it was introduced into the petroleum industry, electrical loggingproved to be a powerful instrument for the delineation of the strata traversedby bore holes and for correlation of such strata. Research and engineeringefforts were mostly devoted at this early time to the design of the techniqueswhich could provide the best data for these purposes.
Later, attention was directed toward the quantitative analysis of electricallogs, in order to obtain information on reservoir characteristics, and inparticular on oil and water saturation. Efforts have been made, accordingly, with a view to improving the already existing methods, or to finding newmethods which could satisfy these new requirements.
Quantitative analysis implies a determination as accurate as possible of thetrue resistivity of the formations. In fact, however, the "apparentresistivity" recorded in front of a given bed by a conventional electricallogging method is frequently quite different from the true resistivity of thisbed. This is due to the combined influence of the mud column, of the adjacentformations both above and below the bed, and of the invaded zone, in which theoriginal fluid has been more or less replaced by mud filtrate.