Two Decades of Electrical Logging

Abstract

The present paper, written at the request of the editor of the JOURNAL OFPETROLEUM TECHNOLOGY, gives a brief history of the various logging methods andof their progressive development during the last 20 years. More generally, itrefers to the different operations performed in drill holes at the end of anelectric cable to locate the permeable formations and to determine theircharacteristics, such as porosity, fluid content, and dip; to procure samplesof the formation by sidewall coring; and to determine data concerning the drillhole itself, as, for example, its diameter and deviation from vertical. Electrical Logging Correlation and Bed Thickness Determination The first log in an oil well was made in 1927 and comprised only aresistivity curve. The sharp contrast in resistivity from one bed to anothersuggested that such a log would be very useful for correlation between wells, aconclusion that was confirmed by field experience. In 1931, the SP log wasadded. This parameter not only improved the qualities of the electric log forcorrelation purposes, but made it possible, in many cases, and particularly insoft formations, to locate permeable beds and to determine their boundarieswith reasonable accuracy. The combination log, comprising one resistivitycurve, generally a short normal, and the SP log, was introduced commercially inthe United States in 1932 and was immediately recognized as a very valuabletool. First Attempts at Quantitative Analysis Until about 1937, the application of electrical logging was, in many cases, limited to correlation and bed thickness determination. During that period, however, attempts were progressively made to detect oil sands by qualitativeinterpretation, especially by comparing the resistivity of a given permeablebed in different wells drilled in the same field. This resulted in valuableconclusions in many cases, and also brought to light the important influencethat mud filtrate invasion has on the apparent resistivities as measured in thedrill hole. Around 1938 the problem of quantitative interpretation received increasedattention, when the first paper dealing with that problem was published. Fromthere on, not only the service companies, but the oil companies themselves, didextensive work to improve the interpretation. Subsequent papers discussingempirical relations between formation factor and porosity, and betweenresistivity of the uncontaminated part of a permeable bed and its oil and watersaturation, brought important and basic contributions to the problem ofquantitative analysis.