Published in Petroleum Transactions, Volume 207, 1956, pages 65–72. Abstract In quantitative interpretation of electrical logs the presence of clay minerals introduces an additional variable which further complicates an already complex problem. Although recognizing the difficulties introduced as a result of the heterogeneity of natural sediments and despite the present incomplete state of knowledge regarding electrochemical behavior of shades, disseminated clay minerals and concentrated electrolytes, it was felt that useful empirical correlations might be obtained from experimental investigation. Six typical sandstone formations, having a wide variety of petrophysical properties, were selected for the study. Approximately 45 samples from each formation were selected to satisfactorily represent the range of pore size distribution within the particular formation. As a matter of general interest, four limestone formations were also included in the investigation. Previously proposed equations relating to resistivity, SP and interrelationship of the two phenomena have, where possible, been tested with data obtained in this investigation. These equations do not satisfactorily describe experimental behavior of samples through all degrees of shaliness or throughout the range of brine solution resistivities normally encountered in logging practice. An empirical equation has been developed which quantitatively relates formation resistivity factor to saturating solution resistivity, porosity, and "effective clay content." This relation is indicated to be uniformly applicable to clean or shaly reservoir rocks. It is shown that both the SP and resistivity phenomena of shaly samples are related to the sample cation exchange capacity per unit pore volume. The independent chemical determination of this parameter is thus a means of determining the "effective clay content" of samples. Some implications regarding theory and electric log interpretation of shaly sands are discussed. Introduction The use of electrical resistivity logs as a means for estimating formation porosity is based upon the original work of Archie.

The wireline formation tester has proved to be a valuable tool for formation evaluation, but some tests still lead to misleading interpretations while others are inconclusive. Longer flow periods and more tests are generally required for evaluation of low-permeability sands. No particular advantages or disadvantages were found for tools with large, small and/or segregated chambers. The methods of interpretation presently in use have proved to be quite accurate; however, care must be exercised to determine that the conditions required for use of these methods are satisfied and that the tool functions properly. Errors in the measurement or recording of recoveries have probably resulted in many misleading interpretations; simply stating all volumes in the same units would speed interpretation and be helpful in recognizing errors. A single chart for interpretation of formation test recoveries from all sizes of chambers is presented. The surface pressure is helpful in the detection of a leaky seal, but it should not be used to correct for losses caused by leaks. Water resistivity data may be misleading when used to predict whether a zone would produce water and hydrocarbons or water alone. The wireline formation tester is a dependable tool for obtaining pressure measurements; however, the estimation of formation permeability from pressure build-up has been found to be qualitative at best. Introduction The wireline formation tester was introduced to the industry in 1955 by the Schlumberger Well Surveying Corp. It proved so successful that thousands of tests have been run to date and most logging companies now offer this service. It is a valuable addition to the array of wireline tools currently being used for formation evaluation and is the only one, other than the side-wall sampler, that obtains physical evidence of formation contents.