Quantitative Interpretation of Electric Logs in Oil-Wet Rocks Proposed Procedure and Example Applications

Quantitative evaluation of electrical surveys recorded in oil wells often provides the best and many times the only means for determining the reservoir parameters used in reserve estimates, well productivity predictions and other aspects of formation evaluation. Should the zone of interest be oil-wet, standard procedures for interpretation will not apply, and a revised method must be employed. Laboratory data and fundamental reasoning were used as a foundation for deriving an "oil-wet interpretation procedure". The purpose of this paper is to present the premises upon which the revised procedure is based, to explain how an oil-wet zone may be recognized from logging surveys and to test the interpretation method through application to field examples. Also, the scope of quantitative log interpretation is extended to include estimates of absolute and relative permeability and predictions of well productivity in terms of the expected water-oil ratio and initial flow potential. Logs from the Springer sand of Southern Oklahoma were used to test the proposed technique, and results of the work provide further evidence that this sand is at least partially oil-wet in certain areas. From the study of these logs, it is concluded that use of the proposed method derives values which are much more reasonable than those calculated with standard techniques; furthermore, predicted well productivities were in reasonable agreement with reported performance in most cases. Although there is as yet little known about the actual conditions of in situ rock wettability, it is believed that the interpretation method proposed in this paper is presently the best available for analyzing zones possessing characteristics such as those exhibited by the Springer sand. Introduction Quantitative interpretation of electrical, radioactivity, acoustic velocity and other types of logging surveys provides a practical and often quite accurate means for evaluating the prospective formations penetrated by wells drilled in the search for oil.