Features of geophysical monitoring of current oil and gas saturation of productive formations

Abstract

The process of developing oil and gas fields requires continuous monitoring of the current oil and gas saturation of productive reservoirs. Monitoring the current oil and gas saturation allows for effective hydrocarbon recovery. Solving this problem under the conditions of a cased hole significantly limits the suite of geophysical methods, which are the main source of information about the conditional characteristics of reservoirs. The most effective geophysical methods for determining the current oil and gas saturation of a productive reservoir in a producing well is deemed to be the methods of pulse neutron logging. In this paper, the authors analyzed the features of this method, the geological and technological conditions for its effective application, and identified factors that significantly affect the reliability of log interpretation. The indications of pulse neutron-neutron logging are influenced not only by the geological characteristics of the studied reservoir. During the operation and production of an oil and gas well, technological factors also appear, which often significantly change the methods’ indications. Scientists have identified most of these factors and proposed ways to account for their influence. In their work, the authors focused on two factors that are often neglected when performing logging using pulsed neutron methods, accordingly, and not taking them into account during interpretation often led to misinterpretation of pulsed neutron-neutron logging readings when determining the current oil and gas saturation. There are cases of reservoir contamination during drilling and cementing of the casing, when an unusually large colmatage zone is formed. Or the near-wellbore zone of the reservoir formation is damaged during well operation. Given the limited radial resolution of pulsed neutron methods, an enlargement in the colmatage zone or damaging of the downhole zone of the reservoir during well operation has a significant impact on the method readings. The authors proposed a comprehensive approach that allows avoiding such errors