Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part II. Interpretation Example, Interpretation Accuracy, and Time-Lapse Technique

Introduction In the second of this two-part treatment of the subject, three main topics will be discussed.The interpretation techniques developed in Part I will be illustrated with an example in a Middle East well. The thermal decay time log was made with a 1 11/16-in. tool lowered through tubing. Neutron and gamma ray logs furnish the necessary porosity and shaliness data. The final results are presented as a computer-produced log.A study is made of the errors that may be tolerated in each of the following items appearing in the equation for water saturation: Slog, Sma, Sh, Sw, Ssh, øe, and Vsh. "Tolerance factors" for each enable the analyst to evaluate the accuracy of actual log interpretation.The time-lapse technique is discussed. In this method two thermal decay time logs are compared, the first one being a reference log run early in the production life of the well. This comparison provides a more accurate evaluation of changes in saturation. The degree of accuracy can be evaluated by means of tolerance factors provided as a result of the error study. Interpretation Example Gamma ray, neutron, decay time (run through tubing), and induction-log conductivity curves over an interval of a typical Middle East well are shown on Fig. 1.