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

Abstract

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.

Quantitative interpretation of pulsed neutron capture logs: Part 1 — Fast numerical simulation

“…When all the components (rock matrix, shale, and fluids) are homogeneously distributed in space, R obeys a linear mixing law, where each component is weighted by its relative volumetric concentration (Clavier et al, 1971a). For instance, one can write the total capture cross section in a clay-free formation as…”

“…The total length of investigation equals 26 cm, which is similar to the result of 26.7 cm reported by Hopkinson et al (1974). Careful consideration of radial length of investigation is recommended to prevent misleading interpretations of PNC logs in invaded formations (Allen et al, 1965), although cased-hole cases are less likely to be influenced by invasion (Clavier et al,1971a) METHOD Equation 7 is the basis for simulating R logs. Finding a solution of this equation based on the linear, first-order Born approximation, as already successfully done for neutron and density measurements by Mendoza et al (2010), is not applicable to our study.…”

“…In addition to identifying pay zones among water-saturated layers, PNC measurements are acquired throughout the life of hydrocarbon-producing wells to monitor variations of saturation with time. Time-lapse monitoring is a widely used application of R logs in the petroleum industry and has received much attention (Clavier et al, 1971b;Nutt and Watfa, 1989;Kimminau and Plasek, 1992;Cowan and Wright, 1999). The extension of time-lapse logging in waterflooded reservoirs is the estimation of waterflood residual oil saturation via the log-inject-log procedure (Richardson et al, 1973;Reedy, 1984).…”

Pulsed Neutron Capture Log Interpretation In Laminated Formations: A Dual-Exponential-Decay Model

“…1 -4 The following well-known expression by Clavier et al 2 (their Eq. 4) includes a shale term ~log= (1-Vsh -¢e)'f"", + Vsh~sh +¢eSw~ +¢e(l-Sw)4tc.…”

Reservoir Monitoring in Low-Salinity Environments With Pulsed-Neutron-Capture and Gamma Ray Logs

“…Uncertainties associated with rock effects can be accounted for by use of a time-lapse method 4 (Fig. 6).…”