Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques

Abstract

Introduction The thermal decay time (TDT) log is based on a measurement of the rate of decay (absorption) of thermal neutrons in a formation. The principle of such logs has been described in detail elsewhere. In Ref. 4 the special features (capture gamma ray detection, Sliding Gate detection system) of the thermal neutron decay time log are described. Chlorine is by far the strongest neutron absorber of the common earth elements, and the thermal neutron decay time, T, is determined primarily by the sodium chloride present in the formation water. Like the resistivity log, therefore, the thermal neutron decay time measurement is sensitive to the salinity and amount of formation water present in the pore volume. Unlike the resistivity log, this log can be run in cased hole. Also, the thermal decay time log is relatively unaffected by drilling and completion conditions for the usual borehole and casing sizes encountered over pay zones. Consequently, when formation water salinity permits, this log can detect the presence of hydrocarbons in formations that have been cased, as well as changes in water saturation during the production life of the well. The log is thus useful for evaluating oil wells, for diagnosing production problems, and for following reservoir performance. With a version of the tool that is 1 11/16 in. in diameter, it is possible to enter a producing well through the tubing under pressure without having to kill the well.

On the development of a non-linear calibration relationship for the purpose of clay content estimation from the natural gamma ray log

“…(1) provided wide overestimation of the Shurijeh clay content, which leads eventually to the misestimating of the original hydrocarbon in place and reserves. Thus, the values of linear gamma ray, potassium, and thorium indexes were modified, using each of the empirically derived non-linear transform equations introduced by Larinov [22], Clavier [23], Steiber [24], Dewan [25], or Bhuyan and Passey [10] (all the relationships are listed in Table 1) to obtain a less erroneous estimation of Shurijeh clay content. According to the Shurijeh age (Early Cretaceous), Larinov calibration for highly consolidated formations was used and based on the data in Fig.…”

“…7 The comparison between the core-measured clay contents and the estimated values from the empirical non-linear calibration relationship Fig. 8 The comparative look at the Shrijeh Formation weight percent clay from the core analysis and different modifications proposed by Larinov [22], Clavier [23], Steiber [24], Dewan [25], Bhuyan and Passey [10] and the empirical calibration relationship (dashed curvilinear lines). Black circle Gas producing well data points, and black up-pointing triangle the non-producing well data points…”

“…empirically derived non-linear calibration equations such as those introduced by Larinov [22], Clavier [23], Steiber [24], Dewan [25], or Bhuyan and Passey [10] (Table 1). These transforms are all in the non-linear form except for that of Bhuyan and Passey [10], which is developed based on the assumption that the estimation of weight percent clay can be modified by multiplying the gamma ray/potassium/thorium index by an empirical correction factor, i.e., C. This factor is basically determined from the weight percent clay content of average shale adjacent to the zone of interest [10], and commonly ranges from 50 to 70 [10,13,14].…”

“…The thorium to uranium (Th/U) ratio varies with the sedimentary processes and products as well as with the depositional environment; therefore, it can be used to distinguish Table 1 Calibration equations used to estimate the clay content from GR, K, and THOR logs * I A is the gamma ray/potassium/thorium index obtained from the standardization of natural gamma ray log and/or its components data. The ρ b and ρ sh are the bulk density, and shale density, respectively Larinov [22] For highly consolidated and mesozoic rocks 0.33(2 2I * A − 1) For unconsolidated tertiary clastics 0.083(2 3.7I * A − 1) Clavier [23] A good empirical compromise between the tertiary and older rock equations…”

Inversion-based petrophysical interpretation of logging-while-drilling nuclear and resistivity measurements

“…In equation B.9, the nuclear derivatives are numerically obtained with SNUPAR calculations and a GR-C sh relationship (e.g., linear; Larionov, 1969;Stieber, 1970;Clavier et al, 1971;etc. ), whereas the conductivity derivative is obtained with an appropriate S w model (e.g., Archie, Poupon-Leveaux) assuming known values of Archie's parameters, connate water resistivity, R w , shale resistivity, R sh , and shale porosity,  sh .…”