Physical and petrophysical laws of radiation fields in nuclear well logging

Nuclear-Geophysical Information-Measurement Systems of a new generation include not only hardware units (logging equipment), but also software units - measurement procedure (metrological support included) and interpretation - algorithmic support components. Quality and completeness of a resulting geological information depend on the adequate knowledge of physical laws of radiation fields in borehole-formation system and petrophysics of reservoirs and in situ thermodynamic conditions. Considered the urgent lines in devising new generation nuclear-geophysical information systems and their application in petroleum geology, following the up-to-date tendencies of geoinformation technologies, particularly the following ones: 1. Switching from empirical cross charts to petrophysical laws having a wider application and requiring a minimum of laborious laboratory petrophysical research; 2. Converting the problems of geological interpretation of nuclear logs in a combination with other logs from a non-formalized (creative) class into an algorithmically solved class on a base of a principle of 'geological intelligence'; 3. Development of interpretation algorithms taking into account the 'anatomy' of neutron and gamma ray fields in a 'borehole-formation' system; 4. Tuning the interpretation algorithms to individual metrological characteristics of measuring systems; 5. Implementing the adaptivity principle in the algorithms of individual and combined interpretation and applying adaptive techniques instead of correction-based ones. These major advances can be illustrated by the following examples: (i) 'Automodel' law of the radial geometrical factor of formation in integral and spectrometric modifications of Natural Gamma Ray Log. (ii) Unified dependence of the NGR double-difference parameter on the relative clayness for reservoirs of various genesis. (iii) Applying the Natural Gamma-Ray Spectrometry as a 'porosity' method. (iv) Unified dependence of the residual water saturation on the porosity for intergranular reservoirs of various genesis. (v) Determination of effective porosity and gas-saturation with the use of neutron and gamma ray logs in open holes as well as in cased ones (particularly, in the absence of reference beds). (vi) Petrophysical filtration method as applied to identifying and estimation of productive oil and gas reservoirs and the quantitative estimation of dynamic porosity and permeability and forecasting variation of these parameters throughout the production period. (author)