Abstract
The thermal neutron porosity is routinely acquired in almost every well. When combined with the density, gamma ray and resistivity logs, the basic petrophysical parameters of a reservoir are evaluated. The design of the thermal neutron tool is simple, but its interpretation is complex and affected by the formation constituents. The most challenging situation occurs when the formation contains elements with high absorption probability of the thermal neutrons. The existence of such elements changes the neutron transport parameters and results in a false increase in the measured porosity. The problem is reported by many users throughout the years. In 1993, higher thermal neutron porosity is reported due to the existence of an iron-rich mineral, Siderite, in the Nazzazat and Baharia formations in Egypt. Siderite and all iron-rich minerals have high thermal neutrons absorption probability. Recently, in 2018, high thermal neutron porosity in Unayzah field in Saudi Arabia is also reported due to the existence of few parts per million of gadolinium. Gadolinium is a rare element that has high probability of thermal neutron absorption. Currently, none of the existing commercial petrophysics software(s) have modules to correct the thermal neutron porosity for such effects. This represents a challenge to the petrophysicists to properly calculate the actual reservoir porosity. In this paper, the effects of the rare elements and other minerals with high thermal neutron absorption probability on the thermal neutron porosity are discussed, and a correction methodology is developed and tested. The methodology is based on integrating the tool design and the physics of the neutron transport to perform the correction. The details of the correction steps and the correction algorithm are included, tested and applied in two fields.
Highlights
The thermal neutron porosity tool is a simple tool in design but rather difficult in interpretation (Asquith and Krygowski 2004; Bassiouni 1994; Ellis and Singer 2007; Serra 1988; Wu et al 2013; Ellis et al 1987)
The results clearly show that very low concentration of this rare element, 3 ppm, for example, will increase the thermal neutron porosity by 8%
The tool is simple in design but rather difficult in interpretation due to its complex physics
Summary
The thermal neutron porosity tool is a simple tool in design but rather difficult in interpretation (Asquith and Krygowski 2004; Bassiouni 1994; Ellis and Singer 2007; Serra 1988; Wu et al 2013; Ellis et al 1987). Formation porosity using the thermal neutron tool is controlled by multiple neutron transport parameters. The thermal neutron diffuses until it is absorbed by the formation elements. Ellis developed the relationships of the slowing down length, Eq 3, and the diffusion length, Eq 4, using the formation porosity and the neutrons absorption probability.
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