Influence of borehole and formation characteristics on elemental standard spectra in geochemical logging

Abstract

Accurate acquisition of elemental standard spectra is one of the key links in geochemical elemental logging. Because elemental standard spectra are affected by borehole and formation characteristics, it is necessary to study the borehole and formation conditions required for the acquisition of ideal spectra. For these, we constructed a model comprising instruments, borehole and formation; applied the Monte Carlo N-Particle Transport Code to simulate elemental standard spectral responses under different borehole and formation conditions; and performed a comparative analysis of these responses. The results demonstrate that, for the standard capture spectra of an element, a fresh water borehole yields spectra with good statistical properties and significant characteristic peaks. For elements with a high capture cross section and prominent characteristic peaks, a borehole of small diameter yields ideal spectra; conversely, a borehole of slightly larger diameter is suitable for elements with a low capture cross section and insignificant characteristic peaks. A formation containing a certain number of pores full of fresh water can yield better standard spectra except for elements that have a giant capture cross section such as Gd. A formation composed of various compounds of an element yields standard spectra with different effects, which means that an elemental compound has to be selected to get an ideal standard spectrum. For the standard inelastic spectrum of an element, the influence of the borehole fluid or pore fluid should be avoided as much as possible except for oxygen; formations composed of an elementary substance, or chlorides or hydrides of an element yields a better standard spectrum than one composed of its oxides, carbonates or other compounds containing elements with a large inelastic cross section. The compactness of the formation influences the characteristic peaks and statistical properties of the standard spectra. The simulation method takes fully into account all kinds of factors influencing borehole and formation in the process of obtaining the standard spectra, which is helpful in obtaining ideal standard spectra on the basis of optimizing formation and borehole conditions.