Neutron absorbing dopant for reducing borehole fluid background in geochemical logging

Abstract

Disclosed herein are methods and system for determining an elemental composition of a downhole formation using neutron-induced gamma ray spectroscopy and doping the borehole fluid with a neutron absorber to remove at least part of the borehole thermal neutron flux before it can capture borehole nuclei and emit gamma rays within a spectral range of interest. For example, a method for determining the elemental composition includes adding a dopant to a borehole fluid, wherein the dopant absorbs thermal neutrons in the borehole, lowering a logging tool comprising a gamma ray spectrometer, emitting neutrons into a downhole environment, generating neutron-induced gamma rays comprising borehole gamma rays and formation gamma rays, detecting gamma rays from the downhole formation, and extracting the elemental composition of the downhole formation. The dopant may not emit gamma rays above 0.5 MeV or their peaks background should be predictable and/or consistent to be removed during data processing.