PULSED NEUTRON LOGGING SYSTEMS FOR DETECTING GAS ZONES

摘要

An illustrative embodiment of the invention includes methods and apparatus for producing inelastic neutron scattering logs of earth formations surrounding a well borehole. The gamma rays emitted by nuclei of at least carbon, oxygen, silicon and calcium which are excited by inelastically scattered fast neutrons are observed by energy separation and are individually logged as a function of borehole depth. The silicon/calcium and carbon/oxygen inelastic gamma ray counts ratios are also logged as a function of borehole depth. Low hydrogen content of the formations may be estimated by detecting a high count rate in each of the four energy windows and then by comparing the Si/Ca and C/O ratios in these regions to the Si/Ca and C/O ratios for a known water sand, the relative abundance of limestone in the low hydrogen content formations may be estimated thus distinguishing gas zones from water saturated low porosity limestone.