A new petrophysical property and measurement, the Fast Neutron Cross Section (FNXS), was introduced for the formation evaluation in recent years (Rose et al., 2015; Zhou et al., 2016). FNXS is defined as the macroscopic cross section of the formation inelastic scattering with incident fast neutrons. It is an independent measurement of the formation which primarily responds to the formation atomic density (the total number of atoms in unit volume), similarly as formation density to the total mass of atoms in unit volume. FNXS log has been successfully used under some complex scenarios to identify gas or CO2 in reservoirs and determine the gas saturation with case studies (Iglesias et al., 2016; Rose et al., 2017).

Simulated log response in a forward formation model of shale and sand, that shows FNXS is sensitive to gas-filled porosity variation, but insensitive to liquid-filled porosity variation, as well as insensitive to matrix or clay volume variation. Compared to usual ways such as neutron, density and neutron capture cross section, FNXS is more effective to identify gas in formation and less affected by shale or other fluids; Cross plotting FNXS with neutron porosity or density log with a normalized scale, the gas "crossover" is a more straightforward way to identify gas or water zone. While determining gas saturation, the relative error of saturation resulted by the uncertainty of clay volume, is much lowered with the approach of FNXS.

FNXS can be measured by a specifically designed pulsed neutron logging tool, by detecting the gamma rays induced from fast neutron inelastic scattering. It can be measured in open hole or through casing. Case studies from PetroChina Changqing oilfield were presented in the paper, showing to identify gas and water with FNXS log and determine the gas saturation in a complex formation sequences, with low porosity and low resistivity shaly sands, coals and tight carbonates; three ways to interpret logs were illustrated: quick look of FNXS overlaid with other logs, cross-plotting in a chart and quantitative gas volume calculation with volumetric linear models. Well testing result proved the new measurement of FNXS worked in challengeable gas reservoir evaluation. In final part of the paper, we discussed the feasibility of applying FNXS to monitoring CO2 saturation changes in EOR or CCS projects.

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