A new formation nuclear property, the fast neutron cross section (FNXS), is introduced to the well logging industry. It is a measure of the formation's ability to interact with fast neutrons. For sigma and porosity, the other two commonly used neutron measurements, certain elements tend to dominate, such as B, Cl and Gd for the sigma measurement and H for the porosity measurement. However, for the FNXS measurement, there is no single element dominating the response. This is explained by the complex dependence of neutron interactions on energy and elemental composition. Therefore, FNXS can provide information independent of the other neutron measurements for formation evaluation applications.
FNXS can be measured by a pulsed neutron logging tool that has been designed for that purpose. The corresponding raw measurements are the detected gamma rays that are induced by fast neutron inelastic scattering. However, the purely inelastic gamma ray events cannot be measured directly and are always mixed with the gamma ray events induced by thermal or epithermal neutron capture. It is difficult to consistently separate inelastic and capture gamma ray events in a wide range of downhole conditions. Several critical innovative tool design features are required to overcome this challenge. The detailed physical processes leading to the detected inelastic gamma rays, which involve both neutron and gamma ray transport, were modeled explicitly using Monte Carlo techniques in a wide range of formation and borehole conditions. It was found that the inelastic gamma ray response is dominated by FNXS and thus can be described approximately by FNXS. This approximation can be improved by introducing additional formation properties such as bulk density and atomic density. The tool measurement is characterized based on laboratory data to provide formation FNXS values, with corrections to account for the hole size and casing impact. The impact of other typically unknown borehole conditions, such as cement variation, standoff, and eccentered casing, is assessed using modeling.
Because FNXS values of the rock matrix and water are in the same range, lower for light oil and much lower for hydrocarbon gas, FNXS can be used for a quantitative gas saturation measurement. It is particularly useful for differentiating gas-filled porosity from very low porosity in cased-hole formation evaluation if openhole density is not available. Log examples are provided to illustrate the FNXS measurement applications and performance.
