Improvements in a Through-Casing Pulsed-Neutron Density Log

Abstract

The practice of measuring the formation density with a standard pulsed-neutron system has been described in the literature 1,2,3,4,5 , and has been successfully applied in many reservoirs. This success leads to the obvious question: Can a system be developed that is optimized for this measurement?To understand which parameters should be measured, characterized, or require improved response, a more exact analytical model is required. The pulsed-neutron density measurement is based on the transport of gamma rays created by inelastic scattering of fast neutrons. The current density algorithm uses a diffusion model for the gamma ray transport process. This simple model does not include the effect of variations in fast neutron parameters on the process of gamma ray production. As a result, an ambiguity is introduced in the density log that is associated with the dynamic nature of the gamma source. The initial distribution of gamma rays is a function of atom density and hydrogen content and must be properly described as a first step toward a more exact transport model.The pulsed-neutron density measurement is analyzed and optimization is achieved using theoretical models, responses from laboratory formations and test wells, and through computer modeling. Along with data supporting an improved analytical model of the m easurement, data is shown for incorporating fast neutron detectors and for optimizing spacings of the gamma ray detectors. As a result of this study, a prototypical detector array is described for improved density response.

Bulk Density Response and Experimental Study of Pulsed Neutron-Gamma Density Logging

“…For the gamma ray transport group, assuming that D γ is the diffusion coefficient of gamma rays, Σ γ is the macroscopic scattering cross section of gamma rays, ϕ γ is the gamma ray flux, and L γ is the gamma ray attenuation length, which is given by L γ =(D γ /Σ γ ) 0.5 , the diffusion equation for gamma rays can be expressed as follows (Odom et al, 1999;Odom et al, 2001):…”

“…However, it is necessary to correct the influence of the variations in induced gamma ray source in gas-bearing or low-porosity formations. Odom et al (2001) developed a cased-hole pulsed neutron- gamma density tool with a fast neutron detector. Jacobson et al (2004) pointed out that neutron capture gamma rays could be used for correction in density calculation.…”