Resolution and Accuracy of Neutron-Gamma Density Measurements Compared to Conventional Gamma-Gamma Density Measurements

Abstract

Abstract Density measurements are widely used for the in-situ assessment of formation porosity. Traditionally, bulk density is measured with a Gamma-Gamma density tool using a radioactive chemical source. Neutron-Gamma density was recently developed to infer porosity with non-chemical sources. It is therefore imperative to quantify the differences in spatial resolution and accuracy between the two measurements and the corresponding impact on petrophysical interpretation practices. We designed a theoretical, albeit practical logging-while-drilling (LWD) Neutron-Gamma density tool and optimized source-detector spacing for maximum accuracy. The tool design comprises a 14 MeV neutron source, two gamma ray detectors and two fast neutron detectors. Counts ratios are used to establish a linear relationship with formation bulk density. This design yields an accuracy of 0.0147 g/cm3 in shale-free formations, and 0.0182 g/cm3 in shales, to be compared to the accuracy of traditional Gamma-Gamma density measurements (0.015 g/cm3). Neutron-Gamma density uses inputs from all four detectors in an effort to set apartCompton scattering effects needed to calculatebulk density. Consequently, the spine-and-rib correction technique is not implemented in real time, leaving the measurement vulnerable to borehole environmental effects and mainly standoff. Standoff should be limited to 0.25 in for light mud, leading to a 0.05 g/cm3 correction ofbulk density. While the vertical resolution of Gamma-Gamma density is approximately 1 ft, results obtained from synthetic cases show that the vertical resolution of Neutron-Gamma density is limited to 2.5 ft. The vertical resolution of Neutron-Gamma density competes against improved depth of investigation. Neutron-Gamma density is less affected by invasion than Gamma-Gamma density. Depth of investigation of Neutron-Gamma density is approximately 9.5 in, i.e. twice as long as the depth of investigation of Gamma-Gamma density (~4 in). The main technical challenge of Neutron-Gamma density is measurement reliability across diverse solid and fluid rock compositions. Synthetic examples show that the tool design accurately resolves formation density in shale and shaly formations, high-density formations such as anhydrite, and formations saturated with low-density gas. In these cases, the accuracy is similar to that of Gamma-Gamma density and the measurement remains reliable for porosity estimates within +/- 1 pu. We introduce a new tool design for a radioisotope-free, in-situ measurement of bulk density based on two-particle transport. This optimized tool design is the first one to provide an accuracy that compares to Gamma-Gamma density in clean and shaly formations. Synthetic cases serve to compare the measurements and provide insight about the conditions when Neutron-Gamma density can be used as an alternative to Gamma-Gamma density while achieving equivalent accuracy and performance.