Deriving Synthetic Bulk Density Using Fast Neutron Cross-Section in a Log-Integrated Approach from Slim Pulsed Neutron Logging in Cased-Hole Environment

Recent advancement in logging technology and data analytics allows measuring a comprehensive set of formation petrophysical properties and rock composition in cased boreholes. State-of-the-art pulsed neutron logging technology and processing algorithms record capture and inelastic elemental spectroscopy for matrix parameters, and detailed mineralogy characterization, total organic content estimation, and carbon/oxygen analysis, simultaneously with formation sigma, neutron porosity, and fast neutron cross-section. The fast neutron cross-section (FNXS) is a new formation nuclear property introduced by the advanced pulsed neutron tool that is independent of thermal and capture cross-section and highly sensitive to gas regardless of hydrogen index. Unlike thermal neutron capture cross-section, for which certain isotopes have extremely high values (such as Cl, B, and Gd), fast neutron cross-sections of all isotopes are more or less similar. Thus, FNXS is approximately proportional to atom density. Therefore, this new nuclear property has functionality similar to that of the bulk density (gamma-gamma density measurement). A local relationship can be defined to convert the FNXS into bulk density when the lithology and fluid properties are known, and calibration is possible. Otherwise, a more comprehensive assessment of bulk density can be performed by integrating FNXS with the other outputs from the slim pulsed neutron logging into a mineral solver. While solving for rock and fluid volumes from the cased-hole logs, a reconstructed bulk density may be derived in a cased-hole environment. This synthetic bulk density can be used by geophysicists to develop synthetic seismograms to properly map formation tops with surface seismic data. Since the pulsed neutron measurements follow linear volumetric law equations, they can be directly integrated into a mineral solver together with the elemental spectroscopy outputs to create a synthetic bulk density, together with the other answers. A blind comparison was done between synthetic bulk density from the cased-hole log-based mineral solver and a measured openhole density, showing a strong correlation in a three-phase fluid reservoir (gas, oil, and water). A synthetic seismogram is an essential tool when geophysicists fine-tune surface seismic data. This seismogram is developed using bulk density and compressional slowness to derive acoustic impedance, where sometimes bulk density is missing. As a result, an old approach to estimate bulk density using Gardner’s equation has certain limitations in complex environments. The new formation nuclear property that is now available in the slim pulsed neutron technology can be leveraged to provide a more robust and quality-controlled synthetic bulk density derived through FNXS integrated with the other pulsed neutron and spectroscopy outputs.