Lithology Correction and Sensitivity Enhancement for LWD Pulsed Neutron Porosity Logging Based on Slowing-Down Length Ratio

摘要: Logging-While-Drilling (LWD) pulsed neutron porosity logging is a key technology for advancing logging techniques, supporting unconventional oil and gas exploration, and achieving sourceless logging operations. However, achieving accurate porosity measurement under these novel logging conditions and in complex geological environments has become a critical challenge. In this study, theoretical analysis was employed to derive lithology correction and sensitivity enhancement formulas based on the slowing-down length ratio under identical instrument structures. A slowing-down length formula with excellent fitting performance was obtained through reasonable approximations. The relationships between the slowing-down length ratio and density under different lithologies and different neutron source conditions were investigated. Monte Carlo (MCNP) modeling was utilized to establish various formation models for auxiliary theoretical analysis and verification. Finally, the correction efficacy was validated by comparing the original data, the lithology-corrected results, and the sensitivity-enhanced results. The results show: the slowing-down length ratio is not affected by the Hydrogen Index (HI). Under different lithologies, the ratio is inversely proportional to density. Under different chemical and pulsed source conditions, the ratio is directly proportional to density. Using the slowing-down length ratio for lithology correction significantly improves porosity calculation accuracy, with errors not exceeding 1 p.u. Using the ratio for sensitivity enhancement can elevate the thermal neutron count ratio response to the level of a chemical source, greatly expanding the dynamic range and improving porosity sensitivity. This enables more accurate formation evaluation and promotes the transition to sourceless, integrated logging technologies.