A fast forward computational method for nuclear measurement using volumetric detection constraints

Owing to the complex lithology of unconventional reservoirs, field interpreters usually need to provide a basis for interpretation using logging simulation models. Among the various detection tools that use nuclear sources, the detector response can reflect various types of information of the medium. The Monte Carlo method is one of the primary methods used to obtain nuclear detection responses in complex environments. However, this requires a computational process with extensive random sampling, consumes considerable resources, and does not provide real-time response results. Therefore, a novel fast forward computational method (FFCM) for nuclear measurement that uses volumetric detection constraints to rapidly calculate the detector response in various complex environments is proposed. First, the data library required for the FFCM is built by collecting the detection volume, detector counts, and flux sensitivity functions through a Monte Carlo simulation. Then, based on perturbation theory and the Rytov approximation, a model for the detector response is derived using the flux sensitivity function method and a one-group diffusion model. The environmental perturbation is constrained to optimize the model according to the tool structure and the impact of the formation and borehole within the effective detection volume. Finally, the method is applied to a neutron porosity tool for verification. In various complex simulation environments, the maximum relative error between the calculated porosity results of Monte Carlo and FFCM was 6.80