IMPROVEMENTS OF FAST MODELING OF LWD NEUTRON LOGS IN ENLARGED BOREHOLES FOR A COMMERCIAL LWD TOOL

摘要

In complex geological environments, interpretation methods based on fast modeling and inversion procedures deliver better estimates of petrophysical properties than conventional methods. Neutron measurements are affected by high-order formation and borehole effects. Their depth of investigation is also very sensitive to porosity, borehole size variations, and fluid and rock properties. Consequently, reliable petrophysical interpretation of neutron logs under complex rock and geometrical conditions requires fast modeling methods.We develop a fast-forward algorithm for a commercial LWD neutron tool. The algorithm is based on perturbation theory, flux sensitivity functions (FSFs), and diffusion flux-difference (DFD) approximations. The DFD method interpolates between Monte Carlo (MC)-derived, base-case FSFs using one-group diffusion models and a Rytov approximation. Diffusion approximations successfully capture sensitivity flux perturbations: neutron porosities simulated using DFD-perturbed and MC-derived FSFs agree within one porosity unit (p.u.) in highly deviated wells, enlarged boreholes, and wells with invasion. They significantly outperform linear interpolation approaches, reducing errors in estimated porosity by as much as 10 p.u.Even when diffusion approximations are applied, perturbation theory may still yield inaccurate results when compared to neutron porosity estimated from MCNP counts for regions exhibiting contrasting properties, and primarily enlarged boreholes. We introduce a new two-step algorithm to improve neutron modeling accuracy to MCNP counts in the presence of standoff. The algorithm is based on two sets of base cases: one for detector counts, and the other for sensitivity functions. On one hand, diffusion flux-difference approximations are used to compute perturbed sensitivity functions for any tool, borehole, and formation configuration. On the other hand, count base cases obtained for 21.6-cm (8.5-in), 24.1-cm (9.5-in), and 26.7-cm (10.5-in) boreholes extend the validity of the Taylor series expansion by minimizing the size of the borehole perturbations.Compared to neutron porosity calculated from MCNP counts, the new algorithm yields relatively low errors in enlarged boreholes. Comparison benchmarks against synthetic and field cases in vertical and deviated wells confirm good agreement with Monte Carlo simulations and field logging data, respectively. For the synthetic cases, vertical and horizontal wells were assumed with the tool penetrating several bed layers with contrasting hydrogen index. Monte Carlo simulations were carried out to simulate neutron detector counts and to compare against results obtained from our fast-forward algorithm. The fast-forward algorithm was then applied to a field case featuring a high-angle well penetrating a Norwegian sandstone with light hydrocarbons; good agreement was obtained with field logs.