Rapid modeling of LWD nuclear measurements acquired in high-angle and horizontal wells for improved petrophysical and geometrical interpretation
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Nuclear logging-while-drilling (LWD) measurements acquired in high-angle and horizontal (HA/HZ) wells are influenced by tool, geometrical, and petrophysical effects. Reliable interpretation of petrophysical and geometrical properties from LWD measurements acquired in thinly-bedded formations requires that gamma ray, density, photoelectric (PEF), and neutron measurements be quantitatively integrated with explicit consideration of their effective volume of investigation (EVOI). One of the effects of different tool EVOIs is false gas density-neutron crossovers across thinly-bedded formations. Also, in the presence of tool eccentricity, azimuthally-varying standoff gives rise to an azimuthally-varying effective depth of investigation (EDOI), which introduces errors in the inference of formation dip. Conventional Monte Carlo simulations of nuclear measurements are computationally expensive in reproducing multi-sector LWD responses in HA/HZ wells. Using linear iterative refinement of pre-calculated flux sensitivity functions (FSFs), we introduce a fast method for numerical simulation of LWD nuclear images in the presence of tool eccentricity along any well trajectory. Our investigation of measurement responses from FSFs motivates techniques to explicitly consider the EVOI of LWD nuclear measurements. Simple radial DOI and standoff corrections suffice for interpretation of gamma-gamma images but are inadequate for neutron responses due to larger EVOI and azimuthal aperture. We introduce a new azimuthal deconvolution method of neutron images to improve bed-boundary detection. Neutron DOI varies significantly with porosity, whereby we correct neutron images for penetration length due to changes of porosity along the well trajectory. In addition, we implement a new method of separate linear iterative refinement on neutron thermal group responses to improve the resolution of neutron images across heterogeneous and thinly-bedded formations. The method reduces shoulder-bed effects and false neutron-density gas crossovers. We corroborate these techniques with rigorous Monte Carlo simulations in vertical and deviated wells. A field example of application conclusively indicates that numerical simulation of LWD nuclear measurements is necessary for reliable estimation of petrophysical properties.