Quantitative Interpretation Of Pulsed Neutron Capture Logs In Thinly-Bedded Formations

Pulsed neutron capture (PNC) logs are commonly used for formation evaluation behind casing and to assess time-lapse variations of hydrocarbon pore volume. Because conventional interpretation methods for sigma logs assume homogeneous formations, errors may arise in thinly-bedded formations when appraising petrophysical properties of hydrocarbon-bearing beds. There exist no quantitative interpretation methods to account for shoulder-bed effects on PNC logs acquired in sandshale laminated reservoirs. Due to diffusion effects between dissimilar beds, sigma logs acquired in such formations do not obey linear mixing laws between the sigma responses of pure-sand and pure-shale end members of the sedimentary sequence. We introduce a new method to rapidly simulate sigma logs in thinly-bedded formations. The method makes use of late-time, thermal-neutron flux sensitivity functions (FSFs) to describe the contribution of multilayer formations to the measured capture cross section. It includes a correction procedure based on diffusion theory that adapts a homogeneous, base-case FSF to simulate the response of vertically heterogeneous formations. Benchmarking exercises indicate that our rapid simulation method yields differences smaller than 2% with respect to PNC logs simulated with rigorous Monte Carlo methods for a wide range of geometrical, petrophysical, and fluid properties. The second part of the paper develops an inversion method to reduce shoulder-bed effects on PNC logs and estimate layer-by-layer capture cross sections based on the previously defined FSFs. We successfully test the estimation method on synthetic examples that include a variety of bed-thickness configurations. Inversion consistently improves the vertical resolution and sigmadefinition of PNC logs across beds thinner than 1.5 ft. Testing on field data confirms the efficiency, reliability, and stability of the inversion procedure. Our fast simulation/interpretation algorithm inverts sigma logs over 100+ ft. intervals in less than 4 minutes of CPU time, and is therefore suitable for joint petrophysical interpretation with other open- and cased-hole logs.