Quantitative interpretation of pulsed neutron capture logs: Part 1 — Fast numerical simulation

Abstract

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 Σ logs assume homogeneous formations, errors may arise, especially 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 Σ logs acquired in sand-shale laminated reservoirs. Because of diffusion effects between dissimilar beds, Σ logs acquired in such formations do not obey mixing laws between the Σ responses of pure-sand and pure-shale end members of the sedimentary sequence. We have developed a new numerical method to simulate PNC rapidly and accurately logs. The method makes use of late-time, thermal-neutron flux sensitivity functions (FSFs) to describe the contribution of multilayer formations toward the measured capture cross section. It includes a correction procedure based on 1D neutron diffusion theory that adapts the transport-equation-derived, base-case FSF of a homogeneous formation to simulate the response of vertically heterogeneous formations. Benchmarking exercises indicate that our simulation method yields average differences smaller than two capture units within seconds of computer central processing unit time with respect to PNC logs simulated with rigorous Monte Carlo methods for a wide range of geometrical, petrophysical, and fluid properties.

2D inversion-based interpretation of logging-while-drilling thermal-neutron and gamma-ray time decays

“…Haley (1995) develops a log response equation to interpret Sigma logs in sand-shale laminated systems. Mimoun et al (2011) develop a rapid simulation method for cased-hole pulsed-neutron measurements and a 1D inversion scheme to estimate layer-by-layer formation Sigma. Both methods are based on thermal-neutron diffusion transport corrections and disregard the effect of mud-filtrate invasion.…”

Fast modeling of borehole neutron porosity measurements with a new spatial transport-diffusion approximation

“…A similar situation of significant flux perturbations has been observed when modeling borehole sigma measurements across bed boundaries between sands and shales. Mimoun et al (2011) show that a diffusion model can be used to approximate such flux perturbations and hence improve the accuracy and reliability of borehole sigma models. Greenspan (1976) highlights several other formulations for calculation of related flux perturbations.…”

Inversion-based petrophysical interpretation of logging-while-drilling nuclear and resistivity measurements

“…These perturbation effects manifest themselves as higher-order terms in the Taylor's series expansion of the differential operator perturbation function (X-5 Monte Carlo Team, 2003). A diffusion correction method for simulating Sigma measurements in the presence of such perturbations was shown to improve accuracy across layers of dissimilar capture cross-sections (Mimoun et al, 2011). Methods that incorporate higher-order dependencies on borehole and environmental effects have also shown to improve accuracy in simulating gamma-gamma measurements (density and photoelectric factor) acquired in the presence of mudcake (Zhou et al, 2009).…”

“…Guo et al (2009) documented the application of a neutron diffusion code (Butler et al, 1992) A similar situation of significant flux perturbations has been observed when modeling borehole sigma measurements across bed boundaries between sands and shales. Mimoun et al (2011) showed that a diffusion model can be used to approximate such flux perturbations and hence improve the accuracy and reliability of borehole sigma models. Greenspan (1976) …”