Rapid forward modeling of logging-while-drilling neutron-gamma density measurements

Geophysics ◽  
2018 ◽  
Vol 83 (6) ◽  
pp. D231-D246
Author(s):  
Mathilde Luycx ◽  
Carlos Torres-Verdín
Keyword(s):  
Monte Carlo ◽  
Bulk Density ◽  
Gamma Ray ◽  
Neutron Transport ◽  
Density Measurement ◽  
Layer By Layer ◽  
Density Measurements ◽  
Gamma Density ◽  
Particle Counts ◽  
Logging While Drilling

Neutron-activated gamma-ray (neutron-gamma) logging-while-drilling (LWD) measurements deliver bulk density estimates without using a chemical source. The assessment of bulk density is based on neutron-induced non-capture gamma rays, corrected for neutron transport by combining particle counts acquired at two gamma-ray detectors and two fast neutron detectors. Particle counts from all four detectors are necessary to deliver one density measurement whose accuracy compares well to that of the gamma-gamma density instruments. Thereafter, borehole environmental effects are mitigated with empirical corrections based on Monte Carlo (MC) modeling. Such corrections should be avoided for standoff values greater than 0.63 cm (0.25 in) because they are no longer independent of formation properties. Neutron-gamma density measurements are also influenced by bed-boundary and layer-thickness effects. Thinly bedded formations, invasion, high-angle/horizontal (HA/HZ) wells, and enlarged boreholes can all mask true formation bulk density when implementing conventional petrophysical interpretation. Although MC methods accurately simulate 3D environmental and geometrical effects, they are computationally expensive and are thus impractical for real-time interpretation. Layer-by-layer bulk density can, however, be estimated using rapid numerical simulations coupled with inversion procedures. We have developed a rapid modeling algorithm to accurately simulate LWD neutron-gamma density measurements. Simulations are based on a theoretical, albeit realistic, LWD neutron-gamma density tool operating with a 14.1 MeV pulsed neutron source. The algorithm uses flux sensitivity functions and first-order Taylor series approximations to simulate particle counts at each detector before they are processed with a density estimation algorithm. Rigorous benchmarks against the Monte Carlo N-particle code in vertical and HA/HZ wells, across diverse solid and fluid rock compositions, thin beds, and in the presence of invasion, yield average density errors of less than 1% ([Formula: see text]) in approximately [Formula: see text] the time required of MC modeling.

A new method for calculating bulk density in pulsed neutron-gamma density logging

Geophysics ◽  
2020 ◽  
Vol 85 (6) ◽  
pp. D219-D232
Author(s):  
Hu Wang ◽  
Wensheng Wu ◽  
Tianzhi Tang ◽  
Ruigang Wang ◽  
Aizhong Yue ◽  
...  
Keyword(s):  
Monte Carlo Simulation ◽  
Monte Carlo ◽  
Pair Production ◽  
Bulk Density ◽  
Density Estimation ◽  
Measurement Accuracy ◽  
Gamma Ray ◽  
Estimation Algorithm ◽  
Gamma Density ◽  
Pulsed Neutron

Formation density is one of the most important parameters in formation evaluation. Radioisotope chemical sources are used widely in conventional gamma-gamma density (GGD) logging. Considering security and environmental risks, there has been growing interest in pulsed neutron generators in place of the radioactive-chemical source in using bulk-density measurements. However, there still is the requirement of high accuracy of the neutron-gamma density (NGD) calculation. Pair production is one of the factors influencing the accuracy of the results, which should be considered. We have adopted a method, based on the difference between the inelastic gamma-ray response of high- and low-energy windows, to reduce the impact of pair production upon calculating the bulk density. A new density estimation algorithm is derived based on the coupled-field theory and gamma-ray attenuation law in NGD logging. We analyze the NGD measurement accuracy with different mineral types, porosity, and pore fluid and determine the influence of the borehole environment on NGD logging. The Monte Carlo simulation results indicate that the improved processing algorithm limits the influence of the mineral type, porosity, or pore fluid. The NGD measurement accuracy is ±0.025 g/cm3 in shale-free formations, which is close to the GGD measurement (±0.015 g/cm3). Our results also show that the borehole environment has a significant impact on NGD measurement. Therefore, it is necessary to take the influence of the borehole parameters into account in NGD measurements. Combined with Monte Carlo simulation cases, we evaluate the application results of the new density estimation algorithm in various model wells.

Physics, applications, and limitations of borehole neutron-gamma density measurements

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. D39-D56 ◽  
Author(s):  
Mathilde Luycx ◽  
Carlos Torres-Verdín
Keyword(s):  
Environmental Effects ◽  
Gamma Ray ◽  
Vertical Resolution ◽  
Neutron Detectors ◽  
Density Measurements ◽  
Gamma Density ◽  
Limited Applicability ◽  
Empirical Density ◽  
Cesium 137 ◽  
Pulsed Neutron

Radioactive chemical sources can pose security, health, and environmental risks when used to estimate rock porosity in situ. The oil industry has been developing solutions to eliminate radioactive chemical sources in borehole nuclear logging. Pulsed neutron generators have successfully replaced chemical sources in neutron tools, but cesium-137 is still mainly used for borehole density measurements. Neutron-activated gamma-ray measurements (neutron-gamma) are a possible alternative to radioactive chemical sources in density tools. Despite recent advances, the measurement faces challenges regarding density accuracy across diverse solid and fluid rock compositions and nonnegligible sensitivity to borehole environmental effects. We have examined a theoretical, albeit realistic, logging-while-drilling neutron-gamma density (NGD) tool operating with two inelastic gamma-ray detectors and two fast neutron detectors. With a strong emphasis on measurement physics and source-sensor design, the tool delivers density accuracies comparable to those of gamma-gamma density (GGD) tools with [Formula: see text] error in shale-free formations and [Formula: see text] in shale and shaly formations. Our work also compares NGD with GGD in terms of depth of investigation (DOI), vertical resolution, and sensitivity to borehole environmental effects to determine optimal logging conditions. NGD accuracy is limited in the presence of standoff. With inputs of caliper and mud type, empirical density corrections can be applied up to 0.64 cm (0.25 in) standoff. NGD also has limited applicability in thinly bedded formations with maximum vertical resolution of 76 cm (2.5 ft). However, the measurement outperforms GGD in the presence of invasion because its DOI is twice as large.

Sourceless Neutron-Gamma Density (SNGD): A Radioisotope-Free Bulk Density Measurement: Physics Principles, Environmental Effects, and Applications

2012 ◽  
Author(s):  
Michael Evans ◽  
Francoise Allioli ◽  
Valentin Cretoiu ◽  
Fabien Haranger ◽  
Nicolas Laporte ◽  
...  
Keyword(s):  
Bulk Density ◽  
Environmental Effects ◽  
Density Measurement ◽  
Gamma Density

Investigation of Incoherent Gamma-Ray Scattering Potential for the Fluid Density Measurement

2014 ◽  
Vol 575 ◽  
pp. 549-553 ◽  
Author(s):  
Rahadi Wirawan ◽  
Mitra Djamal ◽  
Abdul Waris ◽  
Gunawan Handayani ◽  
Hong Joo Kim
Keyword(s):  
Monte Carlo ◽  
Gamma Ray ◽  
Density Measurement ◽  
Experimental Result ◽  
Fluid Density ◽  
Fluid Parameter ◽  
Scattering Measurements ◽  
Scattering Potential ◽  
The Difference ◽  
Ray Scattering

Incoherent gamma ray scattering is a method that can be applied for the fluid parameter characterization. The aim of the present work is to study the potential usage of the incoherent gamma ray scattering measurements to evaluate the fluid density based on the Monte Carlo approach. Enlarging the density of a fluid results in a significant reduction in the intensity of the detected gamma scattering. The difference of the simulation curve slope results in the gamma transmission mode its about 0.02 compared to the experimental result.

Inversion-based interpretation of logging-while-drilling gamma-ray spectroscopy measurements

Geophysics ◽  
2016 ◽  
Vol 81 (1) ◽  
pp. D9-D34 ◽  
Author(s):  
Oyinkansola Ajayi ◽  
Carlos Torres-Verdín ◽  
William E. Preeg
Keyword(s):  
Gamma Ray ◽  
Chemical Elements ◽  
Weight Fraction ◽  
Layer By Layer ◽  
Elemental Compositions ◽  
Gamma Ray Spectroscopy ◽  
Measurement Mode ◽  
Logging While Drilling ◽  
Pulsed Neutron ◽  
Interpretation Method

Neutron-induced spectroscopy measurements are commonly used to quantify in situ elemental compositions of rocks from the processing of measured gamma-ray energy spectra. However, geometric effects on the measured spectroscopy logs, such as thin beds, dipping beds, and deviated well trajectories, can cause shoulder-bed averaging that compromises the assessment of the true layer elemental composition. We have developed an inversion-based interpretation method to evaluate layer elemental compositions from spectroscopy measurements acquired with a commercial 14-MeV pulsed-neutron logging-while-drilling spectroscopy tool. The algorithm is based on a new spectroscopy fast-forward simulation technique, and it estimates layer-by-layer elemental relative yields, weight concentrations, and their uncertainties. Calculations are performed with inelastic and capture gamma-ray spectroscopy measurements that arose from high- and low-energy neutron interactions, respectively. This strategy provides two sets of data that independently validate estimated elemental compositions and can ascertain chemical elements present in only one measurement mode. In laminated formations in which layer thicknesses are appreciably below the vertical resolution of the tool, it is impossible to quantify layer properties with inversion methods. We have therefore developed an additional interpretation method based on a spectroscopy mixing law to estimate elemental compositions within individual laminae. The new inversion-based interpretation methods were successfully verified with two challenging synthetic cases and implemented in two field cases with varying lithology and well trajectories. Our results found that the developed methods reduced shoulder-bed averaging effects on the measured spectroscopy logs by as much as a 0.4 yield fraction and a 0.17 weight fraction. Estimated elemental compositions with reduced shoulder-bed averaging effects improved the calculations in subsequent spectroscopy-based petrophysical interpretation.

Wood Density Determination by X- and Gamma-Ray Tomography

Holzforschung ◽  
2002 ◽  
Vol 56 (5) ◽  
pp. 535-540 ◽  
Author(s):  
A. Macedo ◽  
C. M. P. Vaz ◽  
J. C. D. Pereira ◽  
J. M. Naime ◽  
P. E. Cruvinel ◽  
...  
Keyword(s):  
Bulk Density ◽  
Wood Density ◽  
Wood Species ◽  
Gamma Ray ◽  
Wood Quality ◽  
Density Measurement ◽  
Calibration Procedure ◽  
Density Determination ◽  
Resolution Level

Summary Wood density measurement is related to several factors that influence wood quality. In this paper, a CT image calibration procedure which allows image quantification in terms of dry bulk density is presented for three different X- and gamma-ray energies (28.3, and 59.5, and 662.0 keV). The mass attenuation coefficients measured for a set of eight wood species did not vary significantly, allowing a single calibration for determination of bulk density of air-dried wood samples at each energy. The equation for bulk density calibration obtained was validated using a second set of twelve wood species. Comparison of bulk density determined by CT images, using the calibration procedure proposed, with values obtained by gravimetric methods, presented a very good linear correlation coefficient (R2=0.94). The main advantage of CT imaging over conventional techniques for wood bulk density determination is that it allows detection and quantification of heterogeneities and internal defects. At the sub-millimetric spatial resolution level, it is possible to identify morphological and structural aspects of wood samples.

Inversion-based method for interpretation of logging-while-drilling density measurements acquired in high-angle and horizontal wells

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. D113-D127 ◽  
Author(s):  
Alberto Mendoza ◽  
Olabode Ijasan ◽  
Carlos Torres-Verdín ◽  
William E. Preeg ◽  
John Rasmus ◽  
...  
Keyword(s):  
Field Measurements ◽  
Horizontal Wells ◽  
Inversion Method ◽  
Layer By Layer ◽  
High Angle ◽  
Penetration Length ◽  
Short Spacing ◽  
Density Measurements ◽  
Logging While Drilling ◽  
Dip Angle

We introduce a sector-based inversion method to improve the petrophysical interpretation of logging-while-drilling density measurements acquired in high-angle and horizontal wells. The central objective is to reduce shoulder-bed effects on the measurements. This approach is possible because of a recently developed technique to accurately and efficiently simulate borehole density measurements. The inversion-based interpretation method consists of first detecting bed boundaries from short-spacing detector or bottom-quadrant compensated density by calculating their variance, representative of the measurement inflection point, within a sliding window. Subsequently, a correlation algorithm calculates dip and azimuth from the density image. Depth shifts that vary azimuthally and depend on relative dip angle, together with the effective penetration length of each sensor, refine previously selected bed boundaries. Next, the inversion method combines sector-based density measurements acquired at all measurement points along the well trajectory to estimate layer-by-layer densities. In the presence of standoff, the method excludes upper sectors most affected by standoff to reduce inaccuracies due to borehole mud. To verify the reliability and applicability of the inversion method, we first use forward simulations to generate synthetic density images for a model constructed from field data. Results indicate that inversion improves the interpretation of azimuthal density data as it consistently reduces shoulder-bed effects. Inversion results obtained from field measurements are appraised by quantifying the corresponding integrated porosity-meter yielded by inversion methods in comparison to standard techniques that use simple cutoffs on field-processed compensated density. Integrated porosity-meter of inverted synthetic density measurements increases by 4.6% with respect to noninverted field measurements. Also, integrated porosity-meter obtained from inversion results that include only bottom sectors improved by 65.4% with respect to that calculated with field-compensated, bottom-quadrant density measurements.

Fast modeling of gamma-gamma density measurementsvia gamma-ray point-kernel approximations

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. D57-D72 ◽  
Author(s):  
Mathilde Luycx ◽  
Carlos Torres-Verdín
Keyword(s):  
Monte Carlo ◽  
Cross Sections ◽  
Gamma Ray ◽  
Forward Modeling ◽  
Reaction Cross ◽  
Second Order ◽  
Complex Geometries ◽  
First Order ◽  
Sensitivity Functions ◽  
Gamma Density

Forward-modeling algorithms based on flux sensitivity functions are commonly recognized as fast, reliable, and the most efficient way to implement inversion-based interpretation algorithms for borehole nuclear measurements. Second-order sensitivity functions enhance the accuracy of fast-forward-modeling algorithms in complex geometries: In the presence of standoff, density accuracy is improved up to 70% compared with first-order approximations. However, second-order sensitivity functions can only be generated with the Monte Carlo [Formula: see text]-Particle code for perturbations in bulk density, material composition, and reaction cross sections; therefore, their use is limited to gamma-gamma borehole density measurements. We have developed an alternative method to second-order approximations in complex 3D geometries. It is the first step toward future improvements to simulate borehole environmental effects across arbitrary well trajectories for nuclear measurements based on coupled neutron and gamma-ray transport. The gamma flux-difference (GFD) method quantifies gamma-ray flux perturbations using exponential point kernels and Rytov approximations. Gamma-ray point kernels are corrected for flux buildup and flux perturbations caused by radial heterogeneities, i.e., standoff. Correction coefficients are calculated by flux-fitting 1D radial sensitivity functions yielded by MCNP to the 1D exponential gamma-ray kernel; they depend on standoff and mud density, but they are negligibly affected by formation properties. The GFD method is benchmarked against Monte Carlo calculations. Compared with first-order approximations, it improves simulated density accuracy across regions of significant contrasting properties, up to [Formula: see text] with 3.18 cm (1.25 in) standoff and freshwater mud. The GFD method yields a maximum density error of [Formula: see text] across complex geometries and up to up to 4.45 cm (1.75 in) standoff, similar to that achieved by second-order forward modeling algorithms. Moreover, the principles behind GFD approximations can be adapted to measurements based on coupled neutron and gamma-ray transport.

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