Neutron-gamma Density Research Articles

On density measurement using various radiation sources: Cs-137, X-ray and pulsed neutron

The use of controllable sources for density measurement is gaining more attention as a safer alternative to chemical sources, since chemical sources pose risks to both humans and the environment. Nuclear density tools using electronically controlled sources, such as X-ray tubes and pulsed neutron generators, are being researched as a replacement for radiological gamma sources such as Cs-137. This study, based on the real gamma density tool structure, optimizes the design of the X-ray and pulsed neutron-gamma density tools, providing a reference for the optimization of source-less density tools. Firstly, the study gives a comparison of density measurement using three sources through a combination of theory and simulation results. And the performance of the tools is evaluated using indicators such as detection efficiency, formation density sensitivity, and depth of investigation. Finally, based on the characteristics of different sources, the aforementioned indicators are balanced to obtain the optimal value of the source-to-detector distance, thus the structure of the X-ray density tool and pulsed neutron-gamma density tool are optimized.

Improving the pulsed neutron-gamma density method with machine learning regression algorithms

Formation bulk density is one of the critical parameters for formation evaluation. As a safe and environmentally friendly method, the pulsed neutron-gamma density (NGD) measurement is emerging as an alternative to the traditional chemical source-based gamma-gamma density (GGD) measurement. However, in the NGD measurement, the initial energy spectrum, source intensity, and spatial distribution of the secondary inelastic gamma-ray source vary with formation components. Moreover, the energy of inelastic gamma rays is at the MeV level, leading to a non-negligible pair production effect. All these factors affect the accuracy of the bulk-density measurement. In addition, the impact of borehole configuration on NGD is evident, and correction for borehole effects is essential. To improve density accuracy: first, we forgo exploring an explicit theoretical formula for density calculation and instead treat the NGD mathematically as a regression problem and introduce the machine learning regressor, a powerful and popular tool for solving regression problems, into the NGD for the first time; second, we select features less affected by changes in formation chemical composition as input features. Our results show that the final three tuned machine learning regressors selected from the 39 candidate regressors outperform the optimized polynomial model (an optimization of conventional density calculation models) in both accuracy and generalization ability. They complete density prediction and borehole correction in one step, avoiding complex exploration of borehole effects as in the optimized polynomial model and dramatically simplifying the borehole correction process. Moreover, the GaussianProcessRegressor can complete borehole correction without the standoff information and perform well, which is impossible for the optimized polynomial model, broadening application scenarios.

Bulk Density Response and Experimental Study of Pulsed Neutron-Gamma Density Logging

The pulsed neutron-gamma density logging technique is used to measure the bulk density of formations based on the detection of gamma rays from the inelastic scattering of neutrons in the formations. However, the induced gamma ray source is regarded as a function of neutron transport and cannot be considered a “point” source. Due to the high energy level of gamma rays, the attenuation of inelastic gamma rays is affected by Compton scattering and pair production. Therefore, bulk density can be measured using inelastic gamma rays while considering the effects of neutron transport and pair production. In this article, a novel density measurement method that uses a completely different response model is proposed to improve the accuracy of density measurement. The process of neutron-gamma density measurement is divided into the neutron transport group and the gamma ray transport group in accordance with the neutron-gamma coupled field theory. A novel density estimation algorithm is derived from the diffusion equation and the gamma ray attenuation law. The accuracy and specification of density measurement are investigated through the Monte Carlo simulation and the calibration of test pits. Theoretical and experimental analyses show that the neutron transport and gamma ray transport are not entirely independent of each other in the pulsed neutron-gamma density measurement. The newly developed model can effectively enable the inelastic gamma rays to conform to the gamma ray attenuation law and keep the measurement accuracy at ±0.025 g/cm3. Moreover, neutron-gamma density is insensitive to the porosity and lithology of the formation. The proposed novel algorithm successfully establishes the calculation model for the relationship between inelastic gamma rays and bulk density, providing a new perspective for density measurement in pulsed neutron-gamma density logging.

An alternative method for sourceless density measurement with boron sleeve gamma detectors

Neutron gamma density measurement using dual gamma detectors (NGD-GD) is a popular density measurement method. Compared with other NGD methods, the NGD-GD method has many advantages in tool optimization, cost control, and density precision; however, it is easily affected by the salinity of formation water. To improve the NGD-GD method, the influence mechanism of formation water salinity was discussed, and a feasible improvement scheme using characteristic gamma information was proposed. A special boron sleeve gamma detector was designed to record the capture gamma rays from the boron-10 element. By analyzing the gamma spectrum, pure boron-10 gamma information was extracted from the total capture gamma spectrum for density calculation. Finally, a new NGD method with boron sleeve gamma detectors (NGD-BD) was developed. Results depict that although the NGD-BD method has a slight decrease in density accuracy and precision compared with the NGD-GD method, its performance is significantly improved in high-brine formations, and oil and gas formations. In addition, the NGD-BD method exhibits better performance in terms of density accuracy, density precision, and environmental impact than another NGD method that uses hydrogen capture gamma information. Moreover, the NGD-BD method is easily implemented with low cost and has little impact on existing NGD tools, and hence, it can coexist with other NGD methods to improve logging quality. Overall, the NGD-BD method is a very promising alternative to the NGD-GD method.

Application analysis on the different neutron gamma density (NGD) logging methods

Neutron gamma density (NGD) logging is the most promising alternative to the traditional density logging (GGD), which is of significance for resolving the radiation and safety issues in oil industry. However, due to the different HI correction methods, multiple NGD methods based on the fast neutron, thermal neutron, and capture gamma detection coexist in the well-logging field, and show considerable differences in the tool specifications. To clarify these differences and guide the NGD development, three typical NGD methods using the fast neutron count ratio, thermal neutron count ratio, and capture gamma count ratio (abbreviated as NGD-FC, NGD-TC, and NGD-CC methods) are selected as representatives for comparative study. Using the Monte Carlo simulation, an integrated NGD tool model was established for studying the differences of three NGD methods in the logging responses, data processing methods, and environmental applications. Research shows that, although the three NGD methods have different measurement systems and data processing methods, the three methods can get rid of the HI effect and obtain accurate formation density. The changes of wellbore size and wellbore fluid have similar and significant impact on the three NGD methods and lead to large density errors, especially for the large-size wellbore or wellbore gas conditions. In the different lithology conditions, three methods have good performances, but the NGD-FC and NGD-CC methods have smaller density errors than NGD-TC method. Compared to the other two NGD methods, the NGD-FC method also has a perfect performance in the oil or gas-saturated formation, while NGD-TC and NGD-CC methods have extremely large errors in the gas-saturated formation. Besides, the NGD-FC method are hardly affected by the formation water salinity, the NGD-TC method is slightly affected, while the NGD-CC method is greatly affected. This study can provide a guidance for the tool design, data processing and environment correction of the NGD technology.

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

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.

Neutron transport correction and density calculation in the neutron-gamma density logging

Formation density is one of the most important parameters in formation evaluation. The radioisotope chemical sources are widely used in conventional nuclear logging tools. Considering security and environmental risks, the pulsed neutron generators have successfully replaced the Am–Be source in neutron porosity logging tools. However, the Cs-137 is still mainly used for bulk density measurement. Recently, there is a growing interest in measuring bulk density with the pulsed neutron generators. Although the neutron-gamma density (NGD) is also calculated by gamma-ray attenuation, the neutron transport correction is needed for neutron-induced inelastic gamma rays. A new method of NGD calculation is developed based on the coupled-field theory. This new method does not require additional neutron transport correction. Additionally, a structural model of NGD logging tool is designed by Monte Carlo method, which includes two gamma-ray detectors and two epithermal neutron detectors. According to the proposed method, the corrected inelastic gamma-ray count rate response can eliminate the effect of neutron transport and meet the law of gamma-ray attenuation. The Monte Carlo simulation results indicate that the accuracy of NGD measurement is 0.02 g/cm3, which is close to the conventional gamma-gamma density logging (0.015 g/cm3). The NGD measurement is insensitive to porosity and formation fluids in the pore space.

A comparative study on the neutron-gamma density and gamma-gamma density logging

With the increasing demand for the radioisotope-free operations, using pulsed neutron gamma density (NGD) instead of gamma-gamma density (GGD) has become increasingly important for the development of logging while drilling (LWD) technology. However, due to the different logging mechanisms, there are huge differences between GGD and NGD in the instrument designs, measurement methods, and instrument specifications. To further promote the NGD development, a comparative study on GGD and NGD was carried out from theoretical and simulation aspects. Based on the theoretical methods, the differences between GGD and NGD in the gamma sources, field distribution, methods for density measurement and instrument specifications were systematically elaborated. Then, using the Monte Carlo simulation, the GGD and NGD models were built to quantitatively verify the theoretical results. Last, the detailed performances of NGD and GGD methods in different logging environments were compared. Results showed that the theoretical results are highly consistent with the simulation results. The density sensitivity of NGD is less than half that of GGD, but the depth of investigation (DOI) approaches twice that of GGD. The density precision and vertical resolution of NGD are less than those of GGD. Additionally, compared to GGD, NGD also has an excellent performance in different pore fluid and lithology formations, but it is easily affected by borehole factors. The research can provide quantitative performance evaluations for the NGD replacing GGD in LWD logging.

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

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.

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

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 method for determining density based on gamma ray and fast neutron detection using a Cs2LiYCl6 detector in neutron-gamma density logging

With the increasing demand for radioisotope-free operations, pulsed neutron-gamma density (NGD) has become increasingly important for logging-while-drilling (LWD) development. However, current NGD tools, adopting the multiple-detector array design, are not conducive to the simplification of instrument design and measurement system. To break obstacles, based on the fast neutron-gamma coupled theory, a new density measurement method was proposed. Further, combined with the neutron-gamma simultaneous detection characteristics of the Cs2LiYCl6 (CLYC) detector, an NGD measurement system consisting of a D-T source and one CLYC detector was used. Results show that the new method is capable of determining formation density using a single CLYC detector, which can not only avoid complex instrument systems but also improve density sensitivity. Moreover, the applicability of the new density method was well verified by Monte Carlo simulation. Additionally, the method was successfully applied in a simulated well, and density results are in good agreement with the benchmarked formations. The research provides theoretical guidance for NGD instrument design.

A method to describe inelastic gamma field distribution in neutron gamma density logging

Pulsed neutron gamma density logging (NGD) is of great significance for radioprotection and density measurement in LWD, however, the current methods have difficulty in quantitative calculation and single factor analysis for the inelastic gamma field distribution. In order to clarify the NGD mechanism, a new method is developed to describe the inelastic gamma field distribution. Based on the fast-neutron scattering and gamma attenuation, the inelastic gamma field distribution is characterized by the inelastic scattering cross section, fast-neutron scattering free path, formation density and other parameters. And the contribution of formation parameters on the field distribution is quantitatively analyzed. The results shows the contribution of density attenuation is opposite to that of inelastic scattering cross section and fast-neutron scattering free path. And as the detector-spacing increases, the density attenuation gradually plays a dominant role in the gamma field distribution, which means large detector-spacing is more favorable for the density measurement. Besides, the relationship of density sensitivity and detector spacing was studied according to this gamma field distribution, therefore, the spacing of near and far gamma ray detector is determined. The research provides theoretical guidance for the tool parameter design and density determination of pulsed neutron gamma density logging technique.

Exploring for oil with nuclear physics

Oil↓eld service companies help identify and assess reserves and future production for oil and gas reservoirs, by providing petrophysical information on rock formations. Some parameters of interest are the fraction of pore space in the rock, the quantity of oil or gas contained in the pores, the lithology or composition of the rock matrix, and the ease with which 'uids 'ow through the rock, i.e. its permeability. Downhole logging tools acquire various measurements based on electromagnetic, acoustic, magnetic resonance and nuclear physics to determine properties of the subsurface formation surrounding the wellbore. This introduction to nuclear measurements applied in the oil and gas industry reviews the most advanced nuclear measurements currently in use, including capture and inelastic gamma ray spectroscopy, neutron-gamma density, thermal neutron capture cross section, natural gamma ray, gamma-gamma density, and neutron porosity. A brief description of the technical challenges associated with deploying nuclear technology in the extreme environmental conditions of an oil well is also presented.

Optimized Design of Spacing in Pulsed Neutron Gamma Density Logging While Drilling

Optimized Design of Spacing in Pulsed Neutron Gamma Density Logging While Drilling

Study on the impact of pair production interaction on D-T controllable neutron density logging

This paper considers the effect of pair production on the precision of D-T controllable neutron source density logging. Firstly, the principle of the traditional density logging and pulsed neutron density logging are analyzed and then gamma ray cross sections as a function of energy for various minerals are compared. In addition, the advantageous areas of Compton scattering and pair production interactions on high-energy gamma ray pulse height spectrum and the errors of a controllable source density measurement are studied using a Monte Carlo simulation method. The results indicate that density logging mainly utilizes the Compton scattering of gamma rays, while the attenuation of neutron induced gamma rays and the precision of neutron gamma density measurements are affected by pair production interactions, particularly in the gamma rays with energy higher than 2MeV. By selecting 0.2−2MeV energy range and performing proper lithology correction, the effect of pair production can be eliminated effectively and the density measurement error can be rendered close to the precision of chemical source density logging.

Formation Evaluation-While-Drilling Technology Improves Data Delivery

Young Technology Showcase Today’s drilling engineering teams are continually looking for better ways to mitigate risk and improve health, safety, and environmental conditions at the rig site. Many operators would prefer to acquire a comprehensive suite of petrophysical measurements while drilling—without incurring the risk and cost associated with deployment of chemical nuclear sources. Traditionally, both wireline and logging-while-drilling (LWD) tools have depended primarily on radioisotopes such as americium-beryllium to generate high-energy neutrons for porosity measurements, and cesium to generate gamma rays for bulk density measurements. However, transportation, storage, handling on the rig floor, and potential abandonment of radioactive sources at the wellsite should the tools become stuck require special safety precautions and costly, time-consuming procedures. In remote geographic regions, mobilizing chemical sources can be slow and logistically complex. If a bottomhole assembly (BHA) with a chemical source becomes stuck, complicated fishing procedures can waste precious rig time, and delay eventual production of oil and gas. To meet the growing industry need for an alternative to radioactive chemical sources, Schlumberger recently released NeoScope sourceless formation evaluation-while-drilling solution (Fig. 1). This technology is unique in three ways. It is the first LWD tool to introduce a completely new nuclear measurement to the industry—sourceless neutron gamma density (SNGD). It is the only tool in the industry that provides formation density and neutron measurements without the need for chemical nuclear sources. Finally, it is the first tool to provide collocated density and neutron measurements (Fig. 2). Development and Field Testing LWD tools have typically used two chemical nuclear sources to obtain neutron porosity and bulk density. In partnership with Japan Oil, Gas, and Metals National Corporation, Schlumberger first developed and deployed a pulsed neutron generator (PNG) for commercial standard neutron porosity, spectroscopy, and sigma measurements 7 years ago, eliminating the need for one of the chemical sources (241AmBe). However, development of SNGD was not yet mature enough for commercial use. The only way to acquire a reliable density measurement was still to use a cesium source. Hence, it remained impossible for operators to obtain a comprehensive sourceless suite of petrophysical data while drilling. Ongoing research and engineering led to modifications of existing technology, design of new detectors, and development of sophisticated characterization algorithms and calibrations to provide the first commercial SNGD measurement—a true breakthrough. Since 2005, extensive laboratory modeling and experimentation, as well as more than 200 field tests in more than 30 countries have been conducted to verify and refine the new SNGD measurement. Tests have been carried out in a wide range of formations under many different environmental conditions. Comparisons of the new PNG-based density measurement with conventional bulk densities derived from chemical sources showed good correlations both in carbonates and sandstone formations, onshore and offshore (Fig. 3). Field testing extended through early 2012, and the commercial solution was officially released in June.

Accuracy and borehole influences in pulsed neutron gamma density logging while drilling

A new pulsed neutron gamma density (NGD) logging has been developed to replace radioactive chemical sources in oil logging tools. The present paper describes studies of near and far density measurement accuracy of NGD logging at two spacings and the borehole influences using Monte-Carlo simulation. The results show that the accuracy of near density is not as good as far density. It is difficult to correct this for borehole effects by using conventional methods because both near and far density measurement is significantly sensitive to standoffs and mud properties.