Pulsed Neutron-gamma Research Articles

Spectrometric borehole logging in mineral exploration and mining

Borehole logging tools based on pulsed neutron technology are used to infer geophysical parameters such as bulk density. If fitted with a gamma-ray spectrometer, they may also measure elemental mass fractions of a variety of formation elements. Spectrometric borehole logging tools of this kind are established in the oil and gas industry. With the demand for mineral resources surging, cost- and time-efficient measurement techniques are increasingly needed in mineral exploration. Their ability to detect multiple elements that are building blocks of minerals makes spectrometric borehole logging tools popular in mineral exploration and mining grade control. In this article, the potential of the OreLog pulsed neutron gamma spectrometric downhole logging tool is demonstrated at Erzberg Mountain (Styria, Austria). Erzberg hosts the largest known siderite deposit in the world, and open-pit mining is used to extract iron ore. The aim was to obtain a more detailed and faster geochemical characterization of iron ore deposits prior to blasting. Elemental logs were acquired with highest accuracies for Fe, Ca, and Mn, followed by Si, Mg, K, and S. The tool mapped a 2D cross section of formation element concentration within a few hours, which agrees with orogenetic fault and geologic contact lines. Mineralogical information retrieved from elemental logs confirmed the interlaced mineralogy of the site. It is dominated by a carbonate body, hosting an intergrowth of Mg-rich siderite (sideroplesite) and a solid solution of dolomite/ankerite/kutnohorite. In addition to the tool's potential for enhanced real-time mining control, the benefit of continuous depth-resolved elemental logs is seen in the possibility to deduce a more refined and reliable geochemical and mineralogical model of the deposit. The information-rich data stream opens applications beyond grade control and exploration, such as data assimilation with gravity models or geologic model building.

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.

Experimental Investigation of the Effects of Reactor Neutron-Gamma Pulse Irradiation on SiGe HBTs Under Different Bias Conditions

The degradation characteristics of silicon-germanium heterojunction bipolar transistors (SiGe HBTs) under different bias modes (forward, cutoff and saturation) during irradiation were reported after multiple pulsed neutron-gamma irradiation at room temperature. The radiation-sensitive parameters of the test samples, including base current IB, collector current IC and DC current gain β, were measured and compared before and after every reactor n-γ pulse irradiation. The test results show that IB increased with increasing fluence, and IC slightly increased in the low base-emitter voltage VBE region (approximately from 0.4 V to 0.5 V) and decreased in the high-VBE region (approximately VBE>0.5 V). Moreover, the degradation degree of the test samples was different under different bias conditions. The performance of the test samples under cutoff bias mode displayed the most serious degradation, while those under forward bias mode suffered minimum damage. Meanwhile, the time-dependent annealing characteristics of the DC current gain for SiGe HBTs at various bias conditions were compared and analyzed.

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.

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.

Алгоритм предобработки сигнала импульсного нейтронного гамма-каротажа с учетом статистической зависимости соседних элементов спектра

Алгоритм предобработки сигнала импульсного нейтронного гамма-каротажа с учетом статистической зависимости соседних элементов спектра

A method for evaluating gas saturation with pulsed neutron logging in cased holes

Tight gas reservoirs have been one of the most important field of gas exploration and development, and the dynamic monitoring of gas saturation is of great significance to the adjustment of oilfield development scheme. Pulsed neutron logging technology plays an important role in the evaluation of gas saturation in cased holes. The low porosity of tight gas reservoir presents a new challenge to the application of pulsed neutron logging technology. In order to further investigate the measurement information of pulsed neutron logs to improve the performance of gas saturation measurement, the Monte Carlo numerical simulation method is used to study the gas saturation responses of different parameters obtained by the pulsed neutron-gamma logging instrument with an extra gamma-ray detector added. The results show that the capture count ratio measured by the instrument is mainly affected by changes in the hydrogen index of the formation. The ratio of pure inelastic gamma rays is more sensitive to the variation of formation density. When the porosity of the reservoir is fixed, the gas layer has a low hydrogen index and low density relative to the water layer. By combining the response characteristics of the two ratios, a new gas saturation evaluation parameter (GSEP) is proposed to evaluate gas saturation. The new method displays a higher gas sensitivity compared with the conventional ratio method and macroscopic capture cross section (Sigma) method. In addition, the response characteristics of GSEP in different well logging environments are also studied. It displays GSEP is less affected by the variation of the formation water salinity and clay contents compared with the Sigma method. At last, field examples are presented to validate the effectiveness of the proposed method.

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.

Monitoring of well integrity by magnetic imaging defectoscopy (MID) at the Ketzin pilot site, Germany

At the Ketzin CO2 storage site, Germany, an interdisciplinary, comprehensive monitoring concept focusses on the storage complex, the overburden, the surface and the wellbores. Well integrity as one key requirement for safe storage operation has been monitored by combined video inspection, pulsed neutron gamma logging PNG, magnetic imaging defectoscopy MID and continuous annuli pressure monitoring. The time-lapse MID derived casing thicknesses are within the production specifications, conform to the API standards, and provide no hints to time dependent changes or to any other signs of corrosion. The comprehensive monitoring data indicate well integrity throughout the entire Ketzin life-cycle.

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.

Solid fuel quality control by the pulsed neutron–gamma method

The possibility of controlling the ash content of fuel based on a carbon/oxygen ratio taking into account variable carbon content out of the organic matter was demonstrated. An algorithm was proposed for the determination of the ash content with the use of a pulsed neutron method with the detection of gamma rays from inelastic neutron scattering on the nuclei of carbon, oxygen, and calcium with an instrumental correction for the variable chemical composition of the mineral matter of coals. The procedure was recommended for the operational quality control of coarse dispersed fuel whose chemical composition varies over wide limits.

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

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

A method to evaluate hydraulic fracture using proppant detection

Accurate determination of the proppant placement and propped fracture height are important for evaluating and optimizing stimulation strategies. A technology using non-radioactive proppant and a pulsed neutron gamma energy spectra logging tool to determine the placement and height of propped fractures is proposed. Gd2O3 was incorporated into ceramic proppant and a Monte Carlo method was utilized to build the logging tools and formation models. Characteristic responses of the recorded information of different logging tools to fracture widths, proppant concentrations and influencing factors were studied. The results show that Gd capture gamma rays can be used to evaluate propped fractures and it has higher sensitivity to the change of fracture width and traceable proppant content compared with the exiting non-radioactive proppant evaluation techniques and only an after-fracture measurement is needed for the new method; The changes in gas saturation and borehole size have a great impact on determining propped fractures when compensated neutron and pulsed neutron capture tool are used. A field example is presented to validate the application of the new technique.

Mathematical modeling problems connected with pulsed neutron-gamma logging

The work is devoted to direct and inverse problems of the transport equation in the context of a nuclear geophysical technology based on pulsed neutron-gamma logging of inelastic scattering (PNGL-IS). In the first part of the paper we analyze the distribution of fast neutrons from a pulsed source of 14.1 MeV and study distributions of gamma-quanta of inelastic scattering. The particle distributions are computed by the Monte Carlo methods. In the second part of the paper we consider the problem of evaluating the elemental composition of the rock from the PNGL-IS measurement data. In its solution we use the method of successive approximations over characteristic interactions, which can be classified as a simple iteration; at each iteration step we solve the corresponding direct problem for the system of neutron and gamma-quantum transport equations. The main aspects of the employed method and the results of the numerical experiments that prove the convergence to the exact solution are presented.

Numerical study on determining formation porosity using a boron capture gamma ray technique and MCNP

Formation porosity can be determined using the boron capture gamma ray counting ratio with a near to far detector in a pulsed neutron-gamma element logging tool. The thermal neutron distribution, boron capture gamma spectroscopy and porosity response for formations with different water salinity and wellbore diameter characteristics were simulated using the Monte Carlo method. We found that a boron lining improves the signal-to-noise ratio and that the boron capture gamma ray counting ratio has a higher sensitivity for determining porosity than total capture gamma.

Monitoring of saturation changes and salt precipitation during CO2 injection using pulsed neutron-gamma logging at the Ketzin pilot site

During injection of CO2, monitoring of the subsurface saturation changes is required. For well logging in cased boreholes only a limited number of techniques such as radiometric pulsed neutron-gamma (PNG) logging are applicable. The conventional PNG saturation model mainly considers a displacement process. But during CO2 injection additional processes such as evaporation and salt precipitation are expected to occur as a result of the mutual solubility between brine and CO2. For this purpose an extended PNG saturation model for NaCl-brines is developed and applied to a time-lapse PNG monitoring data set from the Ketzin site. The results show that for the observation well further away from the injection well, the conventional displacement saturation model is valid, with average CO2 saturations below 60%. In contrast, the data from the injection well shows that both evaporation and salt precipitation have occurred. Here, the largest CO2 saturations with values up to 100% are determined locally. The results of the extended saturation model indicate that dry-out regions, where only CO2 and halite with saturations up to 1.4% exist, and maximum halite saturations up to 14.1% occur in the vicinity of the brine levels. The halite saturation distribution in the injection well seems to be controlled by changes in the injection regime associated with changing brine levels, lithological heterogeneities, and capillary effects. PNG monitoring in combination with the extended saturation model is suited to determine displacement and evaporation/precipitation processes for CO2 storage operations.

Surface-downhole electrical resistivity tomography applied to monitoring of CO2 storage at Ketzin, Germany

Surface-downhole electrical resistivity tomography (SD-ERT) surveys were repeatedly carried out to image [Formula: see text] injected at the pilot storage Ketzin, Germany. The experimental setup combines surface with downhole measurements by using a permanent electrode array that has been deployed in three wells. Two baseline experiments were performed during the site startup and three repeat experiments were performed during the first year of CO2 injection. By the time of the third repeat, approximately 13,500 tons of [Formula: see text] had been injected into the reservoir sandstones at about 650 m depth. Field data and inverted resistivity models showed a resistivity increase over time at the [Formula: see text] injector. The lateral extent of the related resistivity signature indicated a preferential [Formula: see text] migration toward the northwest. Using an experimental resistivity-saturation relationship, we mapped [Formula: see text] saturations by means of the resistivity index method. For the latest repeat, [Formula: see text] saturations show values of up to 70% near the injection well, which matches well with [Formula: see text] saturations determined from pulsed neutron-gamma logging. The presence of environmental noise, reservoir heterogeneities, and irregularities in the well completions are the main sources of uncertainty for the interpretations. The degradation of the permanently installed downhole components is monitored by means of frequently performed resistance checks. In consistency with the SD-ERT data, these resistance checks indicate a long-term resistivity increase near the [Formula: see text] injector. In conclusion, the investigations demonstrate the capability of surface-downhole electrical resistivity tomography to image geologically stored [Formula: see text] at the Ketzin site.

Sensitivity study of pulsed neutron-gamma saturation monitoring at the Altmark site in the context of CO2 storage

The injection of CO2 into depleted natural gas reservoirs has been proposed as a promising new technology for combining enhanced gas recovery and geological storage of CO2. During the injection, application of suitable techniques for monitoring of the induced changes in the subsurface is required. Observing the movement and the changes in saturation of the fluids contained in the reservoir and the confining strata is among the primary aims here. It is shown that under conditions similar to the Altmark site, Germany, pulsed neutron-gamma logging can be applied with limitations. The pulsed neutron-gamma method can be applied for detection and quantification of changes in brine saturation and water content, whereas changes in the gas composition are below the detection limit. A method to account for the effects of salt precipitation resulting from evaporation of residual brine is presented.