Gamma-gamma Density Research Articles

Detecting hydrodynamic barriers of reservoirs based on drilling of horizontal wells to refine geological models

During the geological modeling process, it can be problematic to substantiate predicted faults or barriers that are estimated from seismic data and require additional research. However, logging data from horizontal wells (LWD — logging while drilling), whose trajectories intersect predicted seismic faults or hydrodynamic barriers, can be used to validate such barriers.The aim of this study is to substantiate hydrodynamic barriers in several fields in Western Siberia using data from a complex of geophysical surveys conducted while drilling horizontal wells that pass through such barriers. The main research method is azimuthal gamma-gamma density logging and its interpretation to identify hydrodynamic barriers, including tectonic faults, based on the criteria described in A. V. Chashkov's, S. Z. Mutaev's and other scholars' works.This study presents the practical application of the azimuthal gamma-gamma density logging method to substantiate predicted seismic faults, estimated from seismic data and require additional research, using the example of the First field. This model has been examined and accepted by the State Reserves Committee.

Geophysical technology for determining the ground parameters (methods and apparatus)

At the Institute of Geophysics of the National Academy of Sciences of Ukraine a modern technology for ground investigation has been created for solution of the engineering geological problems, including seismic microzonation. Technology is based on the methods and apparatus of radioactive logging (RL), which involves neutron-neutron logging (NNL), gamma-gamma density logging (DL) and gamma-ray logging (GR).
 The paper presents the results of the development of radioactive logging apparatus as an integral part of the new technology. Set of prototype dual-channel tools, namely 2NNL (dual-spacing NNL) and DL+GR, prototype three-component tool 2NNL+DL+GR, surface control and registration console were developed and produced on the basis of the modern elemental base.
 The 2NNL tool makes it possible to determine neutron porosity in two ways: single-sonde method and compensation method. Compensation neutron logging, in particular, aims to determine the porosity in the presence of anomalous neutron absorbers in the rock, and also, in combination with the single-sonde method, to estimate the content of anomalous absorbers.
 GR-channel of the DL+GR tool was equipped the same gamma-ray detector as the DL-channel. Here, the GR-detector performs a dual function: 1) integral registration of natural gamma-ray of ground, 2) account of natural background in the total readings of the DL-detector.
 The three-component radioactive logging tool combines three RL-methods and includes all the advantages of dual-channel tools. The obtaining a results in one trip by this tool is particularly important for relatively deep boreholes.
 According to the experimental results, the optimal intervals between sources and detectors of radiation in the created combined RL tools were established. The importance of adjustment and controlling the signals of sensors of the RL tools is shown; the new console allows to perform these procedures operational.
 The created apparatus increases the productivity and efficiency of logging operations by reducing the number of trips, digital recording, storage and transmission of information, and by using of a computer programs for processing and interpreting the results of borehole measurements. The effectiveness of the developed apparatus, together with the appropriate metrological and interpretation-methodical support, has been demonstrated on specific examples of borehole investigations and confirmed by independent laboratory data.
 The technology allows to determine the following engineering geophysical parameters: total density, dry ground density, content of shale, porosity, volume moisture, water saturation factor, groundwater level, etc.

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.

WELL LOGGING INTERPRETATION FOR KASHIR AND PODOLSK DEPOSITS USING NEURAL NETWORKS

The kashir and podolsk horizons of the S oilfield are represented by carbonate deposits. They are characterized by limestone and dolomite facies. The kashir and podolsk deposits petrophysical model is developed taking into account the variety of void space types and lithological composition. The success of a petrophysical model testing depends not only on the model correctness. It also depends on quality and volume of well logging data. The paper presents a methodology for creating a synthetic curve of gamma-gamma density log using the intelligent analysis module «Neural Networks» of the «Prime» software. The presence of a gamma-gamma density log synthetic allows to clarify the lithology of the section. It will affect the quality of petrophysical model application for well log data analysis. The result of this approach is an additional extraction of effective oil saturated thicknesses, not previously accounted for, due to incomplete well logging data in oilfield S. Comparison of this approach results and development allows to propose an involving priority areas strategy based on the results of clarifying potential zones for drilling due to additional separation of effective oil-saturated reservoirs.

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.

Improvement of the fast simulation of gamma-gamma density well logging measurement

The fast modeling of gamma-gamma density well logging is essential for the inversion techniques of formation properties, which is usually carried out jointly with other logging measurements such as electrical logging. It also can help to adjust the initial geological model in real time during geosteering. The Monte Carlo method is the foremost numerical technique to simulate gamma-gamma density logging measurement. But due to its slow speed, it is not sufficient for inversion or real-time forward modeling. An algorithm to achieve the fast simulation of density logging response is introduced. In the algorithm, a new approximation model is proposed to enable accurate forward modeling of density logging with better efficiency. The Monte Carlo simulation method is utilized as a benchmark to validate the performance of the fast simulation method. The density logging responses under vertical and high-angle well conditions are simulated. The results of the fast simulation show a good agreement with the Monte Carlo simulations in vertical and high-angle wells. In addition, the comparison of density imaging data also confirmed the accuracy of the fast simulation method.

Drillhole rugosity correction for gamma-gamma density tools – A space modelling approach

SummaryThis paper describes a novel approach to solving borehole rugosity for single detector gamma-gamma density logging. Modelling the space investigated by the gamma-gamma density tool provides the basis for deriving an applied rugosity correction.This paper also describes how the same correction models when applied to dual density tool detectors can increase the vertical resolution of the tool revealing otherwise hidden formation heterogeneity.Measurements made with single detector gamma-gamma density tools are accurate non-rugose (smooth) boreholes whereas measurements made in rugose (rough) boreholes are typically biased low, due to the undesirable effects of low-density borehole fluid, water or air sitting between the formation and the source or detector and in the path of gamma rays.

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.

Improvement in the method for borehole caliper measurement based on azimuthal gamma-gamma density well logging

Accurate characterization of the size and shape of the borehole is critical to the effective borehole correction for logging while drilling (LWD) measurements. It is also necessary for the real-time evaluation of geomechanical wellbore stability and for optimizing drilling operation and determining proper completion strategies. The pseudo-caliper measurement can be derived from LWD azimuthal density measurement, where a linear empirical model is employed to calculate the tool standoffs. However, we found that as the tool standoff continues to increase, the model could no longer describe the variation in borehole size accurately. In this paper, the responses of LWD azimuthal density measurement under different logging conditions are studied using the Monte Carlo modeling method. To improve the estimation of borehole geometry, a new model is proposed to determine tool standoffs and measure the borehole caliper. Simulation models with different degrees of borehole enlargement are built to investigate the performance of the model. Compared with the currently used method, the wellbore caliper values calculated using the new method are more consistent with the actual values. A field example with mechanical caliper measured from wireline logs is also presented to validate the effectiveness of the proposed method.

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.

Pulsed-Neutron Comparison of Open- and Casedhole Wells: An Alaskan Case Study

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190062, “A Pulsed-Neutron Comparison Between an Open- and Casedhole Well: An Alaskan Case Study,” by J. Burt, T. Zhou, D.A. Rose, R. Grover, S. Ahmad, and J. Nemec, Schlumberger, and J. Dunston, Hilcorp, prepared for the 2018 SPE Western Regional Meeting, Garden Grove, California, USA, 22–27 April. The paper has not been peer reviewed. This paper compares the results of gas identification and lithology identification using pulsed-neutron spectroscopy in openhole and casedhole environments. Most pulsed-neutron tools are run after casing; this study provides a unique opportunity to examine the effect of casing on spectroscopy by comparing casedhole measurements to measurements taken in the open hole before the casing was run. Introduction Pulsed-neutron logging has evolved over the last 50 years, but the intrinsic physical measurements have remained unchanged, which means that operators cannot obtain a complete picture of the rock and fluids behind casing with conventional tools. However, advances in tool design and a new fast-neutron cross-section (FNXS) measurement provide for an alternative gas-identification technique. Gas in open holes is typically identified from neutron porosity and gamma-gamma density crossover. In casedhole environments, gamma-gamma density measurements are challenging because of the large casing and cement corrections needed. Previous gas identification in casedhole environments has relied on the formation hydrogen index (HI) or neutron porosity (TPHI) log and sigma. In openhole environments, density and neutron porosity crossover is a typical gas identifier, but, in many instances, shale can mask the identification of gas. This is a common problem in some gas reservoirs in Alaska, and it leads to ambiguous interpretations about the gas saturation and potential producibility of different zones. Gas identification in casedhole environments is even more complicated because the density measurement is not commonly available. The FNXS measurement responds primarily to formation atom density, for which most rocks, clays, and liquids have similar values. Comparatively, gas has a low atom density, and its presence will make the FNXS measurement read low. Thus, a gas pay zone can be differentiated from tight zones by the shift toward lower FNXS values. Also, the difference in FNXS between clean lithologies and clay is less than for sigma and TPHI, so FNXS, which is less affected by variable clay content, can be a more-robust gas indicator when variable clay is present.

Near Real-Time Classification of Iron Ore Lithology by Applying Fuzzy Inference Systems to Petrophysical Downhole Data

Fluctuating commodity prices have repeatedly put the mining industry under pressure to increase productiveness and efficiency of their operations. Current procedures often rely heavily on manual analysis and interpretation although new technologies and analytical procedures are available to automate workflows. Grade control is one such issue where the laboratory assay turn-around times cannot beat the shovel. We propose that for iron ore deposits in the Pilbara geophysical downhole logging may provide the necessary and sufficient information about rock formation properties, circumventing any need for real-time elemental analysis entirely. This study provides an example where petrophysical downhole data is automatically classified using a neuro-adaptive learning algorithm to differentiate between different rock types of iron ore deposits and for grade estimation. We exploit a rarely used ability in a spectral gamma-gamma density tool to gather both density and iron content with a single geophysical measurement. This inaccurate data is then put into a neural fuzzy inference system to classify the rock into different grades and waste lithologies, with success rates nearly equal to those from laboratory geochemistry. The steps outlined in this study may be used to produce a workflow for current logging tools and future logging-while-drilling technologies for real-time iron ore grade estimation and lithological classification.

Acoustic televiewer amplitude data for porosity estimation with application to porewater conversion

The ability to detect and quantify rock matrix porosity is explored using acoustic televiewer images and dolostone rock core samples. In particular, an empirical relationship between the acoustic televiewer amplitude response and core-derived total porosity is developed. The performance of the acoustic televiewer amplitude relationship was compared to neutron and gamma-gamma logs, which are traditional nuclear wireline tools commonly used for estimating porosity in dolostones. Acoustic televiewer amplitude results are consistent with the gamma-gamma (density) and neutron (porosity) logs, provide increased vertical resolution, do not require downhole radioactive sources and also provide oriented fracture information. Sample specific porosity and bulk density values from the calibrated acoustic televiewer datasets are then used to improve porewater concentration estimates from rock concentrations.

Solution for geological and technological problems in carbonate sections with developed secondary capacity on the example of Volga-Ural Petroleum and Gas Province deposit

As of today many of the old fields are already depleted. Replenishment of the category of oil and gas reserves from the category of prospective resources proceeds mainly due to revising the state of overor underlying layers in the already developed horizon, rather than by exploiting entirely new deposits. Current methods of allocation of complex reservoirs both according to the data of geological and technological studies and the results of commercial and geophysical studies are relevant. In this paper, solutions of geological and technological problems are considered on the example of the carbonate section of the Volga-Ural Petroleum and Gas Province in absorbing wells. Today, by the methods of borehole geophysics, reservoirs capacity in isotropic environment is effectively calculated. In the presence of fracture and cavity porosity, the standard methods of electrometry prove to be ineffective, even for a qualitative interpretation. Therefore, in the complex reservoir with a secondary porosity space structure, construction of interpretation model is used by the following methods of well logging - neutron method (NM), acoustic method (AK) and gamma-gamma density method (GGM-P). To eliminate absorptions, both standard methods and the latest developments were used. The effectiveness of the use of profile packers in absorbing wells is shown.

A method for inversion of 1D vertical soundings of gravity anomalies

We have developed a method to interpret potential fields, which obtains 1D models by inverting vertical soundings of potential field data. The vertical soundings are built through upward continuation of potential field data, measured on either a profile or a surface. The method assumes a forward problem consisting of a volume partitioned in layers, each of them homogeneous and horizontally finite, but with the density changing versus depth. The continuation errors, increasing with the altitude, are automatically handled by determining the coefficients of a third-order polynomial function of the altitude. Due to the finite size of the source volume, we need a priori information about the total horizontal extent of the volume, which is estimated by boundary analysis and optimized by a Markov chain process. For each sounding, a 1D inverse problem is independently solved by a nonnegative least-squares algorithm. Merging of the several inverted models finally yields approximate 2D or 3D models that are, however, shown to generate a good fit to the measured data. The method is applied to synthetic models, producing good results for either perfect or continued data. Even for real data, i.e., the gravity data of a sedimentary basin in Nevada, the results are interesting, and they are consistent with previous interpretation, based on 3D gravity inversion constrained by two gamma-gamma density logs.

An approach to derive some simple empirical equations to calibrate nuclear and acoustic well logging tools

A set of three pads was constructed from primary materials (sand, gravel and cement) to calibrate the gamma-gamma density tool. A simple equation was devised to convert the qualitative cps values to quantitative g/cc values. The neutron-neutron porosity tool measures the qualitative cps porosity values. A direct equation was derived to calculate the porosity percentage from the cps porosity values.Cement-bond log illustrates the cement quantities, which surround well pipes. This log needs a difficult process due to the existence of various parameters, such as: drilling well diameter as well as internal diameter, thickness and type of well pipes. An equation was invented to calculate the cement percentage at standard conditions. This equation can be modified according to varying conditions.

Impact of focused gamma ray beam angle on the response of density logging tool

The response of the gamma-gamma density logging tool with different focused gamma ray beam angle is investigated by using the MCNP code. This work focuses on the four aspects of spatial distribution and energy spectrum of scattered gamma rays, measurement uncertainty, the depth of investigation, and vertical resolution of the logging tool. The results show that the density measurement accuracy can be improved when a relative larger collimator angle is employed, and the vertical resolution of the tool would not get worse.

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.