Pulsed Neutron Spectroscopy Research Articles

Uranium in Source Rocks: Role of Redox Conditions and Correlation with Productivity in the Example of the Bazhenov Formation

The paper reports comprehensive analysis of different factors affecting uranium content in oil source rocks and the relationship between uranium content and productivity of source rocks. The analysis of data for 13 wells of the Bazhenov Formation (Western Siberia, Russia) was carried out. The uranium content of the rocks was measured by gamma-ray spectrometry on core samples. In order to analyze factors affecting uranium accumulation in source rocks, we studied content and characteristics of organic matter (Rock-Eval pyrolysis), and also mineral, element, and isotope composition of rocks. We have shown that redox conditions at the sedimentation stage have the most pronounced impact on the uranium accumulation in the rocks of the Bazhenov Formation. It was also shown that productive intervals, containing increased amounts of mobile hydrocarbons, are characterized by low (<20 ppm) concentration of uranium. However, the intervals, containing phosphorite minerals may show better reservoir properties and oil saturation at higher concentration of uranium. The analysis of correlations and relationships between uranium content and Rock-Eval pyrolysis indexes (oil saturation index and productivity index) enabled formulation of criteria for selection of oil-saturated intervals using the spectral gamma and pulsed neutron spectroscopy log data.

Petrophysical analysis and mudstone lithofacies classification of the HRZ shale, North Slope, Alaska

Mudstone is vertically and laterally heterogeneous in terms of variations in mineral composition, organic carbon content, and petrophysical properties. The purpose of this study is to classify mudstone lithofacies within the High Radioactive Zone (HRZ) Shale on the North Slope, Alaska, based upon mineralogy, total organic carbon (TOC), and petrophysical properties to interpret their distributions and depositional/diagenetic environments. We integrate core data from six wells and well logs from 18 wells to analyze the variations in rock properties in the HRZ Shale. Analysis of cores at multiple scales of resolution by techniques, including X-ray diffraction (XRD), trace element geochemistry, pyrolysis, and scanning electron microscopy (SEM) are integrated with conventional and advanced wireline logging suites (including pulsed neutron spectroscopy [PNS]) to map vertical and lateral variation in mudstone lithofacies across the study area. Results show that the HRZ Shale exhibits a high degree of vertical and lateral heterogeneities in mineralogy, TOC, and petrophysical properties. This shale formation is composed of 12 lithofacies: pyritic organic mixed shale, organic mixed shale, pyritic mixed shale, mixed shale, pyritic organic mudstone, organic mudstone, pyritic grey mudstone, grey mudstone, pyritic organic siliceous shale, organic siliceous shale, pyritic siliceous shale, and siliceous shale. Quartz abundance is generally higher in the west, and clay proportions increase from the southwest to the northeast of the study area. The TOC content is variable across the HRZ Shale, ranging from 1.5 to 15 wt%. The lithofacies are influenced by depositional conditions, diagenesis, redox conditions, organic matter productivity, and preservation.

An Integrated Petrophysical Characterization of a Siliciclastic Tight Gas Reservoir in Neuquén Basin, Western Argentina

The Río Neuquén Field is located between Neuquén and Río Negro provinces, Argentina. Historically, it has been a conventional oil producer, but it was converted to a tight gas producer from deeper reservoirs. The targeted geological formations are Lajas, which is already a known tight gas producer, and the less-known overlaying Punta Rosada Formation, which is the main objective of the current work. Punta Rosada presents a diverse lithology, including shaly intervals separating multiple stacked reservoirs that grade from fine-grained sandstones to conglomerates. The reservoir pressure can change from the normal hydrostatic gradient to up to 50% of overpressure. There is little evidence of movable water. The key well in this study has a comprehensive set of openhole logs, including pulsed-neutron spectroscopy data, and is supported by a full core study over 597 ft. Additionally, data from several offset wells were used, containing sidewall cores and complete sets of electrical logs. This allowed the development of rock-calibrated mineral models, adjusting the clay volume with X-ray diffraction data, porosity, and permeability with core measurements, and linking the log interpretation to dominant pore-throat radius models from MICP Purcell tests. Several water saturation models were tested, attempting to adjust the irreducible water saturation with NMR and Purcell tests at reservoir conditions. As a result, three hydraulic units were defined and characterized, identifying a strong correlation with lithofacies observed in cores and image logs. A cluster analysis model allowed the propagation of the facies to the rest of the wells (50). Finally, lithofacies were distributed in a full-field 3D model, guided by an elastic seismic inversion. In the main key well, in addition to the openhole logs and core data, a casedhole pulsed-neutron log (PNL) was also acquired, which was used to develop algorithms to generate synthetic pseudo-openhole logs such as bulk density and resistivity, integrated with the spectroscopy mineralogical information and other PNL data, to perform the petrophysical evaluation. This enables the option to evaluate wells in contingency situations where openhole logs are not possible or too risky, and also in planned situations to replace the openhole data in infill wells, saving considerable drilling rig time during this field development phase. Additionally, the calibrated casedhole model can be used in old wells. This paper explores the integration of different core and log measurements and explains the development of rock-calibrated petrophysical and rock type models addressing the characterization challenges found in tight gas sand reservoirs. The results of this study will be crucial to optimize the field development.

Predicting unconventional reservoir potential from wire-line logs: A correlation between compositional and geomechanical properties of the Duvernay shale play of western Alberta, Canada

Unconventional reservoir performance is assessed and quantified via integration of compositional, rock fabric, and static mechanical property analyses that are routinely performed on drill core or cuttings. This approach has several limitations; for example, it can only be used where drill core and cuttings are available, and comprehensive analysis may be cost prohibitive at the full scale of a resource play. In this contribution, we propose a novel workflow that provides a robust correlation between compositional, mineralogical, and geomechanical properties of unconventional shale plays and wire-line log signature. Our approach enables the extrapolation of compositional and mechanical reservoir properties into areas in which drill core is lacking but wire-line logs are available. We illustrate our workflow using a case study from the Duvernay unconventional shale play in western-central Alberta (Canada). Our analysis reveals a high degree of correlation between core-measured mineral components and two wire-line logs: pulsed neutron spectroscopy (PNS) and spectral gamma ray (SGR). In particular, we show that PNS-derived calcium, aluminum, and silicon concentrations and SGR-derived thorium and potassium concentrations may be used to identify silica-rich, clay-rich, and carbonate-rich intervals, respectively, within the reservoir. We show that these intervals exhibit distinct mechanical properties, suggesting that they are also characterized by distinct hydraulic fracturing efficiency. Since well logs are generally more abundant than drill cores, our approach may prove critical in assessing predrill reservoir potential not only in the Duvernay but in similar unconventional plays worldwide.

Self-Compensated Pulsed-Neutron Spectroscopy Measurements

Formation elemental composition and mineralogy measurements, including organic carbon from recently developed spectroscopy tools, provide critical information for formation evaluation in both conventional and unconventional reservoirs. These measurements can be obtained under conditions by using a slim pulsed-neutron tool with two spectroscopy detectors. One primary limitation is that users must manually provide offsets for the elements (silicon (Si), calcium (Ca), and iron (Fe)) present in casing and cement before performing the oxide closure computation to obtain elemental concentrations. This process is time consuming, and the results could be inaccurate and subjective, especially without any local reference. Another limitation is that the formation element signals are smaller in cased hole than in open hole. This increases the noise in the oxide closure-derived environmental yield-to-weight normalization factor (FY2W), which is propagated to all the elemental weight fractions. A self-compensated spectroscopy algorithm was developed to overcome these two limitations. The key breakthrough is the use of raw measurements with very high precision from the two spectroscopy detectors to predict FY2Ws instead of using the oxide closure or inelastic capture (INCP) closure methods. The capture FY2W is mainly determined by the borehole and formation sigma. It can be characterized by using multiple measured apparent sigma values in different timing gates from multiple detectors, which have different sensitivities to borehole and formation sigma. The inelastic FY2W is mainly determined by the borehole and formation geometry and hydrogen index. It can be characterized by using count rate ratios in both burst-on (inelastic) and burst-off (capture) timing gates from multiple detectors. This method reduces the noise in the FY2Ws by an order of magnitude, which improves the precision of all the final elemental weight fractions. Two independent sets of apparent elemental weight fractions can be calculated from the two spectroscopy detectors. The measured elements from the detector with shorter spacing are more sensitive to the borehole environment, including the casing and cement, whereas the ones from the detector farther away are more sensitive to the formation. This enables self-compensation for casing and cement effects. The new processing can be done without user intervention and results in a more accurate, more precise, and less subjective elemental composition and mineralogy. More than 1,600 laboratory measurements in different conditions were used to characterize the algorithm. Several log examples demonstrate the excellent performance of the new compensated spectroscopy measurements.

Pulsed spallation neutron spectroscopy of low dimensional magnets: past, present, and future

The early 1990s saw the first useful application of pulsed neutron spectroscopy to the study of excitations in low dimensional magnetic systems, with Roger Cowley as a key participant in important early experiments. Since that time the technique has blossomed as a powerful tool utilizing vastly improved neutron instrumentation coupled with more powerful pulsed sources. Here we review representative experiments illustrating some of the fascinating physics that has been revealed in quasi-one and two dimensional systems.

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.

Comparison of supervised and unsupervised approaches for mudstone lithofacies classification: Case studies from the Bakken and Mahantango-Marcellus Shale, USA

Quantitative lithofacies modeling is important to understand the depositional and diagenetic history, and hydrocarbon potential of unconventional resources at a regional scale. The complex heterogeneous nature and large data dimensionality of unconventional mudstone reservoirs increase the challenge of lithofacies interpretation by conventional qualitative methods. Quantitative shale lithofacies, which are meaningful, mappable, and predictable at core, well log, and regional scales, can be defined based on mineralogy and Total Organic Carbon (TOC) derived from core analysis and advanced geochemical spectroscopy logs (e.g. Pulsed Neutron Spectroscopy, PNS). However, access to numerous and widespread core samples and geochemical log responses is typically limited by cost and time.We apply different mathematical techniques to ubiquitous conventional well log suites calibrated to rock types, defined by the limited number of wells with high-quality core and geochemical logs. The documented interrelationships between lithofacies and conventional logs are propagated with a quantified degree of accuracy in wells without advanced log or core data. Our study addresses issues of different approaches of quantitative lithofacies classification and prediction techniques from well logs. Various data-driven supervised and unsupervised computational approaches, such as Support Vector Machine (SVM), Artificial Neural Network (ANN), Self-Organizing Map (SOM) and Multi-Resolution Graph-based Clustering (MRGC), are applied and compared to reduce uncertainty of propagating single-well based lithofacies analysis, and efficiently understand geological trends.Two different dataset from the Devonian Bakken and Mahantango-Marcellus Shale formations in North America are used, in order to undertake a comparative assessment of computational techniques for lithofacies characterization. Original shale lithofacies, defined from geochemical logs and core data, are used to compare the results of selected supervised and unsupervised computational approaches. The results show that both Bakken and Mahantango-Marcellus shale members are vertically and laterally heterogeneous, but can be classified into at least five mudstone lithofacies, along with calcareous siltstone and limestone lithofacies. SVM works better than other techniques for lithofacies classification and prediction in reduced computational time, no iteration, and with highly repeatable results. Accuracy of lithofacies prediction increases if the algorithms are supervised with geological rules.

The hierarchical decomposition method and its application in recognizing Marcellus Shale lithofacies through combining with neural network

Abstract The three dimensional (3-D) distribution of shale lithofacies has been approved to be helpful for recognizing shale gas productive areas at basin and regional scales. To build the 3-D model, the successful prediction of shale lithofacies by conventional logs is critical. Thus, a hierarchical decomposition (HD) method for multi-class classification was proposed and developed through decomposing the original multi-class problem into several simpler sub-problems on the basis of the class hierarchy. This method can effectively combine the mathematical methods and professional knowledge, and also overcome several drawbacks existing in the common decomposition methods, such as one-against-all, one-against-one and error-correcting output code. Due to the feature that the misclassification in upper-level sub-problems will be handed down into the lower-level sub-problems, the error control and recovery algorithms were generated in this HD method, including setting higher threshold in upper levels, deciding classes through voting method based on the K -fold cross validation, and re-classifying the meta-class when its child class cannot be decided. By using the HD, the cross validation correct ratio of core data sample and pulsed neutron spectroscopy data sample was improved from 85.7% to 89.0% and from 77.8% to 79.5%, respectively, which strongly supports the HD as a promising method for solving multi-class classification problems. In this paper, the comprehensive methodology of the HD for multi-class classification was explained, and the enhanced recognition of Marcellus Shale lithofacies in the Appalachian Basin was also demonstrated.

Organic-rich Marcellus Shale lithofacies modeling and distribution pattern analysis in the Appalachian Basin

The Marcellus Shale is considered to be the largest unconventional shale-gas resource in the United States. Two critical factors for unconventional shale reservoirs are the response of a unit to hydraulic fracture stimulation and gas content. The fracture attributes reflect the geomechanical properties of the rocks, which are partly related to rock mineralogy. The natural gas content of a shale reservoir rock is strongly linked to organic matter content, measured by total organic carbon (TOC). A mudstone lithofacies is a vertically and laterally continuous zone with similar mineral composition, rock geomechanical properties, and TOC content. Core, log, and seismic data were used to build a three-dimensional (3-D) mudrock lithofacies model from core to wells and, finally, to regional scale. An artificial neural network was used for lithofacies prediction. Eight petrophysical parameters derived from conventional logs were determined as critical inputs. Advanced logs, such as pulsed neutron spectroscopy, with log-determined mineral composition and TOC data were used to improve and confirm the quantitative relationship between conventional logs and lithofacies. Sequential indicator simulation performed well for 3-D modeling of Marcellus Shale lithofacies. The interplay of dilution by terrigenous detritus, organic matter productivity, and organic matter preservation and decomposition affected the distribution of Marcellus Shale lithofacies distribution, which may be attributed to water depth and the distance to shoreline. The trend of normalized average gas production rate from horizontal wells supported our approach to modeling Marcellus Shale lithofacies. The proposed 3-D modeling approach may be helpful for optimizing the design of horizontal well trajectories and hydraulic fracture stimulation strategies.

Optimized Shale-Resource Development: Well Placement and Hydraulic-Fracture Stages

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 162534, ’Optimized Shale-Resource Development: Proper Placement of Wells and Hydraulic-Fracture Stages,’ by Robert L. Kennedy, SPE, Rajdeep Gupta, SPE, Sergey Kotov, SPE, W. Aaron Burton, SPE, William N. Knecht, SPE, and Usman Ahmed, SPE, Baker Hughes, prepared for the 2012 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Europe and Australia have joined the US in expanding recoverable hydrocarbons from unconventional resources, and initial activities are on the rise elsewhere. However, not all wells are producing commercially, and not all hydraulic-fracture stages contribute within the producing wells, suggesting that it is important to target the field’s sweet spots when dealing with shale resources. Therefore, unconventional-resource development based on the current concepts of geometric placement of hydraulic-fracture stages may not be appropriate. Introduction Although significant advancements in technology for the development of shale gas/oil plays in North America (particularly in the US) have been made in recent years, a number of these shale wells still experience relatively low initial productivity. There are two possible reasons for this: Fracture placement does not intersect the natural fractures in the well, and reservoir quality [i.e., total-organic-carbon (TOC) levels, thermal maturity, and remaining hydrocarbon in the source-rock reservoir] is low or nonexistent at the locations where fracture stages have been placed. Many US operators do not invest in methods of reservoir characterization along the horizontal lateral to assess reservoir quality (i.e., no logs run, drilling cuttings not analyzed, and advanced mud logging with addition of gas chromatography not used). However, international operators in China, Latin America, Saudi Arabia, and India, who are just beginning to explore and exploit shale reservoirs, are eager to consider new technologies that have been shown to reduce cost or increase recovery. Well-by-well production data indicate that shale formations have small spots of very productive wells—sweet spots—surrounded by large areas of wells that produce far less gas/oil. Sweet spots are a function of TOC, thermal maturity, thickness, natural fractures, mineralogy, and geomechanical stresses in the area. The first step in identifying sweet spots is from information collected during a basin screening study. It is recommended that such studies be conducted in a new basin or in a different part of a basin than that being developed currently. To begin initial characterization of the reservoir, it is recommended that 3D seismic be conducted over the potential play area. Data from exploration and appraisal wells are used to pinpoint the location of sweet spots through further formation assessment consisting of detailed mineralogical, structural, and geomechanical characterization (pulsed neutron spectroscopy, nuclear magnetic resonance, and acoustic logs). These data are more location-specific, while information gathered through seismic is broad, spread out, and not as accurate as that obtained from an actual wellbore penetrating the formation.

Experimental studies on pulsed neutron sensitivity of a fusion neutron spectrometer

A sensitivity optimized magnetic proton recoil (MPROS) spectrometer for measuring both steady and pulsed fusion neutron spectra in several complicated environments is developed. The proton energy-position projection relationship of the spectrometer is measured by utilizing an imaging plate. The energy response to neutron sensitivity of the MPROS spectrometer is studied through a single event count method on an accelerated steady DT neutron source and simulations by a three-dimensional charged particle transport code. The detection efficiency of the spectrometer to DT neutrons reaches a level of 2×10-5 cm2 through high efficiency parameter settings, therefore the experimental data of high statistic accuracy are obtained on a comparatively weak neutron source. Results from simulations and experiments, such as α particle calibrations, and DT neutron calibrations, achieve good consistency. Based on this conclusion, precise solution technique of measured spectra can be developed to increase the sensitivity and energy resolution of measurements of pulsed neutron spectroscopy.

Marcellus Shale Lithofacies Prediction by Multiclass Neural Network Classification in the Appalachian Basin

Marcellus Shale is a rapidly emerging shale-gas play in the Appalachian basin. An important component for successful shale-gas reservoir characterization is to determine lithofacies that are amenable to hydraulic fracture stimulation and contain significant organic-matter and gas concentration. Instead of using petrographic information and sedimentary structures, Marcellus Shale lithofacies are defined based on mineral composition and organic-matter richness using core and advanced pulsed neutron spectroscopy (PNS) logs, and developed artificial neural network (ANN) models to predict shale lithofacies with conventional logs across the Appalachian basin. As a multiclass classification problem, we employed decomposition technology of one-versus-the-rest in a single ANN and pairwise comparison method in a modular approach. The single ANN classifier is more suitable when the available sample number in the training dataset is small, while the modular ANN classifier performs better for larger datasets. The effectiveness of six widely used learning algorithms in training ANN (four gradient-based methods and two intelligent algorithms) is compared with results indicating that scaled conjugate gradient algorithms performs best for both single ANN and modular ANN classifiers. In place of using principal component analysis and stepwise discriminant analysis to determine inputs, eight variables based on typical approaches to petrophysical analysis of the conventional logs in unconventional reservoirs are derived. In order to reduce misclassification between widely different lithofacies (for example organic siliceous shale and gray mudstone), the error efficiency matrix (ERRE) is introduced to ANN during training and classification stage. The predicted shale lithofacies provides an opportunity to build a three-dimensional shale lithofacies model in sedimentary basins using an abundance of conventional wireline logs. Combined with reservoir pressure, maturity and natural fracture system, the three-dimensional shale lithofacies model is helpful for designing strategies for horizontal drilling and hydraulic fracture stimulation.

Methodology of organic-rich shale lithofacies identification and prediction: A case study from Marcellus Shale in the Appalachian basin

The success of shale gas in North America has attracted increased interest in “unconventional” reservoirs. Two critical factors for shale-gas reservoirs are units amenable to hydrologic fracture stimulation and sufficient natural gas content. The effectiveness of hydrologic fracture stimulation is influenced by rock geomechanical properties, which are related to rock mineralogy. The natural gas content in shale reservoirs has a strong relationship with organic matter, which is measured by total organic carbon (TOC). A 3D shale lithofacies model constructed using mineral composition, rock geomechanical properties and TOC content can be applied to optimize the design of horizontal well trajectories and stimulation strategies. Core analysis data, log data and seismic data were used to build a 3D shale lithofacies from core to well and finally to regional scale. Core, advanced and common logs were utilized as inputs to petrophysical analysis, and various pattern recognition methods, such as discriminant analysis, fuzzy logic, neural network and support vector machine. A limited set of eight derived parameters from common logs were determined as critical inputs for pattern recognition methods. Advanced logs, such as pulsed neutron spectroscopy, are used to determine mineral composition and TOC data improve and confirm the quantitative relationship between conventional logs and lithofacies. Seismic data, interpreted sequence stratigraphy and depositional environments were used as constraints to build deterministic and stochastic 3D lithofacies models and to extrapolate lithofacies from well scale to regional scale.

Multiwell Case Study - Increasing Oil Production in a Mature Field With Rigless Intervention

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 11644, "Increasing Oil Production in a Mature Field by Rigless Intervention-A Multiwell Case Study," by Vivek Garg, ONGC, and Shubhranshu Ashesh (now with Shell), SPE, Kapil Seth, SPE, and Amit Govil, SPE, Schlumberger, prepared for the 2007 International Petroleum Technology Conference, Dubai, 4-6 December. The paper has not been peer reviewed. Reviving production from brownfields is a major focus for oil and gas companies. Workflow and procedures are outlined that were adopted to increase oil production from seven wells by use of rigless intervention on an offshore platform off the west coast of India. Introduction The field was discovered in 1987 and was put on production in 1994. The main reservoir is a Middle Eocene carbonate deposit characterized by vugs, dissolution channels, and fractures (both vertical and horizontal). Hence, high-permeability streaks provide conduits for early water breakthrough, which has been primarily responsible for the decline in oil production. The average water cut in the field is 80%. The offshore platform posed a special challenge because of its proximity to two water-injector platforms. Injection-water accumulation in the drainage zone covered by the wells of this platform was determined by current temperature anomalies and salinity contrast between the formation water and the accumulated injection water in the vicinity of the wells. The challenge was to study the flow dynamics so that additional perforations in the bypassed section would yield water-free oil. Method The strategy included well diagnostics followed by workover intervention to increase oil production and reduce the water cut. Production logging identified the flow profile. Then, pulsed-neutron logging was performed where required to determine hydrocarbon (HC) saturation in the cased-hole environment. Integration of data from openhole logs, production logs, and pulsed-neutron spectroscopy helped identify depleted zones and zones of bypassed oil. Through-tubing isolation plugs were placed to isolate water-producing intervals. Oil in bypassed zones was accessed with additional perforations. Formation-water salinity was calculated from pulsed-neutron logging to identify zones of injection-water breakthrough. Such intervals were avoided when adding new perforations. Seven intervened wells yielded a 53% increase in oil production to 2,708 BOPD. The average water cut of the platform decreased from 80 to 76%. Candidate Selection The objective of the operation was to increase oil production while minimizing water and gas production. Because no rig was available, operations had to be carried out with a modular skid unit, which constrained available tools and methods.

Modern Carbon/Oxygen Logging: Hydrocarbon- Saturation-Determination Techniques

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 90339, "Modern Carbon/Oxygen Logging Methodologies: Comparing Hydrocarbon-Saturation-Determination Techniques," by Ramsin Y. Eyvazzadeh, Oscar Kelder, A.A. Hajari, Shouxiang Ma, and Abdallah M. Behair, SPE, Saudi Aramco, prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. Recent advances in carbon/oxygen (C/O) logging technology have provided measurements that have become more quantitative in reservoir-monitoring projects, especially in mixed-salinity environments. Although innovations have led to major improvements, fluid-saturation profiles provided by different service companies often vary. These variations are caused by differences in tool design and interpretation method. A study was conducted in a large Middle Eastern carbonate field to evaluate and compare accuracy and precision of different types of pulsed-neutron-spectroscopy (PNS) tools in a controlled environment. Introduction An important reservoir-management factor is accurate determination of water saturation. Accuracy is critical in time-based measurements used in tracking reservoir depletion, enhanced recovery, workover strategies, and injection-water breakthroughs. This parameter is even more sensitive in large fields, where small variations in water-saturation values can translate into large hydrocarbon-volume differences. The carbonate field in this study was discovered in the 1950s, and an enhanced-oil-recovery project was initiated in the 1970s. Since then, several types of water, with varying salinities, have been injected. This salinity variation complicates the saturation computations that rely on accurate measurements of formation-water resistivity (i.e., resistivity-tool data). Resistivity measurements are used in empirical equations to calculate fluid saturations on the basis of known formation-water resistivity. In formations that contain fresh water or unknown water salinity, these equations produce answers with large uncertainties. These limitations contributed to development of the PNS tools. The tools use neutron generators to produce high-energy neutrons that collide with different types of elements in the formation. These collisions produce gamma ray signatures that are used to measure the relative abundance of elements. Two of the most important elements are carbon and oxygen. The abundance of these elements is used directly to calculate hydrocarbon volume. C/O logging was introduced in the 1960s. The early technology was cumbersome, difficult to use, and, often, provided unreliable answers. In the 1990s, several new-generation tools were introduced by three major logging companies: Schlumberger, Halliburton, and Baker Atlas. These new tools have high-output neutron generators with better detectors and characterizations that provide more-accurate answers. Even though these tools operate on the same principles, the service companies use different methods, resulting in nonunique solutions. This study examined these differences in two wells. In the first well, saturation data from nuclear-magnetic-resonance (NMR) logs were compared with C/O-log and laboratory core-measured saturation data to determine the applicability of this technology in this field. In the second well, resistivity and capture data were used as references to compare data from three different types of PNS tools.

Combining Pulsed Neutron Spectroscopy and LWD Resistivity for Analysis Behind Casing in Small Holes

Summary Formation analysis from behind casing is gaining more relevance in assessing reservoirs, not only in aging fields but also in new wells completed either without the full complement of openhole petrophysical measurements or with openhole measurements of inadequate quality. Before this technology was available, formation evaluation in freshly drilled holes was dependent on the availability of openhole log measurements. This paper discusses field examples in which reservoirs were evaluated without a full suite of openhole log measurements. Examples from two sidetracks in a mature North Sea field supported by water injection are presented. The small hole size limited openhole data acquisition to resistivity and gamma ray (GR) measurements, which were acquired using logging-while-drilling (LWD) technology. In both cases, pulsed neutron (PN) measurements were recorded through casing to augment the existing data. Porosity and formation sigma (macroscopic thermal neutron capture cross section) measurements were obtained. Fluid saturations were computed using this porosity and the LWD resistivity. Saturations also were computed from the carbon/oxygen measurement, and these agreed with the resistivity-based answers. Lithology analysis was performed using elemental spectral capture yields and was reliable and consistent fieldwide. The results show that, in a mature oil field, PN measurements can provide a complete lithology and saturation analysis in circumstances in which insufficient openhole petrophysical measurements are available.

High-frequency spin waves in YBa2Cu3O6.15.

Pulsed neutron spectroscopy is used to absolute measurements of the dynamic magnetic susceptibility of insulating Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.15}$. Acoustic and optical modes, derived from in- and out-of-phase oscillation of spins in adjacent Cu${\mathrm{O}}_{2}$ planes, dominate the spectra and are observed up to 250 meV. The optical modes appear first at 74\ifmmode\pm\else\textpm\fi{}5 meV. Linear-spin-wave theory gives an excellent description of the data and yields intralayer and interlayer exchange constants of ${J}_{\ensuremath{\parallel}}=125\ifmmode\pm\else\textpm\fi{}5$ meV and ${J}_{\ensuremath{\perp}}=11\ifmmode\pm\else\textpm\fi{}2$ meV, respectively, and a spin-wave intensity renormalization ${Z}_{\ensuremath{\chi}}=0.4\ifmmode\pm\else\textpm\fi{}0.1$.

Distribution of hydrogen vibrations in LaH 2.28 and LaH 2.99

The hydrogen vibrational modes in LaH x samples have been investigated by pulsed neutron spectroscopy. The results are compared with a simple model emphasizing strong HH forces and the lack of stoichiometry in LaH 2.28.