Gamma Ray Spectroscopy Logs Research Articles

Rock classification in the Eagle Ford Formation through integration of petrophysical, geological, geochemical, and geomechanical characterization

Integration of geomechanical, geological, geochemical, and petrophysical characterization is critical to enhance production from organic-rich mudrocks. This paper introduces an integrated rock classification method in the Eagle Ford Formation shales and marls in southern Texas, consisting of organic-rich fossiliferous marine shale deposited during the Late Cretaceous. A joint inversion of triple-combo, spectral gamma-ray, and elemental capture spectroscopy logs was initially conducted to estimate volumetric concentrations of minerals, porosity, and fluid saturation. Effective elastic properties such as Young's modulus and Poisson's ratio as well as minimum horizontal stress were then estimated as a function of depth. Rock classification was finally performed based on geologic texture and geochemical properties, as well as well log-based estimates of petrophysical and geomechanical properties. The introduced method was applied to two wells located in the oil window of the Eagle Ford Formation (average gas-to-oil ratio of less than 2000 MCF/STB). The results of the integrated rock classification demonstrated similar organic richness and petrophysical properties in both wells. However, the geomechanical rock classification derived from in-situ stress profiles suggested higher proportions (55% vs. 34% of net thickness, respectively) of the better completions quality rock class, with lower minimum horizontal stress (MHS), in one of the wells. A 90-day report on the cumulative oil production of this well shows an additional 11,000 bbl (i.e., 20% more oil production) compared to the second well. This observation was accounted for by geomechanical properties and the distribution of rock classes that differentiate reservoir quality and production.

New Analytical Method To Calculate Matrix- and Fluid-Corrected Total Porosity in Organic Shale

Summary Estimation of total porosity from neutron and density porosity logs in organic shale (source rock) is challenging because these logs are substantially affected by fluid and matrix-composition effects. Conventional interpretation of neutron and density porosity logs often includes corrections for shale concentration in which the main objective is to improve the calculation of nonshale porosity in hydrocarbon-bearing zones. These corrections are not desirable in unconventional rock formations because shale pores can be hydrocarbon-saturated. Neutron and density porosity readings across shale zones are sometimes averaged by use of the root-mean-square (RMS) method. We introduce a new and simple analytical expression for total porosity that effectively separates both matrix and fluid effects on neutron and density porosity logs. The expression stems from a new nonlinear mixing law for neutron migration length that is coupled with the linear-density mixing law to calculate total porosity and fluid density. The method is applied in two sequential steps: First, separate corrections for only matrix effects are implemented to enhance the neutron-density crossover for qualitative interpretation of fluid type; second, the coupled equation is used to estimate fluid density and actual porosity devoid of matrix and fluid effects. Calculated porosity and fluid density can be used further to calculate water saturation from density logs. One remarkable feature of this method is the ease with which it can be applied to obtain accurate and reliable results. Application of the method only requires knowledge of single-component nuclear properties and mineral volumetric concentrations. One can obtain nuclear properties from a set of charts for multiple fluid types and minerals provided in this paper, whereas one can calculate mineral concentrations on the basis of available triple combo logs or gamma ray spectroscopy logs. Two synthetic and four field examples (two conventional and two shale-gas reservoirs) are used to test the method. First, we describe an application in a conventional siliciclastic sedimentary sequence in which only shale concentration calculated from gamma ray logs is required to improve the estimation of porosity in shaly sections. Second, we document several applications in which gamma ray spectroscopy logs are used together with a reliable hypothesis for clay type to define mineral properties. Results compare well with nuclear-magnetic-resonance (NMR) and core measurements, whereas the new method outperforms the conventional RMS procedure, especially in the cases of gas-bearing, low-porosity organic shale. The new analytical method can be readily implemented on an Excel spreadsheet and requires minimal adjustments for its operation.

New Method for Determining Mineralogy and Matrix Properties From Elemental Chemistry Measured by Gamma Ray Spectroscopy Logging Tools

Summary Methods for predicting mineralogy from logging-tool measurements have been an active area of research for several decades. In spite of these efforts, methods for predicting quantitative mineralogy including clay types from well-logging data were not fully achieved. The introduction of geochemical logging tools in the 1980s offered promise; however, early versions of geochemical logging tools did not measure elemental chemistry with enough accuracy and precision to enable reliable and quantitative determination of mineralogy. Recent advances in geochemical-logging-tool technology now enable accurate and robust measurements of the chemical elemental concentrations that are needed to determine continuous quantitative and detailed logs of mineralogy. This paper presents a novel approach for determining more accurate and more detailed mineralogy from an elemental spectroscopy logging tool. This work was made possible by three recent developments: the introduction of a new neutron-induced gamma ray spectroscopy logging tool, a new research database consisting of chemistry and mineralogy measured on cores acquired worldwide from conventional and unconventional reservoirs, and a new model-independent inversion method that overcomes the limitations of previous model-dependent methods. The model-independent inversion makes use of the database that includes clean sands, shaly sands, shales, carbonates, and complex mixed lithologies. The database contains laboratory measurements of dry-weight elemental chemistry and mineralogy measured by transmission Fourier-transform infrared (FTIR) spectroscopy. The database is used to derive a model-independent mapping function that accurately represents the complex functional relationship between the elemental concentrations and the mineral concentrations. After the mapping function is determined from the database, one can use it to predict quantitative mineralogy from elemental concentrations derived from the logging-tool measurements. Unlike previous inversion methods, the model-independent mapping function does not have any adjustable parameters or require any user inputs such as mineral properties or endpoints. The mapping function is used to predict continuous logs of matrix densities plus concentrations of 14 minerals (i.e., illite, smectite, kaolinite, chlorite, quartz, calcite, dolomite, ankerite, plagioclase, orthoclase, mica, pyrite, siderite, and anhydrite) from eight dry-weight elemental concentrations derived from the logging tool. The new method was applied to well-log data acquired worldwide in numerous conventional and unconventional reservoirs with a wide variety of complex mineralogies. The predicted mineralogies and matrix densities are generally found to be consistent with core-derived mineralogies and matrix densities.

A preliminary study on the potential of the geothermal resources around the Gulf of Suez, Egypt

The Gulf of Suez is characterized by the presence of many hot springs and deep thermal wells scattered around its coastal areas. So it is considered one of the promised geothermal areas in Egypt. In this study, the main emphasis is to investigate the geothermal potential around the Gulf of Suez using the available logging and geothermometer datasets. The temperature profiles and well logging data of some hot springs and deep wells around or within the coastal area of the Gulf of Suez are used in this study. The temperature profiles are analyzed and some important thermophysical properties are estimated (geothermal gradient, thermal conductivity, heat flow, and specific heat capacity). Such analysis revealed that a medium to high geothermal gradient (22.0–30°C/Km) is given for the Gulf of Suez as a whole, with some spots of much higher gradient in the order of 35.0–44°C/Km (Ras Fanar and Hammam Faraun areas). The compiled thermal plots show that the thick evaporites and rock salt lithology, which is a major constituent in this area, attain the highest thermal conductivity (>3.10 W/m/K) and heat flow (>90 mW/m2) and the lowest specific heat capacity (<0.30 J/kg/K). The available gamma ray and the natural gamma ray spectroscopy logs are used to conduct a radioactive-based heat generation study using the characteristic radioactive nature of some elements like; 238U, 235U, 232Th, and of the isotope of 40K. A good linearity is observed between the heat production (A in microwatt per cubic meter) and the gamma ray (API) along a wide range of datasets (0–150 API) in all wells. The heat production factor increases in the carbonate lithology (up to 3.20 μW/m3) and is proportional to the shale volume. A geothermometer-based study is used to estimate the subsurface formation temperature and heat flow from the geochemical analysis of some water samples collected from the studied hot springs. The estimated thermal parameters are in harmony with the regional thermal regime concluded form logging data. A thermal basin growth study, in relation to the clay diagenesis is conducted concerning the thermal effects that take place with depth giving rise to another clay mineral (illite). Furthermore, a number of 2D thermal–burial history diagrams are constructed for the complied sections of some of the studied areas to show the vertical distribution of the estimated petrothermal properties. A reserve evaluation study is carried out to estimate the economic geothermal capacity of these hot springs to be used as alternative clean source for possible energy production (electricity) and other low-temperature purposes.

Subsurface radionuclide investigation of a nuclear test

This paper reports on an environmental investigation into the vertical distribution of radionuclides from a nuclear test. Dalhart is the name of an underground nuclear test that was executed at the Nevada Test Site at a depth of 2100 ft on October 13, 1988. The test occurred below the static water level of 1667 ft and created multiple radioactive isotopes or fission products. These radioactive isotopes penetrated the surrounding formations and chimney region above the test and were retained there. A 19° 9- 7 8 -inch diameter slant hole was drilled to sample the geologic material in the chimney region above the Dalhart test for the purpose of assessing the distribution of radioactivity in and around the shot site. A 30-ft core recovered from a vertical depth of 1628 ft in the collapsed zone or chimney region and above the original static water level was found to be free of radionuclides. Drilling was completed to a vertical depth of 2156 ft with the present static water level at a vertical depth of 1644 ft. Gamma-ray spectroscopy log measurements, made within the drill pipe while drilling fluid was pumped through this pipe, indicate that radioactive material produced by the test was present from the vertical depth interval of 1746–2156 ft. Side-wall samples acquired from the vertical depth interval of 1721–2089 ft and analyzed in the field contained radionuclides such as 137Cs, 125Sb, 106Ru, plus the natural radioactive background of potassium, uranium, and thorium. These samples were sent to Los Alamos to determine the complete radionuclide content at each depth. These analyses were used with the gamma-ray spectroscopy logging data to determine the subsurface vertical radionuclide distribution at the Dalhart site.

Application of induced gamma-ray spectroscopy measurements: characterization of a North Sea reservoir

Implementation of a correct mineral transform model and strategy on elemental chemistry, derived from gamma-ray spectroscopy logging tools, can provide a continuous accurate measurement of mineralogy through a complete reservoir section. Such mineralogy logs are potentially valuable on their own, especially for the identification of clay minerals in petroleum exploration; however, they can also be used for the determination of other important parameters, such as grain density, porosity, and cation exchange capacity (CEC). In exploration wells where core recovery is poor, such knowledge from nuclear logging can help to extend formation evaluation across these poorly defined zones, maximizing reservoir characterization. A set of reservoir cores from the North Sea (Brent Group) is used to evaluate various models developed for transforming elemental data into geologically meaningful mineralogy. Emphasis is placed on evaluating transform techniques using well constrained laboratory data before applying the models to elemental data derived from nuclear logging tools. This evaluation procedure aims to determine the most suitable transform model, as well as quantifying some of the inherent problems with mineral inversion. Strategies for overcoming such problems are also addressed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Application of Time-Series Analysis to Induced Gamma Ray Spectroscopy Logs From Two Cold Lake Heavy-Oil Observation Wells

Summary Induced gamma ray spectroscopy (IGRS) logs from two cyclic-steam-stimulation observation wells in Cold Lake were analyzed to determine the vertical resolution and repeatability of data derived from gamma rays of inelastic and capture neutron reac tions. Time-series analysis, a technique that uses the Fourier representation of the log data, was used to quantify the vertical resolution and the signal/noise characteristics of various IGRS log curves and to compute the coherence vs. spatial frequency of data collected on multiple IGRS passes. The coherence function ranges from 1.0 for perfect repeatability to 0.0 for incoherent noise. Because no real variations in the measured data were expected for the 12- to 24-hour data-collection period, any deviation of the coherence function from 1.0 is attributed to incoherent noise. Generally, coherence is high at low frequencies (large vertical scales) and low at high fre quencies (small vertical scales). By noting the frequency at which the coherence level decreases to the expected value of random noise, we can quantify the vertical resolution of a log curve. Data analysis from these wells indicates that both the vertical resolution and repeatability of individual capture and inelastic curves differ. We found that the H-yield and capture curves have the highest vertical resolution (≈0.3 m) and the best signal/noise ratios (FSN≈30:1). In contrast, the capture Ca and Si yields are of significantly lower quality (FSN≈2:1). Only a small difference exists between the vertical resolution of the inelastic C (1.0 m) and O (1.3 m) yields, but the FSN the O yield is only one-half that of the C yield. Fortunately, the vertical resolution and the repeatability of the C/O ratio, FCO, are determined primarily by the quality of the inelastic C data. Specific to our application of monitoring heavy-oil saturations during cyclic-steam stimulation in a relatively uniform, high-porosity unconsolidated sand, we developed a simplified FCO interpretation that does not rely on the elemental yields from capture data. This linear model permits us to assess the uncertainties in computed saturations from the statistical noise in the inelastic FCO data.

Developments in elemental concentration logging and applications

It is noted that the past decade has seen a dramatic growth of gamma-ray spectroscopy logging as a tool for solving geophysical problems. Developments have provided absolute elemental concentrations for a larger set of elements, using elemental analysis (including tracers) for solving many specific oil-field problems and providing a basis, through geochemical research, for developing a wide range of applications of borehole elemental analyses, such as chemically described mineralogy. Physical properties have been related to the mineral structure of the formation rock, such as cation exchange capacity, grain density, permeability, and source rock evaluation. Some areas of further development, both in the techniques of elemental logging and in the applications of the data, are discussed. >

Thermophysical properties of the Earth's crust: In situ measurements from continental and ocean drilling

Knowledge of the variation of thermal conductivity and heat production with depth is critical to understanding the thermal state of the crust. Continental and oceanic drill holes provide unique opportunities for the calibration of empirical and theoretical models relating crustal thermal properties to physical and chemical properties. These calibrations are based on more than 30,000 well log measurements of acoustic velocity (both compressional and shear), density, porosity, radioelement concentrations (K, U, Th), and major element chemistry (Al, Si, Ca, Fe, S, Mg + Na), in formations ranging in age from 5 to 620 Ma and in composition from granitic to gabbroic. The mineralogical data available from induced gamma ray spectroscopy logs, when combined with known mineral thermal conductivities in a model for the thermal conductivity of a crystalline aggregate, enable the in situ application of a laboratory estimation technique and yield continuous conductivity profiles. However, mineralogy‐based conductivity models cannot account for the anisotropic conductivity observed in foliated metamorphic rocks. A phonon conduction model, which utilizes acoustic velocity and bulk density measurements, is accurate within ±15% in both isotropic and anisotropic formations of the upper continental crust and in oceanic crustal layers 2C and 3. In oceanic crustal layers 2A and 2B, however, the effects of fracturing on compressional and shear velocities lead to inaccurate results. The heat flow‐heat production relationship, which provides one method of determining subsurface heat production, has well‐documented limitations. Allis (1979) and Rybach and Buntebarth (1982, 1984), on the basis of laboratory measurements, have proposed quantitative relationships between heat production and rock compositional and physical properties. The well log K, U, and Th data indicate that heat production can be predicted from compositional data within a factor of 2 or 3. Predictions of heat production from acoustic velocity and bulk density measurements are only accurate to within an order of magnitude. Three factors account for these results: (1) the natural variability of velocity and density for a given rock composition, (2) the tendency of U and Th to associate with secondary rather than primary minerals, and (3) U and Th redistribution through fluid flow and metamorphism. Thus seismic velocity based heat production profiles can, at best, be taken as first‐order estimates which may misrepresent the complexity of the continental crust. However, the correlation of formations of known heat production with subsurface structures, when combined with a suite of existing techniques for estimating heat production, may lead to more accurate results.

Environmental corrections for a neutron-induced gamma-ray spectroscopy logging system in an air-filled borehole

When a nuclear explosive is tested underground, rock in the immediate vicinity of the explosion is heated to very high temperatures. If any of that rock contains carbonates (dolomite or calcite), these are dissociated and the carbon may combine with oxygen to form carbon dioxide, thereby increasing the hazard of a delayed leak. The authors are trying to adapt a commercial neutron-induced gamma-ray spectroscopy log to estimate carbonate content in emplacement (2-4-m diameter) boreholes at the Nevada Test Site. Tests were conducted and showed that the log is sensitive not only to carbonate content but also to the water content of the formation as well as the gap between the logging sonde and the wall of the borehole. Tests in boreholes have shown good qualitative agreement with the trends in the sample data but generally indicated somewhat higher equivalent CO/sub 2/ content. It appears that the corrections developed in the laboratory may result in an overcorrection. Moreover, other minerals present in the field may decrease the accuracy of the measurement. >

The use of nuclear simulation calculations to support applications in radioactive tracer logging

A method for simulating the pulse height responses of passive gamma-ray spectroscopy logging tools is described, and its application to measurements of radioactive tracer spectra is demonstrated. This method consists of computing the flux at the detector surface, convolving the flux with the detector response computed in a separate step to obtain a base spectrum, convolving the base spectrum with itself to obtain a spectrum representative of the contribution due to random summing, and adding the sum spectrum to the base spectrum to obtain the final spectrum. Comparison of measured and simulated spectra and the Compton ratio derived from the measured and simulated spectra for several radioisotopes demonstrates the accuracy of the simulation method. This method has been used to compute calibration spectra for a large number of radioisotopes for which calibration measurements had not been made.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cased-Hole Logging for Evaluating Bypassed Reserves

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. Cased-hole formation evaluation logs are playing a major role inefforts to increase recovery from existing reservoirs. While the logs availabletoday are significant improvements over those of just a few years ago, caremust still be exercised for successful applications. Application limitationsremain, although some of these are being addressed by ongoing research. Introduction Current oilfield economics, while discouraging some exploration and drillingexpenditures, has also produced a growing interest in re-evaluating existingwells to locate bypassed intervals. In mature producing areas, the potentialreserve additions from bypassed reservoirs can be very large; moreover, theeconomics for recompletion can be quite favorable because of the high chance ofsuccess in adding production for moderate capital expenditures. Cased-hole logsproduction for moderate capital expenditures. Cased-hole logs are playing amajor role in locating and evaluating such bypassed hydrocarbon zones. In viewof this role, a review of the common technology, applications, limitations, andpossible future developments in cased-hole logging is appropriate. Cased-hole logs for looking behind pipe are used in a broad spectrum ofapplications, several of which are indicated in Table 1. Although these usesrange from cement evaluation to monitoring EOR processes, for the purpose ofthis discussion our focus will be on recompletions, and specifically thelocation and evaluation of bypassed oil or gas zones. Technology The logs commonly used in formation evaluation behind pipe are listed inTable 2, along with the primary reservoir properties they measure and anindication of some of the other properties they measure and an indication ofsome of the other formation properties that influence their response. This listis not exhaustive, but it does provide an indication of the types of formationproperties that can be measured by common cased-hole tools. Further informationon the operating principles of these tools is readily available in theliterature. Gamma ray logs include both conventional tools that measure total naturalgamma ray count rates and newer spectral gamma ray tools, which measure theindividual contributions of potassium, uranium, and thorium to the energyspectrum of natural gamma rays. These tools are predominantly lithologyindicators and, as such, are useful in predominantly lithology indicators and, as such, are useful in depth control, correlation, and lithology evaluation(especially shaliness). They also detect the buildup of radioactive scale, which in certain cases can indicate zones of prolonged water movement, such asperforations, watered-out zones, or channels behind pipe. Cased-hole neutron logs are also of two primary types: the olderconventional neutron log that measures capture gamma ray count rates with asingle detector and the newer dual-detector, compensated neutron log thatpresents a log trace scaled in porosity units. These logs are good porosity, gas, and indicators, With proper calibration, quantitative estimates ofporosity and semiquantitative estimates of gas saturation can be obtained, provided that shaliness effects are -properly accounted for. Because of theirgas response. they are widely used in monitoring gas/liquid contacts. Pulsedneutron capture (PNC) logs are the primary tools Pulsed neutron capture (PNC)logs are the primary tools used in detecting oil behind pipe. Firstcommercially available in the mid 1960's, they measure the macroscopic thermalneutron capture cross section, using a pulsed neutron source. Present-daytools can be run through tubing as small as 2 in. [5.08 cm]. Because thechlorine present in salt water has a large thermal neutron capture crosssection, these logs can distinguish high-salinity formation water fromhydrocarbons, and hence can yield quantitative estimates of water saturation ifproperly calibrated. They also respond to gas saturation and properlycalibrated. They also respond to gas saturation and porosity, but theirapplications can be severely limited by low porosity, but their applicationscan be severely limited by low brine salinity, low porosity, or shaliness. Pulsed neutron spectral (PNS) or induced gamma ray spectral logs are alsocommonly known as carbon/oxygen (C/O) or gamma ray spectroscopy logs. Theselogs, which were first commercially available in the mid-1970's, measure theenergy spectra of gamma rays from both inelastic neutron scattering and thermalneutron capture to obtain estimates of the relative concentrations ofparticular elements present in the formation. Various ratios of these elementalpresent in the formation. Various ratios of these elemental components (such asthe C/O ratio) can be used to evaluate oil saturation, lithology, and evenbrine salinity; also, count rate or component yield ratios can be used toestimate porosity. JPT

Laboratory and Field Evaluation of an Inelastic Neutron Scattering and Capture Gamma Ray Spectrometry Tool

Abstract An experimental system for gamma ray spectroscopy logging has been developed which uses prompt and capture gamma radiation induced in formations by 14-MeV neutrons from a pulsed-neutron generator to determine relative concentrations of various elements in the formation. The logging system uses computer-processing techniques based on spectral modeling that has been developed to analyze the inelastic and capture gamma ray data obtained with a borehole spectrometer. The physics of the production of gamma rays from fast-neutron interactions with elemental nuclei in formations is discussed, leading to a simple but realistic interpretation model for the tool's response. This model is confirmed by laboratory and field tests. The relative spectroscopic contributions from carbon, oxygen, silicon, calcium, iron, chlorine, and hydrogen are used for various cased-hole and openhole logging applications. Particular emphasis is placed on the carbon/oxygen ratio used to obtain oil saturation independent of formation-water salinity. Carbon/oxygen ratio determinations made in the laboratory are compared with values predicted on the basis of known lithologies, porosities, and oil-saturation changes. In addition, the spectral contributions from iron, silicon, and calcium are used to interpret lithology; hydrogen, silicon, and calcium contributions are used to determine the effects of porosity; and chlorine and hydrogen contributions are used to investigate salinity changes. Field-test log examples of these elemental determinations are shown.