Gamma-gamma Density Logging 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.

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.

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.

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.

THE SELECTION OF COMPLEX RESERVOIRS OF HYDROCARBONS OPENED ON SALINE FLUSH DRILLING FLUIDS IN DEEP EXPLORATORY WELLS

We have investigated the usefulness of GIS methods (geophysical investigations in boreholes) and GTI (mud logging) in distinguishing low-power numerous collectors HC (hydrocarbons) in deep exploratory wells, complicated by the high caverns and drilled for saline drilling fluid. Limitations of techniques emisions and determine their characteristics leads to skipping of productive intervals and their erroneous selection. Data electrical methods (BKZ (lateral sounding), MKZ (microcerotermes sensing), IR (induction logging), PS (logging potentials spontaneous polarization)) in terms of the mineralized mud and cavernous wellbore distorted, poorly reflect the incision and therefore uninformative. There are no direct characteristics collector - positive increment on the microprobes (MHS (micro-gradient probe), MPZ (micro-potential probe) and clay crust against the collector by DS (inspection), at the substation are not marked minimum values of the net (Neglinnaya) terrigenous formations. Saturated and reservoirs on IR also not differentiated. Readings radioactive methods (COG (neutron gamma logging) and GGC-p (gamma-gamma density logs)) are distorted by the presence of large cavities in the wellbore. For successful execution of exploration drilling designed research methodology perspective on HC intervals. The method is based on an integrated use of all possible methods of GIS, the GTI and direct methods for the steady reservoir and determine their characteristics. Proposed rational complex GIS detailed geophysical studies in Sandstone reservoirs, exploration and appraisal wells drilled in saline solutions.

A logging correlation scheme for the Main coal seam of the North Bohemian brown coal basin, and the implications for the palaeogeographical development of the basin

Abstract A method for correlating the Main coal seam in the North Bohemian brown coal basin by interpreting borehole logs is described. The most important well log was the gamma-gamma density log, but in some boreholes it was also necessary to use the natural radioactivity and electric resistance logs or technological sampling of the core. All these methods, at various levels of sensitivity, reflect changes in the amount of organic matter in the borehole profile. Vertical changes of coal matter content characterize variations of sedimentation conditions. In close boreholes gamma-gamma density logs are easily correlatable so that changes of sedimentation can be correlated over great distances. The method has been used to correlate the main seam along the axis of the basin. The main result of this correlation is that coal-bearing sedimentation occurred uninterruptedly along the main axis of the basin, with the development of swamp and lake conditions. During the swamp conditions peat accumulated over the majority of the present basin area, whilst during the shallow lake conditions the region of clastic sedimentation was controlled by the distance from the inflowing sediment source. The coal-bearing sedimentation cycle started by a levelling of the palaeorelief by infilling the depressions, and was followed an extension of the sedimentation area. Compaction of the previously deposited layers affected the ensuing strata. No significant variations in stratal thickness were found along the length of the main borehole line nor in any of the auxiliary lines whole, indicating that tectonic controls in basin subsidence were unimportant. Thus changes of sedimentation conditions dominantly of climatic conditions caused water levels in the basin area to fluctuate. For many reasons it is concluded that the correlatable boundaries are isochronal. The correlation scheme represents the first really objective correlation of the main coal seam over the area of the whole North Bohemian basin.

Eliminating the Rugosity Effect From Compensated Density Logs by Geometrical Response Matching

Summary A theoretical and experimental effort to understand the effects of borehole rugosity on individual detector responses yielded an improved method of processing compensated density logs. Historically, the spine/ribs technique for obtaining borehole and mudcake compensation of dual-detector, gamma-gamma density logs has been very successful as long as the borehole and other environmental effects vary slowly with depth and the interest is limited to vertical features broader than several feet. With the increased interest in higher vertical resolution, a more detailed analysis of the effect of such quickly varying environmental effects as rugosity was required. A laboratory setup simulating the effect of rugosity on Schlumberger Litho-DensitySM tools (LDT) was used to study vertical response functions. These functions were used to derive matching filters, enabling a significant improvement in the compensated log response in the presence of rugosity. The data served as a benchmark for the Monte Carlo models used to generate synthetic density logs in the presence of more complex rugosity patterns. The results show that proper matching of the two detector responses before application of conventional compensation methods can eliminate rugosity effects without degrading the measurement's vertical resolution. The accuracy of the results is as good as that obtained in a parallel mudcake or standoff with the conventional method. Application to both field and synthetic logs confirmed the validity of these results.

Gamma-gamma density measurements: basic response and environmental corrections

The basic response of a gamma-gamma density logging tool and the environmental corrections to that response have been determined. This has been accomplished through the use of test formation data and Monte Carlo modeling. The basic response has been determined for formation densities of 1.6 g/cm/sup 3/ to 3.1 g/cm/sup 3/. Environmental corrections have been obtained for a full range of borehole conditions. Mudcake corrections in light mud, barite mud, and hematite mud have been obtained for simulated mudcake thicknesses of up to 0.75 in. Borehole size corrections have been obtained for mudcake thicknesses of up to 0.75 in. Borehole size corrections have been obtained for air-filled, water-filled, and mud-filled boreholes with diameters from 6 to 14 in. The response and corrections have been verified downhole through extensive comparisons with core and gravinometer data.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Estimating the block size of an ore body

1. The quantitative characteristics of the block size, found by processing measurements of the network of surface cracks, must be converted to the actual parameters of the block size; as a rough approximation we can recommend a linear transformation of the dimensions of the form (1), (2). 2. Borehole measurements of jointing in fans of blastholes enables us to obtain detailed and very important information concerning the jointing of the ore body; there is promise in the possibilities of operational monitoring, and in the use of this information to correct the method of charging and the scheme of blastholes when necessary. 3. Gamma-gamma density logging is a very promising method (although insufficiently researched in this application) of investigating the block size of the ore body-zones of deconsolidation, as a rule, correspond to regions of marked damage. A further step is the development of a method of measurement and quantitative interpretation of the GGDL curves interms of the jointing structure of the rock. 4. The geometry of single blocks (dimensions, area, volume, density, and shape) and their statistical aggregates (packing configuration, structural levels of jointing, and distribution of block dimensions) are important characteristics of the rock, and largely govern the effectiveness of a technology in particular conditions.

Evaluating Oil Shale by Log Analysis

Abstract Induction, nuclear and sonic well-logging methods were employed in a Green River formation oil-shale analysis and evaluation study conducted in northeastern Uintah County, Utah. The physical and chemical properties of an oil-shale section, previously cored and assayed by conventional methods, were used to evaluate the response of the various logs. The logging program was designed to measure variable properties of oil shale which relate to oil-yield potential in order that a relationship between assay-oil yield and log-determined properties could be identified, thereby permitting a direct determination of yield potential from logs alone.All of the major oil-shale zones and section markers are recognizable on the logs used in the study. The relationship between the response of the Density and Sonic logs and the assay-oil yield in gallons per ton was sufficient to permit the derivation of equations expressing the relationships. These equations can be used to determine the potential oil yield in gallons per ton of an oil-shale zone or section. The Neutron log response distinguishes the rich oil-shale intervals from the lean intervals, but does not appear to permit establishment of a quantitative relationship. The Gamma-Ray and Induction logs indicate only a qualitative relationship to oil-shale yields.Logging oil shale by Gamma-Gamma Density and/or Sonic logging methods will permit a fast, economical and accurate means of evaluating the potential yield of oil-shale deposits. Introduction Considerable sums of money have been allocated to Green River formation oil-shale evaluation during the past years by both private enterprise and Federal and state governments. The result of the work accomplished to date is commendable, but the task of fully exploring and evaluating one of the world's greatest reserves of potential energy for economical exploitation is enormous and much information is still needed.Present methods of evaluation consist primarily of sampling and assaying the oil shale for potential yield. The data received are excellent, but are both costly (inasmuch as it requires funds specifically allocated for the evaluation purpose) and time-consuming (as each representative sample must be assayed in the laboratory by specially trained personnel).At the present time, the Green River formation oil-shale province is one of the most active areas in the Rocky Mountain region for exploratory oil and gas drilling. This activity could play a twofold role and serve the function of spear-heading many oil-shale evaluation programs, as it could provide both cutting samples for assaying, geophysical data and modern well logs.This paper deals with a recent study conducted on Green River formation oil shale in which the application of induction, sonic and nuclear well logs to oil-shale evaluation was tested and proved. THEORY AND DEFINITIONS Oil shale may be described as siliceous marlstone, rich in solid organic matter called "kerogen". Kerogen is only slightly soluble in organic solvents, but it will decompose and yield oil vapors and gas when heated to destructive distillation temperatures at about 800 F. The mineral constituents of Green River formation oil shale are found in essentially uniform proportions with one another. The dominant types are, in the general order of abundance, dolomite, calcite, feldspars and quartz. Kerogen is also essentially uniform in its composition, and is about 80 per cent carbon and 10 per cent hydrogen by weight. The ratio of kerogen to the mineral constituents determines the richness or potential yield of the oil shale. This ratio also determines the total physical and chemical properties of the rock and provides a basis by which oil-shale -richness may be determined from well logs.Because Green River formation oil shale was deposited in a lacustrine environment, the lithology is laterally consistent over wide areas. The vertical section consists predominantly of thin bands of alternating rich and lean oil shale which have been described by Bradley as varves. The varves are occasionally broken by thin bands of volcanic ash and tuff and by zones of lean vugular oil shale.The effect that the thin non-oil shale beds and vugular zones would have on the well log analysis was a prime concern in the study. To cope with this possible problem, the logs used in the study were of two types:logs believed to respond favorably to those properties of oil shale which are dependent upon the kerogen-to-mineral ratio, these logs being the Gamma-Gamma Density log, the Sonic log and the Neutron log; andlogs that would respond to lithologies and properties within the oilshale section not associated with yield that might result in anomalous responses by the yield-measuring logs. Logs of the second type were the Gamma-Ray log, the Induction log and the Caliper log. JPT P. 81^

A chemical correction factor in gamma-gamma density logging

Log-derived rock densities may be in error for two reasons: first, the ratio of A/Z varies for different chemical elements; second, the photoelectric absorption, neglected in calibration, may become appreciable if heavier elements are present. The effects are discussed and the A/Z correction factors calculated. (auth)