Results Of Core Analysis Research Articles

Improvement of Sonic Logging Porosity Calculation Method in South Sulige Block

Abstract South Sulige Block is a lithologic gas reservoir with low porosity and permeability, low pressure and low abundance, of which the heterogeneity is very high and pore structure is complex. Due to the influence of lithology and pore structure, the skeleton value is uncertain and it is hard to give a fixed value. It is difficult to obtain the true porosity of formation accurately by using the traditional method of calculating porosity by Wyllie formula in sonic logging. In this paper, according to the principle of sonic logging and rock response characteristics, combined with core analysis data of this area and adjacent areas, using the acoustic time and core analysis porosity regression formula to calculate porosity first, and then, according to different shale content, the corresponding shale correction is carried out for heavy shale content intervals on the basis of core regression formula, which eliminates the porosity calculation error caused by the influence of shale content. Practical application shows that the porosity calculated by this method is in good agreement with the results of core analysis, and can reflect the reservoir characteristics and production capacity of the formation more truly.

Petrophysics of Low Resistivity Reservoir

Low resistivity reservoirs have long been known in the oil and gas industry as reservoirs that have significant potential and contribution to oil and gas production. This reservoir is very unique because of its relatively low resistivity readings (< 5 Ohm) and often other log responses do not show the way carbonate-containing reservoirs do, so they are often overlooked and not considered as potential carbon reservoirs. The low resistivity reading also causes the Water Saturation (Sw) value to be too high, which contributes to why the reservoir is overlooked. Theoretically, the factors that cause low resistivity readings are high clay/shale content, fine grain size, high salinity air formation, presence of microporosity and presence of conductive minerals which affect resistivity log readings in addition to other things such as low structured reservoirs. and there is a significant slope of the layers. Based on statistic reports, generally in low resistivity reservoirs there is a combination of two or more of the causal factors mentioned. This study aims to identify the characteristics of low resistivity reservoirs in the Upper Cibulakan Formation, West Java Basin, especially from log readings and recognition of what factors support reservoir formation both from the results of core and log analysis as well as selecting combinations for calculating water saturation. From the low resistivity reservoir characteristics, it is hoped that it can be used to find and identify the distribution of similar reservoirs so that potential reservoirs that have been overlooked so far can be identified, discovered and developed which in the end are expected to provide additional contributions both in terms of education and for the oil and gas industry in finding new reserves for oil and gas production in Indonesia.

Wellbore Stability in the Bazhenov Formation Considering the Anisotropy of Elastic Properties of Composing Strata

This paper offers a general approach to the modelling of the wellbore stress state in the Bazhenov formation. In modelling, strata are presented as transversely isotropic objects with a set direction of the anisotropy axis that does not coincide with the vertical line. The paper also presents results of the laboratory Bazhenov formation core analysis. The analysis of core materials yielded correlation dependences for reconstructing stress-strain parameters of a transversely isotropic medium in selected wells. Parameters of the rock anisotropy were analyzed on the basis of standard and non-standard laboratory tests. The solution of the stress state problem in a deformation bounded medium was used to suggest an algorithm of calculating the components of the regional stress state along the well path. RN-SIGMA software tools were used to construct borehole stability models for wells penetrating anisotropic rocks. The effect of the anisotropy of elasticity on a safe drilling mud density window is demonstrated. The results of the calculations show that the anisotropy effects are crucial for calculating a safe drilling window. Examples are given to demonstrate potential effects of the anisotropy in image interpretation as well as in selecting horizontal wellbore drilling directions.

Sedimentation model of the middle Devonian clastic succession of the South Tatar Arch, Pashyian Regional stage, Volga-Ural Oil and Gas Province, Russia

The Pashyian Regional stage (horizon) is the main productive unit of the middle Devonian clastic succession of the South Tatar arch. This article presents, for the first time, maps of the lower and upper parts of the Pashyian, including data on sand-shale ratio, number of sand layers (reservoirs) and thickness, based on the analysis of logging data from 25,000 wells. The maps were created by spatial interpolation of Natural Neighbor and ArcGIS Pro software. The model of sedimentation of the Pashyian Regional stage reflects the interpretation of the plotted maps as well as the synthesis of the results of detailed core investigations (lithological, sedimentological, ichnotextural, petrophysical, etc.) and analysis of archive and published materials. The main points of the proposed model are as follows. The Pashyian sediments were formed in a marine basin, in an environment comparable to that of the middle shelf of modern seas – in an offshore zone dominated by current activity. The basin floor was a relatively flat plateau, on which sandy, silty and clay sediments were simultaneously accumulated. Sediments of all types accumulated during sea transgression. Sea regression caused erosion and destruction of the already formed sediments. Positive landforms of seabed relief, composed predominantly of sandy well-sorted material, comprised autochthonous underwater sand bars, formed by constant currents parallel to the bathymetric contour of the seabed. Underwater sand bars formed extensive systems nearly throughout the entire territory of the modern South Tatar arch. At the same time, allochthonous, poorly sorted, less mature sediments were formed in underwater troughs produced by transversal currents (directed from the shore towards the sea). The proposed model explains the consistent thickness of the Pashyian Regional stage, the mosaic distribution of sand bodies over the area, and the lens-like shape of the sand and siltstone reservoirs. The model can be extrapolated to other stratigraphic intervals of the Devonian clastic succession with similar sedimentological features.

Well-Logging Constraints on Gas Hydrate Saturation in Unconsolidated Fine-Grained Reservoirs in the Northern South China Sea

Recently, drilling wells have encountered rich gas hydrates in fine-grained sediments in the northern South China Sea. Gas hydrate in fine-grained sediments is very heterogeneous, and its physical properties are different from those of oil and gas reservoirs. The reliability of the classical logging saturation evaluation models established for diagenetic reservoirs is questionable. This study used four wells in GMGS3 and GMGS4 to evaluate the effects of the application of three typical methods for evaluating saturation with different principles in the unconsolidated fine-grained sediments: nuclear magnetic logging, sigma logging, and the Archie formula. It was found that the value of the lithologic capture cross-section in sigma logging and the rock’s electrical parameters in the Archie formula affect the accuracy of the model. Therefore, to obtain a reliable saturation value for fine-grained sediments, an innovative method for the calculation of resistivity and acoustic time is proposed to estimate gas hydrate saturation based on logging data, which is most consistent with the results of core analysis. The overall relative error of the verification well was 5.87%, whereas that of the density NMR logging method was 56%, showing that the accuracy of the newly proposed resistivity DT logging method’s saturation formula was significantly improved. Finally, a new model-based cross chart was developed, which can rapidly differentiate gas saturation during drilling.

Sedimentological and morphological evidences of Meltwater Pulse 1B in the Southwestern Atlantic Margin

During the post-LGM deglaciation, global temperatures and rates of sea-level rise were influenced by centennial events driven by climate and ice volume variations, leading to cooling/warming climate stages, sea-level slowstands and meltwater pulses (MWP). Sea level fluctuations have shaped the shelf morphology by drowning coastal features and imposing shifts in coastal/shelf sedimentary environments worldwide. The influence of meltwater pulses in the South Atlantic is still not well established. Thus, here, we investigate morphological and sedimentological records of MWP-1B along two distinct areas of the Eastern Brazilian shelf (Espirito Santo and Abrolhos shelf), far-field sites in the western South Atlantic. Sedimentary core analysis, C14 dating, and multibeam data are combined to provide stratigraphic and morphological evidences of sea-level rise acceleration at the end of Younger Dryas, in the South Atlantic. Results of core analysis indicate a coastal environment around 12 kyrs BP and a major shift in sedimentary facies after 11.5 kyrs BP. In both cores, Unit 1 comprises terrigenous (low carbonate content) and mixed facies, and are interpreted as terrigenous/coastal/estuarine facies. Unit 2 is characterized by carbonate/mixed facies, interpreted as open marine deposits. Terrigenous and mixed estuarine/coastal facies (Unit 1) are replaced by open marine facies (unit 2) at 57.3 m in the Abrolhos Depression core (ABDR) and at 47.6 m in the Espirito Santo paleovalley shelf (PVS core). Morphological evidence shows a paleoshoreline at 60 m water depth associated with five drowned and partially filled incised shelf valleys. A depth x age analysis indicates that the sedimentary environment shift and the depth of paleoshorelines are associated with sea-level slowstand during Younger Dryas, followed by a sea-level rise acceleration, representing MWP-1B. These evidences in the western South Atlantic are combined with other morphological and stratigraphic results presented for South Hemisphere far field sites, indicating that the 60 m isobath is an average water depth marking sea-level stabilization during Younger Dryas, and that MWP-1B is potentially a global event.

Development of HFU-based permeability prediction models using core data for characterisation of a heterogeneous Oligocene sand in the Nam Con Son basin

Core data by both routine and special core analysis are required to understand and predict reservoir petrophysical characteristics. In this research, a total number of 50 core plugs taken from an Oligocene sand (T30) in the Nam Con Son basin, offshore southern Vietnam, were tested in the core laboratory of the Vietnam Petroleum Institute (VPI).
 The results of routine core analysis (RCA) including porosity and permeability measurements were employed to divide the study reservoir into hydraulic flow units (HFUs) using the global hydraulic elements (GHEs) approach. Based on five classified HFUs, 16 samples were selected for special core analysis, i.e., mercury injection capillary pressure (MICP) and grain size analyses for establishing non-linear porosity-permeability model of each HFU based on Kozeny-Carman equation, which provides an improved prediction of permeability (R2 = 0.846) comparing to that by the empirical poro-perm relationship (R2 = 0.633).
 In addition, another permeability model, namely the Winland R35 method, was applied and gave very satisfactory results (R2 = 0.919). Finally, by integrating the results from MICP and grain size analyses, a good trendline of pore size distribution index (λ) and grain sorting was successfully obtained to help characterise the study reservoir. High λ came with poor sorting, and vice versa, the low λ corresponded to good sorting of grain size.

Peculiarities of change in the main formation parameters of the KS and PK suite deposits at the Pirallakhi adasy field

In order to analyze the changes in the parameters of the main reservoir parameters by area and depth at the Pirallahi adasy field, the results of the interpretation of core and logging data were used. For this, the data from well logging and the results of core analysis over the area were collected and analyzed. The results of interpretation of the data of complex well logging studies of wells carried out at the field and the results of analysis of core materials are systematized, statistical analysis is carried out. In the section of each well, reservoirs were identified, characterized by reservoir properties for the KS and PK formations, their porosity, clay content, oil and gas saturation, and permeability were determined. The weighted average values of these parameters for individual wells were calculated and distribution graphs were drawn up. Dependences are established between the parameters of the filtration-capacitive characteristics and correlations are built. The graphs of dependence are built between the parameters characterizing the reservoir characteristics, porosity and clay content, permeability and porosity in the KS and PK formations. As a result of the studies carried out, it can be noted that the Pirallahi adasi field, despite the fact that it has been in operation for a long time, the obtained results of the well logging data interpretation for the KS and PK formations, the identified reservoirs have high properties that the hydrocarbon reserves of the field have not been exhausted. Taking into account the above, it is more expedient to continue drilling and development of new wells in the best possible way, choosing the optimal well network at the Pirallahi adasy field. Keywords: well logging; reservoir characteristics; permeability and porosity; interpretation of core and logging data.

Updated stratigraphic correlations due to lithological composition at the M-II-4 horizon, Aksai field

The results of laboratory-based core analysis and log interpretation are presented in the paper to justify separation of various lithological reservoir types of the productive horizon M-II-4 within limits of the Aksai field. The geological section under study is composed of sandstones, gravelstones, conglomerates with carbonate-clayey cement with clay and sandstone interlayers. According to the results of the work, taking into account all geological and geophysical data, aleuritic-sandy, gravelite-conglomerate with carbonate cement and gravelite-conglomerate with carbonate-clay cement with layers of clays and sandstones types of reservoirs are identified. The results obtained were taken into account when reservoir characterization, recommendations were given for further areas of research work.

Improved ST-FZI method for permeability estimation to include the impact of porosity type and lithology

Permeability represents the flow conductivity of a porous media. Since permeability is one of the most vital as well as the complex properties of a hydrocarbon reservoir, it is necessary to measure/estimate accurately, rapidly and inexpensively. Routine methods of permeability calculation are through core analysis and well tests, but due to problems and weaknesses of the aforementioned methods such as excessive costs and time, these are not necessarily applied on neither in all wells of a field nor in all reservoir intervals. Therefore, log-based approaches have been recently developed. The goal of this research is to provide a flowchart to estimate permeability using well logs in one of Iranian south oil fields and finally to introduce a new algorithm to estimate the permeability more accurately. Permeability is firstly estimated using artificial neural network (ANN) employing routine well logs and core data. Subsequently, it is estimated using Stoneley-Flow Zone Index (ST-FZI) and is compared with the results of core analysis. Correlation coefficients in permeability estimation by artificial neural network and Stoneley-FZI are R2 = 0.75 and R2 = 0.85, respectively. On the next step, an improved algorithm for permeability prediction (improved ST-FZI) is presented that includes the impact of lithology and porosity type. To improve the permeability estimation by ST-FZI method, electro-facies clustering based on MRGC method is employed. For this purpose, rock pore typing utilizing VDL and NDS synthetic logs is employed that considers the porosity types and texture. The VDL log separates interparticle porosity from moldic and intra-fossil porosities and washes out and weak rock-type zones. Employing MRGC method, three main facies are considered: good-quality reservoir rock, medium-quality reservoir rock and bad-quality (non-reservoir) rocks. Permeability is then estimated for each group employing ST-FZI method. The estimated permeability log by improved ST-FZI method shows better match with the measured permeability (R2 = 0.93). The average error between estimated and measured permeability for ANN, ST-FZI method and improved ST-FZI method is 1.83, 1.18 and 0.796, respectively. The increased correlation is mainly due to involving the impact of porosity types on improved ST-FZI method. Therefore, it is recommended to apply this algorithm on variety of complicated reservoir to analyze its accuracy on different environments.

Facies models of the Achimov Formation of East-Urengoiskoe license as the basis for optimizing exploration and field development patterns

The results of sedimentological core analysis of the Achimov Formation (Upper Valanginian, Lower Cretaceous) confirm that it was formed by higher efficiency systems of submarine fans in (relatively) deep marine basin. Lithofacies models of Ach5-6 were generated, well correlation was performed based on the comprehensive analysis of core, well logging and seismic data. Distributary channels and proximal parts of depositional lobes are characterized by the best reservoir properties.

Identifying flow units by FA-assisted SSOM—An example from the Eocene basin-floor-fan turbidite reservoirs in the Daluhu Oilfield, Dongying Depression, Bohai Bay Basin, China

Accurate identification of flow units is essential in oil and gas development. In this study, integrated core, log, and production data from the Eocene basin-floor-fan turbidite reservoir demonstrates a new method to identify flow unit using factor analysis (FA) and supervised-mode self-organizing-map neural network (SSOM). The reservoirs were classified into four types of flow units (I, II, III and IV). Five principal factors were extracted through factor analysis on thirteen evaluation parameters for reflecting the characteristics of basin-floor-fan turbidite reservoirs. Then using the five principal factors as the input, the flow unit prediction model was established based on SSOM. The prediction results of flow unit based on FA-SSOM are consistent with the results of core analysis and test oil conclusion, which have a good classification effect. Therefore, the prediction model based on FA and SSOM provides an effective way for fine reservoir interpretation. The established FA-SSOM model is further compared with Linear Discriminant Analysis (LDA) and Back Propagation (BP) neural network and has the best prediction. This study also sheds light on the remaining oil development by linking the identified flow unit type with daily production data.

Introducing optimized validated meshing system for wellbore stability analysis using 3D finite element method

Finite element method (FEM) as a powerful tool for studying stress and strain status is being extensively employed in geotechnical studies. As the initial and boundary conditions, element type, and meshing system heavily affect the accuracy and precision of the results obtained from FEM, in this research we present a novel approach which is optimized and validated by the results obtained from reality. At first a mechanical earth model (MEM) was constructed using different well logging data, results of core analysis, and drilling reports for one of the central Iranian carbonate reservoirs. Then, a depth range was selected in the pay zone of a vertical well for FEM simulation. The selected depth range consists of two different zones: the upper zone with normal faulting regime and the lower zone with strike-slip faulting regime.After studying different model sizes, mesh densities, element types, and boundary conditions, a cylindrical model consist of a combination of regular and irregular hexahedral elements for far-field region, and fine-grained tetrahedral elements for near-wellbore region was obtained. Two different approaches were selected for FEM modeling, in the first approach a pre-drilled well was considered in the model, and in the second approach the model geometry before drilling without a pre-drilled well was subjected to an initial state of stress, removal of the elements, and loading which simulate the drilling process using a certain mud weight. The results of simulating the state of strains around the wellbore using this meshing system in both FEM approaches had acceptable adoption with drilling data.

Hot water flooding and cold water flooding in heavy oil reservoirs

ABSTRACTMurphy Oil’s Seal asset in the Peace River Oil Sands region in North West Alberta, Canada, has a remarkable capacity for implementation of Enhanced Oil Recovery (EOR) processes. Some wells in Seal have shown strong historical production due to excellent permeability and the foamy solution gas drive mechanism. The reservoir characteristics in this portion are highly variable where determining the most suitable EOR method is very challenging. Two of the potential EOR candidates which have been evaluated for this portion include conventional cold waterflooding and hot waterflooding. Results of EOR core analysis, logging, detailed seismic and petrophysical studies, and history-matched reservoir simulation and forecast indicate that cold waterflooding outperforms hot waterflooding. This is surprising as hot waterflooding is believed to outperform cold waterflooding in most medium heavy oil reservoirs.

Mineralogical modelling and petrophysical parameters in Permian gas shales from the Roseneath and Murteree formations, Cooper Basin, Australia

To estimate the resources of Permian Roseneath and Murteree gas shales in the Cooper Basin, Australia, geochemical analysis, log interpretation and core analysis techniques were combined to conduct mineralogical modelling and define petrophysical parameters of both formations. With the sedimentologic, petrographic, SEM and XRD data derived from analysis of cores and cuttings, a mineralogical model was built for target formations. Moreover, based on the results of conventional core analysis, wireline logging, SEM analysis, XRD analysis, and geochemical and petrographic analysis, a petrophysical model was established for key wells. Then, these models were used to analyse the mineral composition and petrophysical properties of Roseneath and Murteree gas shales. The results show that both Roseneath and Murteree gas shales are composed of clay, quartz, carbonate and kerogen, as well as a small quantity of auxiliary minerals (e.g. feldspar and siderite). According to porosity, permeability, TOC, water saturation, mineral composition and other parameters, it is concluded that Murteree shale has higher potential than Roseneath shale within the basin, except for areas in and around Well Encounter 1 where Roseneath exhibit excellent potential.

Reconstruction of cell homogenized macroscopic cross sections for analyzing fast and thermal coupled cores using the SRAC system

A fast and thermal neutron coupled core adopts blanket fuel assemblies with zirconium hydrides in the core for negative coolant void reactivity. Conventional neutronics calculation methods have been developed for analysis of a fast core or thermal core, in which the coarse-group macroscopic cross sections of fuel assemblies are prepared without including the effect of the surrounding fuel assemblies. However, such methods are not adequate for analyzing fast and thermal neutron coupled cores where the intra-assembly and inter-assembly heterogeneity effects must be precisely taken into account. Recently, a concept of reconstruction of cell homogenized macroscopic cross sections has been proposed to take into account effects of inter-assembly heterogeneities on macroscopic cross sections used in the reactor core analysis and successfully applied based on a Monte Carlo method. In the present study, a reconstruction method of cell homogenized coarse-group macroscopic cross section for analyzing fast and thermal coupled cores is developed based on a deterministic neutronics calculation code system, SRAC. Three types of fixed source calculations for unit assembly cell geometry are performed independently of the specific core layouts and their results are combined with the results of core analysis to produce cell homogenized coarse-group macroscopic cross sections. Numerical results show that the heterogeneity effects can be adequately reflected in the reconstructed macroscopic cross sections with the proposed method. When the number of energy groups is small, the proposed method gives poor results in the transitional energy groups from resonance to thermal energy. Therefore, it is necessary to increase the number of energy groups in this energy range.

Los sistemas de producción lítica en el Paleolítico Superior Final: El caso del nivel Asup del Molí del Salt (Vimbodí i Poblet, Tarragona)

In this paper we present the results of core analysis of Asup level from Moli del Salt site (Vimbodi i Poblet, Tarragona/Spain), dated to ca. 11 ka 14C BP (c. 12700-13000 years cal BP). This analysis was carried out from the methodological proposals raised by various authors as Guilbaud (1995) and Vaquero (1997). With this study, we try to define the operative field to better understand the variability of knapping strategies and the possible continuity relations among them used by the populations who occupied the site during the Final Upper Palaeolithic. This technological variability has been corroborated. In general, the cores present facial hierarchy and the extractions are long and narrow. Thus, there is a significant presence of blade knapping. In addition, we have differentiated six types of siliceous raw materials.

Comparison of core sampling and visual inspection for assessment of concrete sewer pipe condition

Sewer systems are costly to construct and even more costly to replace, requiring proper asset management. Sewer asset management relies to a large extent on available information. In sewer systems where pipe corrosion is the dominant failure mechanism, visual inspection by closed circuit television (CCTV) and core sampling are among the methods mostly applied to assess sewer pipe condition. This paper compares visual inspection and drill core analysis in order to enhance further understanding of the limitations and potentials of both methods. Both methods have been applied on a selected sewer reach in the city of The Hague, which was reportedly subject to pipe corrosion. Results show that both methods, visual inspection and core sampling, are associated with large uncertainties and that there is no obvious correlation between results of visual inspection and results of drill core analysis.

Reducing Uncertainty in Reservoir Model Predictions: from Plume Evolution to Tool Responses

With the transition of geological carbon dioxide (CO2) storage from pilot to industrial scale, risk analysis has become pivotal for assessing the suitability of a storage site in satisfying regulatory requirements during and after injection. Risk analysis is inherently related to uncertainty quantification. This paper extends our previously developed workflows to quantify and propagate uncertainty from geological, geophysical and petrophysical data to project-level performance metrics including capacity, injectivity, and containment characteristics. We perform global sensitivity analysis to quantitatively link uncertainty in the computed performance metrics to the uncertainties in the underlying reservoir parameters. This enables one to identify measurements to reduce uncertainty estimates of reservoir performance metrics and predicted monitoring tool responses. We illustrate this workflow in a pre-injection uncertainty study for the Illinois Basin Decatur Project (IBDP). The reservoir model is built using geostatistical approach based on all available geophysical and petrophysical data including 3D seismic interpretation, results of special core analysis and injectivity tests. We focus on three groups of performance predictions: spatial extent of the CO2 saturation profile, partitioning of CO2, and predicted responses seen with the Westbay* multilevel groundwater characterization and monitoring system installed in the monitoring well. Based on multiple realizations of the reservoir model we evaluate the uncertainty range in pressures and CO2 saturations at the Westbay monitoring zones during the life of the project. Predicted measurements at the monitoring zones covering a depth interval between 1950 m and 2120 m are analyzed. Global sensitivity analysis is used to identify the key petrophysical properties of the reservoir whose uncertainties have the most effect on the uncertainty of the reservoir performance and measurement predictions. Results of this analysis provide valuable insight for targeted site characterization and design of the monitoring program.

Prognostic Importance of Gleason 7 Disease Among Patients Treated With External Beam Radiation Therapy for Prostate Cancer: Results of a Detailed Biopsy Core Analysis

To analyze the effect of primary Gleason (pG) grade among a large cohort of Gleason 7 prostate cancer patients treated with external beam radiation therapy (EBRT). From May 1989 to January 2011, 1190 Gleason 7 patients with localized prostate cancer were treated with EBRT at a single institution. Of these patients, 613 had a Gleason 7 with a minimum of a sextant biopsy with nonfragmented cores and full biopsy core details available, including number of cores of cancer involved, percentage individual core involvement, location of disease, bilaterality, and presence of perineural invasion. Median follow-up was 6 years (range, 1-16 years). The prognostic implication for the following outcomes was analyzed: biochemical recurrence-free survival (bRFS), distant metastasis-free survival (DMFS), and prostate cancer-specific mortality (PCSM). The 8-year bRFS rate for pG3 versus pG4 was 77.6% versus 61.3% (P<.0001), DMFS was 96.8% versus 84.3% (P<.0001), and PCSM was 3.7% versus 8.1% (P=.002). On multivariate analysis, pG4 predicted for significantly worse outcome in all parameters. Location of disease (apex, base, mid-gland), perineural involvement, maximum individual core involvement, and the number of Gleason 3+3, 3+4, or 4+3 cores did not predict for distant metastases. Primary Gleason grade 4 independently predicts for worse bRFS, DMFS, and PCSM among Gleason 7 patients. Using complete core information can allow clinicians to utilize pG grade as a prognostic factor, despite not having the full pathologic details from a prostatectomy specimen. Future staging and risk grouping should investigate the incorporation of primary Gleason grade when complete biopsy core information is used.