Nuclear Instruments and Methods in Physics Research A ] (]]]]) ]]]–]]]

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

Pleas

journal homepage: www.elsevier.com/locate/nima

Petrophysical analysis of limestone rocks by nuclear loggingand 3D high-resolution X-ray computed microtomography

M.F.S. Oliveira a, I. Lima a,b,n, P.L. Ferrucio c, C.J. Abreu c, L. Borghi c, R.T. Lopes a

a Nuclear Instrumentation Laboratory, COPPE-PEN, UFRJ, P.O. Box 68509, 21941-972 Rio de Janeiro, RJ, Brazilb Department of Mechanical Engineering and Energy, IPRJ-UERJ, Nova Friburgo, RJ, Brazilc Geology Department, Geosciences Institute, Rio de Janeiro Federal University, Rio de Janeiro, RJ, Brazil

a r t i c l e i n f o

Keywords:

X-ray

Rock

Nuclear logging

Tomography

Petrophysics

Porosity

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.02.019

esponding author. Tel.: +5521 25627 308.

ail addresses: inaya@lin.ufrj.br, inayacorrea@g

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a b s t r a c t

This study presents the pore-space system analysis of the 2-ITAB-1-RJ well cores, which were drilled in

the S~ao Jose do Itaboraı Basin, in the state of Rio de Janeiro, Brasil. The analysis presented herein has

been developed based on two techniques: nuclear logging and 3D high-resolution X-ray computed

microtomography. Nuclear logging has been proven to be the technique that provides better quality

and more quantitative information about the porosity using radioactive sources. The Density Gamma

Probe and the Neutron Sonde used in this work provide qualitative information about bulk density

variations and compensated porosity of the geological formation. The samples obtained from the well

cores were analyzed by microtomography. The use of this technique in sedimentary rocks allows

quantitative evaluation of pore system and generates high-resolution 3D images (�microns order). The

images and data obtained by microtomography were integrated with the response obtained by nuclear

logging. The results obtained by these two techniques allow the understanding of the pore-size

distribution and connectivity, as well as the porosity values. Both techniques are important and they

complement each other.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

In geophysical well logging, many physical properties can beused together to characterize the geology of the borehole. In thiscontext, the understanding of the petrophysical properties ofrocks such as porosity is very important because this parameterrepresents the direct capacity of storage fluids in the rock. Bydefinition, porosity is the ratio of pore volume to the total bulkvolume of the formation, expressed in percentage; such rate canbe absolute or effective. The porosity of the borehole can becompletely different along its depth because the pore-spacecharacter tends to vary significantly from formation to formation;many kinds of lithology can be crossed by the well. For thisreason, a porosity estimate for each lithology group existinginside the well provides a more reliable analysis and a moreaccurate effectiveness assessment of the pore-space system.

The petrophysical analysis carried out in this study corre-sponds to a single well (2-ITAB1-RJ); it has reached 70 m of depthwith continuous cores and has a recovery of around 50%. This wellwas drilled in the S~ao Jose do Itaboraı Paleontological Park, Rio deJaneiro, Brazil.

Techniques that allow the understanding of the porous distribu-tion within the rocks and its quantification are important for reservoir

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mail.com (I. Lima).

, et al., Nucl. Instr. and Met

characterization purposes. One of the most useful method that can beused in geophysical analysis is 3D high-resolution computed micro-tomography (m-CT), which can provide an investigation of the pore-space in a nondestructive way. This technique could be used in orderto provide a 3D visualization of the inner characteristics of the rocksand also some quantitative data on the pores without making use ofany kind of chemical treatment or sample preparation whatsoever.Many studies have demonstrated the effectiveness of m-CT in under-standing the porosity and pore-size distribution within rocks [1–3].

The goal of this paper is to investigate the pore-space systemof recovered cores of the 2-ITAB1-RJ well by combining twonuclear techniques: 3D high-resolution X-ray microtomographyand nuclear logging. Nuclear logging is widely used in oil industryin order to obtain information on the density and porosity of theformations that have been drilled [4]. From the combination of m-CT and nuclear logging, it was possible to determine the region ofinterest inside the well that would be subjected to 3D quantifica-tion of porosity and pore-size distribution analysis through the3D visualization of the internal structure.

2. Methodology

This paper presents the pore-space system analysis of the 2-ITAB-1-RJ well, which has been elaborated based on two techni-ques: nuclear logging and 3D high-resolution X-ray computed

h. A (2011), doi:10.1016/j.nima.2011.02.019

Table 1Regions of interest inside the 2-ITAB1-RJ well.

Interval Depth (m) /total Resolution (lm) Lithology

I 4–10 1.670.5 53.58 Marls with limestone

II 10–32.5 6.770.3 63.44 Travertine limestone with vugs filled with calcite

III 32.5–50 0.870.06 60.62 Biotite gneiss

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microtomography. Descriptions of well cores and of the techniquesused in this study are presented below.

2.1. Samples

The well named 2-ITAB-1-RJ has been drilled in S~ao Jose doItaboraı, Rio de Janeiro State. The Itaboraı Basin, originated in thelate Paleocene age, is one of the smallest basins in Brazil (around1 km2). The basin was filled by a sequence of clastic andchemically deposited (travertine) limestones that were verticallycut by fissures [5]. The 2-ITAB-1-RJ well is 70 m deep, withcontinuous cores. The samples were collected from the coresbased on the nuclear logging technique. In order to estimateporosity values a total of 40 samples were analyzed and measuredby m-CT. The macroscopic description of the samples used tocorrelate the pore-space system is presented in Table 1.

Fig. 1. Nuclear logging setup.

2.2. Nuclear logging

Nuclear logging has been obtained by two nucleartools—Density Gamma Probe (LSD) and Neutron Sonde (NEUT).These tools were primarily used as porosity logs.

The LSD tool consists of a radioactive source of Cs-137 with3.7 GBq, which emits 662 keV gamma ray photons into theformations. The gamma rays interact mostly by Compton scatter-ing. The number of Compton scattering collisions is relateddirectly to the number of electrons in the formation. Electrondensity is related to the true bulk density (rb), which depends onthe density of the rock matrix material, the porosity of theformation and the density of the fluids filling the pores [4]. Thephotons scattered into the formation are detected by two scintil-lation detectors (NaI (Tl)). This is the most used detector due itsfeature to radiation detection. The detector responds linearly for alarge energy range to gamma rays and it is easy to obtain thecrystal in different sizes.

Neutron Logs respond primarily to the amount of hydrogen inthe formation. This log reflects the amount of liquid-filledporosity. Fast neutrons emitted by americium–beryllium(AmBe-241) source with 37 GBq collide with the nuclei of theformation materials and lose part of their energy. The greatestenergy loss occurs when the neutron strikes a nucleus whosemass is practically equal to its own, which would correspond tothe nucleus of a hydrogen atom. The decrease in speed of theneutrons depends largely on the amount of hydrogen in theformation. The neutrons are detected by a Helium-3 detectorand the counting rate at the detector increases whenever there isa small hydrogen concentration and vice-versa [4]. Nuclearlogging was carried out in a part of the well, with a continuouslogging detection of about 50 m of its depth. The detection speedfor each of the tools was 4 m/min for the LSD and 5 m/min for theNEUT. These tools granted us qualitative information about therock formation. The logs that have been obtained from the2-ITAB-1-RJ well did not provide any quantitative informationabout the well formation. However, it is possible to know the bulkdensity variations and the amount of hydrogen within the well.

Please cite this article as: M.F.S. Oliveira, et al., Nucl. Instr. and Met

2.3. Microtomography technique

The m-CT images were obtained using a Skyscan 1173 scanner.This device consists of a microfocus X-ray source operated at130 kV and 61 mA (spot size o5 mm) and a flat panel detector(2240�2240 pixels). The samples were analyzed using filters ofaluminum (1.0 mm Al) and copper (0.2 mm Cu) in order to correctbeam hardening effects. During the acquisition, the object under-goes a 3601 rotation process, with a fixed rotation step (0.61). Ateach angular position, a transmission image is acquired. The conebeam acquisition saves all these projection images as 16 bit TIFFfiles on the hard disk. When the acquisition is finished, thereconstruction can be started. A 3D cone beam reconstructionalgorithm, such as Feldkamp [6], is used in order to take intoaccount the thickness of the object. When the reconstruction isfinished, an image will be generated and the 3D image will becreated. The porosity estimate is performed using the imagetreatment software CTAns. The two-dimensional image under-goes a treatment procedure where the region of interest (ROI) inthe rocks is appointed. The image is then binarized in grayscaleand the determination of the optimal threshold is carried out. Theporosity is estimated based on the 3D analysis of a binary image

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(Fig. 1) corresponding to the volume of binarised pores found inthe sample. The image treatment software CTAns provides anaverage range of reliability of 95% of the image voxels.

3. Results and discussion

The logs obtained from nuclear logging (LSD and NEUT) shownin Fig. 2 were correlated with the lithological log of the 2-ITAB-1-RJwell in order to allow a better understanding of the porosityvariation within the rock formation. The logs are presented incounts per second (CPS) of the scattering radiation by the forma-tion which returns it to the detector. The main radioactive emis-sions of interest in a borehole geophysics are gamma rays andneutron and the matching of LSD and NEUT logs has supported thedefinition of three regions investigated herein: Region I (4–10 m) ischaracterized by low porosities, which are the response of highcounts per second in NEUT and little decrease in counts in LSD;Region II (10–32.5 m) is characterized by moderate porosities withlow CPS in both LSD and NEUT; Region III (32.5–50 m) withcharacteristic intercalations of moderate and low porosities as wellas in the nuclear loggings.

Region I is composed principally of a mixture of limestone andclay (marls). In this region, a decrease of CPS in LSD from 5 up to10 m can be verified, which means that only few photons couldreach the detector, being thus retained by the rock formation. Inother words this means that many photons have interacted withthe formation, leading to the conclusion that the electronic

Fig. 2. Illustration of data

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density of the rocks is greater than the electronic density of rocksfrom 4 up to 5 m of depth. The electronic density is directlyproportional to bulk density and inversely proportional to poros-ity. Since the density is decreasing, the porosity of rocks isincreasing. The CPS in NEUT expresses the concentration ofhydrogen present in fluids within the pore-space. In the Region I,there is an increase in the counts of neutrons between 5 and 10 mdepth when compared with the counts between 4 and 5 m depth.It means that the hydrogen concentration is rather small, whichconsequently results in a larger amount of solid components, sothe correlation between the nuclear logs shows that the Region Iis formed of rocks with little void space (pores).

In Region II, there has not been much variation in bulk densityand in the hydrogen concentration. The counts here are less thanthe counts detected in the Region I, leading to the conclusion thatthe void space in the Region II is higher than in the Region I. Thelithological log shows that this region is formed by travertinelimestone, marls with limestone and diamictite with marl matrix.In Region III, a metamorphic rock with high bulk density namedbiotite gneiss predominates (Fig. 3).

The m-CT was applied to determine the 3D porosity quantifica-tion (Table 1) and 3D visualization of the pore-size distribution.The porosity estimated by m-CT is the total porosity (ftotal), whichrepresents all of the void spaces in solid components. X-raysinteract with the sample and its composition and density varia-tions. This technique is capable of providing high-resolutionresults (�microns), which means that the precise dimensions ofany void spaces can be detected up to micrometers. A total of

processing in m-CT.

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Fig. 3. Nuclear Logs and lithological log of 2-ITAB-1-RJ sketch scheme.

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1851 2D slices were obtained from each analyzed sample, andthey were used to build a 3D volumetric image. The porosityvalues presented in Table 1 correspond to the average values ofthe samples extracted from each one of the regions, and thedeviation associated with those values is the standard deviation.The porosity distribution is presented in Fig. 4, where it is alsopossible to visualize the connectivity of the pores. The samplespresented in Fig. 4 are representative specimens of the 3D modelof the 2-ITAB-1-RJ well’s pore-space system.

Please cite this article as: M.F.S. Oliveira, et al., Nucl. Instr. and Met

The nuclear logging was used to evaluate the porosity varia-tions of 2-ITAB-1-RJ well; from the nuclear logs it was possible toaccess three regions with important variations. These regionsindicate that 2-ITAB-1-RJ is formed of rocks with moderate andlow pore volume. The use of m-CT to analyze the samplesrecovered by the well allowed the estimation of the porosityvalues for each region and understanding of the pores’ distribu-tion and connectivity. Therefore, both complementary techniquesallowed characterizing the pore-space system of the 2-ITAB-1-RJ

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Fig. 4. 2D Slices and 3D reconstruction of samples at different depths: 7.70, 11.6, 26.90 and 39.0 m from A to D (top to bottom). Middle column presents a 3D

reconstruction of the matrix and the pores; right column shows a reconstruction of the pores’ dsitribution.

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well. The results suggest that the pore-space system consis-ts of pores with low connectivity, lying within depths up to32.5 m, whereas below this point, there is a predomination ofisolated pores, which are heterogeneously distributed up to50 m depth.

Please cite this article as: M.F.S. Oliveira, et al., Nucl. Instr. and Met

4. Conclusions

Both m-CT and nuclear logging proved to be useful and reliabletechniques for the characterization of pore-space system/connec-tivity and its distribution along the well and cores. This method of

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investigation has proven to be a potentially good indicator of thequality of carbonate or siliciclastics reservoirs. The application ofthis technology to the 2-ITAB-1-RJ well allowed the identificationof three distinct regions and their typical pore system.

Acknowledgments

This work was partially supported by Agencia Nacional doPetroleo, Gas Natural e Biocombustıveis (ANP), Conselho Nacionalde Desenvolvimento Cientıfico e Tecnologico (CNPq) and Fundac- ~aode Amparo �a Pesquisa do Estado do Rio de Janeiro (FAPERJ). Wewould like to thank Prof. Sergio Bergamaschi and Anderson Baptista

Please cite this article as: M.F.S. Oliveira, et al., Nucl. Instr. and Met

from University of Rio de Janeiro State for the macroscopic geolo-gical description.

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