INAA ELEMENTAL ANALYSIS OF SHALESAMPLES FROM OIL WELLS IN NIGER DELTA,

NIGERIA.

UGWUMADU CHINONSO. E

(PHY/2008/054)

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF PHYSICS, FACULTY OF

SCIENCE OBAFEMI AWOLOWO UNIVERSITY, ILE–IFE, OSUN STATE, NIGERIA

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE

BACHELOR OF SCIENCE DEGREE (B.Sc.) IN ENGINEERING PHYSICS

FEBUARY 2013

ABSTRACT

In this work, Instrumental Neutron Activation Analysis

(INAA) technique, was used to determine the elemental

concentration of shale samples collected from oil wells in Niger

Delta, Nigeria. Irradiation of the packed samples was carried out

using the Portuguese Research Reactor (PRI) at a neutron flux of

8 × 1011 n/cm2, after which the spectra were acquired using a HPGe

detector, and the elemental concentrations were determined the

K0-IAEA software (version 3.21). A total of eleven samples were

analyzed and the elements detected include As, Ba, Br, Ce, Cu,

Fe, Th, La, Na, Rb, Sb, Sc, Sm, and Zn.

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND TO THE STUDY

Naturally occurring radionuclides are part of the soil from

the earth’s original crust (primordial radionuclides) and also

occur through cosmic ray interactions (cosmogenic radionuclides)

and human activities (anthropogenic radionuclides) (USEPA, 2007).

Some industries make use of radionuclides as part of their

production processes. In an oil rich region, oil and gas

exploration are technological processes that involve the use of

radionuclides. Petroleum (or oil) and natural gas are particular

examples of sedimentary organic matter. The oil and gas industry

makes use of anthropogenic radionuclides (sealed sources) for

logging and exploration processes, therefore oil, gas and oil

field brines frequently contain radioactive materials. These

materials accumulate in pipes used to remove and process

petroleum and natural gas. Radioactive materials could also be

found in products and in the industrial waste ( Eisenbud and

Gesell, 1997).

Radioactivity from oil and gas production is, majorly, of

natural origin. Naturally occurring radioactive elements such as

uranium, radium and radon are dissolved in very low

concentrations during normal reactions between water and rock or

soil. Groundwater that coexists with deposits of oil can have

unusually high concentrations of dissolved constituents that

build up during prolonged periods of water/rock contact. Uranium

and thorium compounds are mostly insoluble in water (Elizabeth et

al., 1994), therefore these compounds remain in the underground

reservoir and sometimes accompany oil and gas that are brought to

the surface. Many oil-field brines are particularly rich in

chloride, enhancing the solubility of other elements including

the radioactive element radium. Radium concentrations tend to be

higher in more saline water. Some of this saline, radium-bearing

water is also extracted with the oil and gas. Some radium and

radium daughter compounds are slightly soluble in water and may

become mobilized when this water is brought to the surface. Thus

nature itself is an obvious source of radionuclides. Assessment

of radionuclides in soils and rocks in many parts of the world

has been on the increase in the past two decades because of their

hazard on the health of the populace ( Mc- Aulay et al., 1988,

Matiullah et al., 2004; Sesana et al., 2006; Veiga et al., 2006;

D. Al-Othmany, 2012; A. S. Alaamer, 2008; A.M. Gbadebo, 2011;

Olise et al., 2010, 2012). This is necessary because

radioactivity in the environment is one of the main sources of

exposure to man while the decay products of 40K, 238U and 232Th

series represent the main external source of radiation to the

human body (Fernandez et al., 1992; Florou and Kritidis, 1992;

UNSCEAR., 2000; Olise et al., 2011

1.2 INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

Neutron Activation Analysis (NAA) is a sensitive analytical

technique useful for performing both qualitative and quantitative

multi-element analysis of major, minor, and trace elements in

samples from almost every conceivable field of scientific or

technical interest. For many elements and applications, NAA

offers sensitivities that are superior to those attainable by

other methods, on the order of parts per billion or better. In

addition, because of its accuracy and reliability, NAA is

generally recognized as the "referee method" of choice when new

procedures are being developed or when other methods yield

results that do not agree. The basic essentials required to carry

out an analysis of samples by NAA are a source of neutrons,

instrumentation suitable for detecting gamma rays, and a detailed

knowledge of the reactions that occur when neutrons interact with

target nuclei.

1.3 Objectives and Scope of Study

1.3.1 Objectives

The objectives of this study are to determine the elemental

content of the samples using INAA technique,

1.3.2 Scope of study

This study shall cover the following:

(a) Collection of sediment samples from oil wells in the Niger

Delta area,

(b) Quantitative determination of the elemental content of the

samples using INAA

CHAPTER TWO

LITERATURE REVIEW

2.1 STUDY AREA

Various works have been carried out in the Niger-Delta area

and the results obtained have been beneficial in one way or

another. Studies in the study area were mostly carried out

using Inductively Coupled Plasma Mass Spectrometry (ICP/MS)

and gamma spectrometric techniques to determine the

physiochemical parameters in order to explain the origin of

heavy metals. Obiajunwa et al. (2002) reported heavy metal

pollution around hydrocarbon production facilities in the

Niger Delta using EDXRF. Samuel et al. (2010), used Gas

Chromatography Mass Spectrometer (GC/MS) to determine

polycyclic aromatic hydrocarbons in sediments and soils from

oil exploration areas of the Niger Delta, Nigeria. Samuel et

al. (2010) reported the presence of polycyclic aromatic

hydrocarbons (PAHs) in sediments and soil from oil exploration

area of the Niger Delta using Gas Chromatography Mass

Spectroscopic (GC/MS) analyses method, Elijah et al. (2008)

also used atomic absorption spectrophotometer to determine

physiochemical parameters. The determination of radionuclides

of organic matter in sediments from the north western Niger-

Delta (Olise et al., 2012), the authors made use of particle

induced x-ray emission (PIXE), which is a nondestructive ion

beam analytical (IBA) method. At present, limited data are

available on the occurrence of radionuclides in sediments of

the Niger Delta area (Ajayi et al., 2010). Also there is a

need to explore the analytical capability of the INAA

technique in terms of its low detection limit.

2.2 ANALYTICAL TECHNIQUE

This study involves the use of INAA for the elemental analysis

of sediments to assess the elemental distribution of radionuclide

found in the sediments. INAA is used to determine the

concentration of trace and major elements in a variety of

matrices. The shale sample is subjected to a neutron flux and

radioactive nuclides are produced. As these radioactive nuclides

decay they emit gamma rays whose energies are characteristic for

each nuclide. Comparison of the intensity of these gamma rays

with those emitted by a standard permit a quantitative measure of

the concentrations of the various nuclides.

The sequence of events occurring during the most common type

of nuclear reaction used for NAA, namely the neutron capture or

(n, ɤ) reaction, is illustrated in Figure 2.1. When a neutron

interacts with the target nucleus via a non- elastic collision, a

compound nucleus forms in an excited state. The excitation energy

of the compound nucleus is due to the binding energy of the

neutron with the nucleus. The compound nucleus will almost

instantaneously de-excite into a more stable configuration

through emission of one or more characteristic prompt gamma rays.

In many cases, this new configuration yields a radioactive

nucleus which also de-excites (or decays) by emission of one or

more characteristic delayed gamma rays, but at a much slower rate

according to the unique half-life of the radioactive nucleus.

Depending upon the particular radioactive species, half-lives can

range from fractions of a second to several years.

Figure 2.1: Diagram illustrating the process of neutron

capture by a target nucleus followed by the emission of

gamma rays

Quarshie et al. (2011) analyzed the concentrations of the

toxic elements As, Cd, Cr, Hg and V in soil and tailings samples

from the Bibiani mining area in the Western Region of Ghana. The

authors used Instrumental Neutron Activation Analysis (INAA) and

reported the impact of mining activity on the Bibiani mining

environment. Vance et al. (1987) determined the concentrations of

17 elements in the nail and hair of 117 subjects from a non-

industrialized environment in the USA, the authors carried out

this analysis using instrumental neutron activation analysis

(INAA). Also Neutron activation analysis was used for measurement

of air pollution in a semi-arid zone in the city of Beer-Sheba,

Israel (Shani et al., 1977)

Sarumi Bolatito (2013) using INAA analyzed soil samples from

Iperindo, Nigeria for the presence of Gold in the samples. Kogo

et al (2009) using INAA reported the presence of major, minor and

trace elements present in Leaf Samples and their effects on the

populations and surrounding of Abuja Metropolis.

CHAPTER THREE

METHODOLOGY

3.1 SITE DESCRIPTION AND SAMPLE COLLECTION

The Niger Delta is located in southern Nigeria, between

latitude 4° and 6° N and longitude 3° and 9° E. It is bounded in the

west by the Benin Flank and in the east by the Calabar Flank, to

the south by the Gulf of Guinea (extending offshore to the

Atlantic) and to the north by older Cretaceous tectonic elements,

such as the Abakaliki Anticlinorium and the Afikpo syncline

(Ejedewa, 1981; Ajayi et al., 2010).

The Tertiary lithosratigraphic sequence of the Niger Delta

consists, in an ascending order, the Akata, Agbada and Benin

Formations. Benin Formation makes up an overall clastic sequence

of about 9,000-12,000-m-thick deposits (Evamy et al., 1978). And

in general, it ranges from the Miocene to recent eras. The

paralic Agbada Formation is a sequence of alternating sandstone

and shales. Oil and gas occurrences in the Niger Delta are

concentrated mainly in the sandstone reservoirs at various levels

of the Agbada Formation (Evamy et al., 1978). Figure 3.1 shows

the map of the study area.

Soil samples were collected from oil well blocks in the

western Niger Delta region of Nigeria. Samples were transferred

into sample bags avoiding cross-contamination of the samples. The

samples were then packed into plastic bags from the areas of

surveillance and brought to the laboratory.

Figure 3.1: Map of Study Area

3.2 SAMPLE PREPARATION FOR INAA

Sample preparation for INAA is essentially limited to

weighing a suitable amount of homogenous powder into small quartz

or plastic vials for reactor irradiation and subsequent gamma

spectrometric analysis. The essential issues are to avoid

contamination by careful selection of the vial material, cleaning

the vials and verification of the blank levels. There must be

tight specification on the vial dimensions and wall thickness in

order to reproduce counting efficiencies and gamma ray

attenuation effect.

At the laboratory, the soil samples were air-dried until

constant weight was achieved (when same weight was observed after

measuring the sample thrice). A pestle and mortar washed with

non-metallic liquid soap and thoroughly rinsed with distilled

water and analar grade ethanol was used in crushing (pulverizing)

the samples. Samples, each of about 150 mg, were weighed into

ultra-pure polyethylene containers using Sartorius Research

Balance (model R 180D). Each polyethylene film-contained sample

was then put in the rabbit capsule and heat sealed. The

polyethylene film and rabbit capsule have been cleaned by soaking

in 1:1 HNO3 and water for three days and rinsed with de-ionized

water.

3.3 CHARACTERIZATION OF IRRADIATION AND COUNTING FACILITY FOR INAA

3.3.1 Characterization of Irradiation Facility

The Portuguese Research Reactor (RPI) is a 1 MW pool-type

reactor with a maximum thermal neutron flux of 2.5×1013 n/cm2/s.

Its irradiation facilities include 7 beam tubes, a thermal column

and two rabbit systems. It was built by AMF Atomics Inc., in the

period of 1959/61 and its design follows closely the one of the

Battelle Research Reactor in the USA (Marques, 2008).

The reactor core configuration (Figure 3.2) consists of

seven standard and five control assemblies, with appropriate

dimensions of (8 × 8 × 60) cm (Fernandes et al., 2010). The core

is reflected by a graphite thermal column, beryllium and light

water. Four dummy assemblies were introduced in order to improve

the thermal hydraulic safety margin. The fuel, dummies and

beryllium reflectors are mounted on a grid plate with a 9 × 6

pattern. Samples are routinely irradiated in the free grid

positions, in the dummies and in cavities at some beryllium

reflectors.

The dummies consist of the same external structure as fuel

assemblies but contain only an aluminium tube instead of fuel

plates in the central region allowing sample irradiation. Neutron

fluence rates were measured in the water channel No. 5 of

standard fuel assemblies S3, S5, S6 and S7, in all dummies, in

the beryllium reflector Be-N, and in the free grid positions.

Figure 3.2: The N2-P1/2 Configuration of the RPI Core.

Be: Beryllium Reflector; C: Control Fuel Assembly Hosting a

Control Rod;

D: Dummy Assembly; S: Standard Fuel Assembly; R: Control Fuel

Assembly Hosting a Regulating Rod

The Free Grid Positions where Measurements were Made Are

Identified in Italics (Fernandes et al. 2010).

3.3.2 Characterization of Counting Facility

There are different types of counting facilities (detectors)

that are used for gamma ray analysis. The more commonly used are

the Sodium- Iodide Thallium doped (NaI(Tl)) scintillator

detectors and the Germanium detectors. The detector that was

considered during this experiment is the germanium detector.

Basically there are two types of germanium detectors namely

lithium drifted crystal of purified germanium detector, and the

high purity germanium detector (HPGe). The high purity germanium

detector was specifically used during this experiment due to its

various advantages, which include the re-warming of samples into

room temperature, the resolving of multiple gamma rays from

samples, high resolution and direct sample analysis for gamma

emitting radionuclides. The usual objective of the measurement by

gamma ray spectrometer is the determination of the number and

energy of the photons emitted by the source. Also the peak and

peak area in the spectra obtained are to be determined. The peak

location is a measure of the gamma energy while the peak area is

proportional to the photon emission rate.

Gamma spectra were acquired with an ORTEC and a CANBERRA

liquid-N2-cooled, High Purity Germanium (HPGe) detector with a

relative efficiency of 30 and 25% respectively and a resolution

of 1.85kev at 1.33Mev gamma ray line of 60Co, connected to a

4096 multi-channel analyser (MCA), associated electronic module

all made by ORTEC and a personal computer. The efficiency curves

of the detector system at near and far source-detector geometries

were determined by standard gamma-ray sources over the energy

range of 59.5-2254keV and were extended to 4000keV

Elementary concentrations were determined with the K0-IAEA

software (version 3.21). Quality control was asserted through

simultaneous analysis of certified reference material: GBW07404.

The reference material was prepared in the same way as the

samples and irradiated simultaneously with them.

3.4 Sample Irradiation in INAA

For irradiation, two schemes were adopted based on the half-

life of product radionuclide. For elements leading to short-lived

activation products, each of the samples was packed and sealed in

7cmₔ rabbit capsules and sent for irradiation in turn in an outer

irradiation channel B4, where the neutron flux is low. The choice

of the outer irradiation channel was to eliminate corrections due

to nuclear interferences caused by threshold reactions.

For elements leading to long-lived activation product,

samples were wrapped in polyethylene films and then packed in a

stack inside the 7cmₔ rabbit capsule and sealed for irradiation

in any of the small inner irradiation channel (i.e. A1, B1, B2

and B3) taking the advantage of the maximum value of thermal

neutron flux in the inner channels.

3.5 DETECTOR ENERGY CALIBRATION

The high purity germanium detector is calibrated using gamma

energy peaks of Co-60, Cs-137 and Eu-152 sources to enable full

energy peak efficiency. The calibration curve produced is the

spectrum which displayed a plot of total accumulated counts in a

channel versus the channel number. The energy range is about 0 to

3000keV.

The area under the gamma ray peak is a direct measure of the

concentration of the radionuclide associated with that gamma ray.

Counts

Figure 3.3: Energy Calibration of Detector Energy

3.6 SAMPLE ANALYSIS IN INAA

Energy (keV)

Elemental determinations of the samples were carried out

at the Portuguese Research Reactor (pool-type reactor; maximum

nominal power: 1 MW), through k0-standardized, instrumental

neutron activation analysis (k0-INAA). The samples were

irradiated for 1hour at a thermal-neutron fluence rate of 1.15 x

1012 n cm-2 s-1 together with one disc of an aluminum-gold alloy

with thickness: 125µm and diameter: 5mm as comparator. An ORTEC

and a CANBERRA, liquid-N2-cooled, high purity germanium detector

(1.85 keV resolution at 1.33 MeV, both; 30 and 25% relative

efficiency, respectively) was used for sample analysis. The

resulting gamma ray produced is subjected into amplification

through the use of an amplifier. The two primary functions of

the linear amplifier in the pulse processing chain are pulse

shaping and amplitude gain. The unit accepts tail pulse as an

input, often of either polarity and produces a shaped linear

pulse with standard polarity and span. The amplification factor

or gain required varies greatly with application. The gain is

normally adjustable over a wide range through a combination of

coarse and fine controls. These can be used to control the output

pulse to avoid distortion due to saturation. The amplifier

response to very large overloading pulse is often a critical

factor in determining its overall performance especially at high

rate. The network can be empirically optimized for a given

application by measuring the pulse height resolution as a

function of time constant. The entire span of the biased

amplifier output is assigned to the range of input pulses between

the bias level and the specified maximum input level. The effect

is to magnify this limited range of input amplitudes by spreading

it over the entire output range. The amplifier is connected to an

analog-to-digital converter.

The job performed by the Analog-to-digital converter is

to derive a digital number that is proportional to the amplitude

of the pulse presented at its output. Its performance can be

characterized by several parameters: the speed with which the

conversion is carried out, the linearity of the conversion or

faithfulness to which the digital output is proportional to the

input amplitude, the resolution of conversion or the fitness of

the digital scale corresponding to the maximum range of

amplitudes which can be converted.

The voltage that corresponds to full scale is arbitrary,

but the analog-to-digital converter used for this nuclear pulse

spectroscopy was compatible with the output of typical linear

amplifier. Shaping requirements with a minimum pulse width of a

few tenth of microsecond was specified for the input pulses for

proper functioning.

The conversion gain of an Analog-to-digital converter

specifies the number of channel over which the full amplitude

span spread. At the lower conversion gain, a smaller fraction of

the multichannel analyser can be accessed at any one time. The

analog-to-digital converter was connected to a 4096 multi-channel

analyser.

The operation of the 4096 multi-channel analyser is based

on the principle of converting an analog signal to an equivalent

digital number. Once the conversion was accomplished, the

extensive technology available for the storage and display of

digital information was brought to bear on the problem of

recording pulse height spectra. As a result, the analog–to–

digital converter (ADC) is a key element in determining the

performance characteristics of analyser. The output of the

Analog-to-digital converter was stored in a computer type memory,

which has as many addressable locations as the maximum number of

channels into which the recorded spectrum is subdivided.

The net effect of this operation was thought of as one in

which the pulse to be analysed passes through the Analog-to-

digital converter and is stored into many location that

correspond most closely to its amplitude.

Samples were measured after decaying for 2 – 4 days

(short/medium lived nuclides) and 4 weeks (long lived nuclides),

and comparators were measured 7 days after the irradiations.

Elemental concentrations were determined with the k0-IAEA

software. Quality control was asserted through simultaneous

analysis of the certified reference material: GBW07404. Reference

material was prepared in the same way as the samples and

irradiated simultaneously with them.

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 Certified Reference Material (GBW07404)

The average elemental concentration of the Certified

Reference Material (GBW07404) used in the validation of the

experimental procedure is represented in table 4.1. The obtained

concentration is in good agreement with the certified reference

values, and the percentage deviation is calculated to be between

1.4% and 22.3%.

4.2 Average Elemental Concentrations of Samples

The average elemental concentration of the samples is

represented in table 4.2 below. The results of the analysis from

the various samples show that those elements with Z ≥ 11 were

detected, being Na to Th. The results obtained depend

significantly on the sample elemental composition and flux of the

neutron as these have strong influence on the statistical

limitations for trace element concentrations. Generally fourteen

(14) elements were analysed for all the eleven (11) samples (G70

– G80).

Although Uranium was not detected in any of the sample,

thorium was detected in all the samples except in G70 and G77; in

which it is below detection limit. Other elements like As, Ba,

Br, Ce, Cu, Fe, La, Na, Rb, Sb, Sc, Sm, and Zn were also

detected. From table 4.2, it is observed that all the samples

have the same trend both in nature and level of elements present.

On the average, Na has the highest concentration followed by Cu,

although Na was below detection limit in some of the samples.

Table 4.1: Elemental concentrations of GBW07404 CertifiedReference Material (ppm)

GSS4 (GBW 07404) Element Concentration Certificate Conc. As 5.88E+01 5.80E+01 Br 3.11E+00 4.00E+00 Ce 1.30E+02 1.36E+02 Co 2.39E+01 2.20E+01 Cr 3.85E+02 3.70E+02 Eu 7.83E-01 8.50E-01 Fe 6.91E+04 7.52E+04 Ga 3.04E+01 3.10E+01 Hf 1.23E+01 1.40E+01 K 8.10E+03 8.55E+03 La 4.75E+01 5.30E+01 Lu 7.22E-01 7.50E-01 Sb 7.11E+00 6.30E+00 Sc 1.94E+01 2.00E+01 Th 2.92E+01 2.70E+01

CHAPTER FIVE

CONCLUSION

Instrumental Neutron Activation Analysis (INAA) technique

has been used to determine the elemental concentration of shale

samples collected from oil wells in Niger Delta, Nigeria. From

the analysis of the samples G70 – G80, the elements detected were

As, Ba, Br, Ce, Cu, Fe, Th, La, Na, Rb, Sb, Sc, Sm, and Zn. It

was also observed that all the samples have the same trend both

in nature and level of elements present.

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