Annex I
GUIDELINES FOR
MANAGING NATURALLY OCCURRING
RADIOACTIVE MATERIALS
IN PRODUCTION OPERATIONS
H. BASHAT, SENV Environmental Advisor
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Annex I
GUIDELINES FOR MANAGING NATURALLY OCCURRING RADIOACTIVE
MATERIALS (NORM) IN PRODUCTION OPERATIONS
SUMMARY
Naturally occurring radioactive materials, NORM, have been known to be present in
varying
concentrations
in
hydrocarbon
reservoirs.
These
NORM, under certain reservoir conditions can reach hazardous contamination levels. The
recognition of NORM as a potential source of contamination to oil and gas facilities has
become widely spread and gaining increased momentum from the industry. The contents of
the Annex which wee extracted mainly from References 1 to 3, address the various
problems with NORM and provides the recommended procedures for managing these
materials.
INTRODUCTION
There are two types of NORM contamination which are commonly known in the oil and gas
operations:
1) Radium contamination which is common to formation water and produces low specific
activity scale known as LSA.
2) Radon contamination which is common to natural gas production.
Both of these elements when accumulate in significant concentration will form serious
health and environmental hazard in addition to the operational problems. Therefore
periodic analyses to detect and identify these contaminants at an early stage is becoming an
acceptable industry practice. The following sections will address each type separately.
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I.
NATURALLY
OCCURRING
PRODUCTION WATER
RADIOACTIVE
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MATERIALS
FROM
BACKGROUND
Naturally occurring radioactive materials NORM in formation water are soluble
radionuclides, may precipitate, under certain operational environment, as low specific
activity scale, known as LSA scales. These scales tend to be barium sulphate and strontium
sulphate which co-precipitate with naturally occurring radium leached out of the reservoir
rock; such scales emit alpha, beta and gamma radiation and this, together with the physical
properties of the LSAS, can give rise to a number of problems if such scales or sludges have
to be removed, handled or disposed.
Once LSA scales are formed within the production system two main problems are
presented: the scale will tend to foul valves and restrict the well fluid stream, and secondly,
the levels of radiation on the outside of the flowline or vessel may be so high that the
surrounding area may have to be designated as a restricted area and be cordoned off.
Scale formation can be prevented with some success by the use of scale formation
inhibiting chemicals. However, if the removal of LSA scale is necessary it can be difficult
and expensive because LSA scales (unlike calcium carbonate scale) are insoluble in
inorganic acids. Scale will either have to be removed (by hand or mechanically), or the
scaled up equipment taken out of service and put into safe storage. Therefore safe systems
of work and proper procedures which recognise the hazards, protect the workers from
harmful exposure, minimise interference with the environment and ensure compliance with
government and international regulations are essential.
1. ORIGIN AND FORMATION OF RADIOACTIVE SCALE
1.1 Naturally occurring radioactive rocks
The main radioelements found in the common sedimentary rocks include potassium,
uranium and thorium and the highest concentrations are normally found in shales as
indicated in Table 1.
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Table 1 – Radioelement concentration in sedimentary rocks
K-40
(ppm)
U-238
(ppm)
Th-232
(ppm)
Sandstones
a) Orthoquartzites
b) Arkoses
1.7
0.45
1.5
1.7
5.0
Shales
a) Grey and green
b) Black
2.9
3.2
8-20
13.1
Limestones
0.4
2.2
1.1
Evaporites
<0.1
The potassium content of shales is a reflection of their clay mineral, particularly illite
content. The high uranium concentrations in black shales are probably due to their
augmented organic content.
Sandstones owe their potassium values to their K-felspar, K-mica and glauconite content.
The uranium content of limestones is held largely within the crystal lattice of calcium
carbonate where the uranium ions substitute for the calcium. Thorium, however, does not
enter the carbonate lattice easily and, in consequence, thorium values tend to be low and
held mainly in the clay and heavy mineral fractions. Uranium and thorium values in
evaporites are, however, very low and restricted to the small detrital silicate mineral
fraction.
1.2 Radioactive deposits
Generally it can be said that the radionuclide enrichment of the formation water occurs due
to the concentration of uranium and thorium-bearing minerals within the source rock.
Subsequent leaching by formation or injection water may result in radioactive deposits in
the production train given suitable conditions. The injected seawater, being normally less
saline than the formation water, may additionally dissolve radioactive salts from the
minerals present in various geological strata. These deposit can take several forms:
Scales
Natural formation water will undergo changes of temperature and pressure as it is coproduced with the oil and gas, and may under certain conditions deposit scale within the oil
production system.
Depending on variations in temperature, pressure, flow and geochemical conditions, these
radioactive salts selectively precipitate in a non-reversible manner on the cement, or pipe
around cased wells, liners, tubings, etc. This is significant from the radiological protection
aspect since the co-precipitation effectively encapsulates the radium in a minerals shield,
thereby ensuring almost complete self-absorption of the alpha particles emitted during the
radioactive decay. Thus, a combination of radioactive leaching by either the natural
formation water or the injected seawater together with the occurrence of injection water
"breakthrough" may lead to deposits of mineral scale containing measurable quantities of
natural radioactivity concentrated by this scaling process.
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Sand and silt deposits
These are potentially a greater problem in topside equipment (e.g. production separators).
This material can act as an absorbtive surface for radionuclides present in the production
fluid. An exchange mechanism between cations can then give rise to radioactive sludges
and deposits.
Radon-222 gas is part of the decay chain of radium-226. In most cases where radioactive
scale is produced radium-226 as well as some of its daughter nuclides including radon-222
may be found entrapped within the scale. Normally, radon-222 would be carried away with
the normal gas. There are fears that where sludges are formed, the entrapped radon-222 gas
may be given off in relatively large quantities, particularly when this sludge is being
disturbed. Therefore precautions must be taken to protect the people working with such
materials.
Iron (rust) deposits
It has been noted that occasionally a matrix like iron oxide in oily-water separators has
exhibited low levels of radiation. The mechanism is unclear, but there is some speculation
that radium-226 could be locked into rust itself.
Lead deposits
In some cases lead deposits occur which have high Pb 210 contents. They can occur with
gas wells producing from carboniferous strata. Often they are characterised by high sodium
chloride concentrations in the formation water. The deposits are usually found in well
tubing and well heads.
2. ENVIRONMENTAL PROBLEMS
Radioactive scales and sludges or contaminated equipment may have to be removed at some
time for storage or disposal. Since this could cause environmental problems strict
precautions have to be taken to prevent the irradiation and contamination of people,
animals, plants and other materials. The problems involved are:
2.1 Radioactive scales tend to be highly insoluble in acids. They contrast with most
common non-active shales (e.g. calcium carbonate) which are readily soluble in acids.
Difficulties experienced in dissolving scale with inorganic acid should prompt to check
for the presence of radioactivity.
2.2 LSA scales invariably emit alpha and beta particles and gamma rays. Their presence in
production systems and equipment can give rise to occupational hygiene problems. A
particular concern is with dust particles which can be released in cleaning operations.
This dust can be trapped in the tissues of the lung and emit alpha particles which can
cause long-term health problems. Where LSA scale is present in production trains or in
items such as tubulars or wellheads, concern mainly centres on any effect that the
gamma radiation could have on those working close by.
2.3 Radioactive scale on the insides of the tubing strings may interfere with the natural
radioactive levels of the surrounding strata, causing anomalies in the readings from
gamma ray logs. The observation of such gamma ray anomalies can be an early
indicator of the presence of radioactive scales.
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3. UNITS OF MEASUREMENT
The units used for Activity, Absorbed Dose and Dose Equivalent have recently changed
following the introduction of SI units and are:
3.1 Activity
The activity of an amount of radioactive nuclide at a given time is the number of
spontaneous nuclear transformations in the time unit. The SI unit of activity is the
becquerel (Bq) equal to 1 nuclear transformation per second.
3.7 x 1010 Bq equals 1 Curie (Ci) exactly
37 M Bq equals 1mCi (millicurie)
37 k Bq equals 1µCi (microcurie)
The units generally used for measurement are Bq/g for solids and Bq/l for liquids and gases.
3.2 Dose
A term denoting the quantity of radiation energy absorbed by a medium. Although the
terms "dose" or "radiation dose" are often sued in a general sense, they should usually be
qualified, for example as absorbed dose, dose equivalent, etc. The dose is still usually
measured in pre SI units. Three pre SI units are used, viz:
3.2.1 the Roentgen which measures the radiation dose in air and is sometimes called the
exposure dose;
3.2.2 the Rad which is a measure of the absorbed radiation dose, and
3.2.3 the Rem which is the unit of dose equivalent. For all practical purposes, it is
assumed that doses measured in Roentgen are equal to doses measured in rads at the
same position in the radiation field.
Only dosimeters for measuring neutrons are normally calibrated in rem units. The unit of
dose equivalent takes into account the fact that some types of radiation, particularly alpha
particles and neutrons, are much more efficient at killing or damaging cells per unit amount
of absorbed dose. The rem dose is related to the rad dose by the following relationship:
Dose in rem
= Dose in rad x QF
(dose equivalent) (absorbed dose) (quality factor)
The value of the quality factor can be as high as 20 for alpha particles and 10 for neutrons.
For gamma and beta particles, it is 1. These units are large and for operational purposes
measurements are made in mR, m rad or mrem, which are 1/1000 of the principal unit.
Under SI units, the Roentgen does not have an equivalent. The unit of absorbed dose
replacing the rad is of the Gray or Joule kg-1 and the unit of dose equivalent replacing the
rem is the Sievert. These SI units are related in the same way as the rad and the rem, i.e.:
Dose in Sieverts (Sv) = Dose in Grays (Gy) x QF
By definition:
1 Gray
= 100
rad
1 Sievert = 100 rem
The SI unit is thus even larger than the existing unit and measurements will be made in µGy
or µSv (1/1,000,000th of the principal unit).
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4. FIELD EQUIPMENT FOR RADIATION DETECTION
Generally there are two broad types of detectors available; contamination meters for
measuring surface radiation such as alpha and beta, and the dose rate meters for measuring
gamma radiation. There are also devices capable of measuring both beta and gamma
radiation simultaneously.
4.1 Contamination meters
These are very sensitive devices for measuring surface radiation. They indicate level of
radiation in "counts per second" and should be able to measure alpha, alpha and beta, and
beta radiation. The ability to measure these three ranges is necessary because if the LSA
scale is damp, if there is moisture in the atmosphere or LSA scale is overlayed with calcium
carbonate scale, then the alpha particles may be absorbed. However, by measuring alpha
and beta emissions together the alpha emissions of the LSA scale may be inferred, i.e. as far
as LSA scale is concerned alpha and beta particles are always emitted together. If an
indication of LSA scale contamination is given by such a meter it must be confirmed by
radiochemical analysis or gamma spectrometry.
Contamination meters should be calibrated against a range of sources of known activity and
of similar isotopic composition; the calibration chart then produced will enable, say, a
reading of five counts per second to be converted to 0.37 Bq/cm2 which would only be true
for that particular meter. However, personnel trained in the use of such meters will soon
become adept at interpreting readings correctly.
Contamination meters, if treated with care, will give an early indication of a contamination
problem, provided that the scale is not shielded.
4.2 Radiation monitors or dose rate maters
These meters are reasonably robust and are used to measure radiation levels throughout
industry. Generally they indicate measurements in Sievert/hr and measure gamma
radiation. They are not as sensitive as contamination meters at measuring levels near
background but are a very useful tool to establish whether or not LSA scale with higher
levels of radiation is present inside pipelines, wellheads, vessels, etc. This is because the
steel will stop the alpha and beta particles but allow a certain percentage of the gamma ray
to pass; levels of radiation inside a steel pipeline or vessels may be in excess of two to three
times those indicated outside, depending on the thickness of the steel.
4.3 Available devices
The available devices to measure gamma and beta radiation in the field equipment and
facilities during operations and maintenance are:
• sodium iodide scintillation counters (SC);
• energy compensated geiger mueller (GM);
• thin window geiger mueller Pancake (PK).
4.4 General considerations
Both types of meter are available from a number of manufacturers world-wide but there are
some basic precautions which must be taken when either or both are used:
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• the meters should be calibrated by a suitable laboratory and either, in the case of a
contamination meter, a conversion chart supplied for that particular meter or, in the case
of a dose rate meter, the meter adjusted to read as accurately as possible across the range;
• meters should be overhauled and calibrated at least once a year;
• if a meter is dropped or damaged it should be recalibrated;
• meters should not be abused and should be switched off and kept securely when not in
use;
• personnel who use such meters should be trained in their operation, be able to interpret
readings properly and be able to recognise when meters may not be working properly;
• indications of LSA scale should be confirmed by radiochemical analysis of gamma
spectrometry so that a completely accurate record of the levels of radiation may be kept.
Note:
LSA scales with high levels of contamination, and therefore posing potentially high health
risks of inhaled/ingested, may not register as such on dose rate meters and ideally, if the
presence of LSA scale is suspected, both kinds of meter should be used together.
5. RECOMMENDED LIMITS
5.1 Dose limits
The basic recommendations of the International Commission on Radiological Protection
(ICPR) are laid down in its publications No. 26 and 30. However the "dose equivalent
limit" recommended by ICPR is 50 mSv over one year according to the defined working
conditions. Table 2 summarises these recommendations.
5.2 Activity limits
When radioactive materials are handled, they should be classed as a radioactive substance
when the specific activity level (the activity per unit of mass) is greater than 100 Bq/g. This
limit only refers to the activity level of the material itself. This must be clearly
distinguished from the limit that is used for decontamination purposes: the allowable
contamination level for alpha emitters on a surface is usually 2 Bqcm-2.
Therefore, when either of these limits is exceeded, proper operation, handling and disposal
are required.
6. OPERATIONAL PROCEDURES – DISPOSAL ASPECTS
6.1 Scale handling
When handling scale or scaled items, during such operations, as when pulling tubing,
entering production separators or produced water skimmers, removal of Xmas trees, valves,
meters and flowlines, etc. The following measures should be considered:
• contain contamination as near as possible to its site or production;
• limit the possibility of ingestion or inhalation;
• control and restrict direct exposure of workers;
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• measure and record the levels of activity where scale is found;
• follow the recommended method of disposal.
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Table 2 – Activity and radiation dose limits
Involvement
Term
Dose limits (1) (2)
for whole body
for individual organs/tissues
Classified
Working condition 'A'
Controlled area
50 mSvYr-1
500 mSvYr-1
workers Non-classified
(1)
(2)
workers Public and workers with no
involvement
(special
Working condition 'B'
(1) requirements
for
Supervised area
(2) transportation)**
3/10
x
50mSv
15 mSvYr-1
3/10
x
500
mSv
150 mSvYr-1
= 1/10
x
50
5 mSvYr-1
= 1/10
x
500
50 mSvYr-1
Derived levels
Hourly limit for external 2000 µSv h-1 (recommended 15
mSv
expose of the whole body (2) maximum for radiographers)
= 7.5 µSvh-1
2000 hours
"Annual limit of intake"* ALI Radium 226 by inhalation is 3/10 ALI, e.g. for Radium 226
of radioactive material (1)
20 kBq
Yr-1 by inhalation 3/10 x 20 kBq
Radium 226 by ingestion is Yr-1 = 6 kBq Yr-1
70 kBq Yr-1
Surface contamination level ALI
(2),
e.g. 3/10
ALI
(2),
e.g.
likely to result
8 Bq cm-2 for Ra 226
2.4 Bq cm-2 for RA 226
Airborne contamination+
ALI
(2),
e.g. 3/10
ALI
(2),
3 Bqm-3 for Ra 226
e.g. 1 Bqm-3 for Ra 226
Radioactive substance
Specific activity
100 Bq g-1 and any other substance having a lower activity
concentration that cannot be disregarded for the radiological
protection of persons at work (2).
Use only classified workers or ALARA***.
Regular
Precautions
non classified workers working monitoring of affected areas.
to a strictly controlled written Contain surface contamination.
system of work (dose limit Prevent
airborne
15 mSv per annum, i.e. 500 hrs contamination.
per year at 30 µSv-hr); contain
contamination.
Controlled disposal of radioactive substances. Occupational
hygiene precautions to prevent inhalation and/or ingestion.
*
**
***
+
(1)
(2)
(3)
5
mSv
=
mSv
=
mSv
= 2.5 µSvh-1
2000 hours
1/10 ALI, e.g. for Radium 226
by inhalation 1/10 x 20 kBq
Yr-1 = 2 kBq Yr-1
1/10
ALI
(2),
e.g.
0.8 Bq cm-2 for Ra 226
1/10
ALI,
e.g. 0.3 Bqm-3 for Ra 226
0.4 Bqg-1 used for pollution
control
and
disposal
authorisations.
ALARA***
and
'not
significantly above background
levels'. Controls on radioactive
substances as pollutants.
Annual limit of intake (ALI): an ALI is the amount of radioactive material which if taken into the body would delivery a
committed dose equivalent to the annual dose limit for either the whole body or individual tissues whichever is the more
restrictive. Each isotope has its own ALI, e.g. Radium 226 by inhalation is 20 BqYr-1.
Transport packages contain limit. On external surfaces of transport packages/containers the limit is 4 Bqcm-2 (3). 70 Bqg1 is limit used in transport regulations (3) (but check with national regulations for applicable limits).
ALARA: as low as is reasonably achievable.
Airborne contamination: very different limits can apply to different isotopes,
e.g. 3 Bq m-3 – Ra 226; 0.01 Bq m-3 – natural thorium
Internal commission on radiological protection limits for intakes of radionuclides by workers, ICRP Publication No. 30, Part
2, Pergamon Press, Oxford, 1980.
EURATOM Directive of the Council about radiation protection of workers and the public.
International Atomic Energy Agency, Safety Series No. 6, Regulations for the Safe Transport of Radioactive Materials
1973, Revised Edition (as amended), Vienna 1979.
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6.2 Containment, disposal and the environment
Disposal of LSA scale and sludges can be difficult and expensive, due to the occupational
hygiene and environmental protection considerations discussed earlier. The following
courses of action should be considered:
• containment;
• disposal.
6.2.1 Containment
If tubulars are pulled and are found to be scaled, then provided that the scale is thin, hard,
tenacious and smooth and offers little resistance to well fluid production, and provided that
the tubing does not need to be reworked, then the scaled tubulars can be re-run back into the
well. Similarly if a vessel is opened up for inspection and is found to contain LSA scales or
sludges which are not interfering with production and the material does not have to be
removed, then the vessel can be closed up again.
6.2.2 Disposal
The decision logic for disposal is presented in Figure 1.
Method of disposal include:
Disposal to the sea
Scales exhibiting levels of activity above background may be disposed of to the sea either
from offshore installations or from onshore facilities with their own direct flushed outfall.
Generally a maximum particle size (say 1 mm) could be specified together with a limit on
the total activity (specific activity times weight) expressed in Gigabecquerels.
Because particles of this size will obey Stokes Law, if this method is employed where there
are reasonably strong tides and currents, there should be no detectable increase in the level
of radioactivity in the sea or the surrounding seabed. In such instances, however, seabed
surveys (much like those for oil-based mud cuttings) may be required.
Disposal on land
Listed below are methods of disposal on land:
• In specially dug pits, abandoned mines and oil wells: where such facilities are
available, burial of scale in mines, pits or abandoned wells are possible methods of
disposal.
• Storage in secure yards or warehouses: in many cases where it is considered too
difficult or expensive to descale such items as tubulars, filter baskets, valves, etc., it may
appear cost-effective to put them into long-term secure storage. Such storage must only
be used with the agreement of the relevant authorities.
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However, the following produced offshore, should be considered:
• the risks of exposing other workers to the hazards of LSA scale are increased (e.g.
seamen, dockers, transport workers);
• if LSA scales are stored onshore, thought must be given to keeping such stores secure for
generations in order to prevent workers from being exposed to risk in the future – the
half-life of Radium 226 is 1620 years;
• if methods such as mixing 'concentrated' LSA scale with cement and then pumping the
slurry into drums or down abandoned wells are used then again additional risks are
incurred by the workers handling the scale.
6.3 Scale inhibition and scale dissolution
6.3.1 Inhibition
Generally, it can be said that scale inhibition of Ba/SrSO4 using the correct programmes,
appropriate solutions and clean suitable equipment, will be successful. Scale inhibitor
squeezes will usually be performed when it is known that barium is present in the formation
water following the first indication of injection water breakthrough. An increasing sulphate
ion count in the produced water is the usual indicator of the onset of breakthrough.
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Otherwise, the produced water must be monitored closely for other indications. Inhibitors
can also be injected into the well fluid stream in the production train to help prevent scale
formation in valves and production manifolds, etc.
6.3.2 Dissolution
Scale dissolution, usually in the production train manifolds, has been attempted, most often
using organic chemicals. Some of these organic chemicals show promise, but dissolution of
the salts has still to be proven effective, due to their almost chemically inert nature.
7. PROCEDURE FOR A FIELD SURVEY
The initial survey for a NORM at a site is typically performed along the exterior of on-line
intact equipment such as vessels, piping, compressors, and other production equipment. Of
the three types of radiation present in NORM (alpha, beta and gamma), gamma rays alone
can penetrate the steel and be detected outside the equipment. As discussed in the detection
equipment section (section 4.3), the SC probe is used to identify areas of potential concern
and the GM probe is used to quantify potential human doses.
Both measurements can be made during the same survey using a meter that can support the
two probes. The results of the survey should be documented and assigned one of the four
categories (A, B, C, D) described in Table 3.
Cut-offs of 2.5, 25 and 500
microSieverts/hour (uSv/hr) are used to define requirements needed to ensure a safe work
environment9 such as limiting access, posting of signs, and other follow-up actions. (Note:
10 uSv/hr = 1 mR/hr). It is recommended that sites with NORM contamination be
resurveyed every two years to identify changing conditions.
Table 3 – Determination of area NORM category
uSv/hr (GM)
Category
Definition
Requirements
<2.5
A
Public access area
None
2.5-5
B
Limited access area
•
•
•
Limit public access
Document findings
Inform maintenance
25-500
C
Regulated area
•
•
•
•
Limit worker access
Post with NORM sign
Train workers
Personal dosimetry
>500
D
High radiation
•
•
•
•
•
•
Limit worker access
Post with "high radiation" sign
Train workers
Personal dosimitry
Work permit required
Notify EA
8. DECONTAMINATION
Any equipment, tools, or personal protective equipment (PPE) that has contacted NORM or
LSAS contaminated surfaces needs to be evaluated to determine whether they have been
contaminated.
Similarly areas where NORM-related work has led to possible
contamination also need to be evaluated.
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The decontamination decision logic is presented in Figure 2. Measurements are made using
the GM probe and the PK probe (direct measurement or a wipe sample depending on the
configuration of the surface). If both measurements are less than the criteria, the material is
not considered to be NORM contaminated.
For materials that do not meet the stated criteria, decontamination and repeat monitoring are
one possible option. The other option is packaging for disposal.
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II.
RADIOACTIVE ELEMENTS IN NATURAL GAS
Occurrence of radon
Natural gas contains small quantities of the gaseous radioactive nuclide Radon-222 formed
from the decay of Radium-226, which is a daughter nuclide of naturally occurring Uranium238. Radon enters natural gas in the earth by diffusion from a formation. Uranium
minerals are often associated with carbonaceous deposits, therefore radon can be expected
to occur in natural gas.
Radon-222 has a half life of 3.8 days and produces upon decay a series of short and long
lived daughter nuclides as shown in Figure (1). When propane is separated from natural
gas, radon tends to be concentrated in the propane process stream since the boiling point of
radon is close to that of propane. Consequently, it is typically enriched in propane by a
factor of the order of 10.
In natural gas condensate the long lived daughters of radon (particularly Lead-210 and
Polonium-210) are generally present.
Figure (1) Decay scheme of the 238U natural series
Notes:
1) Half-lives are indicated in years a, days d, minutes min, and seconds.
2) The nature of the radiation is indicated by ,, and  (only energetic  radiations with
high yield).
Recent reports of radon contaminated buildings through out the world, attest to the wide
distribution of radon in the environment.
Once formed by the radioactive decay of radium-226, radon is free to migrate as a gas or
dissolve in water without being trapped or removed by chemical reaction. Migration
through rocks and soil, radon is produced with natural gas at the wellhead. Table 1 shows
that radon contamination of natural gas is a worldwide problem, and particularly high
concentrations of radon are reported in the US and Canada.
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Table 1 - Radon concentrations in natural gas at the wellhead *
Location of Well
Radon concentration (pCi/L)
Borneo
1 to 3
Canada
Alberta
10 to 205
British Columbia
390 to 540
Ontario
4 to 800
Germany
1 to 10
The Netherlands
1 to 45
Nigeria
1 to 3
North Sea
2 to 4
US
Colorado, New Mexico
1 to 160
Texas, Kansas, Oklahoma
1 to 1,450
Texas Panhandle
10 to 520
Colorado
11 to 45
California
1 to 100
*) From "Radon Concentration in Natural Gas at the Well, UN Scientific Committee on
the Effects of Atomic Radiation; Sources and Effects of Ionizing Radiation, United
Nations, New York City (1977).
When radon-contaminated produced gas is processed to remove the NGL's, much of the
radon is removed also. Radon's boiling (or condensing) point is intermediate between the
boiling points of ethane and propane. Upon subsequent processing, radon tends to
accumulate further in the propylene distillation stream. Table 2 shows the boiling points of
radon, the lighter NGL's, and propylene. As expected radon usually is recovered more
completely in plants with high ethane recovery. The radon is concentrated in the lighter
NGL's and is detected relatively easily with radiation survey meters.
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Table 2 - Boiling points at 760 mm Mercury
°F
Methane
-258.0
Ethane
-124.0
Radon
-79.2
Propylene
-53.9
Propane
-44.4
Butane
+31.1
As long as it is contained and controlled within vessels, equipment, and piping, radon
generally is not a health hazard to employees and the public. Even if radon-contaminated
propane were released, the threat of fire or asphyxiation would far outweigh the hazard of a
short-lived radiation exposure.
NORM in NGL facilities
Although entire natural-gas and NGL systems may be contaminated with NORM, some
facilities will be contaminated to the extent that they present significant decontamination
and disposal problems. Gasoline plants and other NGL facilities will be among the most
highly contaminated areas in a system.
During processing in a gasoline plant, the levels of external radiation from radon in propane
1 ft from a liquids pump may be as high as 25 milli-roentgens (mR)/hr. Radiation levels up
to 6 mr/hr have been detected at outer surfaces of storage tanks containing fresh propane.
Sludges in gasoline plants are often contaminated with several thousand picocuries of lead210 per gram.
Table 3 shows vessels and equipment in NGL service that may be significantly
contaminated with NORM. Although NORM contamination will be general throughout an
NGL facility, the contamination usually will be greatest in areas of high turbulence, such as
in pumps and valves.
Table 3 - Priority areas of concern for high radon and radon decay product
contamination
NGL facilities
De-ethanizers
Stills
Fractionators
Product condensers
Flash tanks
Pumps in liquid service
Piping in liquid service
NGL storage tanks
Truck terminals
Filter separators
Dessicants
Waste pits
Pipelines
Filters
Pig receivers
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Machine shops
In-house Contract
When employees open equipment and vessels, precautions must be taken to prevent
exposure to radioactive contamination. Maintenance procedures should include the use of
respirators and good hygiene to prevent inhalation of radioactive dust. Grinding, if
necessary, should be done wet to minimise dust.
Occasionally, a plant or other facility that has been processing light hydrocarbons,
particularly ethane and propane, is taken out of service and the facility sold or dismantled.
Any equipment with internal surface deposits of NORM must receive special consideration
when scrapped, sold, transferred, or otherwise disposed of, particularly when the facility is
being released for unrestricted use. Analyses for lead-210 usually will be required to verify
the extent of contamination and to determine if special handling is needed. Particularly care
must be used to prevent employee exposure to NORM contamination.
There are potential liabilities involved if contaminated equipment, vessels, and other parts
of the facility are released or sold for unrestricted use without first being cleaned and tested
to be essentially free of NORM contamination according to state and federal regulations.
Much of the material wastes from a facility contaminated with NORM must be handled as
low-level radioactive waste and disposed of accordingly. Contaminated wastes should be
consolidated and separated from non-contaminated waste to keep radioactive waste
volumes as low as possible. Consolidated contaminated wastes should be stored in a
controlled-access area. The area should be surveyed with a radiation survey meter and, if
required, should be posted.
The investigation level
Normally, the amount of radioactivity in the natural gas and its products is insufficient to
cause health hazards during handling and subsequent use by consumers.
However, it is recommended that the radon content of natural gas and Polonium-210 in the
condensate of wells should be monitored prior to production. A record should also be kept
of radon and polonium in gas and condensate from reservoirs which have been in
production for a long period. The results of such measurements should be compared with a
'Derived Investigation Level'. A derived investigation level, as defined by the international
Commission on Radiological Protection (ICRP), is a value of concentration of radioactive
material. It is usually set in relation to a single measurement, which is the resulting
radiation dose to humans sufficiently important to justify further investigation.
It is important to recognise that an investigation level is not intended to be a limit. Should
an investigation level be exceeded, this should be reported to the Central Offices EP Health,
Safety and Environment Department (SIPM-EPO/6) who will contact the radiological
specialists for advice. A close investigation of the (local) circumstances will be required.
The investigation will often be no more than a recognition that the circumstances will not
cause any hazard as the investigation level is based on a 'worst case' estimate. Below the
investigation level, the information need not be further studied by experts.
Calculation of the investigation level
Two types of radioactive exposure to humans resulting from radioactivity in natural gas or
condensate can be identified:
a)
During the use of the natural gas by consumers, e.g. heating and cooking.
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b) Relating to gas handling at gas processing stations.
In both cases, a derived investigation level will be specified.
Natural gas and LPG for Domestic Consumption
When natural gas or LPG is burned in domestic appliances (cooking and heating) radon will
be emitted into the atmosphere and contribute to the radiation level already naturally
present. Radon (and its daughter nuclides which are formed by decay) become attached to
aerosol particles and may subsequently be inhaled.
To calculate the derived investigation level for radon in raw natural gas, the following
'worst case' conditions are assumed:
- Minimal ventilation and maximal invented appliances of LPG in which radon is
enriched.
- The combustion products will all contribute to the radon concentration in indoor air.
The maximum permissible yearly dose to members of the public is 5 mSv/a (milliSievert
per year, a unit for the ionising radiation dose to human beings). As a base for the derived
investigation level, one twentieth of this dose equivalent is taken, i.e. 0.25 mSv/a. This
dose equivalent is not exceeded when the concentration in the natural gas is below 2
kBq/m3 (50 pCi/dm3). For comparison, the average yearly dose from the natural
background and medical radiation is around 3 mSv/a. This level can be considered as the
derived investigation level for radon in raw natural gas taking into account the use of
recoverable LPG when converted into fuel gas.
For use of natural gas as an industrial fuel gas, the same investigation level should be used,
provided that the investigation level for surface contamination is not exceeded.
Contamination of Gas Processing Equipment
Inner parts of gas processing equipment may be contaminated with the long lived daughter
products of Radon-222 as a result of deposition of the solid daughters (particularly the long
lived Pb-210 and Po-210) at places where the stream is dispersed over a large surface or
where high turbulence occurs. An additional effect is the enrichment in the propane stream
(see 3.8.1) which causes an increased chance of contamination in the propane stream
equipment.
Similarly, Polonium-210 present in the condensate may be deposited in pumps, distillation
columns, heat exchangers, etc.
The chance of contamination above a certain level is related to the initial concentration of
radon or polonium in natural gas or natural gas condensate. However, possible enrichment
and the throughput of gas or condensate are also important factors.
The long lived daughters of radon (e.g. Polonium-210) mainly emit alpha radiation which
cannot penetrate steel walls of equipment. Only the short lived Bismuth-214 may be
detectable at the outside of the equipment. In view of the short half-life, however, the
external radiation level will always be minimal and will disappear after shut-down of the
installation.
Thus, radiation hazards, if present, may only occur when opening equipment, by inhalation
or ingestion of the contamination. Before workers enter equipment which has been exposed
to condensate containing polonium or propane containing radon, it may be advisable to
monitor the inner surfaces to avoid the risk of contamination of the personnel involved. For
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a derived investigation on surface contamination of equipment, 4 kBq/m3 (10 pCi/cm2)
should be used.
Since the mechanism for deposition of solid daughters of radon (e.g. Polonium-210) cannot
be quantitatively described, it is not possible to calculate investigation levels for liquid
concentration of radon or Polonium-210 resulting in contamination. However, from
experience, contamination of inner surfaces of equipment is unlikely when the level of
Polonium-210 in natural gas condensate is below 20 kBq/m3 (0.5 pCi/cm3).
Monitoring
Radon-222 in natural gas can be detected using an ionisation chamber. Radon
concentration determination is usually carried out as one of the routine tests during
production testing of new gas reservoirs.
Several methods for determination of Polonium-210 in natural gas condensate are available.
One of the accurate methods consists of extraction of Po-210 from the condensate and acid
destruction followed by plating Po-210 on a silver disc. The alpha activity on the silver
surface is determined using, for example, a surface barrier detector.
For detection of contamination of inner surfaces of equipment, a simple technique
developed by KSLA of applying a film sensitive to alpha radiation is available.
Suggested programme for the control of NORM
The following are suggestions for use in establishing a programme for the control of
NORM contamination.
1) Determine whether there is a NORM contamination problem.
2) Determine areas of potential NORM exposure and contamination.
a)
Make gamma radiation surveys of facilities and equipment.
b) Make wipe tests on accessible interior surfaces of selected equipment and vessels,
especially any in NGL service.
c)
Obtain samples of sludges and scale and analyse for radium and lead-210.
d) Obtain samples of other waste materials, such as dessicants and filters.
e)
Analyse produced water and waste pond water for radium.
3) Establish programmes to ensure personnel safety, products quality, customer
satisfaction, and protection of the environment.
a)
Establish policy on periodic surveys, inspection and maintenance procedures,
product controls, and record keeping.
b) Provide safety-manual material that informs employees and details required
procedures, particularly for maintenance personnel.
c)
Recommend a management and audit system.
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d) Develop plans and procedures for the disposal of contaminated waste materials,
equipment, and facilities.
REFERENCES
1.
Low specific activity scale: origin, treatment and disposal
E&P Forum Report No. 6.6/127, 1988
2.
E.C. Thayer and L.M. Racioppi
Naturally occurring radioactive materials: the next step
SPE 23500, 1991
3.
4.
P.R. Gray "NORM contamination in Petroleum Industry" JPT Jan. 1993.
G.E. Jackson
Formation and inhibition of scale in offshore oil productive systems
Offshore Radioactivity Seminar, OYEZ London, 1983
5.
K.S. Johnson
Water scaling problems in the oil production industry
in Chemicals in the Oil Industry, Ed. P.H. Ogden, 1983
6.
UKOOA Reference Manual on naturally occurring radioactive substances on offshore
installations
UKOOA, London, 1985
7.
W.A. Kolb and M. Wojcik
Enhanced radioactivity due to natural oil and gas production and related radiological
problems
Science of the Total Environment 45, 77-84, 1985
8.
A.L. Smith
Radioactive scale formation
OTC 5081, Offshore Technology Conference, Houston, 1985
9.
10.
11.
12.
P.R. Gray "Radioactive materials could pose problems for the gas industry" Oil &
Gas J. (June 25, 1990) 45-48.
J. Summerlin Jr. and H.M. Prichard "Radiological Health Implications of Lead-210
and Polonium-210 Accumulations in LPG Refineries" J. American Industrial Hygiene
Assn. (1985) 46, No. 4, 202-05.
E&P Form Report no. 6.6/127, 1988. Low Specific Activity Scale.
E.C. Tayler & C.M. Raciopi NORM; "The Next Step", SPE 23500, 1991.
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RADIOLOGICAL UNITS – SI CONVERSIONS
CONVERSION TABLE
Activity units
1pCi
1nCi
1µCi
1mCi
1 Ci
1KCi
=
=
=
=
=
37mBq
37 Bq
37KBq
37MBq
37GBq (37E+09Bq)
37TBq
1mBq
1 Bq
1KBq
1MBq
1GBq
1TBq
=
=
=
=
=
=
0.027pCi
27pCi
27 nCi
27µCi
27mCi
27Ci
= 27fCi
Curies to becquerels
1 pCi
37 mBq
1 nCi
37 Bq
1 µCi
37 KBq
1m Ci
37 MBq
1 Ci
37 GBq
1 kBq
27 nCi
1 MBq
27 µCi
1 GBq
27 mCi
1 TBq
27 Ci
Becquerels to curies
1 Bq
27 pCi
Absorbed dose units
1µrad
1mrad
1 rad
1Krad
1Mrad
=
=
=
=
=
0.01µGy
0.01mGy
0.01 Gy = 10mGy
10 Gy
10 KGy
1µGy
1mGy
1 Gy
10 KGy
1MGy
=
=
=
=
=
100µrad
100mrad
100 rad
0.1Mrad
100 Mrad
Dose equivalent units
1µrem
1mrem
1 rem
1Krem
1Mrem
(H)µrem
1
1
(H)µSv
=
=
=
=
=
0.01µSv
0.01mSv=10Mv
0.01 Sv=10mSv
10 Sv
10 KSv
mrem
1
10
µSv
1µSv
1mSv
1 Sv
1KSv
1MSv
mrem
10
100
µSv
= 100 µrem
= 100 mrem
= 100 rem
= 0.1Mrem
= 0.1Grem
mrem
10
1
mSv
mrem
1
10
mSv
Prefixes
k
M
G
T
kilo
mega
giga
tera
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-
thousand (103)
million (106)
thousand million (109)
million million (1012)
m
µ
n
p
milli
micro
nano
pico
-
thousandth (10-3)
millionth (10-6)
thousand-millionth (10-9)
million-millionth (10-12)
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GLOSSARY
Activity
The quantity of a radionuclide described by the number of nuclear
transformations occurring per unit time (see becquerel and curie).
ALARA
As low as is reasonably achievable.
Alpha particle ()
A charged particle emitted from the nucleus of an atom having a mass and
charge equal in magnitude to that of a helium nucleus, i.e. two protons and
two neutrons.
Becquerel
The SI unit of activity. One Becquerel (symbol Bq) equals one nuclear
transformation per second.
Beta particle ()
Charged particle emitted from the nucleus of an atom, with a mass and charge
equal in magnitude to that of the electron.
Coelestobarite
Ba/Sr(Ra)SO4 solid solution of RaSO4 in Ba/SrSO4.
Contamination (radioactive)
Radioactive material in any place where it is not desired particularly where its
presence may be harmful. The harm may be in inhaling or ingesting the
radioactive material which may cause internal radiation dose.
Controlled area
A defined area in which the occupational exposure of personnel (to radiation)
is under the supervision of the Safety/Radiation Adviser and the dose rate is
above 7.5 µSv/hr.
Counter (Geiger-Muller)
A glass or metal envelope containing a gas and two electrodes. Ionising
radiation causes discharges, which are registered as electric pulses in a
counter. The number of pulses is related to the dose.
Counter (Proportional)
A similar device as a Geiger-Muller counting tube; the intensity of the
electric pulses produced is proportional to the energy of the primary ionising
particles.
Counter (Scintillation)
A device containing material that emits light flashes when exposed to
ionising radiation. The flashes are converted into electric pulses by a photomultiplier.
Curie
The pre-SI unit of activity. One curie (abbreviated Ci) equals 3.7 x 10 10
nuclear transformations per second, i.e. it equals 37 gigabecquerel.
Decay
Disintegration of the nucleus of an unstable nuclide by spontaneous emission
of charged particles and/or photons. It causes the decrease in activity or
radioactive substances.
Detector (Radiation)
Any device for converting radiant energy to a form more suitable for
observation. An instrument used to determine the presence, and sometimes
the amount, of radiation.
Dose
A general term denoting the quantity of radiation or energy absorbed. For
special purposes it must be appropriately qualified. If unqualified, it refers to
absorbed dose.
Collective effective dose The quantity obtained by multiplying the average effective dose equivalent by
the numbers of persons exposed to a given source of radiation. Expressed in
man-sievert. Frequently abbreviated to collective dose.
Cumulative radiation
dose (radiation)
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Dose equivalent (symbol A quantity used in radiation protection. It expressed all radiation on a
H)
common scale for calculating the effective absorbed dose such that biological
effects can be compared. It is defined as the product of the absorbed dose and
the quality factor (see Quality factor and Sievert).
Effective dose
equivalent
The quantity obtained by multiplying the dose equivalents to various tissues
and organs by the risk weighting factor appropriate to each and summing the
product. This procedure makes it possible to compare this number with a
whole-body dose equivalent.
Maximum permissible
dose equivalent (MPD)
The greatest dose equivalent that a person or specified part thereof shall be
allowed to receive in a given period of time. This quantity has been rejected
in ICRP 26.
Dose rate
Absorbed dose delivered per unit of time.
Dosimeter
Instrument to detect ad measure a dose received. For example, a pencil-size
ionisation chamber with a self-reading electrometer, used for personnel
monitoring.
Exposure
A measure of the ionisation produced in air by X or gamma radiation (see
Roentgen).
Gamma ray ()
Short-wave length electromagnetic radiation of nuclear origin (range of
energy from 10 KeV to 9 MeV) emitted from the nucleus.
Gray (symbol Gy)
The unit of absorbed dose. One gray equals one joule per kilogramme.
Half-life (radioactive)
(symbol t1/2)
Time required for a radioactive substance to lose half of its activity by decay.
Each radionuclide has a unique half-life.
IAEA
International Atomic Energy Authority.
ICRP
International Commission on Radiological Protection.
Ionising radiation
Radiation that produces ionisation in matter. Examples are alpha particles,
beta particles, gamma rays, X rays and neutrons.
Irradiation
Exposure to radiation.
Isotopes
Nuclides having the same number of protons in their nuclei, and hence the
same atomic number, but differing in the number of neutrons, and therefore in
the mass number. Almost identical chemical properties exist between
isotopes of a particular element. The term should not be used as a anonym
for nuclide.
Joule
The unit for work and energy, equal to one Newton expended along a
distance of one metre (IJ = 1N x 1m).
Monitoring
Periodic or continuous determination of the amount of ionising radiation or
radioactive contamination present.
Nuclide
A species of atom characterised by the number of protons and neutrons, and
the energy content.
Rad
The pre-SI unit of absorbed dose; equal to 0.01 J/kg (see Gray).
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The emission and propagation of energy through space or through a material
medium in the form of waves; for instance, the emission and propagation of
electromagnetic waves. The term radiation or radiant energy, when
unqualified, usually refers to electromagnetic radiation. Such radiation
commonly is classified, according to frequency, as hertzian, infra-red visible
(light), ultraviolet, X ray, and gamma ray (see Photon).
Background
Radiation arising from radioactive material other than the one directly under
consideration. Background radiation due to cosmic rays and natural
radioactivity is always present. There may also be background radiation due
to the presence of radioactive substances in other parts of the building, in the
building material itself, etc.
External
Radiation from a source outside the human body.
Internal
Radiation from a source within the body (as a result of incorporation and
deposition of radionuclides in body tissues).
Radioactivity
The property of certain nuclides of spontaneously emitting particles or
electromagnetic radiation.
Radionuclide
An unstable nuclide that emits ionising radiation.
Radon
In the context of this report Radon is taken to mean either Radon 222 or
Radon 220 – radioactive gases produced by decay of Ra 226 or Ra 224.
Rem
The pre-SI unit of dose equivalent; equal to 0.01 J/kg (see Sievert).
Risk factor
In connection with ionising radiation, the probability of cancer and leukaemia
or genetic damage per unit dose equivalent. Usually refers to fatal malignant
diseases and serious genetic damage. Expressed in Sv.
Roentgen (R)
The pre-SI unit of exposure. One Roentgen is the dose given by a radiation
field that produces ionisation, due to secondary electrons, of one electrostatic
unit of charge per cm3 (NTP) of air. It is equal to 2.58 x 10 -6 coulomb per
kilogramme of air.
Sealed substance (or source) A radioactive substance sealed in an impervious container which has
sufficient mechanical strength to prevent contact with and dispersion of the
radioactive substance under the conditions of use and wear for which it was
designed.
Shield
A body of material used to prevent or reduce the passage of particles or
radiation.
SI
Abbreviation of "Système International d'Unites", the International System of
Units, recommended for general use.
Sievert (symbol Sv)
The unit of (effective) dose equivalent. The sievert has the dimensions of
joule per kilogramme. The dose equivalent in sieverts is numerically equal to
the absorbed dose in grays multiplied by the quality factor (see Gray and
Quality factor).
Specific activity
Total activity of a given nuclide per unit mass of the specific material.
Tracer (isotopic)
The isotope or non-natural mixture of isotopes of an element which may be
incorporated into a sample to permit observation of the course of that
element, alone or in combination, through a chemical, biological, or physical
process.
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Tritium
The hydrogen isotope with one proton and two neutrons in the nucleus.
(Symbol 3 H or H-3, sometimes T).
X rays
Electromagnetic radiation of which the wave lengths are shorter than those of
visible light. They are usually produced by bombarding a metallic target with
fast electrons in a high vacuum, as occurs in an X ray machine.
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