SCALE DETECTION IN GEOTHERMAL SYSTEMS: THE USE OF NUCLEAR
MONITORING TECHNIQUES
E. Stamatakisa,b,‡ , T. Bjørnstad b, C. Chatzichristos b, J. Muller b and A. Stubosa
aNational
Centre for Scientific Research Demokritos (NCSRD), 15310 Agia Paraskevi, Attica, Greece
bInstitute for Energy Technology (IFE), PO Box 40, NO-2027 Kjeller, Norway
e-mail: manos@ipta.demokritos.gr
ABSTRACT
Various nuclear techniques for the in-situ detection of
mineral scale in actual production systems have been
reported during the last years. Effective scale
management requires on-line monitoring of scaling
tendencies as well as detection and identification of
scale deposits. In that respect, the gamma-ray
attenuation technique and the gamma-tracer emission
technique are examined in this study for their
capability of measuring in real-time the scale
formation under field conditions. Calcite formation has
been investigated in the lab-scale in absence and
presence of scale inhibitors in the course of this work.
A number of runs have been successfully performed
where the desired experimental conditions have been
evaluated and tested using both techniques. The
obtained results are presented here.
1. INTRODUCTION
The production, utilization and/or reinfection of
brine found in geothermal reservoirs are often
hampered by serious and very unique scale
problems. Some of these scale problems are so
severe that entire field operations are
endangered. Effective scale management
requires on-line monitoring of scaling tendencies
as well as detection and identification of scale
deposits.
Laboratory determinations of real-time scale
deposition were recently reported using a
radiotracer technique (Stamatakis et al., 2005a),
an attenuated total reflection sensor (Smith et
al., 2003), a rotating disk electrode technique
(Morizot & Neville, 2000), tapered optical fibers
(Moar et al., 1999), a piezoelectric sensor
(Emmons et al., 1999) and an ultrasonic
technique (Gunarathne & Keatch, 1995). In
addition, various nuclear attenuation techniques
for the in-situ detection of mineral scale in actual
production systems have been also reported
recently. In particular, a handled device was
developed to detect the presence of scale in
‡
To whom correspondence should be addressed
surface piping by measuring the nuclear
attenuation across the pipe diameter and two
field cases were presented in which a dualenergy-venturi multiphase flow meter was used
to detect and characterize scale according to the
attenuation of the nuclear spectrum (Theuveny
et al., 2001). A year later, Poyet and coresearchers presented a gamma-ray attenuation
method, based on continuous triple-energy
gamma-ray attenuation measurements, for the
detection of scale deposition in real-time in
oilfield production tubulars (Poyet et al., 2002).
The last method was actually an extension of
previous work that had been applied to several
field cases of surface pipe scale detected by
dual-energy attenuation measurements (Kevin,
1999). The method enables a simple monitoring
device to detect and characterize scale in its
earliest stages of formation.
The objective of the present work has been to
demonstrate the use and applicability of nuclear
techniques for studying calcite scale formation in
geothermal operations. In that respect, a gamma
transmission (attenuation) technique and a
gamma emission (radiotracer) technique were
designed and evaluated in the lab for the realtime measurements of scale formation under
flow conditions.
2. TECHNICAL DESCRIPTION
2.1. Gamma transmission
2.1.1.
Principles of the method
The attenuation of gamma radiations through
composite materials is of wide interest for
industrial, medical and agricultural studies. The
gamma transmission method is a nondestructive non-intrusive method to measure
mass density in an object by detecting the
attenuation of gamma radiation from an external
gamma source as it penetrates the object
(Fig.1).
Scale detection in geothermal systems: The use of nuclear monitoring techniques
Figure 1 – Gamma transmission method
Figure 2 – Schematic γ-ray narrow beam adsorption
measurement through a composite sample.
Transmission of a mono-energetic beam of
collimated photons through a simple adsorption
sample (as in Fig.1) can be described by the
Lambert-Beer’s equation:
I x  I o e  x
(1)
where, Ix is the intensity of the γ-ray radiation
after passing through a material, Io is the
intensity of the narrow beam monoenergetic γray radiation before passing through a material,
μ is the linear attenuation coefficient (L-1) and
x is the sample thickness.
A quantity more commonly found tabulated is
the mass attenuation coefficient μm=μ/ρ with
dimensions L2/M. The mass attenuation
coefficient represents the penetration and the
energy deposition by the photons in materials.
This can be obtained by the measurement of Io
and combination with the confirmed values of Ix
and x. Research has involved in obtaining this
value for radiological interest, as this value is
characteristic for each element, mixture and
compound.
Equation 1 gives the decrease of the gamma
rays intensity for a homogeneous absorber. If
the beam of gamma rays enters a different
dense medium Eq.1 has to be modified. It can
be easily shown that when a narrow beams of
gamma rays passes through our test session
(aluminum tube) to the forming calcite scale, and
finally, to the liquid phase (see Fig.2), the
attenuation is additive according to:
I xm  I o e
 2  Al xm , Al 2 Ca xm ,Ca l xm ,l
 


 Al
Ca
l

2



(2)
Here, xm is the so-called mass thickness with
dimensions M/L2. The relation between the
linear thickness and the mass thickness is
simply x=xm/ρ.
In case of CaCO3 scaling on the inner walls of
an aluminum tube, the only unkown parameters
are xm,Ca and xm,l. The contribution of the liquid
thickness can be corrected for by calibrating the
system by removing the liquid (mass attenuation
of pure air can be neglected relative to CaCO3).
Thus, xm,Ca can be uniquely determined.
2.1.2.
Experimental system
Figure 3 shows the schematic diagram of the
experimental apparatus, which was designed
and constructed for this study. Twin HPLC
pumps drive two solutions, one containing scale
forming cations (Ca2+) and the other anions
(HCO3-). The solutions are pumped through two
pre-heating tubes, which raise the solution
temperature to the desired test temperature prior
to arriving at the mixing point. The combined
solution, which now has a positive saturation
index, passes through our test session (1m long
aluminium 6061 with 1cm ID) and scale crystals
nucleate and grow on its internal surface. The
temperature is maintained constant by a heating
equipment fixed along the tube.
Figure 3 – Experimental set-up illustration for the
gamma transmission measurements.
To monitor the forming scale, a narrow beam of
gamma rays from an appropriately shielded
radioactive source (133Ba) is directed through the
tube; variations in the transmitted intensity
recorded by a radiation detector positioned on
the opposite side of the tube can then be related
to the variations in the mass per unit area of the
intervening deposited material (see figure 2).
To obtain the final profile of the deposits, the
tube can be moved across the mounted source
and detector, and measurements were collected
at certain intervals at the end of each run.
2.1.3.
Typical results
Typical primary results from this kind of
experiments are given in Figures 4 – 8. For
more detailed results see the work of Stamatakis
and coworkers (Stamatakis et al., 2005b).
Scale detection in geothermal systems: The use of nuclear monitoring techniques
3
1,00
4600
0,90
0,80
scale thickness (cm)
count-rate (cps)
4550
4500
4450
background
tind
4400
0,70
0,60
0,50
0,40
0,30
0,20
0,10
4350
0,00
0
5
10
15
20
25
30
0
10
20
time (hours)
Figure 4 – Gamma transmission as a function of time
at the liquid mixing point for calcite precipitation
30
Position (cm)
40
50
60
Figure 8 – The primary attenuation data converted to
scale thickness distribution across the tube.
0,275
2.2. Gamma emission
0,250
Scale thickness (cm)
0,225
2.2.1. Brief outline of the method
0,200
0,175
0,150
0,125
0,100
0,075
0,050
0,025
0
0
5
10
15
20
25
Time (hour)
Figure 5 – The primary attenuation data converted to
scale growth rate.
The basis of this method is to label the calciumcontaining liquid with the radioactive gamma
emitter 47Ca (in the chemical form of Ca2+). The
radioactive 47Ca behaves chemically identical to
non-radioactive
calcium.
When
calcium
carbonate precipitates, 47Ca will be a messenger
for where the precipitation takes place and to
what degree (amount of precipitate). The
gamma radiation from 47Ca will penetrate the
tube and be detected by external detectors. In
this way, the emission technique is an on-line
non-intrusive and continuous experimental
technique.
The experimental setup is illustrated in Fig.9.
Figure 6 – Scaling rates of calcite precipitation in the
presence and absence of a scale-inhibitor.
7000
6800
background
6600
final
Two energy-dispersive NaI(Tl) γ-detectors are
for the on-line measurements of 47Ca activity.
One is set at a fixed position (position ‘zero’ of
the sand-pack), dedicated to measure
accurately the induction time, while the other (γscanner) is used to monitor the growth of the
calcite deposition along the sand-packs. This
scanner-detector system is computer operated.
6400
cps
6200
6000
5800
5600
5400
5200
5000
0
10
20
30
40
50
Figure 9 – Experimental set-up illustration for the
radiotracer technique.
60
Position (cm)
Figure 7 – Gamma transmission measurements
across the tube.
The main gamma energy of 47Ca is 1297 keV.
However, by including also its Compton
background and lower energies in the counting
window, the sensitivity in the experiment may be
Scale detection in geothermal systems: The use of nuclear monitoring techniques
increased. It is necessary to avoid contribution
from 47Sc, which is the daughter nuclide in the
decay of 47Ca (see Fig. 10). The energy window
for the detectors will therefore be chosen from
350 keV and upwards to avoid the 159 keV γquanta from 47Sc.
4
needed, which can be found in the market. For
example, enriched 46Ca (4.1%) is available to
us, meaning that there is an 1000 times stronger
target for irradiation.
2.2.3. Typical results
Typical results, taken from a previous study
(Stamatakis et al., 2005a) where higher
saturation ratios were used (there was no need
for enriched 46Ca), are shown in Figures 11 - 15.
200
1,0
Ca(47)
tracer background
Δp
175
0,8
Samples are also collected periodically at the
exit end of the sandpack and the activity of 47Ca
(1297 keV) in solution determined in off-line
high-resolution detector gamma-spectrometric
measurements with a HpGe-detector coupled to
a multichannel analyzer.
125
0,6
100
0,4
75
Δp (bar)
Figure 10 – Chart of nuclides
count-rate (cps)
150
50
0,2
25
0
0,0
0
20
40
60
80
100
120
140
Tim e (m in)
Solution temperature Ts, differential pressure
Δp, pH, absolute system pressure p and 47Ca2+
activity (counting rate R) from the two on-line
detectors are logged by computer during the
experiment.
Figure 11 –
deposit growth at the liquid mixing
point along with the differential pressure build-up over
the tube as a function of time.
47Ca
11
0-30min
30-60min
60-90min
90-120min
120-150min
after 4 hours
background
10
2.2.2. The tracer 47Ca
9
has a half-life of 4.54 days, and has to be
produced by thermal neutron irradiation of Ca
(46Ca) in a nearby nuclear reactor (in this case at
IFE). The following nuclear reaction takes place:
46Ca(n ,γ)
th
→ 47Ca (γ-emitter)
 ti
)e
7
6
5
4
3
Natural calcium contains only 0.004% of the
target nuclide 46Ca. For low SR-values and
reasonable uncertainty in the experiments,
application of natural calcium as target material
is insufficient. This argument will become clearer
by studying the information in Eq.3 below
showing the activation equation where the
number of target nuclides (N = 46Ca) is one of
the parameters.
D        (1  e
count-rate (cps)
8
47Ca
 t d
2
1
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
position (cm )
Figure 12 – Distribution of 47Ca-containing scale
along the tube at different time intervals.
(3)
where,
σ = reaction cross section in cm -2
φ = neutron flux (ncm -2s-1)
N = number of target atoms
λ = decay constant (=ln2/T1/2)
ti = irradiation time
td = decay time
In natural Ca there is not sufficient number of
46Ca-atoms to reach a disintegration rate, which
is needed for saturation ratios in the order of 1.5
and slightly above. Calcium enriched in 46Ca is
Figure 13 – 47Ca deposit growth at different inhibitor
concentrations.
Scale detection in geothermal systems: The use of nuclear monitoring techniques
5
turn results in more efficient decisions regarding
scale treatments in the field. Such simulation
tools are currently under validation for use in
field scaling management procedures. In
addition, the gamma transmission technique has
been frequently used to detect scale formation in
surface equipment at the field-scale.
REFERENCES
Figure 14 – Final distribution of
along the whole tube.
47Ca-containing
scale
Emmons D.H., Graham G.C., Holt S.P., Jordan M.M.
and Lorcardel B., “On-Site, Near-Real-Time
Monitoring of Scale Deposition”, paper SPE 56776
presented at the SPE Annual Technical Conference
and Exhibition, Huston, Texas (1999).
Gunarathne G.P.P. and Keatch R.W., “Novel
Techniques for Monitoring and Enhancing Dissolution
of Mineral Deposits in Petroleum Pipelines”, paper
SPE 30418 presented at the Offshore Europe
Conference, Aberdeen UK (1995).
Kevin K., “Operating Experiences for the World’s First
Commercially Installed Multiphase Meters in the
Liverpool Bay Field”, paper presented at the IBC
Conference on Multiphase Flow Metering, London
(1999).
Moar P.N. et al., “Fabrication, modelling and direct
evanescent measurement of tapered optical fiber
sensors”, Journal of Applied Physics, 3395 (1999).
Figure 15 – Calcite scale thickness distribution
across the tube
CONCLUSIONS
Two different nuclear techniques have been
designed and evaluated in the lab for the realtime measurements of scale formation under
flow conditions. Both methods are capable to
visualize the distribution of the scale deposits, a
result that is not readily obtained by methods
commonly used in conventional dynamic scaling
experiments. Furthermore, the techniques are
sensitive to scaling, resulting generally in shorter
induction times compared currently employed
methods (based on pressure drop). The
methodologies can be easily used for the
laboratory investigation of the scaling processes
occurring in geological systems, including
oilfield, geothermal and hydrology applications
and for all kind of mineral scales.
It should be noted that although the nuclear
methods have been evaluated at the lab-scale,
their results are meant to be applicable at the
field scale. Indeed, the quantification of the
earlier occurrence of scale precipitation that
those techniques attain can be directly
implemented in large scale simulators. This in
Morizot A.P. and Neville A., “A Novel Approach for
Monitoring of CaCO3 and BaSO4 Scale Formation”,
paper SPE 60189 presented at the 2nd International
Oilfield Scale Symposium, Aberdeen, UK (2000).
Poyet J-P., Ségéral G., Toskey E., “Real-Time
Method for the Detection and Characterization of
Scale”, paper SPE 74659 presented at the 4th
International Oilfield Scale Symposium, Aberdeen, UK
(2002).
Smith J.K., Yuan M., Lopez T.H., Means M. and
Przybylinski J.L., “Real-Time and In-Situ Detection of
Calcium Carbonate Scale in a West Texas Oilfield”,
paper SPE 80372 presented at the 5th International
Oilfield Scale Symposium, Aberdeen, UK (2003).
Stamatakis E., Haugan A., Dugstad Ø., Muller J.,
Chatzichristos C., Bjørnstad T., Palyvos I., “Validation
of Radiotracer Technology in Dynamic Scaling
Experiments in Porous Media”, Chemical Engineering
Science, 60/5, 1363-1370 (2005a).
Stamatakis E., Muller J., Chatzichristos C., Haugan
A., “Real-time monitoring of calcium carbonate
precipitation from geothermal brines”, technical report,
presented during the European Hot Dry Rock
Association (EHDRA) Scientific Meeting, Soultz-SousForêts, France, 17-18 March (2005b).
Theuveny B., Ségéral G. and Moksnes P.O.,
“Detection and Identification of Scales Using Dual
Energy / Venturi Subsea or Topside Multiphase Flow
Meters”, paper OTC 13152 presented at the Offshore
Technology Conference, Houston, Texas (2001).