1

Scale Detection in Geothermal Systems

The use of nuclear monitoring techniques

E. Stamatakisa,b, T. Bjørnstadb, C. Chatzichristosb,

J. Mullerb and A. Stubosa

aNational Centre for Scientific Research Demokritos (NCSRD), Athens, Greece

bInstitute for Energy Technology (IFE), Kjeller, Norway

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Nuclear experimental methods Gamma transmission experiments

Method Typical results

Gamma emission (tracer) experiments Method Typical radiotracer results

Outline

3

Nuclear based methods

1. Gamma emission based on radioactive tracers added to the flowing and reacting system

2. Gamma transmission based on use of external gamma sources

4

Principles of gamma transmission

x

Gamma source Gamma detector

Absorption sample

Transmission of a mono-energetic beam of collimated photons through a simple absorption sample can be described by Lambert-Beer’s equation x

ox eII is the linear mass absorption coefficient with dimension L-1 (cm-1), x the sample thickness

Io Ix

5

Mass absorption coefficientA quantity more commonly found tabulated is the mass absorption coefficient / with dimension cm2/g. In a composite sample the attenuation is additive according to

)22

( ,,,

m

Ca

CamCa

Al

AlmAl

m

xxx

ox eII

XAl XCa Xl XCa XAl

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E (keV) Iabs(%)

80.998 0.008 34.0 0.3

276.397 0.012 7.16 0.07

302.851 0.015 18.3 0.1

356.005 0.017 62.0 0.8

383.851 0.020 8.9 0.1

133BaThe gamma source used in the present experiment is 133Ba due to suitable energies (see table below) and half-life (10.5 y). Main gamma-ray energies and intensities for 133Ba are:

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Experimental setup

8

Close-up look of -ray source and detector

arrangement

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Preliminary lab. experiments

1 0.7 160 15 1

2 1.5 160 15 1

3-6 20 185 10 1

RUN SR Temp.C

Pres.bar

Flowr.ml/min

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Results from “Run 1”

4350

4400

4450

4500

4550

4600

0 6 12 18 24 30 36 42 48

time (hours)

cou

nt-

rate

(cp

s)

background

Gamma attenuation measurements for calcite

precipitation at the inlet of the tube at 160oC, 15 bars and

SR=0.7 (run 1)

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Results from “Run 2”

4350

4400

4450

4500

4550

4600

0 5 10 15 20 25 30

time (hours)

cou

nt-

rate

(cp

s)

tindback-

ground

Gamma attenuation

measurements for calcite

precipitation at the inlet of the

tube at 160oC, 15 bars and SR=1.5

(run 2)

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Results from “Runs 3-6”

Gamma attenuation measurements for

calcite precipitation in the presence and absence of a scale

inhibitor 10cm from inlet of the tube at 185oC, 10 bars and SR=1.5 (runs 3-6)

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Calcite growth rate

0

0,050

0,100

0 5 10 15 20 25

Time (hour)

Sca

le t

hic

kne

ss (

cm)

0,125

0,025

0,075

Scaling rates (scale thickness as a function of time) of calcite precipitation at the inlet of the tube for run 2 - preliminary results

0,150

0,175

0,200

0,225

0,250

0,275

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Calcite distribution across the tube

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

0 10 20 30 40 50 60Position (cm)

scal

e th

ickn

ess

(cm

)

5000

5200

5400

5600

5800

6000

6200

6400

6600

6800

7000

0 10 20 30 40 50 60

Position (cm)

cps

background

final

Scale thickness distribution across the

tube at the end of run 3

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The 133Ba-source (30 mCi or 100 MBq) gives a typical counting rate of about 4500 cps (counts per second) in tube filled with water (ID = 10 mm) with a detector collimator opening of 4.5x4.5 mm. 

The brine-filled tube reduces the normalized incident intensity from 1.000 to 0.891when corrected for the Al-metal walls.

The increased mass thickness (g/cm2) due to scale obviously leads to an increased attenuation and to a reduction in contrast towards mass changes during the experiment.

Transmission experiments may be used to study calcite scaling in open tubes with the dimensions used here.

Discussion on -transmission

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Principles of the -emission method• CaCO3 scaling may be studied by radio-

labeling of any of the chemical components involved.

• However, for on-line, continuous and non-intrusive detection, gamma-ray emitters are required.

• Neither O nor C have suitable gamma-ray emitting isotopes.

• Ca has only one suitable radioactive isotope, namely 47Ca, with a half-life of 4.54 days.

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Chart of nuclides - How to produce 47Ca

21

20

19

24 25 26 27 28 29 30Neutrons

Pro

ton

s

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Tracer- experimental setup

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• 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 increased.

• It is necessary to avoid contribution from the 159 keV γ-quanta of the daughter radionuclide 47Sc.

• The energy window for the detectors will therefore be chosen from 350 keV and upwards.

On-line detector setup

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• Samples are also collected periodically at the exit end of the sandpack and the activity of 47Ca (1297 keV) in solution is determined in off-line high-resolution gamma-spectrometric measurements with a HpGe-detector coupled to a multichannel analyzer. 

• 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.

Other measured parameters

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Typical tracer results (1)

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140

Time (min)

cps

tind

scales

47Ca background

Add NaHCO3

Environmental background

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Typical tracer results (2)

Typical results from a previous experiment with higher SR:

47Ca deposit growth at the inlet and Δp buildup along the

tube vs. time

0

25

50

75

100

125

150

175

200

0 20 40 60 80 100 120 140

Time (min)

cou

nt-

rate

(cp

s)

0,0

0,2

0,4

0,6

0,8

1,0

Δp

(b

ar)

Ca(47) tracer backgroundΔp

47Ca deposit growth in the presence and absence of a

scale inhibitor

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Typical tracer results (3)

012

345678

91011

0 5 10 15 20 25 30 35 40 45 50 55 60

Position (cm)

cou

nt-

rate

(cp

s)

total Ca(47)

initial Ca(47)

final Ca(47)

precip. Ca(47)

0

2

4

6

8

10

12

0 20 40 60

Position (cm)

mg

r C

aC

O 3

Final distribution of the deposits across the tube

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Position (cm)

cou

nt-

rate

(cp

s)background Ca(47)

0-30 min

30-60 min

60-90 min

90-120 min

120-150 min

after 4 hours

47Ca deposit distribution across

the tube at different time-steps

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Discussion on -emission

The radiotracer 47Ca can be used to study CaCO3 precipitation in tube blocking tests providing the following unique information:

The induction time of CaCO3 scale deposition

Visualization of the spatial distribution (concentration versus position) of the CaCO3 scale deposition

All experiments with tracers showed that the tracer monitoring gives a shorter induction time than monitoring of the pressure dropA novel technique for the determination of MIC, based on the γ-emission method, can be developed.

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Final Conclusions 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.

The techniques are sensitive to scaling, resulting generally in shorter induction times compared to Δp-monitoring.

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.

Their results are meant to be applicable at the field scale; the quantification of the earlier occurrence of scale precipitation that those techniques attain can be directly implemented in large scale simulators.