Scale Detection in Geothermal Systems

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
2
Outline

• Nuclear experimental methods
• Gamma transmission experiments
• Method
• Typical results
• Gamma emission (tracer) experiments
• Method
• Typical radiotracer results

3
Nuclear based methods

• Gamma emission based on radioactive tracers added
  to the flowing and reacting system
• Gamma transmission based on use of external gamma
  sources

4
Principles of gamma transmission
Absorption sample
Gamma source
Gamma detector
Io
Ix
x
Transmission of a mono-energetic beam of
collimated photons through a simple absorption
sample can be described by Lambert-Beers equation
? is the linear mass absorption coefficient with
dimension L-1 (cm-1), x the sample thickness
5
Mass absorption coefficient
A quantity more commonly found tabulated is the
mass absorption coefficient ?/? with dimension
cm2/g. In a composite sample the attenuation is
additive according to
XAl
XCa
Xl
XCa
XAl
6
133Ba
The 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
7
Experimental setup
8
Close-up look of ?-ray source and detector
arrangement
9
Preliminary lab. experiments
Temp. ?C
Pres. bar
Flowr. ml/min
RUN
SR
10
Results from Run 1
Gamma attenuation measurements for calcite
precipitation at the inlet of the tube at 160oC,
15 bars and SR0.7 (run 1)
11
Results from Run 2
Gamma attenuation measurements for calcite
precipitation at the inlet of the tube at 160oC,
15 bars and SR1.5 (run 2)
12
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 SR1.5 (runs 3-6)
13
Calcite growth rate
0,275
0,250
Scaling rates (scale thickness as a function of
time) of calcite precipitation at the inlet of
the tube for run 2 - preliminary results
0,225
0,200
0,175
0,150
Scale thickness (cm)
0,125
0,100
0,075
0,050
0,025
0
0
5
10
15
20
25
Time (hour)
14
Calcite distribution across the tube
Scale thickness distribution across the tube at
the end of run 3
15
Discussion on ?-transmission

• 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.

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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.

17
Chart of nuclides - How to produce 47Ca
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Protons
20
19
24
25
26
27
28
29
30
Neutrons
18
Tracer- experimental setup
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On-line detector setup

• 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 ?-quanta of the daughter radionuclide
  47Sc.
• The energy window for the detectors will
  therefore be chosen from 350 keV and upwards.

20
Other measured parameters

• 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
  ?p, pH, absolute system pressure p and 47Ca2
  activity (counting rate R) from the two on-line
  detectors are logged by computer during the
  experiment.

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Typical tracer results (1)
Add NaHCO3
scales
47Ca background
tind
Environmental background
22
Typical tracer results (2)
Typical results from a previous experiment with
higher SR
47Ca deposit growth at the inlet and ?p buildup
along the tube vs. time
47Ca deposit growth in the presence and absence
of a scale inhibitor
23
Typical tracer results (3)
47Ca deposit distribution across the tube at
different time-steps
Final distribution of the deposits across the tube
24
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 drop
• A novel technique for the determination of MIC,
  based on the ?-emission method, can be developed.

25
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 ?p-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.

26
How to produce 47Ca
Activation equation

• where
• reaction cross section in cm-2
• neutron flux (ncm-2s-1)
• N number of target atoms
• decay constant ( ln2/T1/2)
• ti irradiation time
• td decay time