1. 1 Scale Detection in Geothermal Systems The use of nuclear monitoring techniques E. Stamatakis a,b, T. Bjørnstad b, C. Chatzichristos b, J. Muller b and A. Stubos a a National Centre for Scientific Research Demokritos (NCSRD), Athens, Greece b Institute for Energy Technology (IFE), Kjeller, Norway

2. 2  Nuclear experimental methods  Gamma transmission experiments  Method  Typical results  Gamma emission (tracer) experiments  Method  Typical radiotracer results Outline

3. 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. 4 Principles of gamma transmission x Gamma sourceGamma 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  is the linear mass absorption coefficient with dimension L -1 (cm -1 ), x the sample thickness IoIo IxIx

5. 5 Mass absorption coefficient A quantity more commonly found tabulated is the mass absorption coefficient  /  with dimension cm 2 /g. In a composite sample the attenuation is additive according to X Al X Ca XlXl X Al

6. 6 E  (keV)I abs (%) 80.998  0.00834.0  0.3 276.397  0.0127.16  0.07 302.851  0.01518.3  0.1 356.005  0.01762.0  0.8 383.851  0.0208.9  0.1 133 Ba The gamma source used in the present experiment is 133 Ba due to suitable energies (see table below) and half-life (10.5 y). Main gamma-ray energies and intensities for 133 Ba are:

7. 7 Experimental setup

8. 8 Close-up look of  -ray source and detector arrangement

9. 9 Preliminary lab. experiments 10.7160151 21.5160151 3-620185101 RUN SR Temp.  C Pres. bar Flowr. ml/min

10. 10 Results from “Run 1” Gamma attenuation measurements for calcite precipitation at the inlet of the tube at 160 o C, 15 bars and SR=0.7 (run 1)

11. 11 Results from “Run 2” Gamma attenuation measurements for calcite precipitation at the inlet of the tube at 160 o C, 15 bars and SR=1.5 (run 2)

12. 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 185 o C, 10 bars and SR=1.5 (runs 3-6)

13. 13 Calcite growth rate 0 0,050 0,100 0510152025 Time (hour) Scale thickness (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

14. 14 Calcite distribution across the tube Scale thickness distribution across the tube at the end of run 3

15. 15  The 133 Ba-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/cm 2 ) 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

16. 16 Principles of the  -emission method CaCO 3 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 47 Ca, with a half-life of 4.54 days.

17. 17 Chart of nuclides - How to produce 47 Ca 21 20 19 24252627282930 Neutrons Protons

18. 18 Tracer- experimental setup

19. 19 The main gamma energy of 47 Ca 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 47 Sc. The energy window for the detectors will therefore be chosen from 350 keV and upwards. On-line detector setup

20. 20 Samples are also collected periodically at the exit end of the sandpack and the activity of 47 Ca (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 T s, differential pressure Δp, pH, absolute system pressure p and 47 Ca 2+ activity (counting rate R) from the two on-line detectors are logged by computer during the experiment. Other measured parameters

21. 21 Typical tracer results (1) t ind scales 47 Ca background Add NaHCO 3 Environmental background

22. 22 Typical tracer results (2) Typical results from a previous experiment with higher SR: 47 Ca deposit growth at the inlet and Δp buildup along the tube vs. time 47 Ca deposit growth in the presence and absence of a scale inhibitor

23. 23 Typical tracer results (3) Final distribution of the deposits across the tube 47 Ca deposit distribution across the tube at different time-steps

24. 24 Discussion on  -emission  The radiotracer 47 Ca can be used to study CaCO 3 precipitation in tube blocking tests providing the following unique information:  The induction time of CaCO 3 scale deposition  Visualization of the spatial distribution (concentration versus position) of the CaCO 3 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 γ-emission method, can be developed.

25. 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 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.

26. 26 How to produce 47 Ca Activation equation: where  = reaction cross section in cm -2  = neutron flux (n·cm -2· s -1 ) N =number of target atoms =decay constant (= ln2/T 1/2 ) t i =irradiation time t d = decay time