corrosive species and scaling in wells at olkaria ... - Orkustofnun

The IR spectra of scale samples #1
to # 5 all have IR features similar to
those of precipitated silica gel or
amorphous silica. A broad
asymmetric stretching band is
observed in all the spectra between
3200 cm -1 and 3600 cm -1 . In all the
scales the spectra bands at ~ 3469 to
3554 cm -1 are associated with the
hydroxyl group (OH) of the bound
water in the crystal lattice. Weak
symmetric stretching vibration
bands are observed at ~ 2852 cm -1
and 2848 cm -1 and these represent
C-H groups that could be due to
contamination by grease or oil.
Molecular water (H 2 O) is present in
these scales represented by the
spectra band at 1641 cm -1 . In all the
scales there is a strong peak in the
range ~1099 - ~1033 cm -1 from
bending O-Si-O vibrations due to
antisymmetric Si-O-Si stretches.
The second strongest peak between
~ 469 and 465 cm -1 is due to O-Si-O
bending vibrations and a weaker
Counts
3.4
3.2
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
# 4
# 2
# 3
3469 -3554
2852-2848 1641-1654
1099 -1033
950-970
793-795
465-469
4500 4000 3500 3000 2500 2000 1500 1000 500 0
Wavelength cm -1
FIGURE 47: Infrared spectra of scale samples #1 - # 5
collected from Olkaria well OW-34
band near 793-795 cm -1 is evaluated as due to symmetrical stretching of tetrahedrally co-ordinated Si
and Al. The 1099-1033 cm -1 band has shoulders at ~1200 cm -1 on the low energy side and ~ 950-970
cm -1 on the high energy side. The 950-970 cm -1 peak is particularly characteristic of amorphous silica.
Scanned Electron Micrographs of the scale samples from well OW-34 fluids are presented in
Appendix C together with chemical analysis, shown in Table 2 and a summary of elements calculated
as oxides is presented in Table 3 of Appendix C.
All the scale samples # 1 to # 5 are predominantly composed of Silicon (Si) and Oxygen (O). The
percent silicon composition in the scale samples # 1 to # 3 suggest that silicon composition (Si) ranged
between ~58 and ~37% and that of oxygen (O) varied between 61 and 39%. These samples did not
show any trace amounts of other elements in them. They could represent almost pure amorphous silica
phases (Appendix C). Scale samples # 4 and # 5, though largely silica rich had a higher iron and
aluminium content. Trace concentrations of iron in scale # 4 could have its origin in the pipework or
probably the walls of the separator while that of scale # 5 could have its origin from the pipework in
the discharge flow line.
X-ray diffraction patterns for the scales formed from Olkaria well OW-34 are shown in Figure 48. All
the scales of samples # 1 to # 5 are largely amorphous to X rays. They all depict a broad peak at ~ 23˚
2θ, characteristic of amorphous silica or opaline silicates. Scale sample # 5 has a small diffraction
peak centred at ~ 27˚ 2θ alongwith the broad spectra band at ~ 23˚ 2θ for amorphous silica phases.
The diffraction peak at ~ 27 ˚ 2θ could be due to quartz in the scale sample.
Results of chemical analysis of the scales by ICP are shown in Table 6 of Appendix A together with
analysis of scales formed on coupons at Nesjavellir. Scales # 1 to # 4 have trace amounts of the major
aqueous cations except for calcium which is present in high concentrations in scale # 3 Zinc is
abundant in scale # 1. The source of high calcium (Ca) and zinc (Zn) concentrations in these scales is
hard to establish but could be due to contamination. The calcium may also reflect the presence of a
# 1
# 5
45