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Testing of Scale Inhibitor Efficiency in a Pre-Scaled Environment
Issam Aarag (CLARIANT OIL SERVICES SCANDINAVIA AS)
Dr. Karl Fredrik S. Alnes (CLARIANT OIL SERVICES SCANDINAVIA AS)
Abstract:
Scale inhibitor selection and ranking for a proposed field application can be made more
effective by employing laboratory test techniques that will better simulate and reflect the realfield scaling environment that the inhibitor will encounter on application. One of the most often
overlooked but important parameters with respect to scale inhibitor selection is the effect of
pre-formed inorganic mineral scales located on wetted production surfaces. Pre-scaled
surfaces will impact scale inhibitor performance and ranking in two ways; firstly the scaled
surface will provide an ideal high crystal surface area for attraction and interaction-retention of
bulk scale inhibitor molecules, essentially acting as a scale inhibitor ‘thief zone’, and secondly,
the crystal surfaces will simultaneously present an energetically more favorable environment
for further scale growth and also act as a potential locus for new scale and seed crystal
development to promote more aggressive bulk scaling.
The following technical paper reports the application of the Kinetic Turbidity (KT) Test
technique as part of a conventional scale inhibitor laboratory test suite for ranking and
selection of a downhole scale inhibitor in a slightly unorthodox carbonate scale control
scenario. The KT test allowed for rapid performance screening of the candidate scale
inhibitors using pre-scaled and non-scaled (clean) KT test cells, and demonstrated the
importance of testing using pre-scaled and ‘clean’ wetted test surfaces when determining the
minimum inhibitor concentration (MIC) needed for effective scale control.
Introduction:
The range of scale inhibitor performance tests used to select an inhibitor for a defined
carbonate and / or sulphate scaling scenario generally falls under one of a number of industry
accepted test suite programs. Each component of the test suite contributes specific
performance data, and when viewed together, provides the basis for ranking of candidate
scale inhibitor products according to the specific minimum performance criteria agreed with
the client.
The traditional ‘basic’ scale inhibitor selection test suite usually comprises;
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
Brine and Materials Compatibility Tests

Static Jar Tests

Tube Blocking (Dynamic) Tests

Thermal Stability Tests
For scale inhibitor squeezing, the following additional tests can be included;

Static Adsorption Tests

Formation Damage Coreflood

Isotherm Derivation Coreflood
A range of more exotic and specific scale inhibitor performance tests can be considered for
inclusion that are either particular to the mode of scale inhibitor application and / or potential
for interaction with other materials of construction / production constituents.
All scale inhibitor tests have their own particular merits but tend to be performed using sterile,
solids free synthetic waters created in the laboratory using high purity grade reagents
dissolved according to recipe in distilled or deionized water, and the static jar tests performed
in clean disposable laboratory glassware. The clinical scaling environment created in
laboratory jar and tube blocking tests differs considerably from the actual field situation,
particularly with respect to brines and wetted surfaces; real field produced waters tend to
contain a myriad of additional dissolved and suspended organic and inorganic species, and
real field wetted production surfaces in contact with scaling brines can often present some
degree of mineral scale deposition.
Real field produced waters are far too complex to reproduce exactly within the laboratory,
therefore we need to use an aseptic analogue which is hoped will provide a response in scale
inhibitor performance tests similar to the real field brine. Pre-formed scale presence on wetted
production surfaces can, to a certain degree, be reproduced and assessed in the laboratory
and included as part of the routine scale inhibitor testing program.
Pre-Scaling and Scale Inhibitor Testing:
The impact of pre-scaled surfaces on scale inhibitor performance is a known and well
understood phenomenon, as the presence of solid scale fouling on a wetted surface in
contact with a scaling brine will provide an ideal high crystal surface area for attraction and
interaction-retention of bulk scale inhibitor molecules, essentially acting as a scale inhibitor
‘thief zone’. The pre-scaled surface will also present an energetically more favorable
environment for further scale growth and act as a potential locus for new scale and seed
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crystal development to promote more aggressive bulk scaling.
Pre-scaling can be incorporated into conventional tube blocking performance tests as an
additional variable. There are concerns about establishing repeatability of pre-scaling in tube
blocking tests and this can, on occasion, lead to uncertainty when comparing results from
tube blocking test ranking exercises. Pre-scaling is generally not used in static jar tests for
scale inhibitor performance determination.
Scale inhibitors are designed to rapidly locate themselves at growing scale crystal surfaces
and edges, and in doing so ‘quench’ any further growth at that crystal surface location. The
process of crystal growth however will continue to develop at other points on the crystal
surface as long as super-saturation criteria are met and / or the concentration of bulk free
scale inhibitors falls below a minimum threshold value for that system. If the MIC is artificially
lowered by loss of scale inhibitor through surface adsorption / location then the applied MIC
dose will fail to meet the inhibitor demands of the system and the system will scale – even
though a laboratory derived MIC is being applied. Likewise in a produced water production
scenario where the waters have been laboratory evaluated to show supersaturation but
borderline scaling potential, the presence of the pre-scaled surface may be sufficient to
provide the energetic impetus to initiate scale precipitation processes from the slightly
supersaturated system, and in doing so confounding the initial assessment of the very low
MIC demand for the system.
The KT test provides a capability for performance testing scale inhibitors in the presence and
absence of pre-scaled wetted test surfaces, and also employs a ‘quasi-dynamic’ aspect to the
testing via in-situ stirring. The KT instrument was originally created to provide an alternative
technique for determining scaling onset and development for use in scale inhibitor
performance ranking, without relying on conventional scaling indicators such as changes in
differential pressure (tube blocking test) or changes in brine ion concentration (static jar test).
Scaling Scenario:
Well ‘A’ in offshore platform ‘T’ was configured for downhole scale inhibitor chemical injection,
and the incumbent scale inhibitor ‘I’ was dosed into the well continuously from surface to
provide control of calcium carbonate scale from point of injection and downstream, up-well. A
modest calcium carbonate scaling potential had been identified below the scale inhibitor
injection point in Well ‘A’ which was treated periodically via washing with mineral acid. No
scaling had been identified from wellhead to injection point.
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More recently, calcium carbonate scale build-up had been identified in the well flowline to
separator, and after a period of 8 weeks had accumulated sufficiently to raise the flowline
pressure by 60 psi. Mineral acid washing of the flowline resulted in the pressure returning to
normal. Scale inhibitor ‘I’ was dosed throughout but had failed to provide acceptable
protection from calcium carbonate scaling.
Flowline scaling was identified again after resumption of production and on this occasion a
section of flowline tubing was isolated and removed for inspection. Examination revealed that
calcium carbonate mineral scaling was developing within the tubing and layering was evident
in the 1 cm thick deposit.
A scale inhibitor selection exercise was performed at the request of the operator to identify a
suitable replacement for scale inhibitor ‘I’ for carbonate scale control downhole in Well ‘A’ and
Well ‘A’ flowline. As part of the selection program the operator requested calcium carbonate
scale inhibition performance testing in the presence of pre-formed calcium carbonate scaling
of wetted surfaces.
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Test Water:
Table 1 provides a representative produced water chemistry for Well ‘A’.
Table 1: Well ‘A’ Produced Water Ion Composition
Element
Concentration (mg/L)
Sodium
15700
Potassium
600
Calcium
1200
Magnesium
70
Strontium
55
Barium
1
Chloride
27100
Bicarbonate
300
Sulphate
400
pH
7.0
Scale Inhibitors:
Four off-the-shelf scale inhibitor products were selected and identified here as inhibitors 1, 2,
3 and 4. Inhibitors 1, and 4 were water soluble polymers and inhibitors 2 and 3 were generic
phosphonate scale inhibitor formulations. All were pre-screened for suitability for application
at 130°C via continuous downhole injection via injection line and valve.
Laboratory Test Suite:
The laboratory test suite used in the selection and ranking exercise included brine
compatibility tests, static jar tests for calcium carbonate scale efficacy and also tube blocking
dynamic performance tests. All tests were performed at 130°C using 100% well ‘A’ produced
water composition, identified in Table 1.
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Results:
Brine Compatibility Tests
The brine compatibility tests showed all candidate products were 100% compatible with well
‘A’ produced water at 130°C at all doses across a 24 hour test period, see Figure 1.
Dose
At mix
0.5 h
1h
2h
4h
24 h
Dose
50 ppm
50 ppm
100 ppm
100 ppm
500 ppm
500 ppm
1000 ppm
1000 ppm
1%
1%
5%
5%
10 %
10 %
20 %
20 %
50 %
50 %
90%
90%
Dose
At mix
0.5 h
1h
2h
4h
24 h
Dose
50 ppm
50 ppm
100 ppm
100 ppm
500 ppm
500 ppm
1000 ppm
1000 ppm
1%
1%
5%
5%
10 %
10 %
20 %
20 %
50 %
50 %
90%
90%
At mix
0.5 h
1h
2h
4h
24 h
At mix
0.5 h
1h
2h
4h
24 h
Figure 1: Brine Compatibility Results in Well ‘A’ Produced Water at 130°C. Incompatibility is
indicated via greyscale shading. No incompatibility = white circle.
(Top LHS = inhibitor 1, Top RHS = inhibitor 2, Bottom LHS = inhibitor 3, Bottom RHS = inhibitor 4)
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Static Jar Tests
The static jar tests provided better discrimination of product character with respect to their
calcium carbonate scale inhibition performance at 130°C. Scale inhibitor 3 proved most
effective for calcium carbonate scale control with inhibitors 1 and 2 performing equally and
were therefore tied for second position, and last was inhibitor 4 which showed the poorest
performance of all 4 scale inhibitors for calcium carbonate scale inhibition in well ‘A’ produced
water at 130°C across a 24 hour test period, see Figure 2.
1
2h
2
24 h
2h
3
24 h
2h
4
24 h
2h
24 h
Bl a nk
Control
10 ppm
20 ppm
30 ppm
40 ppm
50 ppm
Figure 2: Static Jar Tests Results in Well ‘A’ Produced Water at 130°C for calcium carbonate
scaling. The degree of scaling is indicated via greyscale shading. No scaling = white
background. (1 = inhibitor 1, 2 = inhibitor 2, 3 = inhibitor 3, 4 = inhibitor 4)
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Tube Blocking Dynamic Performance Tests
The results of the tube blocking test clearly discriminate between the dosed and non-dosed
tube blocking test runs, with the ‘Blank’ duplicates scaling at virtually the same time in repeat
blank runs, see Figure 3 at approximately 28 minutes run time.
Blank 1
Blank 2
Inhibitor 1
Inhibitor 2
Inhibitor 3
Inhibitor 4
8
Differential Pressure (psi)
7
6
20
ppm
5
18
ppm
16
ppm
12
ppm
14
ppm
10
ppm
8
ppm
4
ppm
6
ppm
2
ppm
0
ppm
4
3
2
1
0
0
75
150
225
300
375
450
525
600
675
750
825
Time (minutes)
Figure 3: Tube Blocking Test Results in Well ‘A’ Produced Water at 130°C for CaCO3 scaling.
The tube blocking test results showed that inhibitor 3 performed best, and suggested a
dynamic MIC of approximately 2 ppm. Scale inhibitors 2 and 4 performed similarly, indicating
dynamic MICs of approximately 8 ppm. Last was inhibitor 1 which performed poorly in the
tube blocking test, showing a dynamic MIC of approximately 14 ppm for well ‘A’ calcium
carbonate scale control.
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Test Suite Summary:
The test suite clearly identified inhibitor 3 as best candidate for recommendation as
continuous downhole scale inhibitor replacement to incumbent chemical ‘I’ for deployment in
well ‘A’ for calcium carbonate scale control. Ideally the tests should have been performed
using the incumbent product as reference throughout, however this was not possible and
therefore the test suite was completed in its absence.
Kinetic Turbidity Testing:
All four of the candidate scale inhibitors were performance tested via the KT testing
instrument, an Agilent Cary Win UV configured with a 6x6 Multi Cell Holder Peltier Series II
adapter, see Figure 4. Tests were performed with and without pre-scaling of the test cell to
assess the potential impact on scale inhibitor demand created by pre-scaling the KT test cell.
Figure 4: Kinetic Turbidity Test equipment. Left: The Agilent Cary Win UV instrument. Middle: Is
the 6x6 Multi Cell Holder Peltier Series II adapter. Right: is the cuvette test cell complete with microstirring pip in the base.
In 2012, Baugh et.al. (OTC-23150-MS) presented a new method for screening scale inhibitors
using a multi-cell UV-Vis spectrometer with constant monitoring of turbidity at 500 nm. The
instrument used the same configuration as described in Figure 4, which can heat the sample
cuvettes up to 97°C and also provide constant stirring via magnetic stirring bar. The KT test
presents a useful additional tool for the rapid screening of a range of scale inhibitors in
selection programs. The KT test has since been adopted and adapted to create, a new
method for testing scale inhibitor performance using pre-scaled test cuvettes; the goal being
to mimic scale inhibitor performance in production environments where wetted surface prescaling is suspected.
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KT Test Results:
Scale Inhibitor 1
Scale inhibitor 1 provided a good example with respect to comparing the impact that prescaling might have on scale inhibitor performance, see Figures 5 and 6 for conventional KT
and pre-scaled KT tests respectively. In the absence of pre-scaling, the Blank and Control
test cells behave as expected. The Blank test cell absorbance profile, see Figure 5, shows
rapid significant particle development identified by the sharp rise absorbance. The
absorbance / turbidity declines across the following 2 hour test period as the scale particles
become so large that the stirring bar is incapable of maintaining them in stirred solution, and
they drop to the bottom of the cell. The scale inhibitor cells show scaling for both the 4 ppm
and 6 ppm dosed cells, with scale control established at approximately 8 ppm applied.
0.25
Absorbance
0.2
DI water
0.15
Blank
4 ppm
6 ppm
0.1
8 ppm
10 ppm
12 ppm
0.05
0
0
20
40
60
Time (minutes)
80
100
120
Figure 5: KT Test Results for Inhibitor 1, NO PRE-SCALING.
In the pre-scaled test, the Blank and Control test cells behave as expected, see Figure 6.
Again the Blank absorbance profile declines across the 2 hour test period as the scale
particles form and enlarge and then drop to the bottom of the cell as their mass becomes too
large to keep them in stirred solution, see Figure 6. It is interesting to note that there appears
to be relatively low surface deposition of scale on the cell walls of the cuvette. At 4 ppm dose
of inhibitor 1 the solution turbidity increases with time signifying solid scale precipitation which
does not appear to drop from solution but is either maintained in suspension or adheres to the
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cell walls resulting in no net decrease in turbidity with time as was evident in the Blank. A 6
ppm dose presents a similar story however the profile shows a more gradual increase in
turbidity, see Figure 5. At doses above 6 ppm the KT test response shows a very small
increase in turbidity with time across the 2 hour test period.
As the tube blocking dynamic MIC was determined as approximately 14 ppm, see Figure 3
above, it was decided to dose the pre-scaled cell with 10-18 ppm of inhibitor 1. The inhibitor 1
pre-scaled KT test results show a different performance with the spectra initiating at higher
absorbance values compared to non-pre-scaled analogues and the DI water KT test cell, see
Figure 6. The Blank response is again characteristic of zero inhibition and rises, peaks and
then falls in absorbance with time.
0.16
0.14
0.12
Absorbance
0.1
DI water
Blank
0.08
10 ppm
0.06
14 ppm
12 ppm
16 ppm
18 ppm
0.04
0.02
0
0
20
40
60
Time (minutes)
80
100
120
Figure 6: KT Test Results for Inhibitor 1 WITH PRE-SCALING.
The dosed cell results appear quite challenging to interpret, however the absorbance spectra
from the 16 and 18 ppm dosed samples appears to be very similar in profile, and also appear
to maintain a stable linear profile response after initially experiencing a very slow tapering
decline. The other dosing tests show continued drop throughout the dosing period suggesting
scale formation and loss as the particles grow large enough, see Figure 6. An alternative
interpretation regarding the slow tapering decline is the connection between solution pH, the
pH sensitive pre-scaled coating and the pH of the scale inhibitor dosed into the cell. The
addition of low pH scale inhibitor may exert a pH-effect on the existing carbonate scale
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deposited on the test cell wall, and as such may cause a reduction in absorbance as the scale
film on the cell wall is dissolved with time as the pH equilibrates. In the absence of any other
obvious significant scale related changes then a steady decrease in absorbance with time for
acidic scale inhibitors will need to be seriously considered as the most likely cause of spectra
slow decline. The lower the pH of the scale inhibitor dosed into the cell coupled to actual dose
applied will combine to create the most significant effect on the spectra.
The KT derived MIC therefore roughly correlates with the tube blocking dynamic MIC for scale
inhibitor 1, with the combined MIC approximating to between 14 and 16 ppm applied dose.
Scale Inhibitor 2
KT testing of inhibitor 2 in the absence of pre-scale, showed the Blank and Control cells
behaving as expected, see Figure 7. The control showed rapid particle development which
declined with time across the 2 hour test time period. The scale inhibitor dosed cells indicated
scaling at both 2 and 4 ppm applied doses, see Figure 7, with scale control established at
approximately 6 ppm applied, see Figure 7. The 6 ppm MIC value is reasonably close to the
tube blocking derived 8 ppm ‘approximate’ dynamic MIC for inhibitor 2, see Figure 3.
0.25
Absorbance
0.2
DI water
0.15
Blank
2 ppm
4 ppm
0.1
6 ppm
8 ppm
10 ppm
0.05
0
0
20
40
60
Time (minutes)
80
100
120
Figure 7: KT Test Results for Inhibitor 2, NO PRE-SCALING.
The KT results for tests performed in pre-scaled cells dosed with scale inhibitor 2 are
presented, see Figure 8. The 2 ppm and 4 ppm applied dose tests clearly fail. The 6 ppm and
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8 ppm dosed sample cells appear to increase in absorbance suggesting scale development,
and then reduces to give a level profile for the latter stages of the experiment. The very slight
decline observed for both 6 and 8 ppm dosed tests may be attributed to the pH-effect
mentioned above.
0.14
0.12
Absorbance
0.1
DI water
0.08
Blank
2 ppm
0.06
4 ppm
6 ppm
8 ppm
0.04
0.02
0
0
20
40
60
Time (minutes)
80
100
120
Figure 8: KT Test Results for Inhibitor 2 WITH PRE-SCALING.
The KT test and tube blocking MIC results combined suggest that a dose of 6–8 ppm of
inhibitor 2 will provide protection against calcium carbonate scaling in well ‘A’, even in the
presence of pre-scaled tubing.
Scale Inhibitor 3
For inhibitor 3, the absorbance spectra in Figure 9 indicated that a 1 ppm dose of inhibitor 3
fails, while all other higher scale inhibitor 3 doses pass and show symmetry with the DI water
absorbance profile.
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0.3
0.25
Absorbance
0.2
DI water
Blank
0.15
1 ppm
3 ppm
4 ppm
0.1
6 ppm
0.05
0
0
20
40
60
Time (minutes)
80
100
120
Figure 9: KT Test Results for Inhibitor 3, NO PRE-SCALING.
When pre-scaling is included we see a change in performance for inhibitor 3, see Figure 10.
Doses of 2-8 ppm were used, as a preliminary KT pre-scaled test run performed at lower
concentration identified possible scaling issues at 2 ppm applied dose. The 2-8 ppm dosed
KT tests performed in pre-scaled cells are presented, see Figure 10. All profiles appear to
show the tapered decay pattern that indicates a slow loss of absorbance with time, without
showing the dramatic pulse of scale formation and increase in turbidity at the start of the test
(as per the Blank). It is therefore considered most likely that the tapered spectral profiles
shown here have resulted from the impact of dosing a low pH scale inhibitor into a scaled cell
containing acid soluble scale film on the cell wall surface. The cells were inspected before
and after test and there appeared to be some indications of increased solid accumulation on
the cell floor of the cells dosed with 1-2 ppm scale inhibitor 3, while no increase in cell floor
deposit was evident in cells dosed with higher concentrations of inhibitor 3.
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0.16
0.14
0.12
0.1
DI water
Blank
0.08
2 ppm
4 ppm
0.06
6 ppm
8 ppm
0.04
0.02
0
0
20
40
60
80
100
120
-0.02
Figure 10: KT Test Results for Inhibitor 3 WITH PRE-SCALING.
Combining test results from the dynamic tube blocking test MIC, see Figure 3, and the KT test
derived MIC, a combined MIC of 4 ppm of scale inhibitor 3 would be recommended for control
of scale in well ‘A’.
Scale Inhibitor 4
Scale inhibitor 4 KT testing in the absence of pre-scaling showed the Blank absorbance
profile response to behave as expected, see Figure 11. The Blank cell absorbance shows
rapid particle development which then decays across the following 2 hour test time period.
The DI water absorbance profile and the inhibitor 4 tests all showed the self-same
absorbance profile trend characterized by a slow steady rise in absorbance in all cases, see
Figure 11. This occurrence is difficult to explain.
The pre-scaled test results for inhibitor 4 are presented in Figure 12, and appear to be quite
challenging to interpret. The Blank and DI water test cell responses behave as expected, and
unlike the zero pre-scaled DI water profile, pre-scaled DI water control cell provides a flat
baseline.
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0.25
Absorbance
0.2
0.15
DI water
Blank
4 ppm
6 ppm
0.1
8 ppm
10 ppm
0.05
0
0
20
40
60
Time (minutes)
80
100
120
Figure 11: KT Test Results for Inhibitor 4, NO PRE-SCALING.
0.14
0.12
Absorbance
0.1
DI water
0.08
Blank
2 ppm
0.06
4 ppm
6 ppm
8 ppm
0.04
0.02
0
0
20
40
60
Time (minutes)
80
100
120
Figure 12: KT Test Results for Inhibitor 4 WITH PRE-SCALING.
The 6 and 8 ppm higher applied dose cells absorbance profiles appear to parallel one another
closely while the 4 ppm and 2 ppm tests show quite different profiles with steady decline with
time for 4 ppm and decline with an initial absorbance rise for the 2 ppm dosed cell. Combining
tube blocking and KT test results suggests an MIC of approximately 6 ppm for scale inhibitor
4 for scale control in Well ‘A’.
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Discussion:
The Kinetic Turbidity tests were performed as a supplement to the conventional scale inhibitor
performance testing suite and have provided much additional data regarding the candidate
scale inhibitor chemical performances for control of calcium carbonate scale under well ‘A’
high temperature downhole application conditions, particularly for pre-scaled and non-prescaled tests. In general there was fairly close agreement between the candidate scale
inhibitor MICs determined via conventional Tube Blocking test and then Kinetic Turbidity test
evaluation, and also a looser MIC correlation between pre-scaled and non-pre-scaled KT test
MICs.
The KT technique is still in its infancy and will benefit from increased use during scale inhibitor
selection studies to increase the knowledge base when using this technique and in particular
improve interpretation of absorbance profiles to enhance the scale inhibitor selection process.
The absence of the incumbent scale inhibitor from the testing suite has robbed the study of an
additional dataset which would have allowed comparison in conventional performance testing
against the candidate scale inhibitors, and unfortunately also removed any possibility of
explaining why the incumbent scale inhibitor chemical stopped providing scale control in the
case of well ‘A’.
The KT tests were an add-on and were therefore performed after the initial selection test
program was completed. In retrospect it would have been interesting and helpful to the
discussion to have repeated the tube blocking tests, but on this occasion to have included
some pre-scaling.
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Conclusions:

The Kinetic Turbidity test has been used successfully to enhance a scale inhibitor
selection exercise which was focused on selecting a suitable scale inhibitor species for
continuous application downhole for an unusual calcium carbonate scaling scenario.

The KT technique tested scale inhibitors using both pre-scaled and non-pre-scaled tests.

The KT tests generated MIC results similar to that achieved via conventional tube blocking
test.

The KT test results showed some correlation between tests performed in the absence and
presence of cell pre-scaling.

The KT test technique has shown considerable promise for being included as an add-on
to the conventional scale inhibitor selection program.

The technique is still in its infancy and will benefit markedly through use and subsequent
build-up in the knowledge database.
Acknowledgements:
The authors of this paper want to thank the following people for all their help and assistance
in creating this paper:

Dr. Alex Thornton for proof-reading and all his comments and changes.

Stina Storebråten for doing most of the leg work.

Terje Bratseth for discussions.

Vasan Singaravel for performing the initial tests on the method.
The authors would also like to thank Clariant Oil Services and the Oilfield Chemistry
Symposium committee for permission to publish this paper.
Page 18 of 18