SPE-187994-MS

Scale Resistant Production Tools

Deepak Kumar; Darren Bane; Zhiyue Xu, Baker Hughes Incorporated

Copyright 2017, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Kingdom of Saudi Arabia Annual Technical Symposium and Exhibition held in Dammam, Saudi Arabia, 24–27
April 2017.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents
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Abstract
A novel silicon and oxygen doped diamond like carbon coating was developed to reduce the build-up of scale from oilfield
production tools such as sub-surface safety valves and gas-lift valves. The coating was deposited on 2” x 2” flat metallic
coupons, and its performance was compared with those of commercial coatings conventionally used as anti-scale coating by
the industry, such as fluoropolymer and Ni-P-fluoropolymer coating via custom scale growth experiments at 85F and 150F,
micro-scratch testing for coating-substrate adhesion, and environmental compatibility in downhole brine and oil at temperatures
up to 400F.

The results showed that the doped-DLC coating had the best anti-scale property. Under identical test conditions, the weight of
scale deposit on the doped-DLC coating was substantially (up to 90%) less than other coatings, likely due to its low surface
energy and smooth surface roughness. The abrasion resistance (5 times higher) and environmental compatibility of the doped-
DLC coating was also superior to the other coatings.

Field trial studies of the coating is currently underway on multiple coated sub-surface safety valves and gas lift valves. The
coated valves were put in the same wells and completion configurations in which an operator was having to replace the safety
valves every 5-6 months due to scale plugging, whereas it’s been 8 months since the doped DLC coated valves were deployed
and the coated valves are functioning properly. These results suggest that the coating is preventing or minimizing the scale
build-up from the valves. Analysis of the valves, once they are retrieved from the wells, will be conducted to fully determine
the benefits of the coating.

1. Introduction
Scale can build-up at/or downstream of any point in the oil and gas production system at which supersaturation of scale forming
cations (Ca2+, Ba2+, Sr2+) and anions (CO32-, SO42-) are generated. Supersaturation can be generated by change in pressure,
temperature, pH and alkalinity of single produced/formation water or by mixing two incompatible waters [1]. In general,
whenever an oil and gas well produces water, or water injection is used to enhance recovery, there is the possibility that scale
will form [2]. The deposited scale may adhere to the surface of the production tubing, valves, pumps, and completion equipment
with time and lead to higher frictional losses, reduced flow rates and even malfunctioning of the tools.

The primary methods for mitigating such scale build-up in the oilfield are: (i) acid treatments by HCl, H 2SO4, and chelating
agents like oxalates to dissolve scale, (ii) application of special mechanical or electro-mechanical tools to clean out the scale
via scraping or abrasive jetting actions, and (iii) continuous injection of chemical inhibitors such as PPCA, PAA into the
wellbore. These approaches while very effective, have their own limitations. For example, acid treatments tend to be expensive
and cannot be used on sulfate scales because the rate of dissolution in such case is too slow to make it a commercial solution.
The cost of a coiled tubing unit for mechanical clean-up is high and requires work intervention. Chemical inhibition usually
involves phosphate compounds, inorganic polyphosphates, organic amino phosphates, and organic polymers which are toxic
to the environment. Moreover, the effectiveness of the scale inhibition dies away rapidly with time, requiring continuous
injection of inhibitors which can increases the cost per barrel of the oil. Therefore, an alternative method of preventing scale-
related fouling or lengthening equipment uptime through scale minimization could offer substantial cost savings [3].
SPE-187994-MS 2

An effective and desired approach for reducing scale-build-up includes making the tools less attractive to scale by applying a
hydrophobic coating. Hydrophobic coatings are made out of low surface free energy material, and accumulate less scale because
of being non-wettable to scale causing water and brines [4-5.]. Many attempts have been made to reduce scale formation by
coating surfaces with PTFE or other fluoro-polymers due to their low surface energy and non-stick properties. However, fluoro-
polymers are not suitable for downhole tools such as sub-surface safety valves (SSSV), gas-lift valves (GLV), and electrical
submersible pumps (ESP) due to poor abrasion resistance and poor adhesion to alloys typically used in these tools such as alloy
420, nickel alloy 718 and nickel alloy 625 etc.

Recently, a few studies [4, 7-10, 13] in-connection to bio-fouling (accumulation and adhesion of bacteria) and heat exchanger
fouling have suggested that diamond-like carbon (DLC) coating can be effective against unwanted material deposition. The
properties of the DLC, such as high hardness, chemical inertness, and low surface free energy if doped by optimum amounts
of as F, Si, O, and/or N, are attractive for upstream oilfield applications. Therefore, the present study focused on investigating
the suitability of a proprietary silicon and oxygen doped diamond like carbon (hereon referred as doped DLC) coating for
downhole applications such as sub-surface safety valves (SSSV), gas-lift valves (GLV), and electrical submersible pumps
(ESP). The coating was deposited on metallic coupons, and its performance was compared with those of current commercial
coatings conventionally used as anti-scale coating by the industry, such as fluoropolymer and Ni-P-fluoropolymer coating via
custom scale growth experiments, micro-scratch testing for coating-substrate adhesion, and environmental compatibility in
downhole brine and oil at temperature up to 400F. Field trial of the doped DLC coating are underway on multiple sets of
coated SSSVs and GLVs since 2016. The current paper discusses key results of the laboratory tests and the field trial.

2. Experimental Procedure
2.1 Deposition of doped DLC coating
The doped DLC coating was deposited on alloy 420 and nickel alloy 718 coupons of size 2” x 2” x 0.08” via plasma ion
immersion and deposition (PIID) technique using HMDSO (hexamethyldisiloxane) gas as precursor at average film growth
rate of ~15 nm/min. Prior to the coating the surface of the samples were finished by US grit 600 abrasive paper, followed by
ultrasonic cleaning with isotone/isopropyl mixture for 5 minutes. Ar + ion etching was done for ~15 minutes after the sample
was put in the vacuum coating chamber to remove away any oxide film present on the samples, and thereby improve the
coating-metal adhesion.

2.2 Contact angle measurement

Contact angle of water with the samples was measured using a DSA 20 instrument from Kruss Inc. A small volume, 5 µl, of
water was first pipetted out onto the test coupons (2”x 2”x0.08”) using a micro-syringe. The image of the droplet resting on the
surface was recorded using a digital camera. The contact angle was measured at the triple junction of the solid, water droplet,
and air using image-analysis software provided by the by the instrument manufacturer.

2.3 Scale deposition test

A custom apparatus, shown in Fig. 1, was used to perform the scale deposition test in the laboratory. The apparatus comprised
a 1- liter glass vessel with three ports and a heating jacket for heating the solution to the test temperature. The first port was
used to continuously monitor the pH and temperature of the solution via appropriate probes and a computer. The second port
was used for hanging the test piece using 1/8-inch steel wire, and a condenser was used in the third port to minimize solution
evaporation. A magnetic stir-bar rotated at a constant speed to avoid gravitational settling of the precipitates to the bottom of
the vessel.

The experiment began with the simultaneous pouring of 375 ml of calcium and bicarbonate brine of compositions listed in
Table 1 into the vessel. Typically, within 15-20 minutes of pouring, CaCO3 particles precipitated throughout the solution, and
on the sample. Prior to being poured into the vessel, the two brines were heated to the test temperature (85F and 150F) and
their pH buffered to 6.7 by bubbling CO 2 gas for approximately 1 hour. The solution’s pH was maintained at 6.7 throughout
the experiment via CO2 bubbling. The weight of the test piece before and after the experiment was measured on a physical
balance with 10-4 grams of resolution, and the difference was used to calculate the mass of the scale deposited on the test piece.
Photographs of the samples were taken post-experiment, and selected scale deposited samples were observed under high
magnification scanning electron microscope (SEM).

2.4 Chemical compatibility test

High-temperature chemical and mechanical integrity of the coatings was investigated by aging them in 3% KCl brine at
300F/1000 psi hydrostatic pressure for 1 day, 3 days, and 7 days in a sealed autoclave. Any change in appearance such as
SPE-187994-MS 3

discoloration, blistering, coating delamination, cracking, or degradation in mechanical property such as scratch hardness was
recorded.

3. Results and Discussion

3.1 Characterization of the doped-DLC coating
Figure 2 shows the SEM image of the deposited doped DLC coating. A uniform thin coating of ~1 m thickness with good
adhesion to the substrate is being observed. Since PIID is a non-line-of-sight coating process even microscopic irregularities
or features on the sample, such as the one seen near the top, middle in Fig. 2, was coated nicely.

The chemical composition and the bonding states of carbon, oxygen and silicon atoms in the coating was determined through
x-ray photoelectron spectroscopy (XPS), Fig. 3. Prior to the XPS measurement, about 5 minutes of Ar-etching was done to
remove any surface contamination present in the coating. The atomic concentrations of C, O, and Si in the coating was 55 at%,
14 at%, and 31 at%, respectively. The deconvolution of the C1s peak (Fig. 3a) revealed carbon to be in 3 bonding states: sp2
(C=C, 283.9 eV peak) and sp3 (C-C or C-Si, 284.8 eV peak), and C=O (peak at 286 eV) with area fractions of 63%, 36%, and
1%, respectively. The Si2p peak (Fig. 3c) could also be resolved in two peaks: 101.0 eV and 102.0 eV. The peak 101.0 eV
indicates Si bonded to three carbon atoms and one oxygen atom, and the peak at 102.0 eV suggests the presence of silsequioxane
(RSiO3/2) type of polymeric structure in the coating [11].

The composition, average surface roughness, and thicknesses of the commercial ceramic and fluoro-polymer coatings used to
compare the performance of the doped DLC coating is given in Table 2. The microstructure and other details of these
commercial coating are discussed in an earlier paper [12], and is not repeated in this paper.
3.2 Hydrophobicity of the Coatings
Figure 4 compares the contact angle of water with the coatings. Among the conventional coatings, the fluoro-polymer coatings
showed the highest hydrophobicity. The contact angle (~110 - 115) for these coatings was 15-20% higher than the other
conventional coatings; Ni-P-PTFE (contact angle ~100) and the PT 14 coating (contact angle 94). The doped-DLC coating
was moderately hydrophobic with contact angle similar to that of PT 14 coating.
3.3 Scale Deposition Tests
3.3.1 At 85F for 48 hours
The scale deposition was very much dependent on the surface properties of the sample. This is evident from Fig. 5, which plots
the weight of scale deposited on coatings under identical test condition. For comparison purpose, the result for bare alloy 420
steel is also included in the plot. The doped-DLC coating accumulated approximately 90% less scale than the bare surface, and
hence exhibited superior most anti-scale performance, followed by the fluoropolymer 2 and Ni-P-PTFE coatings. Figure 6
presents the photographs of the scale deposited samples. The results presented in the Fig. 5 can be qualitatively verified from
this figure. The white particles observed in the photographs are deposited CaCO 3 scale. Clearly, the bare steel sample
accumulated maximum amount the scale and the doped-DLC accumulated the least. In addition to scale (white particles) a few
corrosion pits (brown particle) were also noted only on the bare steel sample. This result suggests that the localized corrosion
resistance of the coatings is also better than the alloy 420.

SEM images showing of the highest scaling samples (bare 420 steel and PT-14 in Fig. 5) is compared with that of the least
scaling sample (doped-DLC) in Fig. 7. Two distinct scale morphologies: needle like and porous multi-faceted crystals are
observed on the high scaling (bare 420 steel and PT 14) samples, whereas cuboid crystals are noted on the doped-DLC sample.
SEM/EDS analysis confirmed that these precipitates were CaCO3, i.e. rich in calcium and carbon. Needle-like and multi-faceted
crystals have larger surface area compared to that of a cuboidal crystal, and likely will adhere stronger with the underneath
substrate, suggesting that the adherence of the scale particles was stronger than that on doped-DLC sample. Or in other words,
the doped-DLC coating was ‘non-stick’ in comparison to the bare and PT-14 samples.

3.3.2 At 150F for 1 hour

High temperature scale deposition testing was conducted on selected samples at 150F for 1 hour. Figure 8 shows the
photographs of the scale deposited samples. A more or less uniform coverage of the CaCO 3 scale (white colored precipitates)
is being observed on the entire surface of the PT14 and fluoro-polymer 2 coatings, whereas on the doped-DLC the CaCO3 are
present only in from of discreet patches. It is evident that the doped-DLC coating was the superior anti-scale sample even at
this temperature.
3.4 Downhole chemical compatibility
Select coatings which showed good anti-scale performance in the scale deposition test were subjected to aging test. The aim
was to determine their chemical and mechanical integrity in the downhole condition. Figure 9 shows the images of the coatings
before and after 7 days of aging. As is evident from this figure, the fluoro-polymer 1 and the Ni-P-PTFE coating could not
survive the aging test. Blisters, wrinkles, and even localized peeling-off of the coatings are observed throughout the sample.
SPE-187994-MS 4

Blistering and localized delamination of coatings typically occurs due to presence of defects such as cracks, which allow
chemicals to penetrate the coating or poor coating-substrate adhesion. Thus, this result suggests that a better primer or thinner
coating should have been used for the fluoro-polymer 1 or Ni-P-PTFE composite coating.
On the other hand, the appearance of the doped-DLC coating remained largely unaffected by the aging, Fig. 9. Some
delamination is observed in the doped-DLC coating. But, unlike fluoro-polymer 1 and Ni-P-PTFE coating, the delamination
was concentrated near the holes that were drilled after coating deposition, and not in the middle of the sample. Therefore, it
can be safely assumed that the noticed delamination in this case is result of the coating’s failure during hole-drilling and not
due to aging.

4. Field trial
Laboratory tests (described above in sections 3.1 – 3.4, and in reference 12) concluded that the doped-DLC coating is a
promising anti-scale coating with good downhole chemical compatibility and high abrasion resistance: properties that are well
suited for oilfield tools such as SSSVs and GLVs. Therefore, multiple sets of these tools (two SSSVs and fifteen GLVs) were
coated with ~ 1 m thick doped DLC coating, and were put down hole for field trial for two major operators. For both tools,
the entire flow-wetted ID and OD of the tools were coated.

Figure 10 shows the picture of coated individual components (flow tube, flapper housing, and upper sub) of a SSSV prior to
its assembly. A complete functional test was performed on the coated SSSVs per the industry standard API 14A. The valve
performed well throughout the functional test, demonstrating that the SSSV valve operation and repeatability of the valve
cycling was within the acceptance criteria after the coating application. The coated SSSVs were installed in July, 2016 within
a known scale problematic well in which an operator was having to replace safety valves at regular interval due to scaling
problems, approximately every 6 months. It’s been approximately 8 months since the installation and the SSSVs continue to
function, suggesting that the coating is preventing or minimizing scale-build up from the valve. Analysis of the valves, after
being retrieved from the well, will be conducted to corroborate this conclusion.

Field tests are also being performed on coated gas lift valves or barrier-orifice valves used to inject gas for artificial lift in
known scale problem wells in North Sea oilfield. The valves were installed in the well in December, 2015 and retrieved in
May, 2016, because the operator wanted to move the gas injection point further down the wellbore. A picture of a retrieved
valve and its downhole service condition profile (downhole annulus temperature, annulus pressure, and gas-lift rate) are
presented in Fig. 11. The overall functionality of the valve was tested by the operator in the well 3 times, and during all the 3
times the valve performed well. As is noted in the Fig. 11a, after 6 months of service, the coating was intact and scale (primarily
BaSO4 per the operator) is barely starting to form towards the ID side of the nose. The overall performance of this valve has
been deemed an acceptable improvement by the operator, and the same operator has given repeat orders for the coated valves.

5. Conclusions
A novel silicon and oxygen doped diamond like carbon (doped-DLC) coating was developed in the present study. Through
laboratory tests it was demonstrated that the coating is a promising abrasion resistant, anti-scale coating for upstream production
tools. Under identical test conditions, the weight of scale deposited on doped DLC sample was substantially less than other
coating (up to 90% less than a bare sample). The abrasion resistance and the downhole chemical compatibility of the doped
DLC coating was also better than the other commercial fluoropolymer type of coatings.

For field trial, two sets of SSSVs and 15 sets of GLVs were coated with this doped DLC coating, and were inserted in wells
known to have scale problems in 2016. After 6 months of the field trial, one of the coated GLVs has been retrieved. The coating
was still intact and appears to be effective in preventing the build-up of scale. The scale is slightly starting to form at the nose
of the valve. The coated SSSVs were installed in July, 2016 in a known scale problem wells and completion configurations in
which an operator was having to replace safety valves approximately every 6 months. It’s been approximately 8 months since
the installation and the SSSVs have been functioning well, suggesting that the coating is preventing or minimizing the scale-
build up from the valves. Analysis of the valves, being retrieved from the well, will be conducted to corroborate this conclusion.

Acknowledgements
The authors thank Baker Hughes Incorporated for permission to present this paper.
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References
[1] Moghadasi et al., “Scale deposits in porous media and their removal by edta injection”, Proceedings of 7 th International Conference on
Heat Exchanger Fouling and Cleaning – Challenges and Opportunities, Tomar, Portugal, July 1-6, 2007.
[2] Crabtree M. et al., “Fighting scale-removal and prevention”, Oilfield Review, Autumn 1999, p.n. 3-45.
[3] I.R.Collins, “A new model for mineral scale adhesion”, SPE 74655, 2002.
[4] Zhao et al., “Heat transfer surfaces coated with fluorinated diamond-like carbon films to minimize scale formation”, Surface & Coatings,
Vol. 192, p.n. 77-80.
[5] Kumar D. et al., “Scale inhibition using nano-silica particles”, SPE 149321, SPE Middle East Health, Safety, Security, and Environment
Conference and Exhibition, Abu Dhabi, UAE, April 2-4, 2012.
[6] Zhao et al., “Effect of surface free energy on the adhesion of biofouling and crystalline fouling”, Chemical Engineering Science, Vol. 60,
p.n. 4858-4865, 2005.
[7] Bornhorst A. et al., “Reduction of scale formation under pool boiling conditions by ion implantation and magnetron sputtering on heat
transfer surfaces”, Heat transfer Eng., Vol. 20, Issue: 2, p.n. 6-14, 1999.
[8] Forster M. et al, “Modification of molecular interactions at the interface crystal/heat transfer surface to minimize heat exchanger fouling”,
International Journal of Thermal Science, Vol. 39, p.n. 697-708, 2000.
[9] Dexter S.C. et al., “Influence of surface wettability on the attachment of marine bacteria to various surfaces”, Applied Microbiology, Vol.
30, p.n. 298-308, 1975.
[10] Cheong W.C. et al., “Using nature to provide solutions to Calcerous scale deposition”, SPE114082, SPE International Oilfield Scale
Conference, Aberdeen, UK, 28-29 May 2008.
[11] Veres et al., “Characterization of a-C:H and oxygen-containing Si:C:H films by Raman spectroscopy and XPS”, Diamond and Related
Materials, Vol. 14, p.n. 1051-1056, 2005.
[12] Kumar D. et al, “Reduction in scale build-up from sub-surface safety valve using hydrophobic material coating”, SPE-166218-MS, SPE
Annual Technical Conference and Exhibition, New Orleans, LA, USA, 30 September – 2 October, 2013.
[13] Ahmed M. H. el al., “Study of human serum albumin adsorption and conformational change on DLC and silicon doped DLC using XPS
and FTIR spectroscopy”, Journal of Biomaterials and Nanobiotechnology, Vol. 4, p.n. 194-203, 2013.

Table 1. Composition of br ine used for sc ale depositi on tests at 85  F and 150 F.
NaCl (in g/l) CaCl2.2 H2O (in g/l) NaHCO3 (in g/l)
Cationic brine 33 18.83 –
Anionic brine 33 – 7.37

Table 2. Composition, sur face roughne ss , and thickn ess of th e coatings chosen in the present stud y. Aver age
surfa ce roughnes s w as measured using a laser microscope.
Coating name Average surface Coating procedure
roughness (µin)
Fluoro-polymer 1 32 Spray coating + curing at 400-700F for 0.5 hour
Fluoro-polymer 2 55 Same as above
Fluoro-polymer 2 28 Same as above
PT-14 65 Spray coating + curing at 400F/1hour
Ni-P-PTFE 10 Electroless coating at 100F+ heat treatment at 570F/2.25 hours
Doped-DLC 12 Plasma ion immersion deposition (PIID) at T<300F
SPE-187994-MS 6

Figure 1. Custom laborato r y set -up used for scale deposition experiments.

Figure 2. Cross -se ctional image of the doped DLC co ated sample captur ed under back scatter electron mo de .
SPE-187994-MS 7

Figure 3. XPS spectra deco nvolution of the C1s, O1s, and Si2p of the doped DLC coating.

Figure 4. Contac t angle of w ater w ith the samples under ambient conditions.
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Figure 5. M ass of CaCO 3 s cale deposited on samples tested under identical condition. Each data point is an
aver age of tw o readings.

Figure 6. Pictures of s cale deposited samples at 85  F for 48 hours.

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Figure 7. SEM image of the scale deposited samples at 85  F for 48 hour s.

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Figure 8. SEM image of the scale deposited samples at 150  F for 1 hour.
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Figure 9. Pictures of the coatings before and after 7 days of exposure in 3% KCl brine at 300F and 1000 psi.

Figure 10. Picture of safety valve parts coated with doped DLC.
SPE-187994-MS 12

Figure 11. (a) Pictures of the gas lift valve with the entire ID and partial OD coated with doped DLC after ~6 months of well service,
and (b) details of the service condition (temperature, pressure, and gas lift rate) of the gas lift valve.