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Scale Formation in Multiphase Conditions
Ogbemi Bukuaghangin†, Anne Neville and Thibaut V. J. Charpentier†*
†
School of Mechanical Engineering, University of Leeds, Leeds, UK
*
Corresponding Author
E-mail address: [email protected]
ABSTRACT Scale formation is recognized as one of the major problems affecting
production in the oil and gas industry and is an extensively studied phenomenon.
Nevertheless, to date, very limited work has been carried out on evaluating it in multiphase
conditions. This work aims to study both mineral scale bulk precipitation and surface
deposition in a multiphase environment that can replicate more accurately real conditions
encountered in oil and gas production facilities. A conventional bulk jar test set-up with an
overhead stirrer was used to create an oil/water (o/w) emulsion where various scaling
parameters are studied using a set of bulk and surface analyse techniques such as Inductively
Coupled Plasma (ICP) spectroscopy or Scanning Electron Microscopy (SEM). These
included, but are not limited to:
The influence of multiphase on the Minimum Inhibitor Concentration (MIC) and inhibitors’
efficiency, the oil wettability of surface and its subsequent effect on mineral surface fouling
are addressed. By analysing the change of scaling behaviour observed in singlephase and
multiphase conditions suggestions of the controlling factors in these systems are presented
and discussed.
INTRODUCTION Organic scale inhibitors remain one the most effective means of
preventing the formation of mineral scale within infrastructures transporting oil and gas.
Before considering an inhibitor formulation for a field application an experimental matrix is
usually defined and laboratory tests are carried out in order to evaluate its adequacy for a
specific scaling environment. Despite these efforts, the match between laboratory
performance and the one ultimately observed in the field can be poor. Overcoming these
performance discrepancies ultimately requires a more comprehensive and better
understanding of the behaviour of the scaling process in complex multi-phase and multisurface environments in which they occur. Indeed, real field applications rarely contain only
an aqueous phase and are generally more complex due to the presence of an oil phase
(present either as a bulk phase or as emulsified oil). The presence of such organic phases can
lead to loss of inhibitor performance due to its partitioning from the water to the oil; however
the organic phase can also contribute to reducing surface fouling through preferential wetting.
These antagonistic effects make prediction of surface fouling difficult and lead to the need for
field measurement to refine formulation of scale inhibitors to the specific conditions
prevailing in each application. This study is part of an ongoing program with the long term
goal of establishing a knowledge on mineral surface fouling kinetics and mechanisms in oil
and gas production systems. For two scale inhibitors (diethylenetriamine penta
methylphosphonic acid and polyphosphinocarboxylic acid) and surfaces (stainless steel 316L
and fluoropolymer coating) this study shows how surface fouling in a barium sulphate scaling
environment is directly affected by the presence of an oil phase.
EXPERIMENTAL PROCEDURE
Surfaces. Alloy AISI 316L was used as a reference material; the alloy is commonly used in
the oil and gas industry for surface piping and valve components. Some stainless steel
samples were also coated with a non-stick fluoropolymer coating. The coating was selected
for its industrial relevance in order to study the effect of a low surface energy on mineral
surface fouling phenomena [1]. The values of surface roughness (analysed by white light
interferometry) and surface energy (assessed by contact angle goniometry) of both surfaces
are summarized in Table 1 while water and oil contact angle are given in Table 2
Surface
Surface energy γ (mJ.m-2)
Surface roughness Sa (μm)
AISI 316L
41.3
0.216
Fluoropolymer
14.2
0.943
Table 1 Roughness and surface energy of AISI 316L and fluoropolymer samples
Surface
Water contact angle
AISI 316L
55.5
Fluoropolymer
101.3
Water surface tension (mN/m)
Isopar M surface tension (mN/m)
Isopar M contact angle
87.4
57.1
72.8 [2]
25.7 [2]
Table 2 Water and isopar M surface tension and contact angle value measured on AISI 316L and
fluoropolymer
Scaling brine composition. All scaling experiments were carried out at 80 °C and
atmospheric pressure. Barium sulphate was precipitated spontaneously by mixing 100ml of
North sea seawater and 900ml of formation water (mixing ratio of 10:90). Figure 1 shows
the barite saturation ratio for various mixing ratios of sea water and formation water while
Table 3 summarizes the water composition. Before mixing, the two brine solutions were
filtered through a 0.45µm membrane to remove any form of impurities or crystals. The initial
value of saturation ratio was calculated from the Multiscale prediction software and was
approximatively 114 [3].
Ions
Na+
Ca2+ (from CaCl2.6H2O)
Mg2+ (from MgCl2.6H2O)
K+ (from KCl)
Ba2+ (from BaCl2.2H2O)
Sr2+ (from SrCl2.6H2O)
SO42- (from Na2SO4)
pH
Mixing ratio
Initial saturation ratio
Sea water NSSW (ppm)
Formation water FW (ppm)
10890
31275
428
2000
1366
739
460
654
-269
-771
2960
-5.5 (buffered)
10:90
114
Table 3 Composition of North sea seawater and forties formation water used in this study
Barite formed (mg/L)
450
400
400
350
350
300
300
250
250
200
200
150
150
100
100
50
50
0
Precipitation of Barite (mg/L)
SR (Barite)
SR (BaSO4)
450
0
0
20
40
60
80
100
% NSSW
Figure 1 Barite Saturation Ratio SR, for various mixing ratios of NSSW/ FW at 80ºC, pH 5.5
Scale inhibitors. Diethylene triamine penta methylene phosphonic acid (DETPMP) and
poly-phosphino carboxylic acid (PPCA) were used as scale inhibitors in this study .
The
Minimum inhibitor concentration (MIC) was evaluated at 4 ppm for both inhibitors through
standard laboratory procedure [4]. For the scaling tests, a 1000ppm active solution of
inhibitor was prepared in synthetic North sea seawater and subsequent dilutions were made to
prepare the working standards [5].
Figure 2 Molecular structure of (a): poly phosphinocarboxylic acid (PPCA) and (b): diethylene
triamine penta methylene phosphonic acid (DETPMP)
A buffer solution was prepared by dissolving 34g of sodium acetate tri-hydrate in 240ml of
distilled water. 1.02g of glacial acetic acid was added and final volume adjusted to 250ml
with distilled water. The final pH was approximatively 2.5 and 1 vol.% of buffer was used
during the scaling tests.
Multiphase conditions was achieved using an overhead dissolver stirrer from IKA (model R
1300) designed to create an emulsion under very turbulent flow conditions. Isopar M, an
isoparaffinic hydrocarbon having carbon numbers in range C11 to C16 was added to the
formation water prior experiment and the overhead stirrer was set at 500 rpm throughout the
experiment to maintain the emulsion. Scaling tests were carried out at oil-to-water ratios of
0:100, 5:95, 20:80 and 50:50 by adding 0ml, 53ml, 250 ml and 1000ml of paraffinic oil in
1000ml of scaling brine respectively.
RESULTS AND DISCUSSIONS
Surface scaling at various water:oil ratio. Alloy AISI 316L and fluoropolymer coating
were exposed to a barium sulphate scaling environment at various oil/water (o/w) ratios as
described previously. After each test, the sample was rinsed with distilled water and dried in
an oven for 12 hours. In order to obtain the scaling tendency, the barium sulphate was
dissolved (20 ml EDTA 50g/l, pH 11) and the amount of barium analysed by ICP-OES. The
results are summarized in Figure 3; each test was run in duplicate or triplicate.
Figure 3 Surface barium content at various o/w ratios on (a): Alloy AISI 316L and (b):
Fluoropolymer coating
On stainless steel, the presence of paraffin oil leads to a decrease of barium sulphate
deposited on the surface from 37 ppm in a single phase system down to 21 ppm at a 5:95 o/w
ratio. Although the presence of small quantity of paraffin oil (as low as 5 vol.%) significantly
reduces surface scaling (nearly 45% reduction), further addition oil only has a minor effect on
surface scaling with barium contents of 18 and 16 ppm at o/w ratio of 20:80 and 50:50
respectively. In a single phase scaling environment, the fluoropolymer coating exhibits a
barium sulphate mass gain similar to that measured on stainless steel (31.3 ppm). With such
similar mass gain the benefits of using an antifouling coating such as the fluoropolymer used
in this study were ambiguous. However when the scaling tests are carried out in a multiphase
environment the antifouling performance of the fluoropolymer shows a drastic improvement
over the stainless steel. At o/w ratio of 5:95 the barium content drops to 2 ppm which
represents a significant reduction of nearly 95%. At higher oil concentration the barium
content drops to 1ppm at o/w ratio of 20:80 and 50:50.
Figure 4 shows the scanning electron micrographs of AISI 316L and fluoropolymer samples
subjected to scaling tests in single and multiphase. In the latter case, the fluoropolymer
(Figure 4.d) did not reveal any barite crystals on the surface. Several explanations can be
found to describe more precisely the reduction of mass gain observed. These relate to the
film-forming capability of the emulsion and the interaction between the dispersed oil droplets
and the surfaces where scaling takes place. In a single phase environment, only a solid-water
interface can exist; however by introducing oil in the system, the organic phase will displace
some of the water molecules from that interface thus reducing surface scaling. The
probability of having oil droplets in contact with a solid surface correlate with different factor
such as the nature of the emulsions or the oil content which explain why surface scaling tends
to decrease as the oil content increases [6, 7]. However the likelihood of a film of paraffin oil
wetting the surface does not depend solely on the oil content otherwise both alloy AISI 316L
and the fluoropolymer coating would exhibit similar amounts of barium sulphate. It is then
shown that the probability of forming a stable oil film protecting the surface from mineral
fouling is associated with the Displacement Energy (DE) – a thermodynamic measure of the
ability of a surface to favour oil wetting by displacing water molecules from the interface. DE
is defined as:
Equation 1
𝐷𝐸 = 𝛾𝑊𝐴 × cos 𝜃𝑊𝑆 − 𝛾𝑂𝐴 × cos 𝜃𝑂𝑆
[8]
Where γWA and γOA denotes the water/air and oil/air surface tension respectively while θWS
and θOS denotes the contact angle of water and oil on the surface of interest. Equation 1
shows that the DE is simply the difference between the work of adhesion of oil and water,
respectively, on the solid surface. An o/w emulsion with optimal tendency for oil to wet the
surface should have a negative DE and the more negative the value of DE is, the more readily
such displacement take place. Using the experimental data presented in Table 2, DE was
evaluated at 40 and -28 for AISI 316L and the fluoropolymer coating respectively. Such
displacement energy values show that clean metal surfaces such as AISI 316L samples used
in the current study are polar and thus have an affinity for water. Therefore the latter will
have a low contact angle and will be prone to surface scaling. The fluoropolymer however is
highly hydrophobic and favours the displacement of water and the formation of a stable oil
layer that prevents scale forming at the surface of the sample.
Figure 4 SEM micrographs of alloy AISI 316L subjected to barium sulphate scaling environment
in (a): single phase and (b): multiphase (5% oil) and fluoropolymer coating subjected to barium
sulphate scaling environment in (c): single phase and (d): multiphase (5% oil). Scale bar = 10µm
Surface scaling in multiphase environment in presence of scale inhibitors. Alloy AISI
316L samples have been subjected to the different scaling environments presented in Table 4
for a 2 hour periods. Results are summarized in Figure 4 and Table 4.
Figure 5 Barium scale measured on surfaces. Tests were performed in multiphase condition and
different scale inhibitors were used: (a): PPCA scale inhibitor; (b): DETPMP scale inhibitor.
Experiment Experimental condition
Barium content (ppm)
1
Single phase, no inhibitor
37±1
2
20% oil, no inhibitor
18±1
3
Single phase, 1ppm DETPMP
53±1
4
Single phase, 4 ppm DETPMP (MIC)
2±0.3
5
Single phase, 1ppm PPCA
117±1
6
Single phase, 4 ppm PPCA (MIC)
1±0.1
7
20% oil, 4 ppm PPCA (MIC)
0.6±0.0
8
20% oil, 1 ppm PPCA
162±2
9
20% oil, 4 ppm DETPMP (MIC)
1±0.3
10
20% oil, 1 ppm DETPMP
54±2
Table 4 Experimental parameters and resulting Barium content on measured on surfaces
In the absence of inhibitor, the presence of oil leads to a decrease of barium sulphate
building-up on the surface (from 37 ppm to 18 ppm when 20% oil is added). The addition of
scale inhibitor at the MIC concentration also significantly reduces the amount of scale
measured on the surface. At a concentration of 4 ppm of PPCA the amount of barium
decreases to 1 ppm and 0.6 ppm in singlephase and multiphase conditions respectively. A
similar behaviour is observed with DETPMP with the amount of barium dropping to 2 ppm
and 1 ppm in singlephase and multiphase conditions respectively. However when the amount
of inhibitor added is below the MIC at 1ppm (40% efficiency) the results are exhibiting an
opposite trend. Indeed 1 ppm of PPCA leads to an increase of surface scale build-up to 117
pm and 162 ppm in singlephase and multiphase conditions respectively. In singlephase
conditions such behaviour has already been shown by Graham et.al. and can be explained by
the absence of inhibitor film at the metal surface as well as the lower nucleation energy
barrier of the surface compared to the bulk solution [9, 10]. It is believed that in multiphase
conditions, there are 2 processes taking place and competing with each other. The first one is
the presence of an organic phase wetting the surface which contributes to reduce surface
scaling. The second effect however is the partitioning of the scale inhibitor between the
aqueous phase and the organic phase. The second process can be harmful for surface scaling
if the initial concentration of scale is already low.
The partitioning coefficient (P) is defined as the ratio of concentrations of a compound in a
mixture of two immiscible phases at equilibrium. P is often measured in a water/octanol
system and is a direct measure of the difference in solubility of a chemical in two phases. It is
often used in its logarithm form as shown in equation 2:
Equation 2
[𝑠𝑜𝑙𝑢𝑡𝑒]
log 𝑃𝑜𝑐𝑡/𝑤𝑎𝑡 = log ( [𝑠𝑜𝑙𝑢𝑡𝑒]𝑜𝑐𝑡𝑎𝑛𝑜𝑙 )
𝑤𝑎𝑡𝑒𝑟
[11]
DETPMP has a very low partition coefficient (log P ≈ -3.4) [12] which indicates the inhibitor
will mostly remain in the aqueous phase and therefore remain effective. Such low affinity
with the organic phase explains why the scaling tests at 1 ppm of DETPMP in single and
multi -phases exhibit very similar surface barium content. PPCA however has a greater
affinity for organic solvents and values of log P around 0 have been reported depending on
the molecular weight of the polymer and the pH of the aqueous phase. Such compatibility
with oil is likely to explain the increase of barium content measured in multiphase condition
when PPCA is present at concentration below MIC.
CONCLUSION This work reports on surface scaling in multiphase conditions. Low surface
energy coatings such as the fluoropolymer used in this study do not show great antifouling
properties in single (aqueous) phase scaling tests and the barium sulphate mass gain was
similar to the one observed on stainless steel. In multiphase conditions however, the
efficiency of the fluoropolymer is drastically enhanced with an amount of barium that
dropped by 95 % from 31 down to 2 ppm. Such small amount of scale is similar to the one
observed in presence of scale inhibitor at or above MIC level. The performance of the
fluoropolymer coating tested was attributed to its propensity to favour an oil wetting state and
therefore prevent the test coupons to be in contact with the aqueous phase where scaling
occurs. In presence of scale inhibitors, the ability of the inhibitor to migrate into the organic
phase is of paramount importance especially when concentration falls below MIC levels.
Indeed, the dosage of scale inhibitors below the optimum (MIC) concentration can results in
a dramatic increase of surface scaling and the concentration of scale inhibitors with a high
partition coefficient might be monitored more closely when they reach near MIC level.
Future work will build on the knowledge gained from the current study and extend it by
evaluating the influence of other experimental parameters (e.g. effect of scale and inhibitors
on the stability of emulsions, effect of emulsion size on the kinetic of scale precipitation and
wetting properties, scale inhibitors partition coefficient under various experimental
conditions).
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