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<p class="art-type">Review Article</p>
<p class="art-title">Introduction to Wax Deposition</p>
<p class="art-author"><?php $authors="Muhammad Ali Theyab<sup>*</sup> and Sarah Yousif Yahya"; echo (stristr($authors,$coauthor))?str_replace($coauthor,"<a href='".$extpath."authors/".$courl."' target='_blank'>".$coauthor."</a>",$authors):$authors; ?></p>
<p class="art-affl">
Ministry of Higher Education and Scientific Research, Iraq
</p>
<p class="art-aff"><b>*Corresponding author: <?php $corresponding_author="Muhammad Ali Theyab"; echo ($coauthor!="" && $coauthor==$corresponding_author)?"<a href='".$extpath."authors/".$courl."' target='_blank'>".$coauthor."</a>":$corresponding_author;?></b>, Ministry of Higher Education and Scientific
Research, Iraq, E-mail: <a href="mailto:theyabm@lsbu.ac.uk">theyabm@lsbu.ac.uk</a>
</p>
<p class="art-aff"><b>Received:</b> April 10, 2018
<b>Accepted:</b> May 2, 2018
<b>Published:</b> May 7, 2018</p>
<p class="art-aff"><b>Citation: </b>  Theyab MA, Yahya SY. Introduction
to Wax Deposition. <i>Int J Petrochem Res.</i>
2018; 2(1): 126-131. doi: <a href="https://doi.org/10.18689/ijpr-1000122">10.18689/ijpr-1000122</a></p>
<p class="art-aff"><b>Copyright:</b> &copy; 2018 The Author(s). This work
is licensed under a Creative Commons
Attribution 4.0 International License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the
original work is properly cited.</p>
<p><a href="<?php echo $extpath;?><?php echo $jres['journal_link'];?>/ijpr-1000122.pdf" class="btn btn-danger pull-right" target="_blank">Download PDF</a></p>
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<div class="articlecontent">
<p class="art-subhead">Abstract</p>
<p class="art-para">This work is presents in brief an introduction to wax deposition process to give a
general understand of this problem because of arising the challenges of growing wax
deposition in the hydrocarbon pipelines by increasing production in the cold
environment due to increasing demand for energy. Background and identification of
wax deposition problem in the hydrocarbon pipelines are presented. Mitigation methods
are used to address this problem, such as chemical, mechanical and thermal methods, but despite the available inhibition methods, many oil companies currently suffer from
wax deposition problems and are still looking for a good solution to solve this issue. This research presents some of the case studies of wax deposition around the world and
the methods followed to mitigate wax deposition of each case study.</p>
<p class="art-para">Numerous researchers have used various different types of chemical inhibitors, such
as polyacrylate polymer, copolymer esters, polyethylene, olefin/ester copolymers, ethylene/vinyl acetate copolymers, ester/vinyl acetate copolymers, polymethacrylates, alkyl phenol resins, xylene and toluene, to studying their influence on the rheology of
crude oil and to evaluate the suitable inhibitor for the waxy crude oil that provides the
desired results in preventing wax deposition.</p>
<p class="art-para">The decent understanding and managing of wax deposition phenomena before it
happens is strongly required in order to overcome the challenges in production and
transportation of pipelines in the cold environment.</p>
<p class="art-para"><b>Keywords:</b> Wax deposition, Problem identification, Case study, Chemical inhibitors, Spiral flow.</p>
<p class="art-subhead">Introduction</p>
<p class="art-para">The world demand for energy has led oil companies to expand their operations in
cold environments such as the offshore deepwater and onshore for more reservoirs. During hydrocarbon production in the cold environment, these oil companies are
challenged by wax deposition problem building up on the pipe wall.</p>
<p class="art-para">Crude oil is a complex mixture and contain a main components of saturates (paraffins/waxes), aromatics, asphaltenes, naphthenes and resins. The high molecular
weight paraffin (wax) is naturally responsible for the problems during production and
transportation in the hydrocarbon pipeline systems.</p>
<p class="art-para">The accumulation of wax in the oil pipelines depends on the difference temperature
between crude oil and pipe wall. There are two conditions should be available to occur
wax deposition in the crude oil pipe. Those conditions are involved, the temperature of
the crude oil close to the pipe wall should be less than the wax appearance temperature, and the pipe wall temperature must be lower than the oil temperature <a href="#1">[1]</a>. Wax
deposition process is influenced and controlled by some factors, such as pipe wall
temperature (inlet coolant temperature), flow rate, pressure drop, oil temperature, shear
stress, recirculation time of crude oil and viscosity.</p>
<p class="art-para">At the reservoirs, wax molecules are dissolved in the crude
oil because of the temperatures range between (70 - 150 &#176;C) and
pressures range between (50-100MPa). Though, crude oil flows
through the subsea pipeline resting on the cold environment of
ocean floor at a temperature of 4&#176;C, the temperature of oil in
time decreases below its wax appearance temperature because
of the heat losses to the surroundings <a href="#2">[2]</a>.</p>
<p class="art-para">Wax molecules start to precipitate out of the crude oil
when the solubility of wax decreases drastically as the
temperature decreases. Oil production in deep sea areas and
cold environment onshore has increased significantly due to
the high demand for energy.</p>
<p class="art-para">The exploration and production technologies in deep sea
areas have made deep water drilling economically feasible
and the oil industry has drilled subsea oil wells as far as 160
miles away from the shore. As oil wells are developed further
offshore, wax problems will become more severe and
extensive due to the increased transportation lines on the
cold ocean floor <a href="#3">[3]</a><a href="#2">[2]</a>.</p>
<p class="art-para">In 2015, the global offshore oil production reached 9.3
million barrels per day (including lease condensate and
hydrocarbon gas liquids) from deepwater projects. Deepwater
production has increased 25% from nearly 7 million b/d a
decade ago. Shallow water has been somewhat less expensive
and less technically challenging for operators to explore and
drill, but changing economics and the exhaustion of some
shallow offshore resources has helped to push producers to
deepwater or, in some areas, ultra-deepwater (at depths of
1,500 meters or more) resources. The share of offshore
production from shallow water in 2015 was 64%, the lowest
on record <a href="#4">[4]</a>.</p>
<p class="art-para">This research is showing the challenges arising from the
growing size of wax deposition in the hydrocarbon pipelines
by increasing production in the cold environment. Therefore, some of case studies of wax deposition in the crude oil
pipelines around the world are presented during this research. Many oil companies around the world used different
techniques to reduce wax deposition in the pipelines such as
chemical inhibitors, mechanical and thermal techniques. Most
of the oil companies preferred the chemical inhibitors because
it not needs to stop production.</p>
<p class="art-para">Many researchers, as will see in this work, preferred
chemical inhibitors in their research to reduce wax deposition. While, Theyab and Diaz (2016a, 2016b, 2016c) <a href="#5">[5]</a><a href="#6">[6]</a><a href="#7">[7]</a> were
used the chemical inhibitors such as polyacrylate polymer (C16-C22), spiral flow, and the combination of the chemical
inhibitor with the spiral flow to mitigate wax deposition. The
spiral flow technique, raise interest in the possibility of
increasing the shear rate to prevent wax deposition by mixing
the wax crystals with the crude oil and prevent it to deposit on
the pipe wall. Moreover, undertake a systematic study of
inhibition of wax deposition using the effects of combination
spiral flow with a chemical inhibitor to increase the effect of
the mitigation process.</p>
<p class="art-para">It can be concluded that, a fundamental understanding of
wax deposition phenomena is highly required in order to
overcome the challenges in production and transportation of
pipelines in the cold environment.</p>
<p class="art-subhead">Background of Wax Deposition</p>
<p class="art-para">As early as 1928, wax deposition was reported as an issue
that led to challenges in the crude oil production, transportation and storage <a href="#8">[8]</a>. In 1969, control of wax
deposition in U.S. domestic production annually cost $4.5-$5
million. The problem of onshore wax deposition could be
addressed by relatively simple methods, including the
optimisation of the operating conditions (pipeline size, pressure, etc.), because of easy access and management of
these resources. Heating of the onshore pipeline or mechanical
removal of the wax deposit was used occasionally, but was
generally not as prohibitive <a href="#8">[8]</a>.</p>
<p class="art-para">During the late twentieth century, as the problem of wax
deposition became increasingly challenging, as the production
of petroleum fluids shifted from onshore resources toward
offshore reservoirs around the world <a href="#8">[8]</a>. In Lasmo oilfield in UK, the problem of wax deposition was so severe and frequent that
led to abandon the entire field at a cost of over $100 million <a href="#1">[1]</a>.</p>
<p class="art-para">The severity of the wax problem needs evaluated in the
design of every subsea and onshore development across the
world, including the Gulf of Mexico, the North Slope, the North
Sea, North Africa, Northeast Asia, Southern Asia and South
America <a href="#8">[8]</a>, see Figure 1; about the onshore including oil fields
in North America, South America, Asia, Africa and Europe to
estimate the cost of suitable remediation methods for wax
deposition in the subsea pipeline and to avoid blockage.</p>
<div class="art-img">
<img src="<?php echo $imgpath;?>images/IJPR-122-figure1.PNG" class="img-responsive center-block"/></div>
<p class="art-subhead">Identification of Wax Deposition Problem</p>
<p class="art-para">Wax deposition is, a public problem, a critical operational
challenge and one of the main flow assurance problems in the
oil industry around the world including the offshore and
onshore oil fields. Wax deposition precipitates and deposit on
the cold pipeline wall when the inner wall temperature falls
below the wax appearance temperature and occurs when
paraffin components in crude oil (alkanes with carbon numbers
greater than 20) <a href="#1">[1]</a>.</p>
<p class="art-para">The formation of wax in the pipe during the fluid production
from the bottom hole of the well to the surface can restrict the
flow of crude oil, creating pressure abnormalities and causing an
artificial blockage, as shown in Figure 2, leading to the reduction
or even cessation of production <a href="#9">[9]</a><a href="#10">10</a><a href="#11">[11]</a><a href="#12">[12]</a><a href="#13">[13]</a><a href="#14">[14]</a>. Wax
deposition also leads to formation damage near the wellbore, reduction in permeability, changes in the reservoir fluid
composition and fluid rheology due to phase separation as wax
solid precipitates <a href="#15">[15]</a>.</p>
<p class="art-para">Wax precipitation is impacted by several factors, such as: crude oil composition <a href="#16">[16]</a>, flow rate, temperature gradient <a href="#17">[17]</a>, pipe wall temperature (inlet coolant temperature), crude oil
temperature, shear stress, production time and oil viscosity <a href="#18">[18]</a>.</p>
<div class="art-img">
<img src="<?php echo $imgpath;?>images/IJPR-122-figure2.PNG" class="img-responsive center-block"/></div>
<p class="art-para">Wax deposition can be reduced or prevented by a chemical, mechanical, or thermal remediation method or by a combination
of some of them <a href="#20">[20]</a>. If the wax mitigation methods not working
well (e.g. insulation of pipeline, injection of wax inhibitor, or
combination of both), a gel layer of wax will grows rapidly in
thickness and obstructs the flow of oil because of flow restriction
<a href="#1">[1] </a><a href="#2">[2].</a> Those elimination methods become increasingly
significant as the oil industry operations expanded in deepwater
to greater depths and distances in cold environments, which
poses a great challenge to the industry <a href="#21a">[21a</a>, <a href="#21b">21b]</a> <a href="#15">[15]</a>; <a href="#22">[22]</a><a href="#23">[23]</a><a href="#8">[8].</a></p>
<p class="art-para">Pigging is one of the most typically corrective methods
used in the oilfields. In pigging, a pig passes through the
pipeline to scrape off the wax deposit (pig is a solid material
with the diameter less than the inner diameter of the pipe). Conversely, the pigging method cannot professionally be
used without a proper wax deposition prediction. For example, pigs at times get stuck inside the pipeline in the presence of
thick hard deposits making the situation worse, which
occurred in a Gulf of Mexico pipeline. Production must be
stopped in worst cases in order to replace the plugged portion
of the line, which is estimated to cost approximately
$40,000,000 per incident <a href="#8">[8].</a></p>
<p class="art-para">Alternative remediation method is to use a fused chemical
reaction with controlled heat emission to mitigate wax
deposition. However, in order to successfully use this
technique, it is critical to know the thickness profile and the
wax fraction of the deposit as a function of axial location and
time. If this technique were to be used based on inaccurate
information on the location of wax deposit and its wax
fraction, there could be unwanted local high temperature in
the pipeline due to the failure of re-dissolving wax deposit.</p>
<p class="art-para">Successful wax deposition management will become
more important in the future because new explorations and
productions are being made farther offshore. The wax
deposition management cost to the petroleum production
industry is enormous and will increase both in terms of capital
costs (e.g. preventive methods) and operating costs (e.g. corrective methods) <a href="#2">[2].</a></p>
<p class="art-para">Recently, three technologies are used around the world
to mitigate wax deposition in the offshore and onshore oil, namely, pigging, thermal mitigation and wax inhibitors (chemical inhibitors. Most of the oil companies are using the
wax inhibitors, as a main mitigation method to reduce wax, combined with pigging or thermal insulation.</p>
<p class="art-subhead">Case Studies of Wax Deposition</p>
<p class="art-para">In India offshore field to the west of India, the oil is
transported through a 30-inch, 203-km pipeline from the
offshore platform to a terminal on land. This crude oil has high
wax content and a 30&#176;C pour point. Given that the average
seabed temperature is 22&#176;C, pour point depressant (PPD) is
injected. Specifically, 300 ppm/350 ppm of PPD is injected to
achieve a pour point of 21&#176;C/18&#176;C and therefore maintain the
flow of the oil. The subsea pipelines are regularly pigged. The
cost of PPD injection alone is US$15 million/year. Whereas the
use of PPD has maintained the oil flow, an on-going increase in
the pipeline pressure has been observed over time. This indicates
the presence of wax deposits in the subsea pipeline. According
to a technical survey, approximately 20,000 m? of wax has already
been deposited in the pipeline (which has a volume of 90,226
m?). A combination of PPD and dispersant was being considered
to mitigate the wax deposition and therefore reduce the
frequency of pigging <a href="#24">[24]</a>.</p>
<p class="art-para">Another case study, the Gannet oil field is located
approximately 180 km east of Aberdeen in United Kingdom in a
depth of about 90 m. The Gannet field is located 16 km northeast
of the Gannet Alpha platform. Risers 31 and 32 carry oil from
Gannet. The wax appearance temperature of this oil is
approximately 35.5&#176;C. Wax deposition in the subsea pipeline of
Gannet was estimated to be around 21 m<sup>3</sup> of wax had built up
over a length of 8 km. An appropriate solvent was selected and
injected at a rate of 0.6 m<sup>3</sup>/min. After solvent injection, an
additional 480 m<sup>3</sup>/day of oil could be recovered. Also, the pigging
method was used frequency with the inhibitors to remove wax
deposition in Gannet oil field <a href="#25">[25]</a> <a href="#24">[24]</a>.</p>
<p class="art-para">The Power Play oil field is located in 706 m of water in
Garden Banks block 28, Gulf of Mexico, and began production
in 2008. It is a subsea tieback to a host facility with a subsea
flow loop to enable oil circulation and pigging. The subsea
architecture consists of two 10 km 4.5 &times; 7&Prime; (pipe in pipe) PIP
pipeline pigging loop systems with insulated steel catenary
risers. The wax appearance temperature of the oil is 41.1&#176;C, and the pour point ranges between -5&#176;C and 19&#176;C. Given that
the WAT is high, wax deposition is possible. To mitigate this
risk, a PIP system was applied to minimize heat loss from the
oil. As observed in the northern and southern subsea pipelines
during oil production, a simultaneous increase in the flowing
bottomhole pressure and a drop in the oil production rate
pointed to the possibility of partial plugging as a result of wax
deposition in the pipeline. To mitigate this plugging, xylene
solvent was injected into the pipelines at a rate of between
0.04 and 0.32 m<sup>3</sup>/min. This, combined with an increase in the
pressure, was not able to remediate the wax deposition. Rather, the operation opted to switch the production zone
from the lower zone to the higher zone to increase the flow
rate. This increased the flow temperature to above the WAT
and successfully mitigated wax deposition <a href="#26">[26]</a>.</p>
<p class="art-para">On another hand, Russian oil production is almost
exclusively onshore and suffering from the problem of wax
deposition such as padovskoe oil field, Mukhanovskoe oilfield, Petrukhnovskoe oil field and Urmanskoe oil field. Technological
methods such as pigging and thermal insulation have been
used combined with the chemical inhibitors to reach the
highest efficiency to control the wax deposition in the
pipelines. The chemical inhibitors that have been used to
reduce wax deposition are includes surfactant (nanylphenol, synthanol), aromatic solvents (benzene, xylene, toluene), and
aliphatic alcohol (butano, pentanol, hexanol) <a href="#27">[27]</a>.</p>
<p class="art-para">The wax deposition problem can occur even in the high
temperature areas. For example, Kirkuk &#8211; Ceyhan crude oil
pipeline between Iraq and Turkey is an example for the
onshore pipelines that have wax deposition in winter season
of the year. The average lowest temperature is about 2&#176;C in
winter between north of Iraq and Turkey, therefore, it&#700;s
enough to form and deposit wax inside the pipeline. Kirkuk &#8211; Ceyhan crude oil pipeline is 40-inch in diameter and 1049 km
long, of which 986-km pass through Turkish territory, and it
had a capacity of 700,000 b/d. It was noticed that the wax
mole fraction starts to rise with a value of 0.193 around 300
km from inlet and at the end of the pipeline reaches 0.2225. The pigging method and the aromatic solvents used to
mitigate wax deposition in the pipeline <a href="#28">[28]</a>.</p>
<p class="art-subhead">Discussion</p>
<p class="art-para">Many oil companies prefer chemical additives in analysing
the economics of waxy crude oil production in cold
environments, considering this the best solution to reduce
wax deposition in pipelines due to chemical additives does
not need to stop production for cleaning the pipe but it
considers as an online mitigation method <a href="#8">[8].</a> Some
researchers are stating that there is currently no universal
type of inhibitor can be used for all kinds of crude oil due to
the varying properties of crude oils <a href="#29">[29]</a><a href="#24">[24]</a><a href="#30">[30]</a><a href="#31">[31]</a>. This is an
investigation to understand the wax deposition problem, because of the universal inhibitor may solve the wax
deposition problem and create more problems such as (corrosion) due to varying properties of crude oils and the
different climate.</p>
<p class="art-para">A universal solution would be a both convenient and cost
effective response to the current demand. Presently, most of
the companies have their personalised technique to tackle
the wax deposition. This is not very practical as oil viscosity
changes depending on the geology and geographical
location. If spiral flow technique is adopted universally in the
correct way, it will help to reduce the amount of investment as
well as man power to achieve better results.</p>
<p class="art-para">Researchers have used various different types of chemical
inhibitors, such as polyethylene, ethylene/vinyl acetate
copolymers, copolymer esters <a href="#24">[24]</a><a href="#32">[32]</a><a href="#31">[31]</a>, ester/vinyl acetate
copolymers, olefin/ester copolymers, polymethacrylates <a href="#33">[33]</a>
<a href="#34">[34]</a><a href="#35">[35]</a><a href="#36">[36]</a>, alkyl phenol resins, xylene and toluene <a href="#24">[24]</a><a href="#37">[37]</a>
<a href="#38">[38]</a>, studying their effects on wax appearance temperature, wax content, pour point, and crude oil viscosity using
analytical methods, to evaluate the suitable inhibitor for the
waxy crude oil that provides the desired results in preventing
wax deposition.</p>
<p class="art-para">A small number of researchers have used an experimental
flow loop to study and determine dynamically the efficiency
of wax inhibitors on wax deposition inside the pipe, such as
some of them are <a href="#29">[29]</a><a href="#30">[30]</a>, <a href="#39">[39]</a>.</p>
<p class="art-para">The difference between the analytical methods and the
experimental flow loop systems is the experimental conditions
used in the flow loop deposition test affected the performance
of paraffin inhibitors, indicating that temperature gradients (i.e., oil temperature and inlet coolant temperature) must be
optimized to achieve the highest reduction in wax deposition. While, the experimental conditions in the analytical methods
can be controlled, such as pressure, temperature, and shear
rate, providing accurate results in the analytical of wax
inhibition <a href="#40">[40]</a>.</p>
<p class="art-para"><a href="#29">[29]</a> investigated the effect of chemical inhibitors such as
poly(ethylene-co-vinyl acetate) (EVA) and poly(maleic anhydridealt-1-octadecene) (MA) on wax deposition, using cold finger
apparatus. The wax inhibition percentage of their study was
23.1% using EVA and 7.5% using MA at a coolant temperature of
25&#176;C. Adeyanju and Oyekunle (2014) <a href="#30">[30]</a> investigated the effect
of groups of acrylate ester copolymers of varying alkyl side
chains as wax inhibitors during the flow of crude oil in the flow
loop. Wax inhibition percentages of 25-55% were obtained at
high coolant temperatures above 20&#176;C at a concentration of
5000ppm of the inhibitor. Hoffmann and Amundsen (2013) <a href="#39">[39]</a>
found that about 60%-90% of wax thickness was reduced by
applying different concentrations (125, 250 and 500ppm) of the
commercial inhibitor, and using silicon as an insulation material
during experimental work investigation.</p>
<p class="art-para">In the previous studies, even though many different types
of chemical inhibitors have been used at different
concentration, at different inlet coolant temperatures, there is
still wax deposit on the pipe wall due to the researchers
missed investigate the effect of combining the chemical
inhibitors on wax deposition.</p>
<p class="art-para">A small number of researches mentioned using the spiral
flow and studied its effects in different areas <a href="#41">[41]</a> <a href="#42">[42]</a><a href="#43">[43]</a><a href="#44">[44]</a><a href="#45">[45]</a>
<a href="#46">[46]</a><a href="#47">[47]</a>, however, Theyab and Diaz (2016a, 2016b, 2016c) <a href="#5">[5]</a><a href="#6">[6]</a><a href="#7">[7] </a>were used the technique of spiral flow for the first time as a
wax mitigation method. They built an experimental rig to study
the wax deposition thickness under the single phase and to
study the impact of some factors, such as flow rate, pressure
drop, inlet coolant temperature, crude oil temperature, oil
viscosity, time, shear stress, Polyacrylate polymer <a href="#48">[48]</a> and spiral
flow, that influence and control on wax deposition process. The
spiral flow was generated by inserting a twisted plate inside the
pipe and examined in the test section of the pipe in order to
increase the shear rate and shear dispersion and mitigate wax
deposition <a href="#49">[49]</a>. The results illustrated that the reduction in wax
deposition was 100% after using the influence of bending spiral
flow with polyacrylate polymer at a concentration of 1000 ppm
and 2000 ppm at different time and flow rates. The reduction in
wax deposition was 100% after using the effect of bending the
spiral flow with the inhibitor at a concentration of 500 ppm at
flow rate 4.8 L/min, and the reduction in wax deposition was 94% at the same concentration and flow rate 2.7 L/min.</p>
<p class="art-subhead">Conclusions</p>
<p class="art-para">According to the previous studies mentioned in this work, it can be concluded: <ul class="art-para">
<li> Many oil companies prefer chemical inhibitors in cold
environments, considering this economic way and best
solution to reduce wax deposition in pipelines due to
chemical additives does not need to stop production for
cleaning the pipe but it considers as an online mitigation
method.</li>
<li> There is currently no universal type of inhibitor that can
be used for all kinds of crude oil due to the varying
properties of crude oils, because of the universal inhibitor
may solve the wax deposition problem and create more
problems such as (corrosion) due to varying properties of
crude oils and the different climate.</li>
<li> It needs to undertake a systematic study of inhibition of wax
deposition using chemicals and combinations of chemicals.</li>
<li> Raise interest in the possibility of increasing the shear
rate to prevent wax deposition, using spiral flow as a way
to increase shear rate in the flow.</li>
<li> Undertake a systematic study of inhibition of wax
deposition using the effects of combination spiral flow
with a chemical inhibitor.</li>
</ul>
<div style="clear:both">&nbsp;</div>
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