Wax Precipitation and Deposition Risk : Issues on Ethics and Professionalism

*Corresponding author: nmakwashi.cpe@buk.edu.ng doi: http://dx.doi.org/10.4314/njtd.v19i1.3 ABSTRACT: The deposition of wax on pipelines causes problems that affect oil production rate and the facilities. Several preventive and control measures were employed to manage this problem. However, there is no single technique that is hundred per cent effective for different fields. Therefore, this article provides new interpretation of the associated risk of the problem – such as the impact of ethical and professionalism on wax deposition. It sheds more light on the need for the implementation of sound engineering practices during pipeline design, construction, and operations to reduce wax deposition risks and the associated remediation costs. The laboratory case study revealed that crude oil properties (such as wax appearance temperature (WAT), pour point (PP), and density), standard operating conditions and procedures must be accurate and continuously updated throughout the production life cycle. The results showed that maintaining crude oil temperature above wax appearance temperature (30°C) and at a relatively high flow rate particularly within the turbulent flow region (7, 9 and 11 l/min) provides a safe and uninterrupted production of waxy crude oil (δwax ≈ 0 mm).


I. INTRODUCTION
Flow assurance is key to the optimum oil and gas production and vital during pipeline design, and construction. Flow assurance has been defined according to the study by Hammami and Ratulowski (2007) and Gao (2008) as guarantee of flow of hydrocarbon from the reservoir to the surface processing facilities and point of sale. The oil and gas industries are prone to several challenges regarding flow assurance. This includes wax formation and deposition on the pipeline, asphaltenes agglomeration, formation of gas hydrates, corrosion, scales, and slug flow (Bai and Bai, 2005;Chala et a.l, 2018;Kang et al., 2019). These problems are heightened as the oil and gas exploration and production moves towards deeper higher pressure higher temperature wells (El-Dalatony et al, 2019;Huang et al, 2015).
Many uncertainties affect the oil and gas operations, particularly in managing economics and safety of production, refining, and delivering hydrocarbons. According to Ershaghi (2019), the intrinsic risk and ethical issues confronting the oil and gas fields are related to the uncertainties and associated hazards. Generally, the risk issues, particularly in the upstream sector are higher and are divided into two (Ershaghi, 2019). These include the economic or soft issue and operational or hard issues. The soft issues include discriminating policies related to employment or working conditions, resource evaluation (such as overbooking), negligence on record keeping, and incident documentation and reports. The hard issues include operational processes and decision making. These could directly or indirectly affect the health and safety of the people and the environment and directly cause severe economic losses to the operations (Ershaghi, 2019). However, good ethical and professional behaviour plays a vital role, especially in the design and management operations of a reservoir that is naturally waxy.
Apart from having safe oil and gas production activities, oil industries have other goals of protecting the public, employees, the environment, and the risk of investors' capital in initiating and maintaining the project. In all of these, wax deposition poses a lot of concerns. A typical example is the Staffa oil field, which was operated by Lasmo Oil Company in the UK. The field was abandoned after four years of production because of difficult and recurrent deposition of wax (Nguyen et al., 2001;Singh et al., 2000), which reported more than $100,000,000 losses as a result of wax deposition.
In line with the current practice, it is worth noting that the recent track of ethical issues on production technology has shifted focus from the industrial revolution to the social revolution (Mitcham, 2020). Particularly for waxy crude production where non-compliance to the ethical and professional practices could harm the revenue generation, the life span of the oil field (as in the Staffa oil field), and the environment. Another typical example is the consequential impacts on the aquatic environment and vegetation caused by the use of some chemical wax inhibitors during remediation activities as a result of wax blockages (Bai and Bai, 2005;Ershaghi et al. 20118;Sharma, R. et al., 2020). However, in line with the best ethical and professional practices through proper planning, training and regulation the impact can be substantially reduced. Therefore, one of the objectives of this study is to theoretically evaluate and explore the essential ethical practices required for prevention of wax deposition. This includes conditions that must be met to avoid the devastating effect of wax deposition. Secondly, the paper also presents a practical demonstration of wax deposition issues. Therefore, a pipeline flow loop experiment was designed through which the standard operating procedure was alteredleading to the deposition of wax. The designed was done in accordance with the best professional and ethical practices that allow a smooth flow of hydrocarbon.

A. Wax Deposition Concern During Oil Production
Paraffin wax is a soft, colourless, soluble solid that precipitates as the crude oil temperature drops below the precipitation temperature (Wax Appearance Temperature, WAT) (Soedarmo et al., 2017). WAT is generally dependent on many factors: such as the ambient conditions, the composition of the oil (Saturate, Aromatics, Resins, Asphaltenes (SARA) fractions), and wax content (Wang and Huang, 2014). Wax precipitates as a clusters of crystals molecules that form an interlocking network configuration (Paso et al., 2005). However, the deposition of wax is regarded as a molecular diffusion process of crystalized wax particles on the pipe wall. Therefore, the deposition of wax reduces the pipe flow area, and if not prevented could lead to loss of production through unmanageable pipe blockage (Wang and Huang, 2014;Soedarmo et al, 2017).
Generally, several factors (such as oil temperature and composition, oil flow rate, pipe surface roughness and system pressure) and physical mechanisms (e.g., molecular, and Brownian diffusion) govern the precipitation and deposition of wax crystals on the colder surface (Chi et al., 2017;Harun et al., 2016). Hence, wax deposition is typical in the offshore environment, where the ambient condition is extremely harsh. As the hydrocarbon arrive at the sea environment (< 4 ℃), a thermal gradient occurs between the fluid and pipe wall (Lee, 2009). However, sound professionalism and ethical practices could enhance and improve wax management and remediation.
It should be noted that a pipe is the most used material for the production and transportation of oil and gas, mainly due to safety and economic reasons (Alamri, 2020;Chen et al, 2019). However, at a temperature lower than the WAT of crude oil, wax molecules begin to appear, leading to a deposition problem. According to a study by Shahriar et al. (2012), several factors are significant for the system design and construction of pipelines to produce waxy crude oil such as nature and properties of the fluids, volume flow rate, length of the pipeline, terrain and medium traversed by the pipeline, i.e., soil/water, and climatic conditionsextreme heat/cold. Other includes environmental constraints, standards and regulations governing the design, construction and operation of the pipeline, Seismic conditions, economics, materials, and construction, operation, and maintenance of the pipeline.

B. Existing Practices for Wax Prevention and Remediation
Wax deposition on the pipe wall could be catastrophic if there is no prevention strategy in place. Ethical practices, including adherence and adoptions to standards, are essential in wax management and environmental sustainability in wavy oil production. As wax deposition begins to build-up and eventually plug the pipeline, the reliability of the pipeline is being jeopardized. Currently, there are several wax management strategies used by oil and gas industries that require technical and ethical practices to yield the desired goal. These strategies are categorized as preventive and corrective measures (White et al., 2018;Paso et al., 2005). These measures could be achieved either by chemicals (e.g., wax inhibitors), mechanical (e.g. pigging process), and thermal methods (e.g. steam injection, electrical heating traces) (Bai and Bai, 2005;Paso et al., 2005). Some of these methods are efficient for use in a short distance pipe only. For instance, the thermal method is frequently employed in a short-distance pipe of around 500 m water depth. It is worth noting that most of the prevention techniques are employed to keep the flow line temperatures above WAT. As such, the wax crystals are kept dissolved in the crude oil matrix.
On the other hand, chemical inhibitors such as the wax crystals modifiers are widely used in long-distance pipes (Bai and Bai, 2005;Chala et al., 2018;Paso et al., 2005). Naturally, chemical inhibitors exhibit comparable structure with the molecules of wax. This makes the chemicals form a lattice structure within the crude oil matrix which substitute the wax molecules on the crystal lattices, and hence prevents the networking molecules of crystals (White et al., 2018;Kelland, 2009). However, study by Fan and Llave (1996) revealed that some inhibitors are usually limited to remediation purposes only because of the safety issues (e.g., chlorinated hydrocarbons).
The mechanical methods (e.g., pigging operation) are widely used for remediating affected facilities. This process is complex and requires a shutdown of the entire production and physical removal of wax by a pigging process (Bai and Bai, 2005;White et al., 2018). In addition to the complex nature of this process, in most cases some part of the pipeline is cut-off and replaced with completely new material. Which could result to an oil spill that could contaminate the sea water. Consequently, this could subject the living organism around the sea (the aquatic life, vegetation and even humans) to a severe risk. It is important to note that as of today, there is no distinctive practice that could successfully works for all the different types of crude oil. Similarly, there is no single technique that could be 100% effective in preventing wax deposition. Hence, the need and search for a viable and robust design and development of wax management strategy continues. Which could rely upon effective adherence to the professionalism and ethical practices within the industry.

C. Indicative Factors on Good Practices for Waxy Oil Production
The following key factors are related/highlighted as to the best practices that ensure safe operation and production of waxy oil from the reservoir to processing facilities. The factors are also paramount in selection and application best wax mitigation strategy. These factors include;

1) Material selection
This entails meeting the standard specifications through appropriate material selection. It is vital to ensure proper materials are selected, because wax deposition is as result of decrease in temperature gradient. This implies the choice of materials that are less conductive, non-corrosive and can withstand high pressure and high temperature within acceptable standards and specifications is essential. According Rashidi et al. (2016), a steel material (such as carbon steel and stainless steel) is commonly used for oil and gas pipeline production and transportation due to its strength, toughness, ductility, and weldability. It is also essential to consider the surface roughness, especially in the subsea environment. Lowfriction surfaces provides very low tendency of wax to stick on the pipe walls (Pedersen and Rønningsen, 2003;Rashidi et al., 2016). Design engineers use coated pipe to prevent deposition or at least minimise the rate of deposition.

2) Pipeline design
The most important engineering considerations that should be made while designing an oil and gas pipeline include the pipeline diameter, length, thickness, and even the pipe curvature. The effect of pipe bends and elbows needs to be carefully considered to avoid an increase in the severity of wax deposition. Pipe size should be designed to meet the target reservoir flow conditions (i.e., the flow rate and pressure). Under a laminar condition, higher wax depositions are reported (Makwashi et al., 2019;Wang et al., 2019).

3) Sound engineering practice
Sound engineering judgement must be applied throughout different stages of production and operations especially in matters that are related to occupational health and safety, control, routine checks, repairs, maintenance, instrumentation, constructions, and upgrade (Malins, 1977). This can be successfully achieved particularly for a waxy oil reservoir, through proper pre-start-up safety review (PSSR), planning, and enforcement of regulations.

4) Safety and environmental impact assessment
This is another bedrock of the oil industry within which life and livelihood could be of utmost value. For instance, the Piper Alpha disaster in the North Sea in July 1988 is attributed to unprofessional practices related to unaddressed failure in Safety and environmental considerations. It was reported that gas leakage from plant led to the world greatest oil industry disaster where 167 people lost their lives (Macleod and Richardson, 2018). Similarly, the ethical issues that would ensure the successful flow of waxy crude oil includes.

5) Accuracy of experimental data
Usually, fluid analysis is carried out to determine the fraction of crude oil including wax content. Furthermore, wax deposition models are used for field prediction and hence for design of prevention and mitigation measures, which sorely depend on the experimental results. The experimentation could be through fluid characterisation that defines the oil fraction (e.g., wax, saturate, aromatic, resin and asphaltene) using standard analytical methods and through pipeline flow loop studies pilot plant or laboratory scale level (Paso et al., 2014). The experiment is complex and requires careful study of the developed data obtained through repetitive experimental runs.

6) Social responsibility towards host community
It is crucial for oil and gas company to maintain their real value to the society. This will surely provide enabling and safe environment for the company to carry out their production, transportation, and process activities. Hence, it serves as a powerful tool for creating more meaningful and lasting relationships with key stakeholders and environment (Kanji and Chopra, 2010).

7) Environmental degradation and oil spillage control
Appropriate measure need to be implemented and maintained to ensure the prevention of activities that results in environmental degradation and oil spillage. Wax remediation activities could result to corrosion, unsolicited clearing of vegetation, oil spillage, and acid sulphate leaching. Therefore, the use of inhibitors that are harmful to environment during wax treatment must be avoided at all costs (Alamri, 2020;Chen et al., 2019).

II. MATERIALS AND METHODS
The crude oil sample used in this study was supplied by Roemex Oilfield Service Company, UK. The sample is naturally waxy crude from a well in the North Sea with unknown properties. The sample is characterised (Table 3) to obtain some definite fundamental properties. Figure 1 presents a laboratory flow rig designed in accordance with the best professional and ethical practices that allow a smooth flow of hydrocarbon. However, where the standard operating procedure is not strictly followed, the deposition of wax occurs on the pipe wall, which demonstrates the consequence of non-adherence to professional practices. The laboratory scale facility comprises a jacketed pipe-in-pipe (test section) that is made up of copper material of 15 mm inner pipe and 22 mm outer pipe. The coolant fluid (glycol and water) flow in the outer pipe connected to a chiller while the crude oil flow in the inner pipe.
Four thermocouples are used to monitor the temperature variation within the test section. These includes type K selfadhesive patch and exposed tip thermocouples connected at the inlet and outlet of the pipe. The exposed tip thermocouples are used to monitor crude oil temperatures (T1 and T4). Whereas the exposed tip thermocouples are used to monitor the coolant fluid temperature (T2 and T3) Figure1. Two GC35 pressure sensors (P1 and P2, inlet and outlet) were used to monitor pressure drop across the test section. GC35 has a range between 0 to 75 psi (0 -5 bar). The full experimental procedure is reported in a study by Makwashi (2020).
One of the fundamental and initial practices when dealing with the waxy oil reservoir is fluid characterisation prior to actual production. Therefore, in this study, oil characterisation was done using different standard methods. Similarly, complete characterisation of this oil sample is reported by Makwashi (2020). A shear-controlled Rheometer (Bohlin Gemini II) was used to study rheological properties of the waxy crude oil (such as viscosity, WAT, and PP) which are vital for the calibration of flow loop. The same method Practically, in order to demonstrate the consequence and the severity of unprofessional practices on wax deposition using the pipeline flow loopa total of 26 test matrix was designed (Table 1 and 2). A standard operating procedure was designed to avoid wax deposition following the methods used by Makwashi (2020). The cooling temperature was varied at 10, 15, 20, 25, 30, and 35℃ and the influence of both laminar and turbulent flow was examined at 5 l/min and 9 l/min. Overall, the experiment is conducted at a constant experimental period of 2 hrs. It worth noting that the wax crystals remained dissolved in the crude oil matrix as the oil temperature is maintained at or above WAT and the flow rate at 9 l/min. Wax thickness in the pipeline can be quantified using different techniques; (1)  . The four techniques were comparatively used as shown in section III. However, pressure drop method has proven to give better result with minimum errors (Makwashi et al. 2019;Chi et al., 2017;and Rittirong et al., 2017). Full details of these methods is reported by Makwashi (2020).
where δw is the average thickness of the wax deposit, di is the inside diameter of the pipe and μ is the apparent viscosity of the crude oil.
where represent the inner pipe radius, and M dep is the deposit weight, whereas L is the length of the pipeline test section, and 0 is the excess oil layer due to residual oil and is the density of deposit.
where represent the volume of wax that drops out of the system.
On the other hand, crude oil can be classified as light or heavy oil. A crude oil with API gravities > 33°API is regarded as light crude with lower density. Whereas those with lower API gravity value < 28°API are the heavier crudes usually consisting of more asphaltenes fractions and a small quantity of dissolved gases (Sifferman, 1979). The API of crude oil sample used in this study is 35 ÅPI with density of 835 kg/m3. Usually, API of a crude oil is determined using Eq. (4). o  where is the specific gravity (density of oil/density of water).

III. RESULTS AND DISCUSSION
Generally, the reliability of flow rig experimental results lies in the crude oil properties and the conditions within which the flow rig is configured. This implies that an accurate crude oil characterization is necessary to guarantee smooth crude oil production. Therefore, characterization of crude oil should be carried out prior to the installation of the main production facilities. This allows the field engineers to identify the flow assurance challenges and design appropriate prevention and mitigation methods. The flow rig study carried out in this study was used to develop better understanding of the ethical and professional practices with regards to the wax deposition (prevention and mitigation strategies). Initially, the properties that define the sample crude oil as waxy were measured, these include the API gravity, wax appearance temperature, wax content, and viscosity. Table 3 summarised the properties obtained in this work. It should be noted that the full oil characterization procedure is reported in our previous paper (Makwashi et al., 2021). However, brief descriptions of each standard procedure are given in this study.
Wax content is another parameter that indicate the depositional tendency of a samples. Wax content of the crude oil is obtained through modified UOP46-64 method, also called acetone precipitation method (Hoffmann and Amundsen, 2010;Coto et al, 2011). A study by Holder and Winkler (1965) revealed that a crude oil containing small amount of wax (2% by mass of wax) could cause deposition problem. The crude oil wax content is found to be 19.7 wt.%, which implies that the crude oil highly waxy. Similar results was reported by Makwashi et al, 2021).
The viscometric analysis was carried out using a rheometer at shear rate of 10, 60 and 120 1/s, varied cooling temperature (0 -50℃) and at a cooling rate of 1.0 °C/min. Parameters such oil viscosity, WAT and pour point were measured. Figure 2 show a complete circle of a single experiment captured on a computer. The red line represents the oil viscosity while the varied cooling temperature is represented by green line and the constant shear rate by blue line. Whereas the blue line represents the constant shear rate. As intersection of a baseline drawn from the Newtonian region and a tangent that fitted to the inflexion point represent the WAT of oil, any point of inflexion following the WAT point define the pour point of the crude oil.    Table 3 showed the analysis of result which describe the viscosity of crude oil that varies inversely with shear rate (10, 60 and 120 1/s) and cooling temperature (0 -50℃). It is found that increase in oil viscosity due to a decrease in cooling temperature produces the formation of a gel structure in crude oil. This transformation produces the higher risk of wax precipitation and deposition in the oil pipeline. Roughly, equal wax appearance temperature (35℃) was observed at 10, and 60 1/s whereas at 120 1/s the WAT measured is lower (30℃). This implies that the viscosity of oil drops with increase in shear rate, this causes a decrease in the WAT value because of the decrease in the intermolecular attraction of the wax molecules towards the colder surface.
Therefore, to avoid deposition of wax on the pipeline surface the crude oil temperature must be maintained around 30℃ or higher, particularly at the laminar flow regime. Consequently, any temperature below this condition could potentially cause wax deposition problem, although this depends on the oil flow rate. Hence, during oil and gas production the field and operation engineers are responsible for making proper design and operating the system based on the oil characteristic and other criteria, otherwise, wax deposition problem occurs.    20, 25, 30, 35 and 40 °C. It is observed that wax deposition measured in wax thickness (mm) increased with a decrease in cooling temperature and it reaches the maximum as the temperature drops to 15°C. This implies as the pipe wall temperature drops the crude oil temperature to decreases because of the decrease in the radial temperature gradient.
Below WAT, the crude oil experiences a gel transition, therefore, wax deposition thicknesses are observed to be lower at relatively higher cooling temperature at a low flow rate (e.g., at 30°C and 5 l/m, δwax ≈ 0.4mm). It is observed that above the WAT, both the laminar (5 l/m) (Figure 3a) and the turbulent (9 l/m) ( Figure 5) flow produces zero wax zero wax thickness (i.e., at 35 nd 40°C, δwax ≈ 0mm). Therefore, maintaining the wall temperature above WAT will eliminate any precipitation process of wax crystals. The increase in wax thickness as coolant temperature decreased leads to the formation of a thicker wax layers on the pipe wall thereby reduces the effective pipe diameter.
At a constant lower cooling temperature (15°C) i.e., below the WAT, wax deposition occurs even at a higher flow rate (9 and 11 l/min) ( Figure 6, Table 4). The results showed that wax thickness of 1.3 and 1.5 mm was measured at 9 and 11 l/min respectively. This is because a mass transfer of the wax molecules is created towards the cold surface due to increase in radial temperature gradient and the concentration gradient.
On the other hand, Figure 7 demonstrated that maintaining the crude oil flow at high cooling rate particularly at above or equal to WAT could lower the severity of the deposition. Therefore, as seen in Figure 7 the flow rate was varied from 2 to 11 l/min and at constant cooling temperature of 30°C. The results showed that there is no-wax deposition ( ≈ 0 at 9 and 11 l/min (tubular flow conditions). However,    under laminar condition (e.g., 2 l/min) wax thickness is 2 mm at 15°C and 0.85 mm at 30°C. This implies that at high temperature the solubility of the wax crystals is very high. Whereas, the high flow rate is related to the shear stripping i.e., shear removal effect and short residence time.
Researchers, such as Kang et al. (2019) has represented this effect and proved that there are reduction rate of deposit formation caused by shear forces. NIGERIAN JOURNAL OF TECHNOLOGICAL DEVELOPMENT, VOL. 18, NO.4, DECEMBER 2021   Maintaining the crude oil temperature above wax appearance temperature and at a relatively high flow rate provides a safe and uninterrupted production of waxy crude oil (without deposition). Figure 8(a) below shows a crosssectional view of wax deposited in a pipeline. The deposited wax is measured after 2 hours of ageing and at constant cooling temperature and flow rate of 20℃ (below WAT) and 7 l/min (laminar flow) respectively. In comparison, Figure  8(b) shows an empty pipe after pigging operation. The deposition is as a result of non-adherence to the standard operational parameters, such as operating the pipeline outside the WAT envelope. As shown in Figure 3, to avoid wax deposition based on the temperature envelops the oil temperature need to be maintained at 30°C or higher, although depending on the crude oil flow rate.

IV. CONCLUSION
The sample crude oil was successfully characterized and evaluated using both established techniques and modified approaches. Similarly, the waxy crude oil was experimentally studied using a pipeline flow loop, which was designed and fabricated in the laboratory. Following the experimental analysis and theoretical studies, the essential factors necessary for the uninterrupted production of waxy crude oil were addressed, while addressing the uncertainties and complexity associated with the deposition problem. The study revealed the unethical practices that could possibly lead to the deposition of wax molecules in the pipeline. It has been shown that addressing unprofessional behaviours along with improvement in advancement in technologies could prevent the costly problem of wax crystals in the production system. On the other hand, the laboratory case study has revealed that the standard operating conditions and crude oil properties need to be measured accurately and continuously updated throughout the entire production life cycle.
Not only will this be critical for assisting the flow assurance of waxy crude oil, but it will also significantly enhance the risk assessment of other crude oil fractions and operational problems. Therefore, this works sheds more light on the need for implementation of sound Engineering practices that would provide the best prevention and management strategies for the solid deposit thereby reduces the associated remediation costs.