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Experimental study of water-in-oil emulsion flow on wax deposition in subsea pipelines
Journal of Petroleum Science and Engineering 182 (2019) 106294
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Experimental study of water-in-oil emulsion flow on wax deposition in subsea pipelines
T
Olusiji Ayoade Adeyanjua,*, Layioye Ola Oyekunleb a b
University of Lagos, Lagos, Nigeria Covenant University, Otta, Ogun-State, Nigeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Demulsifier Average absolute deviations Pour point depressants Validated flow facility Dispersed water globules
Experiments were performed to simulate the deposition of wax in the flow of crude oil emulsions in subsea pipeline using a validated flow facility. Two crude samples and their respective formation water from two oil fields in southern Nigeria were used to synthetize two different emulsified crude oil samples (A and B). The trapping of the water globules in the wax deposits dominates the wax deposition rate initially but later the shear removal and the molecular diffusion dominate leading to decrease in the wax deposition rate. As the water (BS& W) composition in the emulsion increases from 10 to 40%, the pour point temperature (PPT) increases from 30 to 41 °C, and the viscosity increases from 40 mPas to 142 mPa at a crude oil temperature of 26 °C. The addition of 45 ppm of long chain acrylate ester co-polymer as pour point depressant depressed the dimensionless wax thickness by 37.5% and 34.3% for the blank emulsified crude oil A and B respectively. The effect of the commercial demulsifier to reduce the wax deposition rate was more successful in crude oil A reducing the wax dimensionless thickness by 58.9% and 43.0% when mixture of 25 ppm of the demulsifier and 45 ppm of long chain acrylate ester co-polymer were added to blank emulsified crude oil A and B respectively. The used demulsifier is more effective in crude oil A compared to crude oil B. Efforts should be made to study the effectiveness of different demulsifiers on each emulsified crude oil before their application for water separation.
1. Introduction Wax deposition in subsea oil pipelines is characterize with many economical, environmental and technical challenges to the petroleum industry (Lee, 2007; Benn et al., 1980; Burger et al., 1981; Majeed et al., 1990; Hamouda and Davidsen, 1995). During crude oil transportation from the production platform/stock tank to export terminals, the crude oils are moved through the ocean floor, which in most cases are at temperature below the crude oil wax appearance temperature (WAT). Due to the recent demand for crude oil as a result of industrial revolution, the oil companies are finding it difficult to meet the ever increasing demand. These had led most field to reach mature stage and oil production to be on the declined. Hence the oil companies are increasingly exploring oil fields in subsea environment in both deep and ultra deep water. The distance the crude oil move in the subsea pipeline ultimately increases, leading to the solidification of the wax particles in the crude oil as the temperature drops below its cloud point (Brown et al., 1993; Hsu et al., 1994). The deposition of the wax particles in the pipeline comes with it many challenges. The deposition of wax can be substantial leading to a complete blockage of the conduit pipeline,
*
which cost millions of dollars in downtime and remediation (Nguyen et al., 2001). The scenario is even more precarious if the crude oil is in form of water in oil emulsion, where water is dispersed as globules in a continuous crude oil phase: since water is produced with crude oil in most subsea fields, the emulsified water is naturally present in crude oil due to uncontrolled agitations and mixing occurring during crude oil productions (Lixin et al., 2004). Several parameters were known to stabilize the interfacial film between dispersed water and continuous oil phase resulting in the stability of the emulsion (Lixin et al., 2004; Wong et al., 2015; Akbari et al., 2016; Akbari and Nour, 2018). Water-in-oil emulsions are formed during crude oil production as formation water accompanies the crude oil. The process is characterized by the intimately dispersion of water droplet in the crude oil Troner et al. (2012). The shear mixing of the fluid during transportations and existence of natural surfactants in the petroleum fluid such as resin, asphaltenes, and wax contribute to the formation of such emulsion (Ramalho et al., 2010). Water in oil emulsions containing high percentage of water are characterized by high demulsification rate during treatments (Venkatesan et al., 2018; Xu, 2017; Troner et al.,
Corresponding author. E-mail address: [email protected] (O.A. Adeyanju).
https://doi.org/10.1016/j.petrol.2019.106294 Received 8 August 2017; Received in revised form 17 July 2019; Accepted 17 July 2019 Available online 18 July 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
2012) The transportations of these crudes in relatively lower temperature and pressure subsea pipeline results in the deposition of solid constituents in the pipeline, the major one of which is the wax deposit (Bidmus and Mehrotra, 2009). Efforts have been made to develop effective chemical to prevent/ reduce the wax deposition during crude oil flow in subsea pipelines. There are three main preventive chemicals used in wax deposition reduction, these are: wax dispersants, wax crystal modifiers, and pour point depressant or flow improvers. Wax dispersants are surface active chemicals that keep the dispersed wax in the precipitated form, preventing them from adhering to the pipeline walls (Pedensen and Ronningsen, 2003). The wax crystal modifiers increases the wax appearance temperature (WAT) thereby reduces the amount of wax deposit while the pour point depressants (PPD) are chemicals that reduce the pour point thereby reducing the rate of gelation of the crude oil (Kelland, 2009). Presently, polymers with long alkyl chain are normally used as wax inhibitors and pour point depressants. The long alkyl chain polymers interfere with the wax crystallization and growth process. Their effectiveness is due to their possession of similar structure to the wax structure, which will allow them to be incorporated into the wax crystal growth, in addition to their possession of similar structure as the wax particles, the polymer is expected to contain structural network which will prevent wax nucleation and aggregation (Jennings and Newberry, 2008). This alteration causes reduction in the wax networks adhesive forces and promotes the formation of smaller wax aggregates, these results in the reduction in the pour point and the viscosity of the crude oil, thereby inhibiting the wax precipitation and deposition in the cooled pipeline. (Pedensen and Ronningsen, 2003). The main chemical group used as pour point depressants are: ethylene copolymers, comb polymers and miscellaneous polymers. The comb polymers are the most effective of the three listed polymers, they consists of polyvinyl backbone with different pendant chains, normally long alkyl chains (Kelland, 2009). In the present study the focus is on the investigation of the effect of dispersed water (globules) in the continuous crude oil phase on wax deposition during crude oil emulsion flows in subsea pipelines. In addition the effect of a long alkyl chain polymer as pour point depressant on wax deposition during emulsified crude oil flow in pipeline was studied in order to understand the mechanism of wax deposition in the flow of emulsified crude oil in real subsea pipeline.
Table 1 Properties of the two Crude Oils and their ASTM Measurement Methods. PROPERTIES
Gravity at 28 °C, API Pour point temperature, oC Wax appearance temperature oC Wax Content, % Viscosity at 28 °C, cp Salt content, (g/m3) Asphaltene (%) Resin (%) Basic Sediment and Water BS&W (%) Saturated Hydrocarbon (%) Aromatic Hydrocarbon, (%)
Measurement Method
CRUDE OIL SAMPLE Oil-A
Oil-B
ASTM-D5002-16 ASTM-D97-66 ASTM D3117 ASTM UOP46-85 ASTM-D445 ASTM-D6470-99 (2015) ASTM-D7-5 (06560) ASTM-D893-69 ASTM-D4007-02
40.5 27 35 37 8.5 17
31.2 18 29 25 8.1 15
0.23 21.5 0.6
0.19 19.4 0.8
ASTM-D2786-91 (2016) ASTM-D3238-95 (2015)
34
28
16.84
27.25
to its higher content of the interfacial active agents. Judging by the low percentage of the basic sediment and water (BS& W) as shown in Table 1 above the two crude oils do not contain any appreciable volume of either free water or water globules (emulsified water). Preliminary investigation was conducted on the two crude oil samples to determine their WAT using a programmable Rheometer that measured the viscosity of the crude at different temperatures under different shear rates (El-Dalatony et al., 2019), The results for crude oil sample A are given in Fig. 1, where the wax appearance temperature (WAT) of the sample oil was determined to be 35 °C (temperature at which the viscosity begins to change with shear rate i.e. when the fluid change from Newtonian to Non-Newtonian fluid). Similar plots were obtained for oil sample B, and its WAT was confirmed to be 29 °C. 2.3. Experimental methods Two crude oils (Oil-A and B) with low basic sediment and water (BS &W) of 0.6 and 0.8% respectively from two oil fields in the Nigerian Niger delta (whose properties are shown in Table 1), mixed together with their respective formation water to form synthesized water-incrude oil emulsion were made to flow in an experimental flow facility fabricated to simulate the flow of crude oil in subsea (cooled) pipelines. The schematic diagram of the experimental flow facility is as shown in Fig. 2.
2. Materials and methods 2.1. Materials Two crude oils code named crude oil-A and crude oil-B from two oil fields in the Nigerian Niger Delta region were used in the study. Equal volumes of long carbon chain acrylate ester co-polymers of carbon number 19, 23 and 27 respectively mixed together are used as the pour point depressant. The co-polymers were used because of the change in the effective long chain acrylate ester polymer for wax inhibition as the operating parameters changes. Hence mixture of the different acrylate ester polymers is imperative (Norland, 2012). A commercial demulsifier was used; it was selected based on its effectiveness in an earlier study, these are mixtures of alkoxylated butyl, amyl and nonyl phenol resin (known for their broad treating range at low temperature for all gravity crude) and alkoxylated poly ethyleneimines, PEI (for improved water clarity) (Hajivand and Vaziri, 2015). Acrylate ester polymers were supplied by Finlab (Nigeria) Limited, while the MON Scientific Nigeria Limited supplied the commercial demulsifier.
2.4. Emulsion preparation In formulating the emulsion, the emulsion samples were prepared in
2.2. Characterizations of crude oil The properties of the two crudes together with the used ASTM measurement methods are shown in Table 1. Overview of the properties shows that the crude oil-A is likely to form a more stable emulsion due
Fig. 1. Viscosity profile of the oil A at different shear rates and temperatures. 2
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
Fig. 2. Schematic diagram of wax deposition experimental flow facility.
2.6. Wax deposition experimental methods
the ratio 2:7 water-oil volumes. In preparing the emulsion 2100 ml of each crude oil sample was mixed with 100 ml of sodium dodecl sulphate (SDS), A homogenizer set to a speed of 10,000 rpm was then used to stir the crude oil and surfactant mixture in a container for duration of 10 min. Thereafter, the dispersed phase i.e. 600 ml of formation water from the separator plant was introduced into the container containing the crude oil and the surfactant in a slow and gentle manner. After the addition of the dispersed phase the mixture is further agitated with the homogenizer at a speed of 8000 rpm for another 10 min to facilitate homogenization between the components of the resulting emulsion. The homogenized mixture was left for five days to enable the free water to settle out of the emulsion. These procedures were repeated three more times in order to have enough emulsified crude that will fill the flow facility. A droplet of the resulting emulsions was placed on a filter paper to ascertain that water-in-oil emulsions are formed (Bhattacharyya, 1992).
The emulsified waxy oil sample was made to enter the test section at a relatively higher temperature (55–60 °C) than the wall/coolant temperature (16 °C) in order to generate wax deposit in the inner section of the flow-line. The pressure differential method was used in evaluating the volume of wax deposited in the simulated pipeline (Chen et al., 1997). After each wax deposition test with the blank oil, the waxy oil was heated up to 65 °C, and passed through the pipe at a rate of 4 L/min. to remove wax deposit and oil thermal history. Then different quantities (20–45 ppm) of pour point depressants (PPDs) sample were injected into the oil reservoir at 65 °C. The oil/pour point depressants (PPDs) mixtures were circulated at a volumetric rate of 7 L/min. for at least 20 min at 65 °C. Then the oil/PPD mixtures were cooled to 45 °C for this study. Before the start of wax deposition process (cooling process) the oil was circulated at about 2 L/min for 25 min to stabilize the flow conditions. Identical wax deposition flow processes were further conducted for the demulsifier added oil. After completion of the wax deposition test with each PPD, the pipe test sections were purged with xylene, and then the wax deposit was pigged and recovered for volume measurement to determine the percentage error. However, wax deposition rate and wax thickness were calculated using the pressure differential techniques proposed by Chen et al. (1997) taken into cognizance the successive change in pipeline diameter as wax deposit are formed.
2.5. Experimental wax deposition test facility The experimental test facility is (Fig. 2) made of mild steel pipe of length 35 cm with an inside diameter of 4 cm. The experimental setup consists of the test section and the reference section. The crude oil temperature was regulated and the oil was pumped through the test section and then through reference section after passing through the liquid mass flow-meter along the flow lines. The test section was covered with a steel jacket in which cold water was pumped from a cooling bath and circulated in the opposite direction to the crude oil flow in the pipeline. The purpose of the test section is to maintain the inner pipe wall at a lower temperature than both the bulk oil temperature and Wax Appearance Temperature (WAT) so as to generate the wax deposit on the inner pipe wall. The configuration of the reference section is completely identical with the test section. However, to prevent wax deposition, the inner pipe wall temperature in the reference section is maintained at a higher temperature than the bulk oil temperature by circulating the heated water through the jacket of reference section (Bidmus and Mehrotra, 2009). Thermocouples (T) were placed both at the inlet and outlet sections of the test and reference tube to determine the temperatures at each point. Thermocouples were also attached to the cooling water tank and crude oil tank. The wax deposited in the previous experimental run was ensured to be removed by flowing hot oil for few hours before the commencement of subsequent experimental runs.
2.7. Wax deposit thickness determination The pressure drop method was used to determine the thickness of the deposited wax (Chen et al., 1997). Once the frictional pressure drop across a pipe section is measured and the flow rate, density and viscosity of the crude oil in the pipe section are determined, the wax thickness present in the pipe wall can be calculated accurately from the following equation: n
(di − 2δ w )5 − n =
2cρL ⎛ μ ⎞ 4Q 2 − n ⎞ ⎜ ⎟ ⎛ ΔPf ⎝ ρ ⎠ ⎝ π ⎠
(1a)
Where ΔPf is the pressure drop, L is the length of pipe section, d is the hydraulic diameter or updated (effective) inside diameter, δw is the wax thickness, Q is the volumetric flow rate, ρ is the fluid density, Where μ is the apparent viscosity of the crude oil. The values of c = 16, n = 1 for laminar flow and c = 0.046, n = 0.2 for turbulent flow were used. Laminar and turbulent flows exist when the Reynolds number NRe are NRe < 2000 andNRe > 3000 respectively (Chen et al. 1997). The flow 3
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
facility used in our study was calibrated using the c and n values as tuning parameters. For laminar flow c = 0.078 and n = 2.3 and for turbulent flow c = 0.034, n = 4.5 were used as tuning parameters, these were validated from the preliminary experiments performed to calibrate the experimental flow facility. The dimensionless thickness at period i (δti) is defined as:
δti =
δ wi ri
And the updated (effective) pipe radius at period i, (ri) is given as,
ri = ri − 1 − δ wi ri-1 = radius of the pipeline (cm) at time period i-1, ri = updated (effective) pipe radius (cm) at time period i. δwi = wax thickness determined from the experimental flow facility at time period i. The pipe radius is updated at every time the dimensionless thickness was calculated.
Fig. 3. Dimensionless wax thickness profiles at different period for emulsified and demulsified crude oil sample A.
3.2. Effect of dispersed water (globules) The molecular diffusion process of the emulsified oil into wax particle being one of the major factors affecting the wax deposition in cooled pipeline (Benn et al., 1980; Bidmus and Mehrotra, 2009; Brown et al., 1993; Hsu et al., 1994; Lee, 2007) can be described by the following five steps. These are: gelation of the crude oil on the cooled pipe, diffusion of crude oil fraction with carbon number greater critical carbon number towards the crude oil-gel interface, internal diffusion of these crude oil fraction through the gelled oil, precipitation of the diffused crude oil fraction in the gelled oil and finally counter diffusion of oil fraction with carbon number less than the critical carbon number out of gel deposit layer (Singh et al., 2000). The counter diffusion process is refers to as wax deposit aging which predominates during the latter period of wax deposition. Fig. 3 shows the changes in dimensionless wax thickness with time for emulsified and demulsified crude oil. From the figure the flow of emulsified crude (containing dispersed water (globules)) is characterized by periodic increase and decrease in the amount of deposited wax this is due to the relatively soft nature of the deposited wax as a result trapped water globules with the oil in the wax deposit. This is also responsible for the initial high volume of wax deposited during the emulsified crude oil flow. The counter diffusion of emulsified oil fraction out of the gelled deposit and shear removal effect were responsible for the relatively lower wax thickness in the latter period of emulsified crude wax depositional process. This is due to higher percentage of the crude with carbon number less than the critical carbon number in the emulsified oil compared to demulsified oil. Fig. 4 confirms the initial soft nature and relatively higher volume of wax (paraffin) formed by the flow of emulsified crude oil compare to what was observed when a demulsified crude oil was used. For crude oil A, the dimensionless thickness of 0.121 (L/L) was observed with the emulsified crude oil after 21 h of flow, compared to a dimensionless thickness of 0.112 (L/L) using the demulsified crude
3. Results and discussions 3.1. Validation of flow facility wax evaluation method The wax evaluation method used in the experimental flow facility was validated by comparing the amount of wax determined from the flow facility and the wax scrapped from the flow facility after each experiment. Table 2 shows the summary of the physical conditions and the results of the experiments performed on different crude oils using the pour point depressant. The determined volumes of wax deposited from the flow facility and the volumes of wax scrapped from the cooled pipe after each experiment were compared. Defining the Average Absolute Deviation, AAD as: N
AAD =
∑i = 1
(Xir − Xie )
N
Xir
*100
(1b)
Where, Xir = Volume of wax deposit calculated from flow facility readings for experiment run i Xie = Volume scrapped from the flow facility for experiment run i N = Total number of experimental runs There is a noticeable lower value of wax volume scrapped compared to the volume of wax calculated from the flow facility reading for all the experimental runs, this is due to shear removal of some deposited wax every period the flow is stopped prior to the scrapping of the deposited wax. The value of the average absolute deviation (AAD) percentage of 9.1% between the experimentally determined volume and the volume of wax scrapped from the flow loop despite the sheared wax deposit confirmed the effectiveness of the wax deposits evaluation method used in the study. Table 2 Experimental Wax Deposition flow facility results. Exp. No.
Oil Sample
PPD (ppm)
Wax Deposited, (cm3) (Experimental)
Wax scooped from flow rig (cm3)
1 2 3 4 5 6 7 8
A A A A B B B B
0 5 10 30 0 10 16 32
48 32 29 18 29 22 17 10
45 30 27 17 27 20 15 8
Fig. 4. Dimensionless wax thickness profiles at different period for emulsified and demulsified crude oil sample B. 4
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
Fig. 5. Viscosity-Temperature profiles at different water percentage at shear rate of 180 s−1.
Fig. 7. Dimensionless wax thickness profiles for Blank emulsified crude oil A at different water cuts.
after the same period of flow. While for crude oil B, a dimensionless thickness of 0.190 (L/L) was observed with the emulsified oil as against a dimensionless thickness of 0.170 (L/L) with the demulsified crude. Hence differences of 8 and 12% respectively for crude oil A and B respectively. The relative higher difference in crude oil B is due to its higher water content. Fig. 5 shows the viscosity profile at different temperature for different percentage of water in the emulsion at a shear rate of 180 s−1, the viscosity of the crudes was observed to increase with the percentage of water in the emulsion but decreases with temperature increases. At a temperature of 26 °C the viscosity increases from about 40 mPas to about 142 mPas, as the percentage of water in the emulsion increases from 10 to 40%, this is more than 300% increment. Fig. 6 shows the plots of pour point temperatures at different water composition (%) for both crude samples. The pour point temperature was observed to increase withe water composition for both crude oils A and B. As the water composition in the emulsion increases from 10 to 40%, the pour point temperature (PPT) increases from 30 to 41 °C for crude oil A, while the PPT changes from 21 to 31 °C for crude oil B. Three parameters determine the amount of wax deposition in the flow of the emulsified oil through the cooled pipeline these are: amount of trapped water globules, the shear rate which determines the shear removal at the deposit layer by the flowing oil along the water-oil interface, and the crudes' diffusion coefficient which determines both the diffusion of crude with higher carbon number into the wax gel deposit and the counter diffusion of lower carbon crude fraction out of the wax gel deposit into the crude oil. The viscosity of the crude increases with the percentage of its water content (Fig. 5), as the viscosity increase is accompanies by increase in shear rate (Fig. 1), this leads to higher shear removal at the crude-wax deposit layer. Simultaneously increase in percentage water content lead to increase in the percentage of trapped water globules in the wax network, leading to increase in volume of deposited wax. Hence the net thickness of the wax deposit depends on which of these two simultaneous mechanisms is dominant.
. Initially, the trapping mechanism of the water globules and molecular diffusion of the emulsified oil into the formed wax crystal gel dominates the wax deposition process judging by the relative increase in the initial wax deposition rate compared to when demulsified oil was used but as the experiment progresses the shear removal effect and the counter diffusion of crude oil fraction with carbon number less than the critical number out of the formed wax crystal gel into the oil phase as a result of higher water content in the emulsion (from 28.6 to 40%) overrides leading to decrease in the rate of wax deposit. These were shown in Figs. 7 and 8 for crude oil A and B respectively. In Figs. 7 and 8, emulsified crude with higher water content initially deposits more wax due to relatively more water globules trapped in the wax gel network during molecular diffusion process. While in the latter period the reverse is the case as more water globule in the wax network results in more water diffusing out of the deposited wax (counter diffusion), this leads to higher viscosity of the flowing crude oil at the crude-wax deposit interface due to higher water content (Fig. 5), and consequently higher shear removal effect (Fig. 1). Figs. 9 and 10 shows the dimensionless wax deposition-time profiles for blank emulsified crude oil, the emulsified crude oil mixed with pour point depressant, PPD (long chain acrylate ester co-polymer) and the blank oil, with both the PPD and the demulsifier for crude oil A and B respectively. 3.3. Effect of pour point depressant (PPD) on wax deposition From the two plots (Figs. 9 and 10), the addition of 45 ppm of long chain acrylate ester co-polymer as pour point depressant succeeded in depressing the dimensionless wax thickness in the emulsified crude oil judging by the reduction in the dimensionless wax thickness from 0.112 (L/L) for the blank emulsified crude oil A to the dimensionless wax thickness of 0.070 (L/L) when pour point depressant (PPD) was added. The pour point depressant successively reduce the pour point temperature of the emulsified crude oil thereby reducing the temperature at
Fig. 8. Dimensionless wax thickness profiles for Blank emulsified crude oil B at different water cut.
Fig. 6. Pour point temperature-percentage water profiles for crude oils A and B. 5
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
where the crude oil flows, making the volume of wax deposit to be relatively lower than the situation where the crude oil occupy the whole cross-section of the pipeline. 4. Conclusion A flow facility that was design and validated to simulate the flow of relatively higher temperature crude oil in subsea pipeline was used to study the effect of emulsified crude oil flow with and without pour point depressant and demulsifier on wax deposition in subsea pipeline. The addition of pour point depressant successfully inhibits the wax deposit for the two crude oil samples used in the study. Addition of demulsifier to the mixture of the emulsified crude oil and the pour point depressant further inhibits the wax deposit for crude oil A, while the inhibition of the wax deposit by the addition of the demulsifier was not as successful for emulsified crude oil B, due to the ineffectiveness of the used demulsifier in separating the dispersed water from the emulsified crude oil B. The viscosity and the pour point temperature of water in crude oil emulsion increase as the amount of water content in the emulsion increases. The increase in viscosity has an antagonistic effect on the wax deposition. This is due to the fact that at constant temperature increase in viscosity is accompanied by higher shear rate (Fig. 1) resulting in the additional shear removal effect of the deposited wax on the wax-crude oil interface. The increase in pour point temperatures result in a synergetic effect on the amount of wax deposition. This is due to the fact that increase in pour point temperature is accompanies by increase in the rate of gelation and deposition of the wax particles in the crude oil flows through the cooled pipeline. Three different mechanisms were identified to determine the rate of wax deposition in the flow of emulsified crude oil in cooled pipeline, these are: Molecular diffusion and counter diffusion of wax molecules in and out of the wax deposit respectively, the trapping of water globules in the deposited wax network and the shearing of the deposited wax due to the shear stress at the wax deposit-crude oil interface. While the trapping of the water globules and the molecular diffusion dominate the wax deposition process at the beginning of the experiment, the shear rate effect and counter diffusion of trapped water/crude fraction with low carbon number override the trapping mechanisms at the later period of the experiment resulting in the relative decrease in dimensionless wax thickness during emulsified oil flow compare to when the oil was de-emulsified before being flow through the flow loop. (Figs. 3, 4, 7 and 8). Thorough de-emulsification of crude oil before their flow through cooled pipeline is a process that needs to be considered for effective wax deposition inhibition in the flow of crude oil in subsea pipelines.
Fig. 9. Dimensionless wax thickness at different time for Blank emulsified crude oil A, oil A with pour point depressant (PPD) and oil A with PPD and demulsifier.
Fig. 10. Dimensionless wax thickness at different time for Blank emulsified crude oil B, oil B with pour point depressant (PPD) and oil B with PPD and demulsifier.
which the emulsified crude oil gel, as gelation of the crude oil precedes the wax deposition, the wax deposition rate is suppressed. Addition of demulsifier to the oil/PPD solution further reduced the dimension wax thickness to 0.046 (L/L). The same result was observed for crude oil B as shown in Fig. 10 the dimensionless wax thickness was reduced from 0.172 (L/L) for the blank emulsified crude oil to 0.112 (L/L) when 45 ppm of pour point depressant was added to the blank emulsified crude oil B and further to 0.100 (L/L) with crude-oil/PPD/demusifier solution.
3.4. Effect of demulsifier on wax deposition Acknowledgements The effect of the used demulsifier to reduce the wax deposition was more successful for crude oil A reducing the wax dimensionless thickness from 0.070 (L/L) when no demulsifier wax used to dimensionless wax thickness of 0.046 (L/L) when 25 ppm of demulsifier was added as against the reduction from the wax dimensionless thickness of 0.112 (L/ L) without the demulsifier to 0.098 (L/L) with the 25 ppm of the demulsifier for crude oil B (Figs. 9 and 10), this is due to the effectiveness of the demulsifier in the de-emulsification of crude oil A compared to crude oil B. Hence, efforts must be made to study the effectiveness of different demulsifiers for each emulsified crude oil before their application on the crude oil for water separation. Addition of the demulsifier to the mixture of the emulsified oil and the pour point depressant makes the emulsified crude oil to be separated to its separate compositions of crude oil and water thereby making the waxy stratified liquid flow to characterized the fluid flow in the pipeline, where the oil will flow in a wavy-like flow on a separated water phase at the bottom part of the horizontal pipe. This makes the wax deposition to form only on the upper part of the horizontal pipeline
The authors thank the Central Research Committee (CRC) of the University of Lagos, Nigeria for providing the funds (CRC No: M2014/ 01) for the conduct of this research. Nomenclature d L N Q ri-1 ri c, n Xir Xie ΔPf 6
hydraulic diameter or updated (effective) inside diameter length of pipe section Total number of experimental runs volumetric flow rate radius of the pipeline (cm) at time period i-1 updated (effective) pipe radius (cm) at time period i tuning parameters Volume of wax deposit calculated from flow facility readings for experiment run i Volume scrapped from the flow facility for experiment run i Pressure drop
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
δw δwi ρ μ Subscripts F i= i-1 ie ir w wi SI metric
wax thickness wax thickness determined from the experimental flow facility at time period i fluid density apparent viscosity of the crude oil
Burger, E.D., Perkins, T.K., Striegler, J.H., 1981. Studies of wax deposition in the Trans Alaska Pipeline. J. Pet. Sci. Technol. 33 (6), 1075–1086. Chen, X.T., Butler, T., Volk, M., Brill, J.P., 1997. Techniques for measuring wax thickness during single and multiphase flow, SPE-38773-MS. In: Proceedings of Society of Petroleum Engineers Technical Conference and Exhibition, San Antonio, Texas, USA. El-Dalatony, M., Jeon, B., Salama, E., Eraky, M., Kim, W., Wang, J., Ahn, T., 2019. Occurrence and characterization of paraffin wax formed in developing wells and pipelines. Enegies 12, 967–989. Hajivand, P., Vaziri, A., 2015. Optimization of demulsifier formulation for separation of water from crude oil emulsions. Braz. J. Chem. Eng. 32 (01), 107–118. Hamouda, A.A., Davidsen, S., 1995. An approach for simulation of paraffin deposition in pipelines as a function of flow characteristics with reference to teesside oil pipeline. In: Proceedings of International Symposium on Oilfield Chemistry, pp. 459–470. Hsu, J.J., Santamaria, M.M., Brubaker, J.P., 1994. Wax deposition of waxy live crudes under turbulent flow conditions. SPE paper No. 28480 In: Proceedings of the SPE 69th Annual Technical Conference and Exhibition Held in New Orleans, LA, USA. Jennings, D.W., Newberry, M.E., 2008. Paraffin inhibitor application in deep-water offshore development. In: Proceeding of International Petroleum Technology Conference Held in Kuala-Lumpur, Malaysia. Kelland, M.A., 2009. Production Chemicals for Oil and Gas Industry. CRC Press, Boca Raton, Florida, USA. Lee, H.S., 2007. Computational and Rheological Study of Wax Deposition and Gelation in Subsea Pipelines. P.hD. thesis. University of Michigan, Ann Arbor, MI.USA. Lixin, X., Shiwei, L., Guoying, C., 2004. Stability and demulsification of emulsion stabilized by asphaltene or resin. J. Colloid Interface Sci. 271, 503–506. Majeed, A., Bringedal, B., Overa, S., 1990. Model calculates wax deposition for North sea oils. Oil Gas J. 88, 63–69. Nguyen, A.D., Fogler, H.S., Sumaeth, C., 2001. Fused chemical reaction 2 encapsultation application to remediation of paraffin plugged pipelines. Ind. Eng. Chem. Res. 40, 50–58. Norland, A.K., 2012. Organic Flow Assurance: Pour Point Depressant Development Through Experimental Design. M.Sc Thesis Submitted to the University of Stavanger, Norway. Pedensen, K.S., Ronningsen, H.P., 2003. Influence of wax inhibitors on wax appearance temperature, pour point, and viscosity of waxy crude oil. Energy Fuel. 17 (2), 321–328. Ramalho, J.B., Lechuga, V.S., Lucas, E.F., 2010. Effect of the structure of commercial poly (ethylene oxide-b-propylene oxide) demulsifier dases on the demulsification of waterin-crude oil emulsion: elucidation of the demulsification mechanism. Quim. Nova 33 (8), 1664–1670. Singh, P., Venkatesan, R., Fogler, H.S., Nagarajan, N.R., 2000. Formation and aging of incipient thin film wax-oil gels. AIChE J. 46 (5), 1059–1074. Troner, A., Agnes, P., Marcia, C., Carlos, I., Luiz, F., 2012. Emulsion inversion using solid particles. J. Pet. Sci. Eng. 96–97, 49–57. Venkatesan, V., Nasal, M., Robin, M., Ginto, G., Murshid, E., 2018. Studies on demulsification of crude emulsion using plant extracts as green demulsifier. Asian J. Appl. Sci. Technol. 2 (2), 999–1004. Wong, S.F., Lim, J.S., Dol, S.S., 2015. Crude oil emulsion: a review on formation classification and stability of water-in oil emulsions. J. Pet. Sci. Eng. 135, 498–504. Xu, B., 2017. Fast and energy-efficient demulsification for crude-oil emulsion using pulsed electric field. Intern. J. Electrochem. Sci. 12, 9242–9249.
fluid period period before i wax scrapped from flow facility wax read from flow facility wax initial wax conversion factors
cp x 1e-03 Pa.s ft x 3.048e-01 m ft3 x 2.831685e-02 m3 Psi x 6.89475 kpa Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.106294. References Akbari, S., Nour, A., Jameri, S., Rajabi, A., 2016. Demulsification of water-in-crude oil emulsion via conventional heating and microwave heating technology in their optimum conditions. Aust. J. Basic Appl. Sci. 10 (4), 66–74. Akbari, S., Nour, A., 2018. Emulsion type, stability mechanisms and rheology: a review. Int. J. Innov. Res. Sci. Stud. 1, 14–21. Benn, P.A., Withers, V.R., Cairns, J.R., 1980. Wax deposition in crude oil pipelines. In: Proceedings of European Offshore Petroleum Conference and Exhibition, pp. 571–578 London. Bhattacharyya, B.R., 1992, Water soluble polymer as water-in-oil demulsifiers, US Patent 5, 100, 582. Bidmus, H.O., Mehrotra, A.K., 2009. Solids deposition during “cold flow” of wax solvent mixtures in a flow loop apparatus with heat transfer. Energy Fuels 23, 3184–3194. Brown, T.S., Niesen, V.G., Erickson, D.D., 1993. Measurement and prediction of kinetics of paraffin deposition. In: Proceedings of Society of Petroleum Engineers Technical Conference and Exhibition, pp. 353–368.
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Experimental study of water-in-oil emulsion flow on wax deposition in subsea pipelines
T
Olusiji Ayoade Adeyanjua,*, Layioye Ola Oyekunleb a b
University of Lagos, Lagos, Nigeria Covenant University, Otta, Ogun-State, Nigeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Demulsifier Average absolute deviations Pour point depressants Validated flow facility Dispersed water globules
Experiments were performed to simulate the deposition of wax in the flow of crude oil emulsions in subsea pipeline using a validated flow facility. Two crude samples and their respective formation water from two oil fields in southern Nigeria were used to synthetize two different emulsified crude oil samples (A and B). The trapping of the water globules in the wax deposits dominates the wax deposition rate initially but later the shear removal and the molecular diffusion dominate leading to decrease in the wax deposition rate. As the water (BS& W) composition in the emulsion increases from 10 to 40%, the pour point temperature (PPT) increases from 30 to 41 °C, and the viscosity increases from 40 mPas to 142 mPa at a crude oil temperature of 26 °C. The addition of 45 ppm of long chain acrylate ester co-polymer as pour point depressant depressed the dimensionless wax thickness by 37.5% and 34.3% for the blank emulsified crude oil A and B respectively. The effect of the commercial demulsifier to reduce the wax deposition rate was more successful in crude oil A reducing the wax dimensionless thickness by 58.9% and 43.0% when mixture of 25 ppm of the demulsifier and 45 ppm of long chain acrylate ester co-polymer were added to blank emulsified crude oil A and B respectively. The used demulsifier is more effective in crude oil A compared to crude oil B. Efforts should be made to study the effectiveness of different demulsifiers on each emulsified crude oil before their application for water separation.
1. Introduction Wax deposition in subsea oil pipelines is characterize with many economical, environmental and technical challenges to the petroleum industry (Lee, 2007; Benn et al., 1980; Burger et al., 1981; Majeed et al., 1990; Hamouda and Davidsen, 1995). During crude oil transportation from the production platform/stock tank to export terminals, the crude oils are moved through the ocean floor, which in most cases are at temperature below the crude oil wax appearance temperature (WAT). Due to the recent demand for crude oil as a result of industrial revolution, the oil companies are finding it difficult to meet the ever increasing demand. These had led most field to reach mature stage and oil production to be on the declined. Hence the oil companies are increasingly exploring oil fields in subsea environment in both deep and ultra deep water. The distance the crude oil move in the subsea pipeline ultimately increases, leading to the solidification of the wax particles in the crude oil as the temperature drops below its cloud point (Brown et al., 1993; Hsu et al., 1994). The deposition of the wax particles in the pipeline comes with it many challenges. The deposition of wax can be substantial leading to a complete blockage of the conduit pipeline,
*
which cost millions of dollars in downtime and remediation (Nguyen et al., 2001). The scenario is even more precarious if the crude oil is in form of water in oil emulsion, where water is dispersed as globules in a continuous crude oil phase: since water is produced with crude oil in most subsea fields, the emulsified water is naturally present in crude oil due to uncontrolled agitations and mixing occurring during crude oil productions (Lixin et al., 2004). Several parameters were known to stabilize the interfacial film between dispersed water and continuous oil phase resulting in the stability of the emulsion (Lixin et al., 2004; Wong et al., 2015; Akbari et al., 2016; Akbari and Nour, 2018). Water-in-oil emulsions are formed during crude oil production as formation water accompanies the crude oil. The process is characterized by the intimately dispersion of water droplet in the crude oil Troner et al. (2012). The shear mixing of the fluid during transportations and existence of natural surfactants in the petroleum fluid such as resin, asphaltenes, and wax contribute to the formation of such emulsion (Ramalho et al., 2010). Water in oil emulsions containing high percentage of water are characterized by high demulsification rate during treatments (Venkatesan et al., 2018; Xu, 2017; Troner et al.,
Corresponding author. E-mail address: [email protected] (O.A. Adeyanju).
https://doi.org/10.1016/j.petrol.2019.106294 Received 8 August 2017; Received in revised form 17 July 2019; Accepted 17 July 2019 Available online 18 July 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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O.A. Adeyanju and L.O. Oyekunle
2012) The transportations of these crudes in relatively lower temperature and pressure subsea pipeline results in the deposition of solid constituents in the pipeline, the major one of which is the wax deposit (Bidmus and Mehrotra, 2009). Efforts have been made to develop effective chemical to prevent/ reduce the wax deposition during crude oil flow in subsea pipelines. There are three main preventive chemicals used in wax deposition reduction, these are: wax dispersants, wax crystal modifiers, and pour point depressant or flow improvers. Wax dispersants are surface active chemicals that keep the dispersed wax in the precipitated form, preventing them from adhering to the pipeline walls (Pedensen and Ronningsen, 2003). The wax crystal modifiers increases the wax appearance temperature (WAT) thereby reduces the amount of wax deposit while the pour point depressants (PPD) are chemicals that reduce the pour point thereby reducing the rate of gelation of the crude oil (Kelland, 2009). Presently, polymers with long alkyl chain are normally used as wax inhibitors and pour point depressants. The long alkyl chain polymers interfere with the wax crystallization and growth process. Their effectiveness is due to their possession of similar structure to the wax structure, which will allow them to be incorporated into the wax crystal growth, in addition to their possession of similar structure as the wax particles, the polymer is expected to contain structural network which will prevent wax nucleation and aggregation (Jennings and Newberry, 2008). This alteration causes reduction in the wax networks adhesive forces and promotes the formation of smaller wax aggregates, these results in the reduction in the pour point and the viscosity of the crude oil, thereby inhibiting the wax precipitation and deposition in the cooled pipeline. (Pedensen and Ronningsen, 2003). The main chemical group used as pour point depressants are: ethylene copolymers, comb polymers and miscellaneous polymers. The comb polymers are the most effective of the three listed polymers, they consists of polyvinyl backbone with different pendant chains, normally long alkyl chains (Kelland, 2009). In the present study the focus is on the investigation of the effect of dispersed water (globules) in the continuous crude oil phase on wax deposition during crude oil emulsion flows in subsea pipelines. In addition the effect of a long alkyl chain polymer as pour point depressant on wax deposition during emulsified crude oil flow in pipeline was studied in order to understand the mechanism of wax deposition in the flow of emulsified crude oil in real subsea pipeline.
Table 1 Properties of the two Crude Oils and their ASTM Measurement Methods. PROPERTIES
Gravity at 28 °C, API Pour point temperature, oC Wax appearance temperature oC Wax Content, % Viscosity at 28 °C, cp Salt content, (g/m3) Asphaltene (%) Resin (%) Basic Sediment and Water BS&W (%) Saturated Hydrocarbon (%) Aromatic Hydrocarbon, (%)
Measurement Method
CRUDE OIL SAMPLE Oil-A
Oil-B
ASTM-D5002-16 ASTM-D97-66 ASTM D3117 ASTM UOP46-85 ASTM-D445 ASTM-D6470-99 (2015) ASTM-D7-5 (06560) ASTM-D893-69 ASTM-D4007-02
40.5 27 35 37 8.5 17
31.2 18 29 25 8.1 15
0.23 21.5 0.6
0.19 19.4 0.8
ASTM-D2786-91 (2016) ASTM-D3238-95 (2015)
34
28
16.84
27.25
to its higher content of the interfacial active agents. Judging by the low percentage of the basic sediment and water (BS& W) as shown in Table 1 above the two crude oils do not contain any appreciable volume of either free water or water globules (emulsified water). Preliminary investigation was conducted on the two crude oil samples to determine their WAT using a programmable Rheometer that measured the viscosity of the crude at different temperatures under different shear rates (El-Dalatony et al., 2019), The results for crude oil sample A are given in Fig. 1, where the wax appearance temperature (WAT) of the sample oil was determined to be 35 °C (temperature at which the viscosity begins to change with shear rate i.e. when the fluid change from Newtonian to Non-Newtonian fluid). Similar plots were obtained for oil sample B, and its WAT was confirmed to be 29 °C. 2.3. Experimental methods Two crude oils (Oil-A and B) with low basic sediment and water (BS &W) of 0.6 and 0.8% respectively from two oil fields in the Nigerian Niger delta (whose properties are shown in Table 1), mixed together with their respective formation water to form synthesized water-incrude oil emulsion were made to flow in an experimental flow facility fabricated to simulate the flow of crude oil in subsea (cooled) pipelines. The schematic diagram of the experimental flow facility is as shown in Fig. 2.
2. Materials and methods 2.1. Materials Two crude oils code named crude oil-A and crude oil-B from two oil fields in the Nigerian Niger Delta region were used in the study. Equal volumes of long carbon chain acrylate ester co-polymers of carbon number 19, 23 and 27 respectively mixed together are used as the pour point depressant. The co-polymers were used because of the change in the effective long chain acrylate ester polymer for wax inhibition as the operating parameters changes. Hence mixture of the different acrylate ester polymers is imperative (Norland, 2012). A commercial demulsifier was used; it was selected based on its effectiveness in an earlier study, these are mixtures of alkoxylated butyl, amyl and nonyl phenol resin (known for their broad treating range at low temperature for all gravity crude) and alkoxylated poly ethyleneimines, PEI (for improved water clarity) (Hajivand and Vaziri, 2015). Acrylate ester polymers were supplied by Finlab (Nigeria) Limited, while the MON Scientific Nigeria Limited supplied the commercial demulsifier.
2.4. Emulsion preparation In formulating the emulsion, the emulsion samples were prepared in
2.2. Characterizations of crude oil The properties of the two crudes together with the used ASTM measurement methods are shown in Table 1. Overview of the properties shows that the crude oil-A is likely to form a more stable emulsion due
Fig. 1. Viscosity profile of the oil A at different shear rates and temperatures. 2
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O.A. Adeyanju and L.O. Oyekunle
Fig. 2. Schematic diagram of wax deposition experimental flow facility.
2.6. Wax deposition experimental methods
the ratio 2:7 water-oil volumes. In preparing the emulsion 2100 ml of each crude oil sample was mixed with 100 ml of sodium dodecl sulphate (SDS), A homogenizer set to a speed of 10,000 rpm was then used to stir the crude oil and surfactant mixture in a container for duration of 10 min. Thereafter, the dispersed phase i.e. 600 ml of formation water from the separator plant was introduced into the container containing the crude oil and the surfactant in a slow and gentle manner. After the addition of the dispersed phase the mixture is further agitated with the homogenizer at a speed of 8000 rpm for another 10 min to facilitate homogenization between the components of the resulting emulsion. The homogenized mixture was left for five days to enable the free water to settle out of the emulsion. These procedures were repeated three more times in order to have enough emulsified crude that will fill the flow facility. A droplet of the resulting emulsions was placed on a filter paper to ascertain that water-in-oil emulsions are formed (Bhattacharyya, 1992).
The emulsified waxy oil sample was made to enter the test section at a relatively higher temperature (55–60 °C) than the wall/coolant temperature (16 °C) in order to generate wax deposit in the inner section of the flow-line. The pressure differential method was used in evaluating the volume of wax deposited in the simulated pipeline (Chen et al., 1997). After each wax deposition test with the blank oil, the waxy oil was heated up to 65 °C, and passed through the pipe at a rate of 4 L/min. to remove wax deposit and oil thermal history. Then different quantities (20–45 ppm) of pour point depressants (PPDs) sample were injected into the oil reservoir at 65 °C. The oil/pour point depressants (PPDs) mixtures were circulated at a volumetric rate of 7 L/min. for at least 20 min at 65 °C. Then the oil/PPD mixtures were cooled to 45 °C for this study. Before the start of wax deposition process (cooling process) the oil was circulated at about 2 L/min for 25 min to stabilize the flow conditions. Identical wax deposition flow processes were further conducted for the demulsifier added oil. After completion of the wax deposition test with each PPD, the pipe test sections were purged with xylene, and then the wax deposit was pigged and recovered for volume measurement to determine the percentage error. However, wax deposition rate and wax thickness were calculated using the pressure differential techniques proposed by Chen et al. (1997) taken into cognizance the successive change in pipeline diameter as wax deposit are formed.
2.5. Experimental wax deposition test facility The experimental test facility is (Fig. 2) made of mild steel pipe of length 35 cm with an inside diameter of 4 cm. The experimental setup consists of the test section and the reference section. The crude oil temperature was regulated and the oil was pumped through the test section and then through reference section after passing through the liquid mass flow-meter along the flow lines. The test section was covered with a steel jacket in which cold water was pumped from a cooling bath and circulated in the opposite direction to the crude oil flow in the pipeline. The purpose of the test section is to maintain the inner pipe wall at a lower temperature than both the bulk oil temperature and Wax Appearance Temperature (WAT) so as to generate the wax deposit on the inner pipe wall. The configuration of the reference section is completely identical with the test section. However, to prevent wax deposition, the inner pipe wall temperature in the reference section is maintained at a higher temperature than the bulk oil temperature by circulating the heated water through the jacket of reference section (Bidmus and Mehrotra, 2009). Thermocouples (T) were placed both at the inlet and outlet sections of the test and reference tube to determine the temperatures at each point. Thermocouples were also attached to the cooling water tank and crude oil tank. The wax deposited in the previous experimental run was ensured to be removed by flowing hot oil for few hours before the commencement of subsequent experimental runs.
2.7. Wax deposit thickness determination The pressure drop method was used to determine the thickness of the deposited wax (Chen et al., 1997). Once the frictional pressure drop across a pipe section is measured and the flow rate, density and viscosity of the crude oil in the pipe section are determined, the wax thickness present in the pipe wall can be calculated accurately from the following equation: n
(di − 2δ w )5 − n =
2cρL ⎛ μ ⎞ 4Q 2 − n ⎞ ⎜ ⎟ ⎛ ΔPf ⎝ ρ ⎠ ⎝ π ⎠
(1a)
Where ΔPf is the pressure drop, L is the length of pipe section, d is the hydraulic diameter or updated (effective) inside diameter, δw is the wax thickness, Q is the volumetric flow rate, ρ is the fluid density, Where μ is the apparent viscosity of the crude oil. The values of c = 16, n = 1 for laminar flow and c = 0.046, n = 0.2 for turbulent flow were used. Laminar and turbulent flows exist when the Reynolds number NRe are NRe < 2000 andNRe > 3000 respectively (Chen et al. 1997). The flow 3
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
facility used in our study was calibrated using the c and n values as tuning parameters. For laminar flow c = 0.078 and n = 2.3 and for turbulent flow c = 0.034, n = 4.5 were used as tuning parameters, these were validated from the preliminary experiments performed to calibrate the experimental flow facility. The dimensionless thickness at period i (δti) is defined as:
δti =
δ wi ri
And the updated (effective) pipe radius at period i, (ri) is given as,
ri = ri − 1 − δ wi ri-1 = radius of the pipeline (cm) at time period i-1, ri = updated (effective) pipe radius (cm) at time period i. δwi = wax thickness determined from the experimental flow facility at time period i. The pipe radius is updated at every time the dimensionless thickness was calculated.
Fig. 3. Dimensionless wax thickness profiles at different period for emulsified and demulsified crude oil sample A.
3.2. Effect of dispersed water (globules) The molecular diffusion process of the emulsified oil into wax particle being one of the major factors affecting the wax deposition in cooled pipeline (Benn et al., 1980; Bidmus and Mehrotra, 2009; Brown et al., 1993; Hsu et al., 1994; Lee, 2007) can be described by the following five steps. These are: gelation of the crude oil on the cooled pipe, diffusion of crude oil fraction with carbon number greater critical carbon number towards the crude oil-gel interface, internal diffusion of these crude oil fraction through the gelled oil, precipitation of the diffused crude oil fraction in the gelled oil and finally counter diffusion of oil fraction with carbon number less than the critical carbon number out of gel deposit layer (Singh et al., 2000). The counter diffusion process is refers to as wax deposit aging which predominates during the latter period of wax deposition. Fig. 3 shows the changes in dimensionless wax thickness with time for emulsified and demulsified crude oil. From the figure the flow of emulsified crude (containing dispersed water (globules)) is characterized by periodic increase and decrease in the amount of deposited wax this is due to the relatively soft nature of the deposited wax as a result trapped water globules with the oil in the wax deposit. This is also responsible for the initial high volume of wax deposited during the emulsified crude oil flow. The counter diffusion of emulsified oil fraction out of the gelled deposit and shear removal effect were responsible for the relatively lower wax thickness in the latter period of emulsified crude wax depositional process. This is due to higher percentage of the crude with carbon number less than the critical carbon number in the emulsified oil compared to demulsified oil. Fig. 4 confirms the initial soft nature and relatively higher volume of wax (paraffin) formed by the flow of emulsified crude oil compare to what was observed when a demulsified crude oil was used. For crude oil A, the dimensionless thickness of 0.121 (L/L) was observed with the emulsified crude oil after 21 h of flow, compared to a dimensionless thickness of 0.112 (L/L) using the demulsified crude
3. Results and discussions 3.1. Validation of flow facility wax evaluation method The wax evaluation method used in the experimental flow facility was validated by comparing the amount of wax determined from the flow facility and the wax scrapped from the flow facility after each experiment. Table 2 shows the summary of the physical conditions and the results of the experiments performed on different crude oils using the pour point depressant. The determined volumes of wax deposited from the flow facility and the volumes of wax scrapped from the cooled pipe after each experiment were compared. Defining the Average Absolute Deviation, AAD as: N
AAD =
∑i = 1
(Xir − Xie )
N
Xir
*100
(1b)
Where, Xir = Volume of wax deposit calculated from flow facility readings for experiment run i Xie = Volume scrapped from the flow facility for experiment run i N = Total number of experimental runs There is a noticeable lower value of wax volume scrapped compared to the volume of wax calculated from the flow facility reading for all the experimental runs, this is due to shear removal of some deposited wax every period the flow is stopped prior to the scrapping of the deposited wax. The value of the average absolute deviation (AAD) percentage of 9.1% between the experimentally determined volume and the volume of wax scrapped from the flow loop despite the sheared wax deposit confirmed the effectiveness of the wax deposits evaluation method used in the study. Table 2 Experimental Wax Deposition flow facility results. Exp. No.
Oil Sample
PPD (ppm)
Wax Deposited, (cm3) (Experimental)
Wax scooped from flow rig (cm3)
1 2 3 4 5 6 7 8
A A A A B B B B
0 5 10 30 0 10 16 32
48 32 29 18 29 22 17 10
45 30 27 17 27 20 15 8
Fig. 4. Dimensionless wax thickness profiles at different period for emulsified and demulsified crude oil sample B. 4
Journal of Petroleum Science and Engineering 182 (2019) 106294
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Fig. 5. Viscosity-Temperature profiles at different water percentage at shear rate of 180 s−1.
Fig. 7. Dimensionless wax thickness profiles for Blank emulsified crude oil A at different water cuts.
after the same period of flow. While for crude oil B, a dimensionless thickness of 0.190 (L/L) was observed with the emulsified oil as against a dimensionless thickness of 0.170 (L/L) with the demulsified crude. Hence differences of 8 and 12% respectively for crude oil A and B respectively. The relative higher difference in crude oil B is due to its higher water content. Fig. 5 shows the viscosity profile at different temperature for different percentage of water in the emulsion at a shear rate of 180 s−1, the viscosity of the crudes was observed to increase with the percentage of water in the emulsion but decreases with temperature increases. At a temperature of 26 °C the viscosity increases from about 40 mPas to about 142 mPas, as the percentage of water in the emulsion increases from 10 to 40%, this is more than 300% increment. Fig. 6 shows the plots of pour point temperatures at different water composition (%) for both crude samples. The pour point temperature was observed to increase withe water composition for both crude oils A and B. As the water composition in the emulsion increases from 10 to 40%, the pour point temperature (PPT) increases from 30 to 41 °C for crude oil A, while the PPT changes from 21 to 31 °C for crude oil B. Three parameters determine the amount of wax deposition in the flow of the emulsified oil through the cooled pipeline these are: amount of trapped water globules, the shear rate which determines the shear removal at the deposit layer by the flowing oil along the water-oil interface, and the crudes' diffusion coefficient which determines both the diffusion of crude with higher carbon number into the wax gel deposit and the counter diffusion of lower carbon crude fraction out of the wax gel deposit into the crude oil. The viscosity of the crude increases with the percentage of its water content (Fig. 5), as the viscosity increase is accompanies by increase in shear rate (Fig. 1), this leads to higher shear removal at the crude-wax deposit layer. Simultaneously increase in percentage water content lead to increase in the percentage of trapped water globules in the wax network, leading to increase in volume of deposited wax. Hence the net thickness of the wax deposit depends on which of these two simultaneous mechanisms is dominant.
. Initially, the trapping mechanism of the water globules and molecular diffusion of the emulsified oil into the formed wax crystal gel dominates the wax deposition process judging by the relative increase in the initial wax deposition rate compared to when demulsified oil was used but as the experiment progresses the shear removal effect and the counter diffusion of crude oil fraction with carbon number less than the critical number out of the formed wax crystal gel into the oil phase as a result of higher water content in the emulsion (from 28.6 to 40%) overrides leading to decrease in the rate of wax deposit. These were shown in Figs. 7 and 8 for crude oil A and B respectively. In Figs. 7 and 8, emulsified crude with higher water content initially deposits more wax due to relatively more water globules trapped in the wax gel network during molecular diffusion process. While in the latter period the reverse is the case as more water globule in the wax network results in more water diffusing out of the deposited wax (counter diffusion), this leads to higher viscosity of the flowing crude oil at the crude-wax deposit interface due to higher water content (Fig. 5), and consequently higher shear removal effect (Fig. 1). Figs. 9 and 10 shows the dimensionless wax deposition-time profiles for blank emulsified crude oil, the emulsified crude oil mixed with pour point depressant, PPD (long chain acrylate ester co-polymer) and the blank oil, with both the PPD and the demulsifier for crude oil A and B respectively. 3.3. Effect of pour point depressant (PPD) on wax deposition From the two plots (Figs. 9 and 10), the addition of 45 ppm of long chain acrylate ester co-polymer as pour point depressant succeeded in depressing the dimensionless wax thickness in the emulsified crude oil judging by the reduction in the dimensionless wax thickness from 0.112 (L/L) for the blank emulsified crude oil A to the dimensionless wax thickness of 0.070 (L/L) when pour point depressant (PPD) was added. The pour point depressant successively reduce the pour point temperature of the emulsified crude oil thereby reducing the temperature at
Fig. 8. Dimensionless wax thickness profiles for Blank emulsified crude oil B at different water cut.
Fig. 6. Pour point temperature-percentage water profiles for crude oils A and B. 5
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O.A. Adeyanju and L.O. Oyekunle
where the crude oil flows, making the volume of wax deposit to be relatively lower than the situation where the crude oil occupy the whole cross-section of the pipeline. 4. Conclusion A flow facility that was design and validated to simulate the flow of relatively higher temperature crude oil in subsea pipeline was used to study the effect of emulsified crude oil flow with and without pour point depressant and demulsifier on wax deposition in subsea pipeline. The addition of pour point depressant successfully inhibits the wax deposit for the two crude oil samples used in the study. Addition of demulsifier to the mixture of the emulsified crude oil and the pour point depressant further inhibits the wax deposit for crude oil A, while the inhibition of the wax deposit by the addition of the demulsifier was not as successful for emulsified crude oil B, due to the ineffectiveness of the used demulsifier in separating the dispersed water from the emulsified crude oil B. The viscosity and the pour point temperature of water in crude oil emulsion increase as the amount of water content in the emulsion increases. The increase in viscosity has an antagonistic effect on the wax deposition. This is due to the fact that at constant temperature increase in viscosity is accompanied by higher shear rate (Fig. 1) resulting in the additional shear removal effect of the deposited wax on the wax-crude oil interface. The increase in pour point temperatures result in a synergetic effect on the amount of wax deposition. This is due to the fact that increase in pour point temperature is accompanies by increase in the rate of gelation and deposition of the wax particles in the crude oil flows through the cooled pipeline. Three different mechanisms were identified to determine the rate of wax deposition in the flow of emulsified crude oil in cooled pipeline, these are: Molecular diffusion and counter diffusion of wax molecules in and out of the wax deposit respectively, the trapping of water globules in the deposited wax network and the shearing of the deposited wax due to the shear stress at the wax deposit-crude oil interface. While the trapping of the water globules and the molecular diffusion dominate the wax deposition process at the beginning of the experiment, the shear rate effect and counter diffusion of trapped water/crude fraction with low carbon number override the trapping mechanisms at the later period of the experiment resulting in the relative decrease in dimensionless wax thickness during emulsified oil flow compare to when the oil was de-emulsified before being flow through the flow loop. (Figs. 3, 4, 7 and 8). Thorough de-emulsification of crude oil before their flow through cooled pipeline is a process that needs to be considered for effective wax deposition inhibition in the flow of crude oil in subsea pipelines.
Fig. 9. Dimensionless wax thickness at different time for Blank emulsified crude oil A, oil A with pour point depressant (PPD) and oil A with PPD and demulsifier.
Fig. 10. Dimensionless wax thickness at different time for Blank emulsified crude oil B, oil B with pour point depressant (PPD) and oil B with PPD and demulsifier.
which the emulsified crude oil gel, as gelation of the crude oil precedes the wax deposition, the wax deposition rate is suppressed. Addition of demulsifier to the oil/PPD solution further reduced the dimension wax thickness to 0.046 (L/L). The same result was observed for crude oil B as shown in Fig. 10 the dimensionless wax thickness was reduced from 0.172 (L/L) for the blank emulsified crude oil to 0.112 (L/L) when 45 ppm of pour point depressant was added to the blank emulsified crude oil B and further to 0.100 (L/L) with crude-oil/PPD/demusifier solution.
3.4. Effect of demulsifier on wax deposition Acknowledgements The effect of the used demulsifier to reduce the wax deposition was more successful for crude oil A reducing the wax dimensionless thickness from 0.070 (L/L) when no demulsifier wax used to dimensionless wax thickness of 0.046 (L/L) when 25 ppm of demulsifier was added as against the reduction from the wax dimensionless thickness of 0.112 (L/ L) without the demulsifier to 0.098 (L/L) with the 25 ppm of the demulsifier for crude oil B (Figs. 9 and 10), this is due to the effectiveness of the demulsifier in the de-emulsification of crude oil A compared to crude oil B. Hence, efforts must be made to study the effectiveness of different demulsifiers for each emulsified crude oil before their application on the crude oil for water separation. Addition of the demulsifier to the mixture of the emulsified oil and the pour point depressant makes the emulsified crude oil to be separated to its separate compositions of crude oil and water thereby making the waxy stratified liquid flow to characterized the fluid flow in the pipeline, where the oil will flow in a wavy-like flow on a separated water phase at the bottom part of the horizontal pipe. This makes the wax deposition to form only on the upper part of the horizontal pipeline
The authors thank the Central Research Committee (CRC) of the University of Lagos, Nigeria for providing the funds (CRC No: M2014/ 01) for the conduct of this research. Nomenclature d L N Q ri-1 ri c, n Xir Xie ΔPf 6
hydraulic diameter or updated (effective) inside diameter length of pipe section Total number of experimental runs volumetric flow rate radius of the pipeline (cm) at time period i-1 updated (effective) pipe radius (cm) at time period i tuning parameters Volume of wax deposit calculated from flow facility readings for experiment run i Volume scrapped from the flow facility for experiment run i Pressure drop
Journal of Petroleum Science and Engineering 182 (2019) 106294
O.A. Adeyanju and L.O. Oyekunle
δw δwi ρ μ Subscripts F i= i-1 ie ir w wi SI metric
wax thickness wax thickness determined from the experimental flow facility at time period i fluid density apparent viscosity of the crude oil
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fluid period period before i wax scrapped from flow facility wax read from flow facility wax initial wax conversion factors
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