Accepted Manuscript The effect of pressure on wax deposition from wax-solvent mixtures with natural gas Qing Quan, Wen Ran, Lu Yang, Ge Gao, Shouxi Wang, Jing Gong PII:
S0920-4105(18)30709-5
DOI:
10.1016/j.petrol.2018.08.040
Reference:
PETROL 5220
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 23 April 2018 Revised Date:
7 August 2018
Accepted Date: 15 August 2018
Please cite this article as: Quan, Q., Ran, W., Yang, L., Gao, G., Wang, S., Gong, J., The effect of pressure on wax deposition from wax-solvent mixtures with natural gas, Journal of Petroleum Science and Engineering (2018), doi: 10.1016/j.petrol.2018.08.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The Effect of Pressure on Wax Deposition from Wax-Solvent Mixtures with
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Natural Gas Qing Quana,*, Wen Ranb, Lu Yangc, Ge Gaod, Shouxi Wanga, Jing Gongd.
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College of Petroleum Engineering, Xi’an Shiyou University, Xi’an Shannxi, 710065, China
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Shaanxi Application of Physical & Chemistry Institute, Xi'an, Shaanxi Province, 710065,
China. c d
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CNOOC Petrochemical Engineering Co., Ltd, Jinan Shandong, 250000, China.
.Beijing Key Laboratory of Urban Oil & Gas Distribution Technology, China University of
Petroleum, Beijing, 102249, China.
Abstract
Wax deposition occurs when the temperature of the pipe wall falls below the wax
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appearance temperature. This deposition reduces the effective flow area of the
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pipelines, leading to a significant pressure drop, which increases the transportation
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consumption and leads eventually to complete blockage. Most wax deposition studies
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have neglected the influence of natural gas, but natural gas is present in the actual
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pipelines and must be accounted for in field operations.
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A self-developed cold-finger apparatus was used to investigate the effects of pressure on wax deposition with natural gas (with 89% methane). The wax solvent
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mixtures consisted of a wax (C22–C37) dissolved in a paraffinic solvent (C11–C15). The
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wax deposition experiments were performed with 5, 7, and 10 mass percent wax
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solutions, for pressures ranging from 1–6 MPa. The experimental results indicated
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that the maximum amount of wax is deposited at ordinary pressure. The amount of
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wax deposited increased at pressures ranging from 1 MPa to 2 MPa and decreased
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thereafter (at pressures ranging from 2 MPa to 6 MPa). In addition, the pressure
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corresponding to the maximum wax deposition varied with the wax content.
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Furthermore, the wax crystal morphology was observed using a polarizing microscope,
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and the morphology was quantitatively described via the fractal dimension. The
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analysis showed that the fractal dimension decreased with increasing pressure.
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This study confirms that natural gas plays a vital role in wax deposition studies,
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especially those aimed at predicting the wax deposition in actual pipelines, using a
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wax deposition model based on the laboratory experiments. *corresponding author,
[email protected]
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Key Words: Wax deposition; pressure; natural gas; cold-finger 1. Introduction During pipeline transportation of waxy crude oil, the waxes will crystallize and
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aggregate to precipitates on the wall of the pipe, when the oil temperature is lower
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than the wax appearance temperature (WAT). The problems resulting from wax
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precipitation, such as the reduction in the effective flow area, lead to an increase in the
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pressure loss and decrease in the pipeline transmission capacity and, in turn, economic
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losses. Owing to the increased difficulty of onshore oil exploration in recent years,
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offshore oil development is gradually becoming a global trend. Currently, marine
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petroleum production accounts for ∼30% of the total global production. The low
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undersea environment temperature leads to severe wax precipitation and
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sedimentation in the submarine oil pipeline. Therefore, the marine petroleum
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pipelines and equipment are exposed to more severe conditions than those associated
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with onshore oil exploration. In addition, owing to the harsh environment, offshore oil
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field development costs are high and accidents are difficult to handle. Therefore, the
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paraffin deposit problem of submarine pipeline systems must be resolved.
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The composition of crude produced from a subsea well is highly complex. This
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crude consists of oil, gas, water, and other impurities under high pressure. Oil-gas
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multiphase flow transportation technology is widely used in the submarine pipeline
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employed in the development of offshore oil fields. However, submarine
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transportation technology suffers from the drawback of multiphase flow wax
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deposition. This mode of deposition has been considered in a few studies, but studies
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focused on oil-gas wax deposition under high pressure are rare.
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Jennings and Weispfennig (2005) studied the influence of the shear and
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temperature on wax deposition by using a cold finger. Tinsley and Prud'Homme (2010)
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used a visual cold plate to perform a single-phase wax deposition experiment, where
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the influence of temperature, wax content, and shear on wax deposition was evaluated.
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The experimental results showed that, compared with low shear speed, high shear
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speed yields smaller wax crystals, and the addition of polymer can change the wax
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temperature difference of wax deposition. According to this concept, wax deposition
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occurs only when the difference between the oil temperature and the ambient
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temperature is greater than the critical temperature difference. Single phase wax
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deposition has been extensively investigated, but (to date) studies of multi-phase wax
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deposition are few. Employing oil and water cold-plate experiments, Cole and Jessen
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(1960) determined the influence of different material wetting characteristics on wax
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deposition. Couto et al. (2006) used a cold finger to investigate the law of wax
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deposition in an oil-water two-phase system. Zhang et al. (2010) used the cold finger
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technique to assess the influence of cold-finger temperature, water content,
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dispersion-phase size and distribution as well as other factors on wax deposition of a
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W/O emulsion. Zougari (2010) improved the traditional cold finger experimental
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apparatus and performed wax deposition experiments under high pressure and high
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shear. Stock tank oil and live crude were used in the experiments. The experimental
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results revealed that the wax deposition rate decreases with increasing shear stress.
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Under the same experimental conditions, the wax deposition rate of live crude is
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lower than that of stock tank oil. Vieira et al. (2009) determined, via high-pressure
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differential scanning calorimetry, the influence of pressure on wax crystal
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precipitation. The results revealed that the WAT value and the enthalpy of
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crystallization both decrease with the use of natural gas pressurization, but the WAT
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value increases during nitrogen pressurization. Methane had the greatest influence on
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the low molecular weight hydrocarbon chain. The fractions heavier than C3,
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contained in the gas mixture, were more efficient at solubilizing the wax with lower
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molar masses than those with higher molar masses. High-pressure differential
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scanning calorimetry was also used (Juyal et al., 2011) to investigate wax
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crystallization precipitation. The experimental results obtained were similar to those
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reported by Vieira et al. (2009), where the WAT value decreased during light
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hydrocarbon gas pressurization. Daridon et al. (2002) observed, by means of a
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simulated oil (with different components) exposed to pressures ranging from
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atmospheric pressure to 100 MPa. Li and Gong (2011) used a polarizing microscope
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to monitor the changes in WAT and WDT of waxy crude under high pressure. The
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results revealed that, for the given simulated oil, both values increased with increasing
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pressure. The WAT and WDT values also varied with the simulated oil components,
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indicating that these values depend on the pressure, composition, and carbon number
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distribution.
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In this work, the influence of pressure, temperature, shear rate, and water content
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on wax deposition are systematically investigated by using a self-developed
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experimental device. These experiments are performed under high pressure with
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mineral spirit and paraffin as the experimental medium. The cold-finger experimental
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results indicate that the largest amount of wax is deposited under ordinary pressure.
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The amount of wax deposition increases initially (when the pressure increases from 1
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MPa to 2 MPa) and then decreases thereafter. The pressure corresponding to the
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maximum wax deposition varies with the wax content. Furthermore, the wax crystal
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morphology is observed by using a polarizing microscope, and this morphology is
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quantitatively described via the fractal dimension. The analysis shows that the fractal
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dimension decreases with increasing pressure.
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2. Experimental section
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2.1 Waxy oil sample
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Solvent oil D80, colorless with low aromatic content and mild odor, it has low
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viscosity, high flash point and non-detectable BTEX content, (see Table 1 and Fig. 1
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for corresponding characteristics and carbon number distribution, respectively), was
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used in the experiments involving de-aromatization and desulfurization.
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The paraffin used in the experiments is a multi-component sliced wax (see Fig. 2
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for High Temperature Gas Chromatography (HTGC) analysis results). As the Fig. 2
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shows, the wax consisted mainly of components lying between C21 and C38. Natural
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The main aim of the experiments is to determine the influence of pressure on
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wax deposition, and experimental parameters such as the pressure, temperature
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difference, wax content, and shear rate were considered. In these experiments, for a
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given wax content of 7%, the pressure was increased from 0 to 6 MPa in 1 MPa
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intervals.
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Values of 15, 18, and 20°C were selected for the temperature difference between
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the oil and the cold-finger. For comparison of the results, the oil and cold-finger
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temperatures associated with different wax contents were set to 6°C higher than the
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WAT value. The WAT of the oil with different wax content is plotted in Fig. 3.
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According to the Fig. 3, the oil experimental temperature is 35°C.
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2.2 Experimental apparatus
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The apparatus used in the laboratory experiments was self-designed in-house.
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The device consisted of a gas pressurization system, temperature control system,
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autoclave, control system, and observation window, as shown in Fig. 4. The gas
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pressurization system, including a gas cylinder, reducing valves, and a gas booster
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pump, is mainly used to increase the pressure of the experimental medium. The
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temperature control system includes a cooling bath and a heating bath. The cooling
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bath controls the temperature of the cold finger through water circulation, and the
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heating bath is used to control the temperature of the oil in the autoclave. The main
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part of this experimental device, i.e., an autoclave, is a stainless steel tank equipped
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with an anchor impeller and cold-finger. The anchor impeller is equipped with a
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transmission that rotates with speeds ranging from 60~150 r/min by turning the knob
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on the control panel. This impeller can shear the oil and stir the oil-water emulsion in
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the autoclave. During the experiment, two identical cold-finger provide the attached
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surface for the sediment. The fingers can be lifted from the autoclave for scraping of
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the sediment to be weighed. The temperature and pressure sensors are installed in the
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autoclave, and the measured values are shown on the control panel. The experimental
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autoclave by a pipeline.
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2.3 Experimental steps and method
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Prior to the start of the experiments, the device was subjected to air tight tests (leak tests). After placing the oil in the autoclave, the experimental medium was heat
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treated for 2 h at 50°C in a heating bath, thereby completely dissolving the wax
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crystal and relieving the shear history of the oil. The cooling bath (set to 35°C) was
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then started and the heating bath temperature was also set to 35°C. When the oil temperature shown on the control panel reached a set value, the stirring paddle was started and modulated to the required speed (in a static experiment, the stirring paddle is unnecessary). The inlet valve was then opened to increase the pressure after closing the decompression valve and confirm that the bolts on the lid of the autoclave have been tightened. The inlet valve was closed after the pressure in the autoclave reached the pre-set value. After 1 h of standing, the cooling bath should be modulated to the experimental temperature. This will generate a temperature difference between the oil and the cold finger. After two hours of standing, the cold finger reached the set value. Using the decompression valve, the gas in the autoclave should then be deflated prior to the end of the experiments. The sediments covering the cold-finger were gathered and weighed. Samples of these sediments were then analyzed by HTGC.
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Owing to the height difference in the deposition covering the cold-finger, the
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results were compared in terms of the deposition weight per unit area, w, which is
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given as follows:
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M- Wax deposition weight; A- Surface area of wax deposition. 3. Results and Discussion
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3.1 Effect of pressure on wax deposition
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Cold-finger experiments of oil (wax content: 7%) are conducted under the
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following conditions: pressure: 1 MPa–6 MPa (increased in intervals of 1 MPa),
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temperature difference: 15°C, 18°C, and 20°C. Each experiment gets average value on
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two consecutive experiments within 0.01g/cm2 error. As Fig. 5 shows, the amount of
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deposit associated with each temperature difference increases when the pressure is
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increased from 1 MPa to 2 MPa and decreases thereafter. The maximum amount of
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deposit occurs at a pressure of 2 MPa. When natural gas is used for pressurization, the
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amount of deposit formed is less than that formed at atmospheric pressure, owing to
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the low hydrocarbon solubility of waxes. With oil wrapped in the loose structure, the
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sediment under pressure is soft. Only a small amount of gas escapes from the
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sediment immersed in the oil. A pressure of 2 MPa yields the maximum wax deposition (see above).
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The aforementioned experimental results indicate the following: (1) when the
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pressure is increased, the WAT of the oil and, in turn, the amount of wax precipitation
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decrease, due to increased solubility of the waxes. At low to moderate pressures, this
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mechanism has only a modest effect on the oil near the cold-finger. The wax
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concentration gradient and the amount of wax sediment are therefore larger than those
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occurring at atmospheric pressure. When the pressure is increased, the solubility of
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the wax plays an increasingly important role in the wax precipitation of oil near the
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cold-finger, leading to a decrease in the amount of sediments. (2) The oil wrapped in
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the sediments plays the key role in the amount of sediments obtained. (3) The curve
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describing wax precipitation (i.e., the precipitation curve of wax), which consists of
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many components with different precipitation temperature ranges, is influenced by the
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pressure. For example, Derakhshan and Shariati (2012) reported that the WAT decreases
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initially and then increases with methane pressurization, as indicated by changes in
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the slope (see Fig. 6). When the pressure is low, the curves of wax precipitation may
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cross each other, consistent with an increase in the amount of wax deposition. The
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curve shifts to the left when the pressure is continuously increased and the amount of
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deposition decreases gradually.
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The pressure dependence of the deposit amount is further investigated through a
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series of follow-up experiments. A set of experiments is performed under the
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following conditions: shear speed: 40 r/min, pressure: 1 MPa–6 MPa, temperature
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difference: 20–35°C. The results obtained (see Fig. 7) are similar to those of prior
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experiments (performed without shear). As shown in the Fig. 7, the amount of deposit
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increases initially with increasing pressure and then decreases (as observed in
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and subsequent decrease result from oil wrapped in the sediments, the deposit will be
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hard, since the amount of oil decreases with shearing. The trend describing the
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pressure dependence should therefore differ from that shown in Fig. 7, if the oil
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influences the amount of deposit formed. Nevertheless, the same trend is observed in
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the present and previous experiments, suggesting that the oil has negligible effect on
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the deposition.
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Considering the complex compositions of the D80 oil and mixed wax added to
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the oil, n-tetradecane (C14) and n-tetracosane (C24) are used as the solvent and solute,
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respectively, in the experiments. The resulting mixture contains 7% of the
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n-tetracosane and has a WAT of 20°C. Furthermore, the cold-finger and oil
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temperatures are set to 5°C and 25°C, respectively. The results obtained for pressures
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ranging from 1 MPa to 6 MPa are shown in Fig. 8.
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The amount of sediment generated in the single crystal wax experiments
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decreases gradually with increasing pressure. At a pressure of 3 MPa, the amount is
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extremely low, with only a thin layer (resulting from natural gas dissolving some of
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the wax) enveloping the cold-finger. The amount of deposit decreases sharply with
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increasing pressure of up to 2 MPa and changes gradually or slightly thereafter (see
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Fig. 8), owing to negligible sediment deposition at high pressure. A comparison of the
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results obtained from mixed wax and single crystal wax experiments indicates that the
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amount of sediment increases initially and then decreases, owing to changes in the
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precipitation of the mixed waxes.
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3.2 Contents of oil on deposition under pressure
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The influence of pressure on the deposition is further investigated via cold-finger
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experiments, where the pressure is varied for fixed wax contents of 5% and 10% oils.
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Each group of experiments is performed twice to ensure the veracity of the results. Fig.
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9 shows the results obtained for the following experimental conditions: temperature
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difference: 20°C, pressure: 1-6 MPa.
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(when the pressure is increased from 1 MPa to 2 MPa) and decreases thereafter (from
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2 MPa to 6 MPa; see Fig. 9), with the maximum occurring at atmospheric pressure.
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The maximum amount deposited for these wax contents is realized at pressure values
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of 3 MPa and 1 MPa.
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As shown in Fig. 9, the deposition amount exhibits the same pressure
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dependence (regardless of the wax content), but the pressure corresponding to the
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maximum amount deposited varies with the wax content. The precipitation curves
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(see Fig. 10), and the effect of natural gas on wax-crystal precipitation vary with the
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wax content. The natural gas required to overcome the heat flow associated with point
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A shown in Fig. 3, and the pressure corresponding to the peak value of deposition
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both increase with increasing wax content. These results indicate that the pressure
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corresponding to the maximum deposition increases with increasing wax content.
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For a rough surface, the sediments under pressure are soft and loose-structured.
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Significant amounts of gas escape, when the sediments are placed in oil, and wrapped
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oil flows out during sampling. Under such conditions, for an acicular or overlapped
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sheet structure, the wax crystal becomes loose with increasing pressure, as shown in
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Fig. 11 (for wax content of 7% at △T=15°C). However, quantification of the changes
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in the wax-crystal form and structure via these photos is impossible. Therefore, the
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fractal dimension is introduced as a parameter for quantifying the characteristics of
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the wax crystal.
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The laws describing the dependence of the wax-crystal fractal dimension on the
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pressure were analyzed via the box-counting method, which yields three wax contents,
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as shown in Table 3. The accuracy of the results is ensured by averaging the fractal
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dimension obtained from six images of the wax crystals.
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As Table 3 shows, the fractal dimension decreases with increasing pressure and
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reaches a maximum at ordinary pressure. Moreover, for pressures of 1 MPa, 2 MPa,
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and 3 MPa, the dimension increases slightly with increasing wax content (5%, 7%,
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and 10%) of the oil. The maximum dimension occurs at a pressure and wax content of
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1 MPa and 10%, respectively, which represent the optimal values for dissolving gas in
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oil with different wax contents. The fractal dimension decreases, owing to the destruction induced by the escape
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of gas molecules from the sediments. In addition to the influence of physical
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chemistry conditions, the wax crystal morphology is also associated with the wax
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molecule structure, as seen in Fig. 12. The n-alkane molecules are easily transformed,
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leading to relatively easy arrangement and accumulation of these molecules during the
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process of crystallization, resulting in a highly stable crystal structure. In contrast, the
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crystallization (i.e., transformation) of i-alkanes yields a defective and unstable
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structure, which is influenced by the branched chains. Wax crystals, formed by
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cycloalkanes with molecules too voluminous to transform, exhibit the worst stability.
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For a low number of n-alkanes, the number of chains branching from the wax
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molecules increases with increasing carbon number. Thus, crystals formed by wax
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with a smaller carbon number are relatively stable (compared with those formed by
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wax with a larger carbon number). The study by Vieira et al. (2009) illustrates that
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natural gas exerts a greater influence on low molar mass hydrocarbons than those with
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high molecular weight. When the pressure of natural gas is increased, gas molecules
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of light hydrocarbons influence the crystal separation of wax with a small carbon
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number, and further influence the stability of deposition.
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Moreover, some oil is wrapped in the structure of the waxy crystal. During
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pressure release and sampling, gas dissolved in the wrapped oil escapes, thereby
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damaging the wax-crystal structure. The damaged structure may become loose,
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leading to a reduction in both the non-linearity degree and the fractal dimension.
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When the pressure is increased, the amount of gas dissolved in the oil wrapped in the
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wax-crystal structure, and the severity of escaping-gas-induced damage to the
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structure (when exhausting for release pressure) both increase. This leads to a
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decrease in the fractal dimension.
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Conclusions The main aim of this work is to determine the effect of pressure on deposition.
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This effect is revealed by considering, from both the macroscopic and microscopic
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points of view, the influence of pressure on the amount of deposit and the wax crystal
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morphology.
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These experiments are performed on a cold-finger device. The results indicate
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that the amount of deposit increases when the pressure is increased from 1 MPa to 2
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MPa and decreases thereafter (for pressures ranging from 2 MPa to 6 MPa). The
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pressure corresponding to the maximum deposition increases with increasing wax
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content. For example, wax contents of 5%, 7%, and 10% correspond to pressures of 1
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MPa, 2 MPa, and 3 MPa, respectively. The largest amount of sediment is obtained
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under atmospheric pressure, owing to the low hydrocarbon solubility of the wax
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molecules. The precipitation curves of mixed wax are influenced by the pressure, and
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the rate of wax separation around the cold-finger, leading to deposition that decreases
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with increasing pressure. Furthermore, the wax precipitation curves of oil with
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different wax contents reveal that the separation and crystallization of wax are
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influenced by increasing pressure of the natural gas. The number of gas molecules
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required for changing the rate of wax separation increases with increasing wax
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content. Hence, the pressure corresponding to the maximum deposition increases with
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increasing wax content.
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The influence of pressure on the separation of wax molecules is investigated by
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observing wax crystals with polarizing microscopes and quantitatively characterizing
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this influence through a fractal box-counting dimension method. This method is
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applicable as the proportion of wax molecules with large carbon number increases
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during the aforementioned separation. The complexity of the branch structure
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increases with increasing carbon number. Furthermore, the stability of the wax-crystal
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structure decreases (and likelihood of structural collapse increases) with decreasing
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fraction of n-alkanes. Gas molecules are dissolved in the oil wrapped in this structure
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the oil, and the severity of escaping-gas-induced damage to the structure (when
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exhausting to release pressure) increase. This leads to a decrease in the fractal
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dimension.
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Acknowledgment
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The authors thank the financial support from the Program for Youth to Innovate
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on Science and Technology in Xi’an Shiyou University, the National Natural Science
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Foundation of China (51704236).
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References
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Jennings, D. W., Weispfennig, K., 2005. Effects of shear and temperature on wax deposition: coldfinger investigation with a Gulf of Mexico crude oil. Energy & fuels. 19(4), 1376-1386. Tinsley, J. F., Prud'Homme, R .K., 2010. Deposition apparatus to study the effects of polymers and asphaltenes upon wax deposition. Journal of Petroleum Science and Engineering. 72(1), 166-174. Santos, J. D. S. T. D., Fernandes ,A. C., Giulietti, M., 2004. Study of the paraffin deposit formation using the cold finger methodology for Brazilian crude oils. Journal of Petroleum Science & Engineering. 45(1), 47–60. Cole, R. J., Jessen, F. W., 1960. Paraffin deposition. Oil and Gas Journal. 58, 87-99. Couto, G. H., Chen, H., Delle-case, E., Sarica, C., Volk, M., 2006. An investigation of two-phase oil/water paraffin deposition. Offshore Technology Conference, Houston, Texas, USA. Zhang Y., Gong J., Wu H., 2010. An Experimental Study on Wax Deposition of Water in Waxy Crude Oil Emulsions. Liquid Fuels Technology. 28(16), 1653-1664. Zougari, M. I., 2010. Shear driven crude oil wax deposition evaluation. Journal of Petroleum Science & Engineering. 70(1-2), 28-34. Vieira, L. C., Buchuid, M. B., Lucas, E. F., 2009. Effect of pressure on the crystallization of crude oil waxes. II. evaluation of crude oils and condensate. Energy & Fuels. 24(4), 2213-2220. Juyal, P., Cao, T., Yen, A., Venkatesan, R., 2011. Study of live oil wax precipitation with high-pressure micro-differential scanning calorimetry. Energy & Fuels. 25(2), 568-572. Daridon, J. L., Pauly, J., Milhet, M., 2002. High pressure solid–liquid phase equilibria in synthetic waxes. Physical Chemistry Chemical Physics. 18(18), 4458-4461. Li, H., Gong, J., 2011. The pressure effect on rheological behavior of water-content waxy crude oil. Petroleum Science and Technology. 29(13), 1344-1352.
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Derakhshan, T., Shariati, A., 2012. Prediction of wax precipitation temperature in petroleum
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reservoirs upon methane injection. Journal of Petroleum Science and Engineering. 98, 1-10.
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Fig. 1. Carbon number distribution of D80
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Fig. 2. Carbon number distribution of sliced wax
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Fig. 3. DSC curves associated with a wax content of 7%
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Fig. 4. Diagram of experimental apparatus
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Fig. 5. Wax deposition of different pressure with different temperature
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Fig. 6. Prediction DSC curve under pressure condition
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Fig. 7. Changes in deposit mass with pressure under shear conditions (∆T=15°C, 40
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r/min, wax content=7%)
Fig. 8. Variation of single crystal wax deposition with pressure (∆T=20°C, C24, wax content=7%)
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Fig. 9. Change of Wax deposition with pressure at different wax contents oils
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Fig. 10. DSC curves obtained at different wax contents
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Fig. 11. Microscope images of wax crystals at different pressure
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Fig. 12. Paraffin crystallization model
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Table captions
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Table 1 Main physical properties of D80
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Table 2 Composition of natural gas
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Table 3 Fractal dimension of wax crystal under different pressures
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ACCEPTED MANUSCRIPT Highlights: 1. The influence of pressure, temperature, shear rate, and water content on wax deposition is systematically investigated using a self-developed experimental device. The experiments are performed under high pressure with mineral spirit and paraffin as
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3. The maximum deposition associated with a given pressure condition increases
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4. When the pressure is increased, the amount of gas dissolved in the oil wrapped in the wax crystal structure, and the severity of escaping-gas-induced damage to the structure (when exhausting to release pressure) increase. This leads to a decrease in
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