Estimation of gas void formation in statically cooled waxy crude oil using online capacitance measurement

International Journal of Multiphase Flow 75 (2015) 257–266

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International Journal of Multiphase Flow j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j m u l fl o w

Estimation of gas void formation in statically cooled waxy crude oil using online capacitance measurement Areeba Shafquet a,⇑, Idris Ismail a, Azuraien Japper-Jaafar b, Shaharin A. Sulaiman b, Girma T. Chala b a b

Electrical and Electronic Engineering Department, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia Mechanical Engineering Department, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia

a r t i c l e

i n f o

Article history: Received 1 December 2014 Received in revised form 23 April 2015 Accepted 18 June 2015 Available online 27 June 2015 Keywords: Dual plane Gas voids Normalized capacitance Static cooling Waxy crude oil

a b s t r a c t Waxy crude oil is one of the massively produced petroleum reserves in the oil and gas industry. It possesses two different natures at different conditions. At a temperature above its wax appearance temperature it functions as a Newtonian fluid. Moreover, when it flows in a subsea environment where the temperature drops, the occurrence of non-Newtonian fluid would appear. However, it is mandatory to prevent the pipelines from building up with the solid wax layer, or else a strong paraffinic gel structure would plug the flow of the waxy crude oil. This gel accumulation results in the emergence of thermal shrinkage where gas voids consequently occur. A Malay Basin waxy crude oil has used for this study to investigate the gas void formation in statically cooled waxy crude oil using an online Electrical Capacitance Tomography (ECT) measurement. This paper has been focused on the development of a non-invasive, non-intrusive approach for the visualization of waxy crude oil by applying the ECT system. The online inspection of the behavior of waxy crude oil is important so that the appropriate action can be taken during the gas void formation within a gelled crude oil. An experimental study was conducted on a 1.18 in. circular flow loop rig at distinct temperatures to assess the behavior and formation of gas voids formed due to thermal shrinkage. A capacitance sensor with dual planes consisting of eight electrodes per plane was installed on the rig to capture the images and measure the raw capacitance. Gas voids within a range of 8–14% were observed within the gelled crude oil based on different cooling temperatures. It has been found from the analysis that the gas voids follow an increasing trend with a decrease in temperature and cooling rate. Ó 2015 Elsevier Ltd. All rights reserved.

Introduction In the oil and gas industry, waxy crude oil represents about 20% of the petroleum reserves. It is often explored in deep water where the temperature of ambient sea water is generally low. In these circumstances, the waxy crude oil undergoes major problems, such as an obstruction in a pipeline because of the precipitation of paraffin waxes and deposition of wax components on cold surfaces that leads to the formation of a waxy-oil gel (Lee, 2008; Dimitriou et al., 2011). There are two foremost situations that greatly affect the behavior of the waxy crude oil is: first, when at reservoir conditions with higher temperature and pressure, the paraffin’s solubility is quite high such that the wax molecules become completely dissolved and the crude oil acts as a Newtonian fluid. However, second, as it flows in seabed pipelines having colder ⇑ Corresponding author. Tel.: +60 13 609 1899. E-mail addresses: (A. Shafquet).

[email protected],

[email protected]

http://dx.doi.org/10.1016/j.ijmultiphaseflow.2015.06.005 0301-9322/Ó 2015 Elsevier Ltd. All rights reserved.

surfaces (at a temperature of 5 °C), the temperature would drop below its wax appearance temperature (WAT) and the wax will crystallize and deposit on the pipe wall. This temperature drop could accompany gelling and cause significant non-Newtonian behavior (Ekweribe, 2008; Aiyejina et al., 2011). In certain conditions, the pipeline carrying crude oil flows are confronted with maintenance or emergency shut downs that may result in crude oil in being in a static condition. Under this situation, the temperature within the pipeline deteriorates considerably as in subsea installations or in arctic regions (Davidson et al., 2004). Below the WAT, the paraffin crystals instigate out of the crude oil and create a strong paraffinic gel that can cause an entire pipeline to be plugged and confine the flow (Venkatesan et al., 2005). This paraffin component deposits more on the internal wall of the pipeline and gradually becomes thicker and harder with time as the temperature declines further. This action would diminish the flow area and completely block the pipeline, which would result in considerable economic loss for the pipeline operator (Ekweribe et al., 2008). Therefore, it is vital to know the behavior

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of waxy crude oil so that an appropriate corrective action can be taken to remediate the gelled waxy layer before the pipeline is entirely plugged and the flow of the waxy crude oil cannot be restarted (Frigaard et al., 2007). Once this kind of situation occurs, then the restarting of the gelled crude oil is a major concern. Therefore, several remediation techniques were adopted to restart the flow of crude oil without any difficulty. As the wax gelation may occur simultaneously with wax precipitation and deposition, the main goal is to maintain the pipeline temperature to be above the WAT and the pour point (PP) temperature in order to minimize the accumulation of wax deposits. The most commonly used conventional method for the removal of a wax deposit is mechanical pigging (Ruiz, 2010). The conventional methods used to predict the volume fraction of the gelled waxy oil consist of invasive techniques which are not viable for applications in the real industry. As those methods are not capable of providing a real time visualization of the condition of the waxy crude oil within the pipelines which are located in deep sea areas. At times, the crude oil undergoes a prolonged period of shut down that makes the waxy crude oil harder. Due to this cooling process, the gel endures thermal shrinkage (change in the pipe volume with cooling) and forms gas void in a waxy-oil gel affecting the pipe volume and making the crude oil compressible and multiphase fluid. When this condition occurs inside a pipeline, the shrunken waxy crude oil produces an empty space or void. A study on the thermal shrinkage by Phillips et al. (2011) came out with a result which proved that there were gas voids produced by the cooling process of the crude oil in the flow line (Phillips et al., 2011). The gas voids’ appearance may affect the compressibility of the gelled crude oil since there are spaces for the gelled crude to move after some amount of pressure is applied. Margarone et al. (2010) also confirmed this statement by stating that gelled crude behaves as an incompressible high viscous fluid (Margarone et al., 2010). Vinay et al. (2009) observed that gas voids during shut down conditions occupied the total pipe volume in the range of 4–8% (Vinay et al., 2009). According to Vinay et al. (2007), the cooling ratio and flow rate significantly affected the position and volume of the gas voids available in the gelled crude oil (Vinay et al., 2007). Hénaut et al. (1999) tested the thermal shrinkage of waxy crude oil by using X-ray scanning. From the experimental results, it was found that the change in the volume of the gelled waxy oil would result in the formation of void spaces, which can be varied in various shapes and sizes depending on the cooling rate and temperature (Hénaut et al., 1999). It is, therefore, necessary to have a comprehensive understanding on the behavior and structure of a gel formed under static cooling conditions, including the thermal shrinkage and gas void analysis. The measurement of gas void formation due to shrinkage has a significant importance in the process industry for sustainable operations as it helps in restarting the process of a gelled crude pipeline. Any kind of errors in measurement can cause serious industrial accidents or economic loss. Hence, it is considered as a crucial parameter for pipeline designing systems in process applications. If the design is not up to the mark and causes any accident, a large amount of expenditure is required for the replacement, installation and production of the pipelines. Thus, it is required to measure and compute the gas void formation in statically cooled waxy crude oil accurately for safety, efficiency and quality assurance in process applications (Huang et al., 2003). The conventional method limitations for restarting the flow could be overcome and provide more consistent measurements by using the following non-intrusive measuring techniques, such as Magnetic Resonance Imaging (MRI), X-ray tomography, ultrasonic, and optical methods (Dong et al., 2003). These imaging techniques are considered as reliable and promising for multiphase flow measurements and possess valuable advantages. However,

due to some restrictions (e.g. expense, hazards, and offline system) they are not favorable for online applications in a real industry. In addition to this, online monitoring of wax deposition in subsea pipelines has also been observed by applying the heat pulse technique which reveals substantial advantages over conventional techniques (Hoffmann et al., 2011). Hence, this study aims to develop a low cost, non-invasive, non-intrusive laboratory scale online method to measure the formation and behavior of gas voids in a statically cooled waxy crude oil by using the Electrical Capacitance Tomography (ECT) system. In the oil industry, it is important to take an online measurement of gas-oil flows. Thus, ECT is productive in measuring the multiphase flow elements directly without splitting them into singular elements. The most significant advantage of ECT over other techniques (as mentioned earlier) is that it can produce cross-sectional distribution which can help in investigating more influential parameters of the multiphase flow (Liu et al., 2005). In the current manuscript, the estimation of gas voids within gelled crude oil as a result of thermal shrinkage that appears during static cooling is investigated by applying an online ECT measurement. The installation of a dual-plane ECT sensor on a flow rig computes the distribution and volume of the gas voids. The visualization techniques and raw data analysis are used to quantify the gas voids’ volume fraction at the measured plane. Besides that, the accuracy of the gas void measurements can be further improved by combining both the image reconstruction and image thresholding onto the measured signal.

Overview of ECT sensor & system ECT is a well-established imaging technique that can provide permittivity distribution in a cross-section of images in real time (Yang, 2001). It is based on measuring a capacitance from the boundary of a cross-section and then reconstructing tomogram images, using a suitable algorithm (Chen et al., 2010). The capacitance measurements are taken from a multi-electrode ECT sensor surrounded by a vessel or pipeline. It can provide a cross-sectional image and can calculate important parameters, such as gas holdup, void fraction and others. The ECT system is considered as the best among the available tomographic techniques because of its exorbitant speed ability (Ahmed and Ismail, 2008). It is evaluated as a ‘‘soft field’’ technique and, therefore, requires intricate image reconstruction due to the non-linear relationship between the capacitance and the permittivity distribution. The commonly used image reconstruction algorithm for a non-iterative technique is the Linear Back Projection (LBP) algorithm (Ortiz-Aleman et al., 2004). Although its reconstruction accuracy is not very good, it has the advantage of being simple, fast and essentially a qualitative image reconstruction procedure. It is applicable for dynamic processes like multiphase flow and extensively used for online image reconstruction. An ECT system generally consists of three main units: a capacitance sensor, data acquisition system and a computer. Fig. 1 shows a schematic of an ECT sensor. The electrodes were fixed on an external wall of an acrylic pipe having a 30 mm inner diameter (ID), 35 mm outer diameter (OD) and a length of 1200 mm. It was comprised of two adjacent planes having 8 electrodes in each plane of 67 mm length and 10 mm width. The electrodes were fabricated using a flexible copper-coated plastic laminate having an 18 lm copper foil thickness. The distance between the two planes of ECT sensor is an optimum calculated distance based on the precedent researcher recommendations i.e., according to Yang (2010). The measuring electrodes were shielded by a copper sheet all around the tube, above and below the electrodes, in an

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259

Experimental setup A summary of the experimental method and materials is provided in the following sub-sections. Material A waxy crude oil sample from the Malay Basin was evaluated in this study. The physical properties of the waxy crude oil are as follows: (a) wax content in this particular type of crude oil was 18 wt%, (b) wax appearance temperature (WAT) was approx. 38.5 °C, (c) pour point (PP) temperature was around 36 °C, and (d) density of the crude oil was approx. 850 kg/m3. The properties of the oil indicated that it was very heavy/black oil that possessed a high viscosity. Apparatus

Fig. 1. Schematic of dual plane ECT sensor.

axial direction of 10 mm and a grounded aluminum screen was used to prevent it from experiencing external interference and noise. The sensor was provided with several insulating layers of non-conductive tape and epoxy resins in order to protect it from thermal affects. The ACECT data acquisition system used in this study is a high-speed imaging system which can be used for capturing the images and computing the capacitance measurements of non-conducting flows. The excitation frequency of the ACECT system was set to 180 kHz and the excitation amplitude at 14 Vp–p. The capacitance measurements were normalized and reconstructed to an imaging area of 64  64 pixels of the pipe. The data collection rate can be adjusted up to 140 frames per second.

The experiments were carried out on an experimental rig for a waxy crude oil flow loop available in the Mechanical Engineering Department of Universiti Teknologi PETRONAS. A schematic of the experimental rig for the waxy crude oil flow loop is presented in Fig. 2. The flow loop was designed to study the effects of thermal shrinkage on gas void formation in statically cooled waxy crude oil with different operating conditions in real time. It consisted of a horizontal acrylic pipe fitted inside the cold water bath (chilled water test section). The test section (i.e., ECT sensor) was made up of an acrylic material (i.e., detachable), which helped in the visualization of gas voids in a waxy crude oil flow entering the test section, supported by a flange at two ends. The temperature of the chilled water bath was kept controlled and a thermocouple was attached to monitor its temperature. A differential pressure transducer was connected to the inlet and the outlet of the horizontal test section that provided the pressure across the entire system. Two thermocouples, one at the inlet and another at the outlet of the test section, were also installed to give the average

Fig. 2. Schematic of an experimental setup for waxy crude oil flow loop.

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temperature of the test section. A crude oil tank having a volume of 50 l with a stirrer and heater installed on the tank was able to maintain the desired temperature of the crude oil, ensuring that all waxes were dissolved homogeneously in the crude oil. The crude oil pump had been used for pumping the crude oil into the horizontal test section at varying flow rates where the temperature and pressure of the fluid were measured. The pump was of a gear type with a flow rate of 35 l/min and viscosity of 0.05 Pa. The Coriolis flow meter was used in this flow loop to measure the mass flow, volume flow and density measurement. The piping of flow loop consisted of stainless steel with an ID of 30 mm. ECT sensor calibration method It is a common practice that prior to connecting an ECT sensor to the flow loops, it is calibrated by filling the sensor with two reference materials and measuring inter-electrode capacitance values with different permittivity materials. This calibration method describes the two extreme points of the measurement range for the ECT system (Yang et al., 2004). Once the ECT system was calibrated, then it was ready to capture an online measurement. The change in the capacitance obtained from the raw data is usually later normalized. An initial calibration of an ECT sensor was performed when the sensor was filled with air at ambient temperature as a low calibration material and then, the sensor was filled with crude oil at 80 °C (for high calibration having a higher permittivity material) within the sensing region. The calibration data was stored in a separate file and was loaded before the start of each experiment. Experimental methodology

80

Temperature (°C)

70

Temp Cooling rate (°C/min) Poly. (Temp ) Poly. (Cooling rate (°C/min))

y = 0.0031x2 - 0.8397x + 75.288 R² = 0.983

60 50 40 30 20

y = -3E-06x3 + 0.0007x2 - 0.0617x + 2.3271 R² = 0.9257

10 0 0

25

50

75

100

125

2.60 2.40 2.20 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 150

Cooling rate for statically cooled waxy crude oil Temperature has a significant effect on the rheological properties of waxy crude oil. The cooling process of the waxy crude oil started from 75 °C (i.e., quite above its WAT) in order to eliminate any kind of thermal history. The cooling rate was calculated from the slope of the temperature versus the time plot and found to be varied from 0.4 °C/min at lower temperatures and 2.6 °C/min at higher temperatures. Fig. 3 shows the plot of temperature profiles and cooling rates of the statically cooled waxy crude oil in the test section during the experiment. Due to the slow cooling rate and small diameter of the pipe (test section), the gelled waxy crude oil was considered to be cooled homogenously. During rapid cooling (2.6 °C/min), the formation of gas voids due to shrinkage was located at the core of the test section. Whereas, during slow cooling (0.4 °C/min) the gas voids were formed along the inner wall of the test section. Cross-correlation velocity measurement The principle of the cross-correlation method for the velocity measurement is shown in Fig. 4. It is a measure of the similarity of two signals in which the time-lag function is applied to one of them. It is significant to select an appropriate distance between the two planes for cross-correlation. According to Yang (2010),

Cooling rate (°C/min)

An important prerequisite for reliable experiments is an appropriate experimental procedure. From the study, it shows that, this type of crude oil usually split into heavier and lighter parts because of a density difference if it was preserved for a long period of time. Therefore, prior to beginning an experiment, initially, all the wax inside the flow loop had to be dissolved by running the trace element heater and crude oil tank heater at 75 °C and 80 °C (i.e., well above its WAT). At this point of temperature, the crude oil became a Newtonian fluid and keeping the temperature constant was necessary in order to ensure a good homogeneous temperature distribution former to the initiation of the flow. The inlet and outlet valves of the test section were fully opened to allow crude oil to flow into the test section and back to the crude oil tank. Once the crude oil within the entire flow loop is consistently distributed, the stirrer of the crude oil tank was turned on as to maintain fine homogeneity characteristics during heating. As soon as the crude

oil started to move into the test section and flowed through it without any hindrance, then both the inlet and outlet globe valves were immediately closed to stop the flow for static cooling condition. When the valves were closed, the crude oil pump and trace heater also closed and allowed static cooling within the test section. The water bath then filled up with chilled water (i.e., at 5 °C) to provide a cooling medium for the statically cooled waxy crude oil. The online measurement of the statically cooled waxy crude oil had been varied from the temperature of 75 °C to 10 °C; thus, observing the properties and characteristics of waxy cooled crude oil in a dual plane ECT sensor. The measurement was initialized once the calibration data file had been loaded successfully by selecting the same measurement parameters as at the time of the calibration. Data for the raw capacitance and the ECT tomogram images were collected for every 5 °C change in temperature; the cooling time was also recorded, simultaneously. After the experiment was finished, the test section was drained from the chilled water, the ECT sensor was cleaned, and the online data files were saved for further analysis.

Time (min) Fig. 3. Temperatures recorded and cooling rate profile of the statically cooled waxy crude oil.

Fig. 4. Basic principle of flow velocity measurement using cross-correlation.

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if the distance is too long, a pattern will change from the Plane 1 to Plane 2, resulting in poor cross-correlation. And if the distance is too short, the time resolution would be poor because the number of samples for a pattern to flow from Plane 1 to Plane 2 is limited. Therefore, a common rule has to be applied such that the distance between the two planes should be equal to the diameter of the sensor to assure a good cross-correlation (Yang, 2010). The waxy crude oil was heated at a temperature of 75 °C so that all the effect of thermal history was eliminated. The thermal history is accredited to different heating and cooling pre-treatments that the crude oil has experienced during experiments. So, in order to remove any of its effect from the waxy crude oil it is preheated higher than its WAT. After this, the entire flow loop was standardized by running at a minimum permissible velocity which could generate a visible homogeneous flow before reaching the steady state for statically cooled waxy crude oil. At this point of temperature, the crude oil was acting as a Newtonian fluid with no thermal history and wax formation within the test section. Hence, the velocity profile was considered to be uniform. The time delay between the output signals of the two sensors can be found by calculating the cross-correlation function of their time records x(t) and y(t) over a measurement period (or integration time) T. The resulting cross-correlation function is given by (1) (Beck and Plaskowski, 1987):

Rxy ðsÞ ¼

1 T

Z

T

xðtÞyðt  sÞdt

ð1Þ

0

where Rxy(s) is the value of the cross-correlation function when the upstream signal y(t) is delayed by time s. The transit time of the flow between the two sensors can be found by calculating the time-lag (sm) at which the cross-correlation function is maximum. Since the distance between the upstream and downstream sensors (L) is known, the average velocity (v) of the flow passing through can be found from (2) (Beck and Plaskowski, 1987):

m¼

L

sm

ð2Þ

where sm is the time delay from the cross-correlation function. It must be stated that sm is the time delay and cross-correlation obtained from the number of time delays/lags which is dimensionless. So, the sampling time between each sample must be multiplied to the number of lags to obtain sm. By searching the peak of the cross-correlation function the transit time sm can be obtained from (3) (Beck and Plaskowski, 1987):

s m ¼ j  Dt

ð3Þ

where j corresponds to the peak of the cross-correlation function and Dt is the time interval between each two samplings. Then, the velocity can be calculated using (2).

bðx; yÞ ¼



0

if Iðx; yÞ < Tðx; yÞ

1 if otherwise

ð4Þ

In the relationship between the gray levels of the reconstructed images and the medium distribution, the medium can be regarded as the gas phase (appearing due to thermal shrinkage) at a given pixel. So, the cross-sectional gas void fraction of waxy crude oil can be calculated by (5) (Dailiang et al., 2004):

a¼1

PN

i¼1 Ai  100% Apipe

ð5Þ

where Ai is the area of the ith pixel and Apipe is the total cross-sectional area of the pipe. The average void fraction can also be obtained by using a normalized inter-electrode capacitance measurement. Results and discussion Calibration of a sensor Calibration is an important step before any application of an ECT system, in order to obtain accurate data between inter-electrode pairs. Hence, for this study the raw capacitance measurements for low calibration, Cl, were obtained by using air (i.e., a low permittivity material), followed by the raw capacitance measurements for high calibration, Ch, by using a higher permittivity material (such as, crude oil at 80 °C). Fig. 5 shows the line graph of the low versus high calibration obtained from the dual plane ECT sensor measurement. The graph shows the comparison between the low vs. high voltage of Plane 1 and low vs. high voltage of Plane 2. VL1 indicates the lowest value of voltage as it was expected because of the low permittivity material and VH2 specifies the highest calibration voltage because of the high dielectric constant value. Image reconstruction The experiments were carried out in a Mechanical Engineering laboratory of Universiti Teknologi PETRONAS. The online measurements of the statically cooled waxy crude oil were varied in the temperature range of 75–10 °C. Once the crude oil temperature reached its measured temperature value, such as 75 °C, then the acquisition system started to capture the tomogram images and capacitance measurement. This process continued until the last measurement was recorded at 10 °C. The data for the normalized capacitance was collected at every 5 °C change in temperature. Table 1 shows the reconstructed tomogram images obtained from Plane 1 and Plane 2 at different cooling temperatures of the waxy crude oil. The images were reconstructed from the capacitance data signals obtained from the two planes using the linear back projection

Image analysis This study aimed to investigate the gas void formation in statically cooled waxy crude oil which is considerably substantial in many industrial applications. The methodology considered to refine the ECT images after reconstruction is an implementation of an image processing technique; namely, thresholding which is used to obtain the volume fraction. The basic concept of thresholding is presented in (4) where each and every intensity of pixels I(x, y) in the gray scale image would be converted into a binary value, b(x, y) depending on the threshold value, T(x, y). The process to perform an image thresholding by using the most commonly used method, Otsu, can be found in Otsu (1979). It helps to convert the gray scale image into a binary image and then retrieve the total pixel values for the gas voids and calculate their percentage.

261

Fig. 5. Voltage line graph obtained from the calibration of the ECT sensor.

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Table 1 Image reconstruction for statically cooled waxy crude oil at different temperatures for Plane 1 & Plane 2. Temperature

10 °C

⁄

20 °C

30 °C

40 °C

Plane 1 LBP

Key 100%

Plane 2 LBP

Temperature

50 °C

60 °C

70 °C

75 °C

Plane 1 LBP

0%

Plane 2 LBP

⁄

Note: At 20 °C, the tomogram images for Plane 1 and Plane 2 show some noise which could be due to external interference, such as the signal to noise ratio.

(LBP) reconstruction algorithm. It is non-iterative image reconstruction algorithm, which is fast and can increase the accuracy for online measurement. The cross-sectional images indicates that the lower permittivity region (that is, a combination of gas voids and solid wax) was located along the inner wall of the pipe, while the higher permittivity region (i.e., liquid crude oil) was located at the middle of the test section. The images at 10 °C have shown an increased percentage of lower concentration present within the cross-section representing the waxy crude oil as a non-Newtonian fluid. However, at higher temperatures, such as 75 °C, when the crude oil was behaving as a Newtonian fluid, the percentage of lower concentration was relatively smaller. The color scale for the reconstructed tomogram images has the blue representing the 0% or lower permittivity region and red representing the 100% or higher permittivity region. The images shown in the figure were grouped from different frames of the measurement. The measurement principle it follows was that when the first electrode was energized, then electrodes number 2 till 8 would act as detecting electrodes. As electrode 2 was energized, then electrodes 1, 3, 4, 5, 6, 7 and 8 would remain detecting electrodes and this continued until the last electrode, 8, was energized and the capacitances from electrode 1 up to 7 were measured. Therefore, each plane would have a total of 56 normalized capacitance measurements for a single frame (i.e., producing one tomogram image). Application of the distribution models on statically cooled waxy crude oil In the data capture process, the measured values of the inter-electrode capacitances for the ECT sensor for each image

frame and the pixel permittivity values derived from them, were normalized to lie between the values 0 and 1, where 0 corresponded to the values measured at the lower permittivity calibration point and 1 corresponded to the values measured at the upper permittivity calibration point. This was carried out using the reference data in the calibration file which was generated during the calibration process. The normalized capacitance (Cn) was computed using (6) (Yang et al., 2004):

Cn ¼

Cm  Cl Ch  Cl

ð6Þ

where Cm was the measured capacitance. Using the Cn as mentioned in (6) can reduce the systematic errors in the measurement system. The application of distribution/capacitance models can support the characterization of the materials in the sensor cross-section. The distribution models used were the series, parallel and Maxwell models (Yang et al., 2002). The results for the normalized values using the parallel, series and Maxwell models are shown in Fig. 6. The selection of the model for the particular application has a significant effect on the calculated measurements. Fig. 6(a)–(c) shows the plot between the normalized capacitances for the adjacent, 1-adjacent and opposite electrode pairs obtained from Plane 1 and Plane 2 versus the temperature in °C for the different distribution models. Fig. 6(a) shows the graph for the Cn-adjacent versus the change in temperature for Plane 1 and Plane 2. It shows that the Cn-adjacent had a similar trend with all the models except the measurement at 20 °C for Plane 2, which indicates the value was higher than the normalized range of 0–1 because of the soft-field effect of the sensor. It also demonstrates that the Maxwell model lay in between the parallel and series

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model; whereas, the parallel model had the highest set of measurements while the series model displayed the lowest range of measurements for both the planes. However, Fig. 6(b) specified that the series model had the highest range of values while the parallel had the lowest and the Maxwell model lay in between the parallel and series for the two planes. It also reveals the error in the measurement at 20 °C for Plane 2 due to the soft-field effect or the external interference of the signal. Lastly, the opposite pair of electrodes as shown in Fig. 6(c) signifies the best possible result for Plane 1 and Plane 2 having similar trends of data and the range of Cn was also within the normalized range of 0–1. By applying the three different distribution models on the results obtained the capacitance could be related with the change in the temperature of the waxy crude oil. It also helped in finding the best model that was appropriate for this study, which was the Maxwell model. As it is a combination of the parallel and series models. Image analysis for gas void formation in statically cooled waxy crude oil The reconstructed tomogram images obtained by using LBP were converted from the gray scale into the binary scale by

performing the thresholding algorithm on the images. Therefore, the accuracy of the gas void measurements were further improved by combining both the image reconstruction and image thresholding algorithm on to the measured signal in order to improve the quantification of the gas void formation in the statically cooled waxy crude oil. The color scale for the threshold images represented only two colors, such as black represented zero and white indicated one. After the thresholding, the gas voids were able to be calculated as mentioned in Section ‘Image analysis’. As this manuscript has focused on the estimation of gas void formation due to the thermal shrinkage of waxy crude oil at static cooling by using an online ECT measurement. Precedent research established that the gas voids is usually calculated when the waxy crude oil reached its gelation point and becomes multiphase fluid until the minimum cooling temperature. For the available flow loop condition, the minimum achievable temperature is 10 °C. Therefore, the gas voids for statically cooled waxy crude oil were estimated between the temperature ranges of 38 and 10 °C. Table 2 presents the images obtained after applying the Otsu’s global thresholding method on to the reconstructed tomogram images for Plane 1 and Plane 2.

Plane 1

Plane 2

1.10

y = 2E-05x2 + 0.0005x + 0.9753 R² = 0.9912

1.05 Parallel P1 Series P1 Maxwell P1 Poly. (Maxwell P1)

1.00 0.95 10

15

20

25

30

35

40

45

50

55

(a) Adjacent Pair for Plane 2

2.20

Normalized Capacitance (Cn)

Normalized Capacitance (Cn)

(a) Adjacent Pair for Plane 1 1.15

60

65

70

Parallel P2 Series P2 Maxwell P2 Poly. (Maxwell P2)

2.00 1.80 1.60 1.40 1.20 y = -9E-05x2 + 0.0084x + 1.1306 R² = 0.0242

1.00 0.80

75

10

15

20

25

30

0.88

Normalized Capacitance (Cn)

Parallel P1

Series P1

Maxwell P1

Poly. (Maxwell P1)

0.84 15

20

25

30

35

40

45

50

55

60

65

70

0.20 30

35

40

45

70

75

0.96 Parallel P2 Series P2 Maxwell P2 Poly. (Maxwell P2)

0.94 0.92 0.90 10

15

20

25

30

35

40

45

50

55

(c) Opposite Pair for Plane 2

0.40

25

65

0.98

(c) Opposite Pair for Plane 1

y = 2E-05x2 - 0.0085x + 0.9145 R² = 0.9589

20

60

1.00

Temperature (°C)

0.80

15

55

y = -1E-05x2 + 0.0015x + 0.9452 R² = 0.2956

1.02

75

Parallel P1 Series P1 Maxwell P1 Poly. (Maxwell P1)

10

50

Temperature (°C)

1.00

0.60

Normalized Capacitance (Cn)

y = 9E-06x2 - 0.001x + 0.9108 R² = 0.9426

10

45

(b) 1-Adjacent Pair for Plane 2

0.92

0.86

40

1.04

Normalized Capacitance (Cn)

Normalized Capacitance (Cn)

(b) 1-Adjacent Pair for Plane 1

0.90

35

Temperature (°C)

Temperature (°C)

50

Temperature (°C)

55

60

65

70

75

60

65

70

75

Parallel P2 Series P2 Maxwell P2 Poly. (Maxwell P2)

0.95

0.75 y = 6E-05x2 - 0.0083x + 0.8907 R² = 0.9377

0.55

0.35 10

15

20

25

30

35

40

45

50

55

60

65

70

75

Temperature (°C)

Fig. 6. Temperature versus normalized capacitance (obtained from parallel, series and Maxwell models) plot between (a) adjacent, (b) 1-adjacent and (c) opposite electrode pairs for Plane 1 and Plane 2.

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Table 2 Image thresholding of statically cooled waxy crude oil for Plane 1 & Plane 2. 10 °C

Plane 1 y = -5E-07x3 + 4E-05x2 -0.0012x + 0.9807 R² = 0.5831

30 °C

Plane 2 Poly. (Plane 1)

Void Fraction (%)

Temperature

0.973

Plane 1 threshold image

0.972

Poly. (Plane 2)

0.971 y = -3E-07x3 + 2E-05x2 -0.0006x + 0.9745 R² = 0.9437

0.97

Plane 2 threshold image

0.969 10

15

20

25

30

35

40

Temperature (°C) Fig. 8. Void fraction obtained using the Cn-Maxwell for Plane 1 and Plane 2.

Fig. 7 shows the percentage of gas voids within the circular horizontal acrylic pipe installed on the flow loop obtained from the statically cooled waxy crude oil. The gas voids formed in the waxy crude oil were observed due to the occurrence of the thermal shrinkage during static cooling condition. As soon as the WAT reached (at 38.5 °C), the appearance of wax crystals was eminent. Therefore, the percentage of gas voids had been estimated between the temperatures of 38 °C and 10 °C. The percentage was calculated using the image from Table 2 based on Otsu’s image thresholding method. It shows 8% (Min.) for 38 °C and 14% (Max.) for 10 °C. The measured values of gas void considered to be uniform and indicates the range of measurements for the total pipe volume of the flow loop. Nevertheless, it is the ranges of gas void corresponding to the thermal shrinkage at different cooling temperatures. An increase in the percentage of the gas void fraction was observed from the two planes. On applying the 3rd order polynomial regression analysis on Plane 1 and Plane 2, the relationship between the gas void fraction and temperature were defined by the following expression (7):

Y ¼ A 0 þ A 1 x þ A 2 x2 þ A 3 x3

ð7Þ

where Y = a, x = temperature and A0, A1 and A2 were the coefficients of the polynomial whose values were mentioned in Fig. 7. The correlation coefficient for Plane 1 and Plane 2 was found to be R2 = 0.994 & R2 = 0.996, respectively. The range of the gas voids for Plane 1 was from 8–11% while for Plane 2 it lay in the range of 12–14% within a temperature range of 38 °C–10 °C. The trend of the plot for the two planes was quite similar to each other and indicates that on decrease in temperature, the gas void formation was increased. The difference in the percentage of gas voids for Plane 1 and Plane 2 was due to the change in the percentage of the gas voids volume traveling from the upstream (Plane 1) to the downstream (Plane 2) plane.

Gas Void Fraction (%)

15.00

Plane 1 y = 7E-05x3 -0.003x 2 -0.0611x + 14.901 R² = 0.9961

14.00

Plane 2 Poly. (Plane 1)

13.00

Poly. (Plane 2)

12.00 y = -7E-05x3 + 0.0045x2 -0.1959x + 12.7 R² = 0.9938

11.00 10.00 9.00 8.00 10

15

20

25

30

35

40

Temperature (°C) Fig. 7. Percentage of voids volume obtained via image thresholding for Plane 1 and Plane 2.

Void fraction using normalized capacitance The average voidage was calculated by using the normalized inter-electrode capacitance measurements. It was obtained by summing all the normalized capacitance values found from one image and dividing these by the sum of the normalized capacitances when the sensor was filled with the higher permittivity material. It was written in mathematical expression as follows (8):

a¼

N 1X ðC n =C k Þ N n¼1

ð8Þ

where Cn represented the normalized capacitance measurement, Ck was the capacitance when the sensor was filled with a higher permittivity material and N was the number of electrodes. Based on the analysis of the distribution model in Section ‘App lication of the distribution models on statically cooled waxy crude oil’, the Maxwell model was selected as an appropriate model for this application. Therefore, Fig. 8 presents the graph plotted between the temperature in °C and void fraction obtained from the Cn-Maxwell for Plane 1 and Plane 2 in %. The volume ratio using the Cn-Maxwell was estimated between the temperatures of 38 °C and 10 °C. It could be observed from the graph that the two planes specify almost a similar trend of measurement except at 20 °C. As mentioned earlier, there was an effect of external interference noise on the measurement at 20 °C; thus, it gave an error in measurement. The range of the volume fraction obtained from the Cn-Maxwell measurement was from 0.97 to 0.973 for Plane 1 and Plane 2. Velocity measurement using cross-correlation Fig. 9 shows the plot of two capacitances data signals measured from the Plane 1 and Plane 2 of the ECT sensor, having 8-8 electrodes installed on an external wall of the sensor, can compute the axial velocity. The graph was plotted by taking an average of 100 frames of measurement for a total of 56 capacitance measurements obtained at 75 °C. It shows that the two data signals were identical to one another and one plane followed the other. Fig. 10 shows the probability density function (pdf) with a normal distribution applied on the Plane 1 and Plane 2 average capacitance measurements. It can be seen that the distribution was symmetric for both the planes. On the application of normal distribution fit, Plane 1 had obtained standard deviation of 0.294 which shows that capacitance values were closer to the mean (1.029) of the curve. Whereas, for Plane 2, the standard deviation was 0.581 which was comparatively higher than the Plane 1and it depicts that the capacitance values were far away from the mean (1.056) value. As the standard deviation measures a dispersion and shows

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265

Fig. 9. Capacitance signals of neighboring electrodes (for axial velocity). Fig. 11. Simulation result for the (a) cross-correlation (CC) and the (b) generalized cross-correlation with the phase transform (GCC-PHAT) between two signals.

Fig. 10. PDF with a normal distribution applied on the Plane 1 and Plane 2 measurements at 75 °C.

the spread of data from the mean value. However, data on both the curves had normal distribution and 68.26% of all observations fall within one standard deviation (one-sigma) of the mean which was considered more reliable and consistent. Based on this analysis, it can be concluded that the two curves were in good

agreement to each other and as such was able to be applied for the cross-correlation. Fig. 11 shows the simulation result of MATLAB for the cross-correlation (CC) and the generalized cross-correlation with the phase transform (GCC-PHAT) between the two capacitance signals obtained from the Plane 1 and Plane 2. The x-coordinate denotes the frame lag and the y-coordinate represents the resulted cross-correlation. It was shown in the figure that the peak of the cross-correlation using xcorr was recorded at a delayed frame of 55. Whereas, GCC-PHAT gave the correct time delay of 49 with a sharpened peak compared to the xcorr result. The cross-correlation was good for the signal with no noise but if the signal was noisy, then generalized CC could remove it and give a distinct peak at the correct time delay. By using the information and the frames per second rate, we were able to acquire the transit time, which in this case was 0.747 s. This time was recorded at 75 °C when the crude oil was heated (i.e., quite higher than its WAT) so that all the effect of thermal history was eliminated. After this, the entire flow loop was standardized prior to the initiation of static cooling, and the waxy crude oil was behaving as a Newtonian fluid. The flow loop was run at a minimum permissible velocity which could generate a visible homogeneous flow in the test loop.

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It demonstrated that the waxy crude oil at this point of temperature, showed an adequately steady velocity over an entire test section cross-sectional area. Thus, the velocity of the flow was obtained after the estimation of the transit time and the distance between the two sensors using (2). The distance, L, between Plane 1 and Plane 2 was 3.5 cm, and the acquired transit time, sm, was 0.747 s; the estimated velocity then, was 4.69 cm/s. Therefore, under this condition it was considered based on the observation in the flow loop that the velocity profile was uniform and the crude oil was free from any kind of thermal history.

Conclusion A dual plane ECT sensor was successfully installed on a laboratory scale waxy crude oil flow loop for imaging and analyzing the formation of gas voids due to the thermal shrinkage in a statically cooled waxy crude oil. The reconstructed tomogram images obtained using the LBP algorithm showed good agreement with the physical observations taken through the transparent test section. The rate of the temperature and cooling rate had a substantial effect on the formation of gas voids. This study has estimated the online measurement of gas voids that were formed due to the thermal shrinkage in a waxy crude oil. It was measured from 38 °C (which specified 8%) until 10 °C (indicate 14%) that considered to be uniform for the total pipe volume of the flow loop by using an image thresholding process. The distribution models have been applied to calculate the normalized capacitance and compute the gas void by selecting the appropriate model. Therefore, the Maxwell model was selected as a significant model from the observations and also applied for the estimation of the gas void by using Cn-Maxwell. The measurement system discussed in this study is non-invasive, low-cost and has a quick response. It can provide an acceptable way of incessant online measurement and monitoring of the waxy crude oil flows. This study measures the gas voids using an online ECT system for waxy crude oil flows that is important for achieving safety, process efficiency, protection and quality assurance in the industrial applications. Furthermore, this paper is a part of an on-going research and the additional experiments are under way in order to extend the application of ECT for gas voids quantification in different conditions.

Acknowledgement The authors would like to thank Universiti Teknologi PETRONAS (YUTP Funding having Cost Centre No. 0153AA-A24) for sponsoring the research and providing the test facilities.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijmultiphaseflow. 2015.06.005.

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