Induction Time of Hydrate Formation in Water-in ... - ACS Publications

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Induction time of hydrate formation in water-in-oil emulsions Haimin Zheng, Qiyu Huang, Wei Wang, Zhen Long, and Peter G. Kusalik Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01332 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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Industrial & Engineering Chemistry Research

Induction time of hydrate formation in water-in-oil emulsions Haimin Zhenga, Qiyu Huanga*, Wei Wanga, Zhen Longa, Peter G. Kusalikb a

b

Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing 102249;

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

Abstract: Blockage of pipelines due to hydrate formation is a major problem for subsea flow assurance. Induction time for hydrate formation from the multiphase system within a pipeline is a critical parameter to determine whether hydrates may form at a given time. In this work, the induction time for hydrate formation in water-in-oil emulsions was investigated under different conditions. For this purpose, an autoclave with an on-line viscometer was designed and built. Based on the viscosity variation observed in the experiments during hydrate formation, a new avenue for defining induction time is proposed, which should be more convenient for determining the hydrate formation time in some pipelines. As hydrate formation in emulsions is more complicated than in pure water, the effects of several factors were considered in this study, including water cut of the emulsions, shear rate, driving force, and memory effect. Additionally, wax precipitation is also a common problem in subsea pipelines and can impact flow assurance when hydrate formation and wax precipitation both occur. Consequently, the effect of wax solid particles on hydrate formation was also considered in this work. The presence of wax particles is observed to impede hydrate formation. In this work, it is determined from induction time that the hydrate formation is initialed at the water-oil surface for water-in-oil emulsion. Moreover, the memory effect can shorten induction times of hydrate formation due to the remaining small CO2 bubbles at the surface of water droplets. Keywords: hydrate, induction time, emulsion, viscosity, wax 1. Introduction With the exploration and development of oil fields moving into deepwater, multiphase (crude oil, gas, and water) transportation has been widely used to deliver the petroleum fluid from offshore oil wells to onshore locations. However, there are several challenges involved in the transportation of these multiphase systems [1] [2]. Hydrate formation in multiphase pipelines is one of the serious problems that should be considered. Hydrates are crystalline compounds in which water molecules form cages and enclose small gas molecules such as carbon dioxide, methane, ethane, and hydrogen sulfide. Hydrate formation can cause partial or total blockage of the pipeline resulting in significant costs, lost time, and safely concerns. The traditional solution for this issue is to add a thermodynamic hydrate inhibitor such as methanol or monoethylene glycol [3]. However, this method requires using a large amount of inhibitor and is expensive. Nowadays, low dosage hydrate inhibitors are being widely used, such as kinetic hydrate inhibitors [4] and anti-agglomerants [5]. Kinetic hydrate inhibitors interfere with the nucleation process by dramatically extending the hydrate induction time and the nucleation process [6]. Hydrate anti-agglomerants allow hydrates to form but keep hydrate particles

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small and dispersed in the liquid, so that they do not agglomerate together and not lead to hydrate plugs [7]. These small hydrate particles can be transported as hydrate slurry which has a higher viscosity compared to the emulsion or multiphase flow without hydrates [8]. Thus, it is essential to have a good estimate of the induction time or the hydrate formation time, especially in crude oil, to avoid transportation accidents when the multiphase flow enters hydrate formation regions. Moreover, the oils in the multiphase fluids often deposit waxy material, called paraffin or wax, in low temperature subsea pipelines. The wax deposition can also cause partial or even total blockage of the multiphase pipelines. As the oil and gas in multiphase pipelines are transported at low temperature and high pressure conditions, formation of gas hydrates and wax deposition may occur together. The formation of one solid may influence the formation and thermodynamic behavior of the other solid [9]. Mohammadreza has built a multiphase integrated thermodynamic model and experimentally investigated the mutual effects of waxes and hydrates [10]. They have investigated multicomponent mixtures and analyzed their effects on hydrate formation. The amount of heavy alkanes decreased in the liquid as the waxes precipitated out. This resulted in more light hydrate forming components being present in the liquid system, which promoted hydrate formation. However, in practical applications, the interplay of other factors besides components can affect the hydrate formation. For example, the formation of solid wax (hydrate) particles may provide the nucleation site for hydrate (wax). So the nucleation or induction effect of wax solid particles on hydrate also needs to be studied. The hydrate induction time may be influenced by the formed wax solid and it is crucial to know whether hydrate formation is promoted or impeded. The hydrate induction time may be defined in two different ways, one is the time from the initial target state to the time that nucleation has occurred, and the other is time from the initial target state to the time that crystals become visible [11]. The first one is more precise definition while the second one is a more practical definition. In an actual experiment, it is hard to measure the time when nucleation has occurred at hydrate formation conditions (high pressure and low temperature) with available instruments. Although it is also difficult to observe the appearance of hydrate crystals and it may vary with different observers, it can be reasonably determined by extrapolation. Monitoring the changes in pressure and temperature is the typical method for determining induction time, because the hydrate formation reaction consumes gas and is exothermic [12][13][14][5][16][17]. Therefore, in most studies, the induction time is marked from the establishment of the target pressure and temperature conditions to the moment when there is both a sudden pressure decrease and temperature increase. Many researchers have studied the hydrate induction time for gases in the pure water [12] [14][15] [18][19] . The main influencing factors are subcooling, driving force, memory effect, stirring rate, and gas composition [18][20][21][22][23][24]. However, hydrate formation in crude oil or in water-in-oil emulsion may not be the same as the process in the pure water, as for example there will be differences in the heat and mass transfer [25][26][27][28][29] [30]. So it is essential to study the hydrate induction time in water-in-oil emulsion. In a multiphase pipeline, it is critical to know that the multiphase viscosity increases when the phase has changes [31][32]. According to Mills’ law [33], the viscosity of an emulsion or slurry increases with increased particle size. Therefore, there will be a sudden increase in the fluid viscosity when the hydrate crystal growth reaches its critical size [8] [34]. In these experiments, the induction time is marked by the sudden increase in the measured fluid viscosity during the hydrate


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formation. Although this moment does not exactly coincide with the appearance of the first hydrate crystal, this definition is practical and convenient in experiments, and the conclusions remain unaffected [35]. In general, viscosity is more important than hydrate particle size for subsea multiphase pipeline transportation [36][37]. As hydrate formation in emulsions is more complicated than that in pure water, a greater number of influencing factors should be considered in these multiphase systems. In this work, a high pressure autoclave with an on-line viscometer was set up. The on-line viscometer was used to measure the viscosity during hydrate formation from water-in-oil emulsion. The induction time was determined from the measured viscosity. Several influencing factors were considered in this study including the water cut of emulsions, shear rate, driving force, and memory effect. Furthermore, the effects of solid wax particles on the induction time for hydrate formation were also investigated. 2. Experimental 2.1. Apparatus The apparatus employed in this experiment is shown in Figure 1. It consists of a stainless steel autoclave, a gas flowmeter, a temperature control system, a pressure control system, a stirring device, a high pressure on-line viscometer, and a data logger. The autoclave has a height of 20 cm, an inner diameter of 16 cm, and was designed to resist pressure up to 15 MPa (HAS-II, Beijing, China University of Petroleum). The gas flowmeter with a range of 0 to 1000 mL/min was installed at the entrance of the autoclave to display and record gas consumption. A thermostatic bath was used to control the autoclave temperature which ranged from -20 °C to 100 °C with an accuracy of ±0.01 °C and a circulating pipe of the thermostatic bath encircled the autoclave. A thermal resistance detector with an accuracy of ±0.01 °C and a pressure transducer with an accuracy of ±0.01 MPa were used to measure temperature and pressure in the autoclave, respectively. An anchor agitator with a stirring rate from 0 to 800 rpm was installed on the lid of the autoclave. The anchor agitator was driven by the rotation of a ceramic magnet above the autoclave. An on-line viscometer (VISCOpro2000+SPC301) was mounted on the wall of autoclave. The on-line viscometer could work at high pressure up to 12 MPa and had a range of 0.1 cP to 1000 cP with an accuracy of ±0.8% of the measured values. The pressure, temperature, flowrate, and stirring rate were recorded every 20 s, and the viscosity values were recorded every 60 s, via a data acquisition system.

Figure 1. Schematic diagram of the experimental apparatus



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2.2. Materials All of the experiments were performed with 99.99% pure CO2. De-ionized water (from a water purification system) was used throughout the study. The oil used in the experiment was -10# diesel. The wax used in this experiment was a C16 to C30 paraffin mixture. 2.3. The preparation of emulsions The emulsions were prepared by stirring a mixture of de-ionized water and -10# diesel with an IKARW20 Digital stirrer at 800 rpm. During the emulsion preparation, the open part of the sample container was tightly covered to prevent evaporation of the light hydrocarbons. All of the emulsions were prepared at the same temperature of 60 °C. Before the hydrate formation experiment, it was necessary to ensure that the emulsions were stable and able to maintain their initial droplets size and distribution. Emulsions prepared with different water cuts were kept at the room temperature (24 °C) for 24 hours and then observed under the microscope. After standing for 24 hours, there was no obvious coalescence and almost no change from the initial emulsion droplets size and distribution. 2.4. Experimental procedure The experimental procedure in this study is given as follows. Before the experiment, the autoclave and on-line viscometer were cleaned first with de-ionized water and then petroleum ether. The thermostatic bath was adjusted to a uniform temperature. The desired water-in-oil emulsions were prepared prior to the experiments. When the autoclave temperature attained a uniform temperature, the as-prepared emulsions were added into the clean autoclave. The autoclave was covered and sealed with the lid. The stirrer in the reactor was started and the on-line viscometer started measuring the viscosity. At the same time, the autoclave was cooled until it reached the target temperature. Upon reaching the target temperature, CO2 was added into the autoclave until the target pressure was achieved. Here, target temperature and target pressure refer to these constant values at which hydrate nucleation occurs (see Figure 2). The gas consumption was measured by a gas flowmeter. The temperature, pressure, and viscosity were recorded via a data acquisition system automatically during each experiment. The beginning of induction time was recorded as the moment when the values of temperature, pressure, and viscosity all attain constant values. Initial nucleation or induction times were determined by the viscosity increase due to hydrate particle formation. 3. Results and discussion The viscosity variation during a typical hydrate formation process is presented in Figure 2. At 36 min, both temperature and pressure reached the desired experimental conditions. The viscosity had also reached a constant value at this point. Thus, this time was recorded as the beginning of the induction period. Temperature in the autoclave began to increase at 64 min signaling that hydrates had begun forming. It can be seen that viscosity was also very sensitive to hydrate formation, showing an increase also at 64 min. Thus, the induction time for this experiment was 28 minutes. Induction times determined by viscosity variation were generally consistent with those determined by temperature, which indicates that viscosity variation is a suitable parameter for defining the hydrate induction time unless otherwise stated, all the induction times reported here were obtained from viscosity variation. The experiments were performed here were at constant pressure conditions, which means that CO2 was added into the autoclave as the CO2 was consumed. Consequently, the pressure showed a slight decrease during hydrate formation point (eg. 67 min), as the amount of CO2 added to the autoclave was slightly less than the amount of CO2 consumed


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to form hydrates. Subsequently the pressure returned to its target value. During the process of hydrate formation, the viscosity increased to a maximum value, after which it decreased slightly. The schematic illustration of the hydrate formation process in water-in-oil emulsion is shown in Figure 3. The viscosity variation was due to the effective volume of dispersed phase after hydrate formation. During the hydrate formation, some continuous (oil) phase was entrapped and immobilized between the hydrate particles (shown in Figure 3c). These factors lead to an increase in the effective volume fraction of dispersed phase. Viscosity increased with the increase in the effective volume fraction of dispersed phase, as seen from the Mills’ model in Equation (1) and Equation (2) [33].

µemulsion = µoil (1 − Φ ) / (1 − Φ / Φ max )2

(1)

µslurry = µoil (1 − Φ eff ) / (1 − Φ eff / Φ max )2

(2)

where, µemulsion is the viscosity of emulsion; µslurry is the viscosity of ice/hydrate slurry; µoil is the viscosity of continuous (oil) phase; Φ is the volume fraction of dispersed phase in emulsion; Φeff is the effective volume fraction of dispersed phase in ice (hydrate) slurry or suspension, which is related to ice or hydrate aggregation; Φmax is the maximum packing, Φmax=4/7. The slight decrease in viscosity after the peak was due to the hydrate particle aggregates breaking up and rearranging into more compact structures under the stirring conditions (shown in Fig 3d) [8] [38]. In this experiment, no anti-agglomerant was added to the emulsion, so the hydrates aggregated and the samples froze quickly after that. Thus, it can be seen here that it is very dangerous if the hydrates are formed in multiphase pipelines without an anti-agglomerant.

Figure 2. Pressure, temperature, and viscosity versus time in the hydrate formation process



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Figure 3. Schematic diagram of the hydrate formation process, (a) water-in-oil; (b) hydrate particle formation; (c) hydrate particle aggregation; (d) hydrate particle rearrangement 3.1. Water cut In this work, water cut refers to the water volume fraction of the fluid mixture. The effects of water cut on the induction time were investigated at a fixed operating condition with the pressure of 1.6 MPa, stirring rate of 300 rpm, and temperature of 2.5 °C. Both induction time and nucleation rate will be used here to analyze the effects of water cut. The relationship between induction time Tinduction and nucleation rate, α , is given by the relationship:

Tinduction ∝

1

α

(3)

In theory, the hydrate nucleation probability should be proportional to quantity of water [39]. Consequently, one might expect the induction time to be inversely proportional to the water cut (see Case 2 discussion below), as shown by the blue dotted line in Figure 4 that uses the pure water value as a reference point. The experimental results (red line in Figure 4) confirm this prediction in oil-in-water emulsions (water-rich), while significant deviations are observed in water-in-oil emulsions (oil-rich). The average diameter of water droplets in water-in-oil emulsions and the average diameter of oil droplets in oil-in-water emulsion are shown in Figure 4 as the blue and magenta lines, respectively. It can be seen that the average diameter of water droplets increased with the increasing water cut in water-in-oil emulsions. Moreover, the average diameter of oil droplets decreased with increasing water cut in oil-in-water emulsions. In the analysis below, we consider the two cases separately, water-in-oil and oil-in-water emulsions. Case 1: For the water-in-oil emulsion, which is an oil rich liquid, hydrate nucleation is predicted to occur at the surface of water droplets, and it is further assumed that cross nucleation was not important in determining the induction time. Then the relationship between induction time and water cut can be described by the following equations:

α ∝ Swater = n ⋅ S droplet

(4)

where Swater is to the total water-oil surface area, Sdroplet refers to the water-oil surface area of one


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droplet, and n is the total number of droplets. Since n is dependent on the amount of water and the size of the droplets, we see immediately that:

S water ∝ (

X water X ) ⋅ rdroplet 2 = water 3 rdroplet rdroplet

(5)

where Xwater refers to the water cut, and rdroplet refers to the radius of water droplets. As induction time is inversely proportional to nucleation time, we can obtain the following relationship:

Tinduction ∝

rdroplet X water

(6)

Since the average size of water droplets (rdroplet) was observed to increase with the increasing water cut, Xwater, the experimental induction time of hydrate showed a smaller decrease than the one predicted by the increasing water cut alone. We can now write the relationship from Eq. (6) as:

Tinduction = A ⋅

rdroplet X water

(7)

The constant coefficient A can be determined from data points. The black dash line in Figure 4 shows the calculated induction times using a value for A determined from an average over the five values for water-in-oil emulsions. Case 2: For oil-in-water emulsion, which is water rich, water is the continuous phase and the nucleation is assumed to occur in the water bulk. Then the hydrate nucleation rate should be proportional to the water cut. This relationship is given by:

α ∝ X water

(8)

As induction time is inversely proportional to nucleation time, it can be expected that:

Tinduction ∝

1 X water

(9)

This behavior is represented by the blue dotted line in Figure 4. It can also be seen in Figure 4 that the size of oil droplets increases slightly with increased oil fraction of the oil-in-water emulsion. Following an approach similar to that used for Eq. (5), the total water-oil surface area can be represented by:

Swater ∝

1− X water rdroplet

(10)

then Eq. (7) becomes:

Tinduction = A⋅

rdroplet 1− X water

(11)

The black dotted line in Figure 4 represents the calculated induction times for hydrate nucleation in oil-in-water emulsions using Eq. (11) and the value of A determined for oil-rich emulsions. However, it can be seen that the induction time (nucleation rate) is not correlated to the water-oil surface area and remains essentially proportional to the water cut in the water-rich region.



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Figure 4. Induction time versus water cut in emulsion at 2.5 °C, 1.6MPa, and 300 rpm. Region A: water-in-oil emulsion (oil rich), Region B: the phase inversion point, and Region C: oil-in-water emulsion (water rich). The calculated and theoretical induction times are described in the text. Another set of experiments was performed to verify the dependence of the hydrate induction time on water-oil surface area in water-in-oil emulsions. First, 30% water cut water-in-oil emulsions with different droplet sizes were prepared at different stirring rates. The blue line in Figure 5 shows that the average diameter of water droplets decreased with increasing stirring rate during the emulsion preparation process. The experimental results show that the induction time decreased roughly in proportion with the decreasing diameter of water droplets as predicted by Eq. (7). Thus, it can be concluded that the hydrate formation is initiated at the water-oil surface for water-in-oil emulsions.

Figure 5. Induction time for 30% water-in-oil emulsion prepared in different stirring rate, at 2.5 °C, 1.6MPa, and 300 rpm 3.2. Stirring rate Two series of experiments examining dependence on stirring rate were conducted in this study, one with pure water and the other with 20% water cut emulsions. Both series of experiments were performed at the same temperature (2.5 °C) and pressure (1.6 MPa) conditions. As the experiment was performed at a constant pressure, CO2 gas was continuously fed into the


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autoclave from the gas bottle as CO2 was consumed in the autoclave. In the water-in-oil emulsions, an initial hydrate shell was formed on the surface of water droplet. During further growth of the hydrate, the CO2 gas diffused into the inside surface of the shell and water diffused to the outer surface of shell [40]. More vigorous stirring could promote CO2 gas dissolution and dispersion into the solution and enhance the mass transfer rate [23]. According to Sloan [41], the collision of a hydrate particle with another water droplet could seed hydrate growth in the droplet. This phenomenon is called cross nucleation. Cross nucleation is one possible mechanism of hydrate formation. Higher stirring rates should increase the probability of hydrate particles colliding with water droplets and thus promote cross nucleation. However, it can be seen in Figure 6 that there was an upper limit above which higher stirring rate did not further promote hydrate formation. The higher stirring rates will produce higher shear force, which will lead to heavy collisions of liquid with the walls. These heavy collisions appear to hinder hydrate formation and extend the hydrate induction time. The dual effects of stirring rate resulting in both promotion and hindrance of hydrate formation can be seen in Figure 6. For pure water, the promotion threshold was around 300 rpm, while it was around 500 rpm for water-in-oil emulsion. Another phenomenon that can be seen in Figure 6 is that the induction time decreased more sharply after 300 rpm for the water-in-oil emulsion. The dispersion and diameters of the water droplets in the emulsion were observed via a microscope after stirring for 90 min at stirring rates of 200, 400, and 600 rpm. Water dispersions were found to be more uniform and the average diameters of droplets were slightly smaller after stirring for 90 min at 400 rpm and 600 rpm, but no change was observed after 90 min of stirring at 200 rpm. The more uniform dispersion of smaller droplets should give rise to a larger water-oil surface area and hence promote hydrate nucleation, resulting in shorter induction times. The behavior of the induction time for the water-in-oil emulsion at low stirring rates can be seen to arise from cross nucleation while the sharper change in induction time at moderate stirring rates is due to the combined effects of cross nucleation and enhanced nucleation from enlarged water-oil surface area [42][43] .

Figure 6. Induction time at various stirring rates in a 20% water-in-oil emulsion and pure water, at 2.5 °C, 1.6MPa 3.3. Temperature and pressure conditions



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In previous research [14][44], subcooling and supersaturation effects on hydrate formation were studied. Subcooling was defined as the temperature difference between equilibrium and the current experimental conditions [13]. Supersaturation is the ratio of actual pressure to the hydrate equilibrium pressure [44]. However, there may be differences between theoretical (calculation) and experimental equilibrium conditions [45][46]. In this work, the temperature and pressure were reported instead of subcooling and supersaturation because the hydrate formation or equilibrium conditions in the water-in-oil emulsions were undetermined. To study the effects of the temperature conditions, the experiments were performed at 1.6 MPa and induction times were measured at temperatures of 2.5 °C, 2.0 °C, 1.5 °C, and 1.0 °C. Another set of experiments was conducted at 1.8 MPa, and the induction times were measured at the temperatures of 3.5 °C, 3.0 °C, 2.5 °C, 2.0 °C, and 1.5 °C. Detail information about CO2 hydrate equilibrium temperature in pure water is provided in Table S1, from which the relative subcooling can be easily calculated. The sample investigated was an emulsion with 20% water cut prepared at a stirring rate of 300 rpm. As can be seen in Figure 7, induction times became shorter as the temperature decreased and followed a roughly linear trend as expected. This result indicates that hydrate crystals nucleate more easily at lower temperature conditions due to the stronger driving force. Moreover, since the hydrate formation is an exothermic process, at a lower temperature, the heat released during hydrate formation can be removed more quickly at a lower temperature. [13][47][48] In the work of Meindinyo et.al [39] study of hydrate formation in pure water, hydrate nucleation or growth rate decreased in a roughly linear and inversely proportional relationship with increasing temperature, which is consistent with the experimental results of this work which spanned a limited range of temperatures. Thus, temperature appears to exert the same effect on hydrate induction times in both pure water and emulsions.

Figure 7. Induction time at different temperatures with pressure of 1.8 MPa and 1.6 MPa in a 20% water-in-oil emulsion, at 300 rpm


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Figure 8. Induction time in different pressures at the temperature of 3 °C and 1 °C in a 20% water-in-oil emulsion, at 300 rpm Next, to study the effects of the pressure conditions, experiments were again performed with the emulsion with a fixed water cut of 20% and stirring rate of 300 rpm. Here, the temperature was kept at 3 °C, and the experimental pressures of 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, and 2.0 MPa were investigated. A second set of experiments were carried out at 1 °C, and the corresponding experimental pressures were 1.3 MPa, 1.4 MPa, 1.5 MPa, and 1.6 MPa. Figure 8 shows that the induction time decreased with increased in pressure. A higher pressure corresponds to a stronger driving force, which in turn will increase the rate of the nucleation process [24]. A rational reason for this behavior is that more gas dissolved in the solution in the higher pressure condition [23]. Since gas solubility increases at higher pressure [49][50], the concentration of dissolved gas in the solution is greater at higher pressure. According to the simulations of Liang and Kusalik [51][52], gas composition is a critical parameter for determining nucleation rates. As observed here, the induction time was shorter at higher pressure, which corresponded to larger gas concentration. In Zhou’s [53] work of hydrate formation in pure water, induction time also decreased and followed a roughly linear trend with the increasing pressure. Pressure has the same effect on hydrate induction time in both emulsions and pure water. Based on the results of this work and the previous work of Zhou et.al [53] work. 3.4. Memory effect To investigate the memory effect, induction times were recorded for hydrate formed from ‘fresh’ water and hydrate formed from water which had already been used for a previous hydrate formation experiment. Hydrate formed from ‘fresh’ de-ionized water was called the first hydrate formation and hydrate formed from reused water was called the second hydrate formation. It can be seen from Figure 9 (a) and (b) that the induction time for second hydrate formation was shorter than that for the first formation. This indicates that the memory effect can decrease the hydrate induction time and facilitate the hydrate formation. To help understand this effect, water-in-oil emulsions before the first and second hydrate formation were observed under a microscope. Microscopic images of the emulsions are shown in Figure 10. The images in Figure 10(a) and (b) show a sample from the fresh emulsion at different scales, while the images in Figure 10 (c) and (d) show a sample from the emulsion that was prepared with diesel and melted hydrate. It can be seen in Figure 10(a) and (b) that the emulsions



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show just water droplets in the oil and no other inclusions. However, in Figure 10 (c) and (d), many small bubbles can be observed in addition to the water droplets and oil in the emulsion. The previous hydrate particles are fully dissociated, having been at atmospheric pressure for a long time, and there are no residues remaining in the emulsion. In Figure 10 (c) and (d), the small bubbles are mostly assembled around the water droplets. This indicates that the small bubbles must be CO2 gas from the hydrate particles. Since nucleation occurs when the gas concentration in the solution reached a critical threshold [54][55], in emulsions where CO2 bubbles remain, it will be easier for CO2 to reach this critical threshold and promote hydrate nucleation. The experimental results are consistent with predictions from simulation results that areas locally richer in gas would nucleate more rapidly [55].

Figure 9. Viscosity variation versus time during the first and second hydrate formation. (a) 1.6MPa, 3 °C, 20% water cut, 300 rpm (b) 2MPa, 4 °C, 10% water cut, 200 rpm

Figure 10. Microscopic images of emulsions before the first (a), (b) and second (c), (d) hydrate formation corresponding to the experiments shown in Figure 9 3.5. Effect of wax


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The effect of the presence of wax solid particles on hydrate formation was investigated. The wax used in this experiment was a paraffin mixture from C16 to C30. Wax appearance temperature (WAT) was measured by differential scanning calorimetry (DSC). In the DSC test, a test specimen with wax and a reference specimen with air were heated to 80 °C and then cooled to -20 °C. The hydrate formation temperature must be below the WAT to ensure that the wax was in solid state and not in liquid state when the hydrate formed. The measured WAT was 30.5 °C, which is considerably higher than the hydrate formation temperature. The presence of wax particles has a significant effect on the emulsion viscosity, so it is difficult to determine the hydrate induction time via viscosity variation. Consequently, in the wax effect experiments, the hydrate formation was determined by temperature variation. Figure 11 (a) and (b) show the temperature changes during the hydrate formation process with different wax content. In the experiments, the initial temperature was 35 °C, which was higher than the WAT, and then the temperature was cooled to the target temperature. So the wax precipitated during the cooling process. The temperatures cooled to the target temperature. Since hydrate always formed below 10 °C, it can be confirmed that wax was present in solid state during hydrate formation. The experiments in Figure 11 (a) were all conducted at 2.2 MPa, 20% water cut, 300 rpm stirring rate, and a target temperature of 3.5 °C. The experiments in Figure 11 (b) were at 3 MPa, 20% water cut, 300 rpm stirring rate, and a target temperature of 7.5 °C. Here, the emulsions were prepared after wax was dissolved into the diesel, so, the water cut was relative to total volume of diesel and dissolved wax. It can be seen in Figure 11(a) that the hydrate was formed most rapidly in the emulsion without wax, followed by the emulsions with 3%, 5%, and 8% wax content, respectively. There was no hydrate formation observed in the emulsion with 10% wax content over 1000 min. Figure 11(b) shows similar behavior. The hydrate formation was slower with more wax content and no hydrate was observed to form in the system with 12% wax. Figure 11 (a) and (b) show somewhat different temperature trends because of the difference between the target temperature and equilibrium temperature. The current experimental results were not consistent with the earlier hypothesis that the wax solid can provide nucleation site and promote hydrate formation [56]. In this series of experiments, all of the emulsions were with the same water cut, which meant that there was the same water-oil surface area or same number of hydrate formation surface sites. However, the hydrate induction time increased with increasing wax content, which indicated that the presence of wax particles provided possible modes of action including adsorption on the water-oil surface or encasement of the water droplets. Moreover, the cross nucleation could also be decreased. Further sets of experiments were performed in order to clarify how the presence of solid wax particles was slowing hydrate formation. It was also found that the hydrate induction time decreased as the water cut increases with fixed wax content, suggesting that the water droplets became encased in wax. These findings will be presented in detail in a forthcoming publication. Figure 12 illustrates the effects of the solid wax particles on hydrate formation. As shown in Figure 12(b), at intermediate wax content, parts of the water-oil surface are covered by wax particles. Thus, the area of nucleation sites may be decreased, and the probability of cross nucleation is reduced as shown by the red dashed arrows (between droplets A and B, and B and D). In Figure 12(c), there is enough wax to fully encase the droplets thus essentially preventing hydrate nucleation. In Mohammadreza’s [10] phase equilibrium calculations, the formation of solid wax particles led to a decrease in the content of heavy alkanes and an increase in light hydrate components in the



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liquid, which resulted in a higher required pressure to form hydrate after wax solid particles formation. While the change in composition is the main factor in simulation, in the present experiments the pressure of CO2 was essentially constant, no changes in components to be considered in this experiment. Here, CO2 gas was used to form hydrate and wax formation had essentially no effect on the CO2 component. The formed wax solid exerted the dominant effect on hydrate formation. In practice, both changes in composition and formation of solid wax particles can impact hydrate formation. Therefore, the effects of both factors should be considered in the analysis of the wax effect on hydrate formation in the real multiphase pipelines.

(a)

(b) Figure 11. Temperature variation during the hydrate formation process with various amounts of wax present. (a) corresponds to experimental conditions of 2.2 MPa, 20% water cut, 300 rpm, and a target temperature of 3.5 °C, while (b) data corresponds to experimental conditions of 3 MPa, 20% water cut, 300 rpm, and a target temperature of 7.5 °C, respectively. Note the discontinuous time scale axes. The green dashed lines represent equilibrium temperatures (see Sloan for more details [57]).


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Figure 12. Schematic diagram of water droplets with different wax content: (a) no wax; (b) some wax; (c) enough wax to encase water droplets. The blue arrows mean successful cross nucleation, red dashed arrows mean unsuccessful cross nucleation. 4. Conclusions The effects of water cut, stirring rate, temperature, pressure, memory effect, and wax content on the induction time of hydrate formation in water-in-oil emulsion were studied. The induction time of hydrate formation was determined by viscosity variation. As expected, the induction time decreased with either decreasing temperature or increasing pressure. Although the induction time decreased with increased water cut, the experimental induction times were always higher than the theoretical prediction in water-in-oil emulsion based solely on water cut. The induction times appeared to depend on droplet size and water cut. After the phase inversion point (i.e. in oil-in-water phase), the induction times appeared to depend solely on water cut. This phenomenon indicated that hydrate initially nucleated at the water-oil surface in water-in-oil emulsions but formed in the water bulk in water-rich emulsions. At low stirring rate, the induction time can be seen arise from cross nucleation. The change in induction time became sharper at moderate stirring rate due to the combined effects of cross-nucleation and enhanced nucleation from increased water-oil surface area. Memory effect was observed as the induction times were found to be shorter during the second hydrate formation than the first one as some CO2 bubbles were found to remain in the emulsion. Hydrocarbon gases will be used in future work to provide further insights into real flow assurance scenarios [58]. Solid wax particles in the emulsion were observed to prevent hydrate formation and the preventive effect was higher with greater amounts of wax.

Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (No.51374224 and No.51534007), which are gratefully acknowledged. PGK is grateful for the financial support of the Natural Sciences and Engineering Research Council of Canada (Grant No. RGPIN-2016-03845).

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