Experimental Investigation on the Gelation Process and Gel Structure

Dec 15, 2016 - emulsions, and their gelation ultimately. Existing research has revealed that the GT2,4 or the gel point5 of a water-in-waxy crude oil ...
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Experimental Investigation on the Gelation Process and Gel Structure of Water-in-Waxy Crude Oil Emulsion Guangyu Sun,*,†,‡ Chuanxian Li,† Fei Yang,† Bo Yao,† and Zuoqu Xiao† †

College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong Province 266580, China Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao, Shandong Province 266580, China

‡

ABSTRACT: The W/O emulsion formed by waxy crude oil tends to gelatinize under low-temperature gathering conditions, which may harm the flow assurance of multiphase transportation. This study mainly focused on the wax-precipitating gelation process of crude oil emulsions, and the structural differences of the emulsions with different water fractions at their gelation temperatures (GTs). First, the changes of wax appearance temperature (WAT) and precipitated wax amount at GT with variable water cut were investigated by DSC. Then, the specific structural properties of these different water cut emulsions at their own GTs were investigated by viscoelastic, yielding behavior, and structural breakdown experiments. On these bases, the gelation mechanism of a waxy crude oil emulsion was deduced. The WAT of a waxy crude oil emulsion was found to rise, evidently with the addition of dispersed water, verifying that water droplets provide necessary nucleation sites for paraffins and thus make them more prone to precipitate. When temperature was cooled down to their respective GTs, the accumulative precipitated wax amount within the emulsions was obviously reduced with increasing water cut. The combination of this phenomenon and further microscopic experiments proved the difference in gelation mechanism. These rheological tests manifested that both storage modulus and yield stress reached their minimum values at about 30% water cut when all emulsions were kept at their respective GTs, while the flow behavior index and the structural breakdown rate came to their maxima at the same water cut. These results are different from what have been known about the rheological properties at the same temperature below the gel point, demonstrating that the structural strength of the emulsion with about 30% water cut is the weakest at the GT. A conclusion could be finally deduced from the above research that, with the dispersed water increasing, the gelation mechanism of a waxy crude oil emulsion is changed, from a spanned wax crystal network structure which traps separated water droplets to an interlinking droplet flocs structure whose interface is adsorbed by a small amount of wax crystals.

1. INTRODUCTION Both the producing area and the output of waxy crude oil have been increasing with the continuous exploitation of petroleum resources worldwide. During gathering and transportation in oil fields, crude oil and water are mainly transported in the form of emulsion. Since the polar fractions in crude oil, such as resins and asphaltenes, may act as natural surfactants, the emulsion tends to be in good stability. As is known to all, the rheological properties of waxy crude oil emulsion are affected significantly by temperature.1 Once the temperature drops below the wax appearance temperature (WAT), wax molecules in the stable waxy crude oil emulsions will crystallize and precipitate out due to depressed solubility and consequent supersaturation. The precipitated wax crystals will adsorb on the droplet surface, or interlink with each other in liquid oil and further form a spongy network structure,2,3 inducing poor flowability of the crude oil emulsions, and their gelation ultimately. Existing research has revealed that the GT2,4 or the gel point5 of a water-in-waxy crude oil emulsion is notably increased with the addition of dispersed water. However, all the present research focuses on the evolution of the rheological properties with different water cut at the same temperature,2−10 while the differences of the structural characteristics among different water cut emulsions at their own GTs have not been investigated yet. Besides, the specific reason which causes a crude oil emulsion to gelatinize is also not clear. Since the produced water content in a well generally increases over its © XXXX American Chemical Society

lifetime, it would be very useful to investigate the increasing impact of dispersed water on both the gelation and gel rheology of a crude oil emulsion.11 Besides, the gel point or pour point is usually taken as the sole principle for transportation safety judgment in current petroleum industry standards.12 For example, according to “The operation regulation of crude oil pipeline” (SY/T 55362004) in China, the principle of safe and economical transportation is set as follows: the lowest inlet temperature of a hot oil pipeline should be 3 °C higher than the gel point of the transported crude oil. In fact, there may be differences in rheological properties (such as yield stress) among different waxy crude oils or crude oil emulsions at their gel/pour points.13 Therefore, considering the gel/pour point as the only principle may bring threats to flow assurance. Meanwhile, with the continuously rising requirement of energy conservation, lowering the operating temperature at each production sector is gradually becoming an inevitable trend, so it is necessary to study the gelation state of waxy crude oil and its emulsions for determining the operating temperature scientifically. Based on the above considerations, this study mainly focuses on the gelling process of waxy crude oil emulsions with different water cuts, and the rheological properties at their GTs, Received: September 6, 2016 Revised: December 15, 2016 Published: December 15, 2016 A

DOI: 10.1021/acs.energyfuels.6b02253 Energy Fuels XXXX, XXX, XXX−XXX

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once. The mixture was then stirred with an IKA RW20 digital stirrer (IKA group, Germany) equipped with a four-blade paddle at 50 °C. The microstructure of the prepared crude oil emulsions was observed with an OLYMPUS BX51 polarizing microscope (Olympus Co., Japan) equipped with a Linkam LTS350 automatic thermal stage (Linkam Scientific Instruments Ltd., UK). The temperature control accuracy is ±0.1 °C. The microscopic pictures were taken with a CoolSNAP 3.3 M digital CCD camera (Roper Scientific, Inc., US) designed for microscopy. Every emulsion was sampled twice for microscopic examination, and 8 photographs from different visual fields were taken for each sample by moving the glass slide, in order that 16 microscopic images could be acquired from one certain water cut emulsion. Then the amount and size of the droplets in these photographs were analyzed by the software ImageJ (National Institute of Mental Health, US) to obtain the Sauter mean diameter (D32). In order to get similar D32 values among different water cut emulsions, the droplet size analysis was utilized. First, the emulsion was prepared at a certain stirring speed and its D32 was analyzed; then the speed was adjusted according to the D32. Ultimately, the emulsions with basically the same D32 were prepared. In addition, for ensuring the repeatability of rheological measurement results, no demulsification needs to be guaranteed during the entire protocols. For this reason, the prepared different water cut emulsions were put into colorimetric tubes and then maintained at 50 °C in the water bath. Only if there was no free water separated in 6 h could the emulsion be used in rheological experiments. Meanwhile, the sample in the measuring system was checked after every test so as to make sure that no instability or even demulsification happened during tests. 2.3. Rheological Tests. Note that all rheological tests for the waxy crude oil and its emulsions were performed with an AR-G2 stresscontrolled rotational rheometer (TA Instruments, US) equipped with a coaxial cylinder measuring system. (1) GT measurement. The GTs of waxy crude oil and its emulsions were determined by small amplitude oscillatory shear (SAOS).19,20 First, the sample was loaded into the cylinder and kept at 50 °C for 10 min. Then, it was set to be cooled at a constant rate of 0.5 °C/min. At the same time, the SAOS was applied on the sample until the storage modulus rose to be equal to the loss modulus. The temperature corresponding to this condition is defined as the GT. During the SAOS test, it has to be sure that the test is performed in the linear viscoelastic region and no damage is made to the sample structure, so the oscillating strain amplitude was set to be 0.0005. The oscillation frequency was set as 1 Hz. (2) Viscoelasticity measurement. After the sample was loaded into the cylinder and kept at 50 °C for 10 min and then cooled to the GT at 0.5 °C/min, it was maintained at this temperature for 45 min, so the structure could fully develop. Afterward, the SAOS was carried out to obtain viscoelastic parameters. In the rest of the rheological tests below, the procedures before structure development at their GTs were the same as the ones described in this section. (3) Yield behavior measurement. The yield stress and yield strain at GTs were determined by the controlled shear rate mode. The constant shear rate of 1 s−1 was applied until the structure of the sample yielded. The maximum stress during the shearing is defined as the yield stress, and the corresponding strain at this moment is defined as the yield strain.6,21,22 (4) Stepwise shear rate test. The stepwise increasing shear rates were loaded to the samples at their respective GTs. The step values were set as 1 s−1, 2 s−1, 4 s−1, 8 s−1, 16 s−1, 32 s−1, and 64 s−1, respectively, and the duration at each shear rate was 10 min.

so that the differences of the gelation mechanism and the gel structure between different water cut emulsions could be figured out. Accordingly, the scientificity of the treating gel/ pour point as the operation principle could be assessed, thereby serving the flow assurance of multiphase transportation better. In addition, this study could also provide valuable information for the low-temperature gathering technology design in frigid and low-production oilfields.

2. EXPERIMENTAL SECTION 2.1. Materials. A waxy crude oil produced from an offshore oil field in China was utilized for preparing emulsion. Distilled water was used as the water phase, with no emulsifying agent added during the preparation. In order to prevent volatilization during emulsion preparation and subsequent tests, the waxy crude oil was stirred for 2 h at 80 °C in a beaker to eliminate light ends. Then, to ensure the repeatability of experimental results, the crude oil was pretreated before emulsion preparation, so that the thermal and shear history could be removed. The pretreatment procedures are as follows. First, the uniformly mixed crude oil was divided evenly into 125 mL of ground stopper bottles. Then the sealed bottles were heated to 80 °C and kept at this temperature for 2 h in a water bath. Finally, the samples in the bottles were cooled quiescently and maintained at room temperature for 48 h before usage.14 The basic physical properties of the pretreated waxy crude oil are listed in Table 1. The WAT was measured with a DSC 821e

Table 1. Physical Properties of the Pretreated Waxy Crude Oil Used for Emulsion Preparation physical parameter

value

density at 20 °C (kg/m3) GT after 50 °C heat treatment (°C) WAT (°C) wax content (wt %) resins content (wt %) asphaltenes content (wt %)

872.1 30.2 40.12 14.33 5.92 1.04

calorimeter (Mettler-Toledo Co., Switzerland). The accumulative precipitated wax content at a certain temperature was determined as follows.15−18 First, the releasing heat due to wax crystallization was calculated by integrating the area surrounded by the heat flow curve and the baseline from WAT to the designated temperature in the DSC thermal chart. Then the precipitated wax content in this temperature range was obtained through the division of the releasing heat by the paraffin crystallization enthalpy, as demonstrated in eq 1 WAT

CW =

∫T

dQ

W

Q̅

(1)

where CW is the concentration of precipitated wax (i.e., the mass ratio of precipitated wax crystals to the sample), wt %; TW is the temperature at which the accumulative precipitated wax content needs to be determined, °C; dQ is the releasing heat due to wax crystallization from (T + dT) to T, J/g; and Q̅ is the average paraffin crystallization enthalpy, which depends on the composition of paraffins, J/g. For complex paraffin mixtures in waxy crude oil, the enthalpy is suggested to be 210 J/g.15,17 The contents of resins and asphaltenes were determined according to the ASTM D4124-09 “Standard Test Method for Separation of Asphalt into Four Fractions”. 2.2. Emulsion Preparation and Stability Assurance. After the crude oil and water were separately maintained in a water bath at 50 °C for 30 min, they were added into a 200 mL beaker sequentially according to their volume ratio, with the total volume being 50 mL. The crude oil was added first, and then the water was poured in all at

3. RESULTS AND DISCUSSION 3.1. Effect of Water Droplets on the Wax-Precipitating Gelation Process of Waxy Crude Oil Emulsions. The B

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WAT is markedly increased by the addition of water. This conclusion is supported by a recent study of Piroozian et al.26 but does not agree with the experimental result by Li and Gong.27 According to the result of Li and Gong, the change in WAT for different water cut emulsions does not exceed 0.15 °C, which indicates that the effect of water droplets on WAT is insignificant. The specific WATs of different water cut emulsions in this study are listed in Table 2. When the water

thermodynamic behavior during the cooling process was analyzed for different water cut emulsions by DSC test. Take, for example, the crude oil emulsion with 40% water cut. As demonstrated in Figure 1, there is a minor-scale increase in heat

Table 2. WATs of the Waxy Crude Oil Emulsions with Different Water Cuts

Figure 1. Heat flow curve of the waxy crude oil emulsion with 40% water cut.

water cut (vol %)

WAT (°C)

0 10 20 30 40

40.12 40.37 41.86 43.01 43.83

cut exceeded 40%, the heat flow began to show an unsteady state. As a result, the WAT could not be accurately acquired. The wax disappearance temperature (WDT) of the original waxy crude oil was further tested to be 42.18 °C. This WDT is 2.06 °C higher than its WAT because of the existence of supercooling in the WAT test, but it is still lower than the WAT of the emulsions with water cut higher than 30%. This means that the wax in the crude oil crystallizes at a lower temperature than that in the emulsions even if there is supercooling in the emulsions. That is to say, the increase of WAT does not result from the decrease of supercooling degree in the emulsions, but truly from the increment of the paraffin crystallization temperature. From the increase of wax crystallization temperature with water cut, it can be deduced that the water droplet interface provides necessary nucleation sites for wax molecules, making the precipitation a heterogeneous nucleation process. Actually, there has been a microscopic experiment which directly proved the existence of wax crystals on the oil−water interface of waxy oil emulsions.3 The result in this study offers another piece of strong supporting evidence for certifying the precipitation of wax crystals on droplets. In fact, the reason why wax molecules precipitate on the droplet interface is that the interface is covered by asphaltene molecules.28,29 The covering asphaltene molecules self-aggregate and form a cross-linking network structure on the interface.30,31 It is the self-aggregated asphaltenes on the interface that provide more available sites for wax molecules to crystallize, which ultimately leads to the rising of the WAT. Next, the GTs of the crude oil emulsions were determined according to SAOS results, as shown in Figure 3. The change of the GT with water cut matches well with previous research.2,4 Note that the rheological parameters of the waxy crude oil and its emulsions mentioned hereinafter were all measured at their respective GTs. According to the wax-precipitating heat flow curves and GTs of the waxy crude oil emulsions, the precipitated wax contents of different water cut emulsions could be calculated by eq 1 when they just reach the gelatinous state. The precipitated wax contents of the crude oil and its emulsions at their respective GTs are listed in Table 3. It is clearly demonstrated that the required precipitated wax amount for gelation is diminished with the increase of dispersed water droplets. Specifically, paraffins accounting for up to 1.254 wt % of the sample have to

flow when the temperature is lowered to about 44 °C. This is attributed to the exothermic crystallization process of paraffins. When the temperature is further cooled down to −19 °C or so, a sharp exothermic peak emerges. The heat flow rises to an extra high level. This phenomenon is only possessed by the crude oil emulsions with water cut above 20%, but not by the original waxy crude oil, implying that water droplets in the emulsions are frozen at this temperature. In comparison with the common fact that water freezes at 0 °C, there is quite a high degree of supercooling. In the study by Oh and Deo,23 the water droplets in the model oil emulsion freeze at around 0 °C. In the study by Matsumoto et al.,24 the water droplets in the silicone oil emulsion freeze when the temperature is lowered to −3.6 °C, but the precondition is that 0.1 mL of ice is added into the emulsion as crystal nuclei, so it is safe to say that the supercooling degree is more than 3.6 °C. Moreover, Rensing et al. found out that ice nucleation forms in water-in-crude oil emulsions after being kept at −10 °C for 50−120 min.25 As can be seen, the supercooling degree becomes high when the continuous phase is crude oil. It is believed that the decrease of the freezing point could be ascribed to the partial dissolution of some components of crude oil in water droplets. By comparing the flow curves of the original waxy crude oil and its emulsions in Figure 2, the conclusion reveals that the

Figure 2. Heat flow curves and WATs of the waxy crude oil and its emulsions. C

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Figure 5. Water droplet distribution in the waxy crude oil emulsions: (a) 20% water cut and (b) 50% water cut.

interlinking with each other. As dispersed droplets gradually increase, the amount of precipitated wax crystals at the GT declines accompanied by smaller particle size. At the same time, the droplets begin to aggregate. Hence, the intrinsic gelation mechanisms for waxy crude oil and high water cut emulsion are different. 3.2. Effect of Water Droplets on Structural Strength at the GT. (1) Viscoelasticity of the gel structures at their GTs. After the gel structures reached equilibrium at their own GTs, the storage modulus and loss angle were determined by SAOS, whose results are illustrated in Figure 6. It is observed that a

Figure 3. Evolution of GT with the water cut of the waxy crude oil emulsion.

Table 3. Precipitated Wax Contents of the Crude Oil and Its Different Water Cut Emulsions at Their Respective GTs water cut (vol %)

weight ratio of precipitated wax crystals to oil phase (wt %)

weight ratio of precipitated wax crystals to sample (wt %)

0 10 20 30 40

1.254 1.030 0.983 0.956 0.857

1.254 0.914 0.764 0.641 0.486

be precipitated for the original crude oil to reach the gelatinous state. This agrees with the result of Kané et al.32 that crude oil gelation appears for crystals around 1−2% by weight when the oil is cooled in quiescent conditions. By contrast, when the dispersed water fraction reaches 40%, wax crystals accounting for only 0.486 wt % of the emulsion sample could cause gelation, which certainly implies that the gel structure of the emulsion differs from that of crude oil. The difference in gel structure could also be told through microscopic photographs. The polarized light was set to observe the morphology of wax crystals, while the direct light was set to observe the distribution of droplets. The results are displayed in Figure 4 and Figure 5, respectively. It can be seen that the wax crystal amount in the crude oil is abundant at its GT, and the size of the crystals is relatively larger due to their

Figure 6. Evolutions of the storage modulus and loss angle of the waxy crude oil emulsions with water cut at their respective GTs.

turning point in the storage modulus emerges at the water cut around 30%. When the water cut is lower than 30%, the storage modulus of the gel structures at their GTs decreases gradually with increasing water fraction. This is an easy-to-understand phenomenon because the precipitated wax amount decreases with increasing water cut. However, when the proportion of dispersed water exceeds 30%, the storage modulus manifests an upward trend with the addition of the water phase. This interesting phenomenon indicates that some changes happen in the gel structure when the water cut is increased. In the meantime, the loss angle of the emulsions at their own GTs rises monotonically with increasing dispersed water, implying that the viscidity and, namely, the energy dissipation feature are intensified. (2) Yield behavior of the gel structures at their GTs. The yield stress and yield strain of the waxy crude oil and its emulsions at their GTs were measured by the constant shear rate loading method. Similar to the storage modulus, the yield stress also shows a turning point at about 30% water cut, as illustrated in Figure 7. These coincident experimental phenomena signify that the structural strength of the samples at their own GTs weakens first with the increase of water droplets, because the structural strength depends on the wax amount in the gel network,33 and then is enhanced after the

Figure 4. Precipitated wax crystal images of the waxy crude oil and its emulsions at their own GTs: (a) the waxy crude oil, (b) the emulsion with 20% water cut, and (c) the emulsion with 50% water cut. D

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Figure 7. Evolutions of the yield stress and yield strain of the crude oil emulsions with water cut at their respective GTs.

Figure 9. Stress response of the waxy crude oil and its emulsions under the stepwise shear rate loading at their GTs.

dispersed water reaches a certain volume fraction. This may have something to do with the formation of droplet flocculation inside the crude oil emulsions. Different from the variation trend of the yield stress, the yield strain of the samples at their GTs rises with increasing water cut all the way. This means that a larger deformation is required during the pipeline restart process for the inside fluid to yield and flow; that is, the start-up process is retarded. When the temperature is further cooled to 28 °C (2.2 °C below the gelation temperature of the waxy crude oil), both the storage modulus and the yield stress turn to increase monotonically with increasing water content, as demonstrated in Figure 8, which is distinct from the results at their GTs. This monotonic increasing trend of the storage modulus and the yield stress agrees with previous studies,2,4−7,9,10 confirming the enhancement of gel structure by dispersed water droplets when the wax precipitates at the same temperature. However, when the crude oil emulsions just reach their gelation states, the waxprecipitating conditions are different, and the gel structures are composed of different amounts of wax crystals and water droplets. As the water content rises, the amount of wax crystals decreases in the gel structure, whose integrated effect makes the gelled emulsions show the unique nonmonotonicity in Figures 6 and 7. 3.3. Effect of Water Droplets on Structural Breakdown and Apparent Viscosity at GTs. (1) Structural breakdown behavior. The waxy crude oil and its emulsions were sheared by the stepwise shear rates at their GTs, and the stress responses were recorded, as shown in Figure 9. Although the yield stress in this test still presents the trend of first decreasing and then increasing, the shear stress after yielding is elevated monotoni-

cally with the increase of water cut at each shear rate step. As we all know, the elasticity of the samples can be ignored after the shear-induced structural breakdown, so the total shear stress is almost entirely composed of viscous stress. In other words, the apparent viscosities at all shear rates are increased with the addition of water cut, which means that the viscidity of the samples is intensified with increasing water cut at their GTs. In order to further quantitatively compare the structural breakdown kinetics of the crude oil emulsions, the flow curves in Figure 9 were described by the elasto-viscoplastic structural kinetics model proposed by Teng and Zhang.34 The stepwise shear rate loading is a very useful test method for gaining a model parameter. The shearing time of 10 min at each shear rate was set to include both the rapid structural breakdown information at the initial moment of shearing and the nearly steady state information after a period of shearing. Additionally, the purpose of the step changes in shear rate was to embody the structural breakdown processes at different shear rates. This is also an efficient means to uncover more information about the structural breakdown process so that the structural kinetics model could express the physics more comprehensively. The parameters of the model were calculated from the stepwise shear rate loading experiment by using the data of total shear stress τ, shear rate γ̇, and shearing time t. A MATLAB program with the nonlinear least-squares method was developed to obtain the model parameters via fitting to the data. The two kinetic equations for the structural parameter λ and the elastic strain γe were numerically solved by the fourth order Runge− Kutta discretization method. As a result, the elastic and viscous parameters along with the kinetic parameters during the structural breakdown process could be obtained. The equations

Figure 8. Evolutions of the viscoelastic properties and yield behaviors of the waxy crude oil emulsions with water cut at 28 °C: (a) storage modulus and loss angle, and (b) yield stress and yield strain. E

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Table 4. Fitted Model Parameters of the Crude Oil and Its Emulsions at Their GTs from the Stepwise Shear Rate Tests water cut

G0

Δk

k

n1

n2

a

b

m

p1

p2

(vol %)

(Pa)

(Pa·sn1)

(Pa·sn1)

(−)

(−)

(s−1)

(Pa−m·sm−1)

(−)

(−)

(−)

(%)

0 10 20 30 40 50 60

464.7 182.3 135.1 73.6 124.9 135.7 327.0

9.287 10.417 11.300 11.392 19.132 27.527 35.227

0.674 0.762 0.852 0.968 1.260 1.739 2.396

0.859 0.862 0.882 0.893 0.872 0.861 0.789

0.994 0.963 0.995 1.041 1.067 1.254 1.237

0.1386 0.1501 0.1621 0.1734 0.1497 0.1366 0.1296

0.762 0.753 0.767 0.770 0.744 0.760 0.963

55.04 14.84 20.00 16.78 17.78 238.16 430.59

1.068 1.259 1.144 1.184 1.304 1.521 3.119

0.59 0.63 0.52 0.48 0.46 1.14 1.42

8.66 6.42 6.00 1.69 2.30 2.34 2.31

10−3 10−3 10−3 10−3 10−14 10−14 10−14

reason why the viscosity of the crude oil emulsion is unable to reach equilibrium lies in the existence of water droplets. To validate the speculation, the crude oil emulsions were sheared at the constant rate of 0.5 s−1 for hours at 46 °C, which is higher than the WATs, with the purpose of excluding the influence of wax crystals. The results are displayed in Figure 10,

of the model are given in eq 2, and the values of the model parameters are listed in Table 4. ⎧ τ = λG0h(γ )γ + (1 − λ) ·(λΔk + k)γ ṅ 1 e e ⎪ ⎪ 1 h(γe) = ⎪ 1 + p1 ·γe p2 ⎪ ⎪ ⎪ dλ 1 m = ⎨ n2 [a(1 − λ) − bλϕ ] + γ dt 1 ⎪ ⎪ dγe ⎪ = [g1 − (1 − g1)sλ]̇ λ ̇ ⎪ dt ⎪ p2 ⎪ g1 = e−(p1·γ ) ⎩

× × × × × × ×

AARDs

(2)

As revealed by the average absolute relative deviations (AARDs) in Table 4, the elasto-viscoplastic model could describe the structural breakdown process quite well. The shear modulus G0 also shows the trend of first decreasing and then increasing with the increase of water cut, which is in line with the trend of the storage modulus determined by SAOS. Moreover, the consistency coefficients k and Δk are notably increased with increasing water cut, proving again the enhancement of viscidity. By contrast, the kinetic index n1 of the samples presents the trend of first increasing and then decreasing, with the maximum value appearing at 30% water cut. This evolution manifests that the crude oil emulsion with 30% water cut turns out to be the least non-Newtonian fluid at its GT. When the water cut is lower than 30%, wax crystals precipitate more in the gel structure at GT; when the water cut is higher than 30%, more water droplet flocs form in the gel structure. Both situations make the non-Newtonian flow behavior of the samples more evident. In addition, the maximum value of the shear-induced breakdown kinetic parameter b also appears at 30% water cut, manifesting that the gel structure with 30% water cut breaks fastest at its GT when the same shear condition is loaded. Compared with the breakdown kinetic parameter b, the structural buildup rate a is rather slow during the shearing process. (2) Apparent viscosity. As illustrated in the stepwise shear rate experiments in Figure 9, the apparent viscosity is increased with the addition of dispersed water at their GTs. Nevertheless, the equilibrium viscosity was not reached yet during the 10 min of shearing at every shear rate step. Therefore, further tests were conducted to extend the shearing time, but they display that the apparent viscosity of higher water cut emulsions still could not reach equilibrium. Existing research makes it clear that the apparent viscosity of waxy crude oil can reach equilibrium under constant shear rate loading condition.35,36 Hence, it can be speculated that the

Figure 10. Change of apparent viscosity with long time shearing for the crude oil emulsions with different water cuts at 46 °C and 0.5 s−1.

from which we can see that the apparent viscosity of lower water cut emulsions (e.g., 10% and 30% in the Figure) could reach equilibrium. The viscosity of the 10% water cut emulsion even shows a very slightly upward trend with time, which may result from the volatilization of the still remaining light components in the oil phase during the long time shearing. However, for the emulsion containing 50% water, its apparent viscosity cannot reach equilibrium even after 9 h of shearing. The emulsion was checked after the test, and no oil−water separation was found, proving that the reduction of the apparent viscosity was not caused by demulsification. Obviously, the evolution of the viscosity of the 50% water cut emulsion in Figure 10 can be divided into two stages. When the shear is just applied to the sample, its apparent viscosity falls off sharply. This may be at the droplet floc breaking stage during which the interlinkages between droplets are disconnected by the shearing action. The more and more dramatic decrease of the viscosity in this stage with increasing amount of water droplets contributes to support this viewpoint. The following stage is characterized by the slow reduction of the apparent viscosity. The gradual coalescence of the droplets under long time shearing may be responsible for this phenomenon. As we know, the droplets in higher water cut emulsions are inclined to coalesce because of the larger amount of droplets and the smaller distance between these droplets. On the contrary, the droplets in lower water cut emulsions are smaller in number and the contact between droplets is less likely to take place, so the possibility of coalescence is small. F

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Energy & Fuels 3.4. Discussion on Gelation Mechanism. According to Visintin et al.,2 the gelation process of waxy crude oil emulsion can be stated as follows. First, the wax particles are strongly adsorbed at the oil−water interface with decreasing temperature, forming Pickering emulsions. Then, the flocs of solid paraffin continuously grow on water droplets or between them. At last, gel behavior emerges when the entire volume is spanned by a wax crystal network and the dispersed water is entrapped in it. This is the gelation process when waxy crude oil emulsion is cooled to a very low temperature. In the study by Visintin et al., the GTs of the crude oil emulsions were around 30 °C, while the gel structure was studied at the low temperature of 5 °C. From another point of view, Paso et al. maintained that van der Waals attractive forces give rise to droplet flocculation when dispersed water droplets are rich in emulsion, and the droplet flocculation network can immobilize a significant portion of the continuous phase.6 The experimental results in this study suggest that the gelation mechanism of the waxy crude oil emulsion is essentially changed with the continuous addition of dispersed water. For waxy crude oil, there is no doubt that its gelation is owing to the formation of a wax crystal network structure.37 When waxy crude oil is emulsified by more and more water, the amount of dispersed droplets per unit volume is accordingly increased, and thereupon, the required wax crystals for the emulsion to reach the gelation state are lessened. Figure 11 can

The WAT of the waxy crude oil emulsions rises evidently with the increase of water cut, indicating that water droplets provide nucleation sites for paraffins, which makes wax crystals more prone to precipitate. At the GTs of the waxy crude oil emulsions, the precipitated wax amount is obviously reduced with increasing water cut. These phenomena, along with the microscopic experiments, prove the difference in the gelation mechanism between different water cut emulsions. At their respective GTs of the waxy crude oil emulsions, both the storage modulus and the yield stress reach their minimum values at around 30% water cut, while the kinetic index n1 and the structural breakdown rate b in the structural kinetics model reach their maximum values at this water cut. These results demonstrate that the structural strength of the emulsion with around 30% water cut is the weakest when all emulsions are kept at their own GTs. By combining the studies of the wax-precipitating gelation process and the rheological properties of the waxy crude oil emulsions, it can be deduced that the gelation mechanism of waxy crude oil emulsions changes with the addition of dispersed water, from the formation of a structure dominated by a wax crystal network which entraps the separated water droplets to formation of one dominated by the droplet flocs whose interface is adsorbed by wax particles.

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Corresponding Author

*Tel: 86-532-8698-1224. Fax: 86-532-8698-1822. E-mail address: [email protected]. ORCID

Guangyu Sun: 0000-0002-3598-3807 Fei Yang: 0000-0002-0705-0006 Figure 11. Schematic of the gelation mechanisms of waxy crude oil emulsions with different water fractions: the small black dots represent wax crystal particles; the big blue filled circles represent water droplets; the red empty circles surrounding the droplets represent the oil−water interface; the gray background represents the continuous liquid oil phase.

Notes

be used to illustrate the change of gel structure with dispersed water fraction. At low water fraction (Figure 11a), to reach the gelation state, more wax crystals need to be precipitated to form a network structure which can entrap water droplets. By contrast, water droplets form a flocculation structure at high water fraction (Figure 11c), so only a small amount of wax crystals is needed whose function is enhancing the oil−water interface by adsorbing at the droplets. For waxy crude oil emulsions with middle water fraction (Figure 11b), neither a strong wax crystal network structure nor a water droplet flocculation structure exists in the emulsions when the gelation state is reached. The oil−water interface just contributes to the formation of the gel structure by providing bonding sites for the precipitated wax crystals,7 so the framework of the gel structure is weak, showing the weakest elasticity and the lowest yield stress.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the China Postdoctoral Science Foundation (No. 2016M592270) and Shandong Provincial Natural Science Foundation (No. ZR2016EEB19) is gratefully acknowledged.

4. CONCLUSION The wax-precipitating gelation process of waxy crude oil emulsions with different water cuts and the structural characteristics at their respective GTs were investigated in this study. On this basis, the gelation mechanisms of different water cut emulsions were discussed. G

NOMENCLATURE τ = total shear stress, Pa γ̇ = shear rate, s−1 γ = total shear strain, dimensionless t = shearing time, s λ = non-negative scaled structural parameter, varying between the value of 0 for a completely broken-down structure and 1 for fully developed structure, dimensionless G0 = shear modulus of the completely structured material (λ = 1), Pa γe = elastic strain of the continuous network structure, dimensionless Δk = structure-dependent consistency, Pa·sn1 k = completely unstructured consistency, Pa·sn1 n1 = kinetic index which describes the viscous stress’s dependence on shear rate, dimensionless p1, p2 = parameters related to the viscoelastic property, dimensionless n2 = positive constant, dimensionless a = kinetic constant for structural buildup, s−1 b = kinetic constant for shear-induced breakdown, Pa−m·sm−1 DOI: 10.1021/acs.energyfuels.6b02253 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels ϕ = rate of energy dissipation, which is defined as ϕ = τγ̇ in simple shear flow, Pa·s−1 m = dimensionless constant s = characteristic time with the value of 1 s

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DOI: 10.1021/acs.energyfuels.6b02253 Energy Fuels XXXX, XXX, XXX−XXX