Practical Modification of Tannic Acid Polyether Demulsifier and Its Highly Efficient Demulsification for Water-in-Aging Crude Oil Emulsions.

In order to break the aging crude oil (WACO) emulsion of the offshore platform more effectively, a highly active isocyanate, polyaryl polymethylene isocyanate (PAPI), was selected to modify the pilot-scale tannic acid demulsifier. In the addition of PAPI, its molecular weight and viscosity dramatically increased, while its relative solubility, hydroxyl number, and cloud point exhibited an opposite direction, showing an increase in hydrophobicity. After adding the above modified demulsifier, a remarkably improved water removal of WACO emulsion accompanied by a notable reduction of the water content in the oil phase monitored by the Karl Fischer method was observed. Demulsification on the offshore platform demonstrated that the best water removal was achieved when the proportion of PAPI is 1.5 wt %. Its demulsification efficiency reached 95.7%, which was 25.6% higher than the 76.2% of unmodified demulsifier. In addition, a positive correlation between viscoelasticity of emulsion and demulsification performance was found by only adjusting the parameters of the rheometer. This method may be utilized to characterize the demulsification performance by any rotary rheometer. The pilot-scale demulsification experiment demonstrated that the water removal can reach 98.14 vol % and residual water content was only 0.55 vol %. These results further confirmed the excellent demulsification performance of the modified demulsifier toward the WACO emulsion in production.

Introduction

In order to improve the crude oil recovery, polymer flooding has been extensively applied. High concentration of oily sewage is produced in demulsification, and the collected crude oil in the addition of a water-cleaning agent and other additives is the main source of aging crude oil emulsions. Containing various oilfield additives[1] and chemical changes caused by oxidation,[2−4] water-in-aging crude oil (WACO) emulsion is fairly stable;[2,3,5] therefore, demulsification of WACO emulsion is a great challenge, especially for the offshore platform. Moreover, resulting from the actual need in the crude oil production, the concentration of the additives is adjusted in a specified range. This behavior inevitably has a negative effect on the demulsification of WACO emulsion. Therefore, a fast and efficient demulsifier is needed. A tannic acid polyether demulsifier has been proven to be efficient in breaking the WACO emulsion of the offshore platform in our previous work.[1] According to the optimum formula, a 700 kg scale-up production of the demulsifier is carried out. Demulsification performance can be further improved by the reaction of the demulsifier with tricarboxylic acid or isocyanate such as toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI), as reported in elsewhere.[6,7] Modification increases molecular weight and consequently the demulsification performance.[7,8] In order to reduce reaction steps and production costs, a modifier with high activity is necessary for the pilot-scale product.

Having a high reaction activity, isocyanate can react at low temperature and shorten the reaction time. Moreover, the product in a concentration of 50 wt % can be directly used as a demulsifier without any purification. Polyaryl polymethylene isocyanate (PAPI) or polymethylene polyphenyl polyisocyanate (pMDI) is a brown viscous liquid (150–250 mPa·s) and is a main material for the synthesis of polyurethane foam.[9,10] Containing a high average functionality (∼2.7),[11] PAPI is also taken as a cross-linking agent to modify starch.[12] Compared with TDI, PAPI has less toxicity and easier to store without interaction with each other. As a liquid, PAPI is easily miscible with organic solvents such as xylene at room temperature without the disadvantage of solid MDI, which crystallizes easily at low temperature. More importantly, PAPI has multiple benzene rings. These benzene rings can increase the π–π interaction between demulsifier molecules and interfacial active substances, reducing the stability of the interfacial film and finally improving the demulsification performance.[13] In order to make full use of the above advantages, PAPI is an ideal material to modify our demulsifier.

In this article, the influence of PAPI on demulsification performance and properties including hydroxyl number, relative solubility number (RSN), molecular weight, and viscosity is studied. Whether this modified demulsifier can be applied to actual oilfield production is evaluated by simulated equipment. In addition, viscoelastic measurement of model crude oil in demulsification is performed by a specified Physica MCR301 rheometer (Anton Paar, Austria) or AR-G2 stress-controlled rheometer (TA, USA).[14−21] Meanwhile, whether other rheometers without above fixtures like a HAAKE MARS III rheometer can be applied to demulsification research through simple parameter adjustments has been tried in this article.

Compared with other demulsifiers, this demulsifier has the following advantages: (1) Though the price of tannic acid is higher, its dosage is less than 0.5%, which has little effect on the price of the demulsifier, and therefore, the effect of the total cost is weak. (2) Because of the small addition and low water content in the oil, remarkable economic benefits can be obtained, such as transportation cost of the submarine pipeline, pipeline anticorrosion cost, and post-treatment cost of crude oil. (3) As a polyether with similar chemical composition to other demulsifiers, it does not affect the existing process. In addition, containing degradable tannic acid components, it has no impact on the environment.

Results and Discussion

Structure Characterization

Various molecular weights are given in Table . All molecular weights increase with the increase in the content of PAPI. This behavior suggests that the addition of PAPI promotes intermolecular linkages. For DP25, its molecular weight cannot be obtained due to the difficulty to pass through the 0.22 μm membrane. This phenomenon suggests a strong cross-linking network at this dosage. The highest Mη also illustrates this phenomenon.

Table 1

Molecular Weights of Demulsifiers as a Function of PAPI

demulsifier	content of PAPI (%)	Mn	Mw	Mw/Mn	Mη
DP		6420	12362	1.93	4136
DP10	1.0	6984	16244	2.33	7055
DP15	1.5	7064	16386	2.33	10501
DP20	2.0	7690	17536	2.28	12917
DP25	2.5				15103

RSN Value, Hydroxyl Number, and Viscosity

RSN as well as hydrophile–lipophile balance (HLB) is a parameter to characterize the hydrophilic and lipophilic properties of a surfactant. Different from the HLB value calculated by hydrophilic groups of PPO-PEO polyether, the RSN value can only be measured by toluene and dimethoxyethane. Figure a shows a negative relation between the RSN value and the content of PAPI. In the addition of PAPI, hydrophilic hydroxyl groups are replaced by hydrophobic benzene rings, leading to a striking decrease in hydroxyl number as shown in Figure b. As expected, with the continuous addition of PAPI, the hydrophilicity of demulsifier molecules decreases, resulting in the reduction of the RSN value. Even a small amount of PAPI is added, the RSN value has an obvious reduction. Moreover, the remarkable increase in viscosity further confirms the modification reaction, as shown in Figure b.

Figure 1

RSN value (a) and hydroxyl number and viscosity (b) of demulsifiers as a function of PAPI.

Linear Viscoelastic Regime

Figure a presents the elastic modulus (G′) and viscous modulus (G″) dependence on stress. For all samples, G″ remarkably dominates G′, implying that all demulsifiers display a behavior of viscous liquids. The phenomena that four samples except DP25 have similar G′ as exhibited in Figure b indicate that they do not form strong networks. In contrast, the strongest G′ of DP25 accounts for the formation of the cross-linking network. Both the highest viscosity and Mη further support this phenomenon. The larger network structure of DP25 exhibits solid-like behavior, which makes it having a short linear viscoelastic regime. A good linear viscoelastic regime is observed for all samples between 0.7 and 60 Pa.

Figure 2

Elastic and viscous moduli of modified demulsifier as a function of stress (a) and elastic modulus dependence on the content of PAPI (b) at fixed stress.

Effect of Modification on Cloud Point

The hydrogen bond formed by the ether bond and the adsorbed water is broken at elevated temperature, resulting in the worse solubility of polypropylene oxide (PPO)-polyethylene oxide (PEO) polyether. When the temperature is elevated enough to cause a change of the solution from clear to turbid, the intersection is taken as the cloud point.[22,23]Figure a demonstrates the temperature dependence of turbidity. For DP, a linear-ship appears between its turbidity and temperature in the experimental range, and thus, no cloud point is found.

Figure 3

Turbidity curves of aqueous demulsifier solutions at 5 g·L–1 (a), the relationship between RSN and cloud point (b), and turbidity curves at various concentrations of DP15 (c). The insets are the cloud points at different contents of PAPI (b) and concentrations of DP15 (c).

The cloud point decreases with the increase in the content of PAPI as shown in Figure a. The addition of PAPI reduces the hydroxyl number and thus the cloud point. Consequently, the cloud point also reflects the hydrophobicity and hydrophilicity of polyether, as shown in Figure b. A high RSN value means strong hydrophilicity of polyether and thus a high cloud point. As a result, a positive relationship between cloud point and RSN value is observed. Figure c displays the turbidity curves of DP15 at concentrations ranging from 1 to 10 g·L–1. Consistent with other references and our observation,[1,24] concentration has a negative impact on the cloud point. The cloud point results from the phase separation caused by the aggregation of micelles.[25] Obviously, increased concentration of polyether is prone to forming a large size micelle,[22] making phase separation easier and thus reducing the cloud point.

Effect of Modification on Demulsification in Laboratory Test

Demulsification of WACO emulsion and their photographs are presented in Figure . The water removal increases rapidly with demulsification time and slows down after 90 min. The water removal of the modified demulsifier is clearly higher than that of the unmodified DP, which may be the contribution of increased Mw.[7,8] The enriched branches may be another factor contributing to demulsification improvement.[7,23] In addition, benzene rings both in modifiers and demulsifier also contribute to the demulsification. Enhanced aromaticity of the modified demulsifier molecule is beneficial to its π–π interaction with asphaltene.[13,26] These aspects help demulsifier molecules to reduce the stability of the interfacial film and finally enhance the demulsification performance.

Figure 4

Water separation of WACO emulsion at 70 °C and 100 mg·L–1 (a) and at different contents of PAPI (b), and photographs of DP25 at different times (c) and different demulsifiers at 150 min (d).

As illustrated in Figure b, the water removal is in the order of DP15 ≈ DP20 ≈ DP25 > DP10 > DP. This order reveals that high dosage of the modifier is beneficial to demulsification. However, the high viscosity restricts its fluidity and prevents its application at high dosage of the modifier like DP25. Furthermore, an amount of crude oil attaches on the glass tube due to the interaction between its polar groups and hydroxyl groups existing on the glass surface. This phenomenon is also found in our experiments and other references.[1,27−29]

The water content in demulsification is monitored by the Karl Fischer method at an interval of 10 min. Figure a exhibits the opposite variation of these two curves. With a remarkable increase in water removal of DP25, the water content in crude oil also maintains a rapid downward trend. After 90 min, the water removal increases slowly accompanied by a slow decline of water content. As expected, a good relationship between water removal and water content is maintained in the whole demulsification as demonstrated in Figure a. Though the water removal of DP25 is higher than that of DP, the gap of water content is continuously shortened with the demulsification time until a similar result appears, which is 6.38 and 6.52 wt %, respectively, as depicted in Figure b.

Figure 5

Water removal and water content of DP25 (a) and water contents of DP and DP25 (b).

Demulsification Measurement on Offshore Platform

Resulting from long storage, the crude oil is prone to occurring chemical reactions,[2,3,5,30,31] which have an opposite effect on demulsification. Therefore, it is necessary to re-evaluate the platform. Based on the results of the laboratory test and consideration of field applications, DP15 and DP20 are selected. These results are shown in Figure . The water removal of the BH demulsifier is only 27.3% at 60 min, while that of DP is 76.2%. The demulsification efficiency reaches 95.7% for DP15 and 89.8% for DP20. Moreover, significant differences of separated water among the four demulsifiers are found in Figure b. Due to the poor performance of the BH demulsifier, WACO emulsion on the offshore platform cannot be treated. The modified demulsifier can break the WACO emulsions more effectively than the unmodified one. Among all demulsifiers, the demulsification performance is the best at a dosage of 1.5 wt % PAPI. It is noted that the results of platform demulsification are better than those of laboratory evaluation. The possible explanation is that the aging of WACO emulsion in the laboratory leads to the formation of a more stable emulsion and hard demulsification.

Figure 6

Water removal of WACO emulsions being produced on the offshore platform at 70 °C and 100 mg·L–1 (a) and their photographs at 60 min (b). BH is a current demulsifier on the offshore platform to break crude oil emulsions.

Effect of Modification on Interfacial Activity

As shown in Figure , the interfacial tensions (IFT) of all demulsifiers decrease at elevated concentration. The hydrophobic chains tend to attach with the crude oil, while the hydrophilic groups are adsorbed at the oil–water interface, which helps in IFT reduction. Therefore, the higher the concentration of demulsifier, the more demulsifier molecules absorbed on the interface, and the lower the IFT. Compared with DP at fixed concentration, DP20 and DP25 increase IFT, while DP10 and DP15 have an opposite effect toward IFT. The former is attributed to the increased hydrophobicity and formation of the cross-linking network,[32] while the latter may result from the improved branches at low PAPI dosage.[33,34] Though the concentration of crude oil in kerosene is only 30%, the crude oil without the demulsifier is prone to adhering on the glass tube of the interfacial tensiometer with its strong adhesion, leading to a failed IFT measurement. Figure b displays the correlation among RSN, interfacial tension, and water removal obtained from Figure at 60 min and 100 mg·L–1. With the decrease in RSN, the interfacial tension first decreases to the minimum and then increases. Meanwhile, the water removal demonstrates an opposite change. The surprising aspect of these data is that the minimum interfacial tension and the maximum water removal appear simultaneously at an RSN value of 10.95, indicating that both high interfacial activity and suitable RSN value are beneficial to the improvement of demulsification performance.[7,35]

Figure 7

Interfacial tension as a function of concentration (a) and correlation among RSN, interfacial tension, and water removal at 60 min and 100 mg·L–1 from Figure (b).

Rheological Behavior of WACO Emulsions

All oscillatory shear results in demulsification are demonstrated in Figure . Different from the result of G′ > G″ obtained from simulated crude oil in the literature,[36,37] in the whole measurement, a more viscous liquid rather than an elastic one is found since G″ > G′.[38] It is well accepted that G′ and G″ are related to the properties and stability of emulsions.[39] High values of G′ and G″ mean a stable emulsion.[40] Moreover, G′ is taken as a good indicator of the interaction and cross-linking of interfacial molecules.[19,21,36,41]

Figure 8

G′ (a) and G″ (b) of WACO emulsions as a function of time at 70 °C and 100 mg·L–1. The inset is the viscoelasticity of the blank.

The initial rapid reduction of G′ of all samples can be attributed to environmental changes caused by the addition of the rotator, such as temperature changes from room temperature to selected temperature and torque required by the equipment to reach the selected shear rate.[39] For the blank, G′ continuously increased from 500 s, probably caused by the combination of initial rearrangement and consolidation of the asphaltenes aggregated at the interface.[36,39] It is reported that the asphaltene had an important contribution to G′;[20] therefore, the newly formed network between rearranged asphaltenes leads to a more viscoelastic film under the oscillation of the rotator.

On the addition of the demulsifier, G′ declines in the form of a straight line. This behavior suggests that the demulsifier presents interfacial activity soon after the addition. The demulsifier molecules are absorbed onto the oil–water interface and compete with the natural surfactant and added additives, resulting in the formation of a less elastic interface with a lower value of G′.[39] As more molecules diffuse to the interface, the viscoelasticity continues to decline accompanied by a rapid reduction of the stability of the interfacial film. This behavior promotes the coalescence of water droplets leading to the change of the emulsion’s properties. This phenomenon can be observed by a decrease in viscosity at the macroscale as observed by Kolotova et al.[38] and a reduction of G′ at the microscale as exhibited in Figure a and other references.[14,15,21]

Since there exists a linear-ship between G′ and demulsification time, the slope of this line represents the demulsification speed, while the plateau modulus reflects the demulsification ability. The negative slopes of DP and DP15 are 5.38 × 10–5 and 8.57 × 10–5, respectively, simulated by Origin software, while the plateau moduli are around 1.5 and 1.3 Pa. These data provide a higher demulsification speed and demulsification ability of DP15 than those of DP. Consequently, this phenomenon further confirms the conclusion that the demulsification performance of the modified demulsifier has been significantly improved.

Resulting from the greatly decreased number and size of water droplets in demulsification as observed in our previous work,[1] the coalescence is hard to occur, and the water removal becomes slow until the appearance of a plateau. Therefore, the intersection between the declined line and the plateau can be regarded as the end of demulsification. The G′ of DP enters the plateau from 6000 s, and G″ is 5500 s. The results of the offshore platform also demonstrate that DP does not enter the stable stage of water removal in the test range (3600 s) as shown in Figure , consistent with the rheological results of DP. The times that G′ and G″ of DP15 enter the plateau are 4000 and 3000 s, respectively, in good agreement with the result that the water removal of DP15 changes little in water removal after 50 min as revealed by Figure .

Figure 9

Schematic demulsification process of WACO emulsions: (a) the separation of collided water droplets and the water separation of WACO emulsions in the addition of the demulsifier (b) and modified demulsifier (c).

In addition, the order of G′ is blank > DP > DP15, and G″ also shows the same order. Therefore, this order builds relationships among three samples in viscoelasticity. The decline of viscoelasticity of emulsion means a dramatic change existing at the oil–water interface. Obviously, this change is caused by the breakup of the interfacial film after the demulsifier is added. Furthermore, the degree of this change is closely related to the demulsification performance of the demulsifier. As a result, a conclusion can be drawn that the lower the viscoelasticity reduced by the demulsifier, the more severe the damage of the oil–water interface, and the more striking the demulsification effect toward WACO emulsions. This conclusion is also consistent with the Kang group on the effect of the demulsifier on the rheology of crude oil.[14]

The above discussion demonstrates that the rheometer equipped with a coaxial cylinder and a rotator can be employed to measure viscoelasticity of crude oil emulsion in demulsification by adjusting parameters. Moreover, this method can even be expanded to characterize the instability of emulsions.

Effect of Different Methods on Demulsification Performance in Simulated Application

Based on the excellent demulsification performance of DP15, an amount of 50 kg in a concentration of 50 wt % xylene solution is produced to treat the WACO emulsions in the production of the oilfield. These demulsification results at different methods are listed in Table . At all methods, the water removal is over 94% or even 98% in the presence of DP15, and therefore, their water content is less than 1.5%. The water content reaches 0.55% at the addition of DP15, much lower than that of 1.54% reached by electric stripping. Moreover, a considerable decrease in water content is observed when these two methods are used simultaneously, showing the existence of a synergistic effect. These data demonstrate that the water content can be reduced from 30% to less than 1% in 40 min in the presence of DP15. This low water content has great significance in reducing the transportation cost of offshore platforms. Obviously, a conclusion can be made that DP15 is an effective demulsifier in breaking the WACO emulsions in production.

Table 2

Demulsification Performance at Different Methods

method	water content (vol %)	water removal (vol %)
electric stripping	1.54	94.79
DP15	0.55	98.14
electric stripping and DP15	0.44	98.51

Effect of Dehydration Time on Demulsification Performance in Simulated Application

Table lists the demulsification data at different times. Resulting from the dynamic production, a small fluctuation in water removal and water content is observed. These data reflect that there is no significant change in water removal over time, indicating that the demulsification performance can reach the optimal value within 40 min.

Table 3

Demulsification Performance at Different Times

time (min)	water content (vol %)	water removal (vol %)
40	0.55	98.14
50	0.72	97.56
60	1.03	96.52
70	0.67	97.73

Demulsification Mechanism of Modified Demulsifier

In summary, combining the above discussion and previous research, the schematic demulsification process of WACO emulsion is shown in Figure . Elastic deformation occurs on the contact surface when a protected water droplet collides with another. Having strong elasticity, these two water droplets are easily separated from each other without any changes observed by Feng et al.,[42] as exhibited in Figure a. Once the DP demulsifier is added, a remarkable decrease in elasticity leads to a deterioration of deformation recovery at the interface. With the size reduction of asphaltene aggregation in the presence of the demulsifier observed by Atta et al.,[43] the interfacial film is easy to be raptured. In addition, there are also existing hydrogen bonds and weak π–π interaction[13,26] between the aromatic rings and hydrophilic polyether groups in demulsifier molecules and the asphaltenes, which contains a polycyclic aromatic hydrocarbon core as revealed by Schuler et al.[44] Therefore, the asphaltenes are easily replaced by demulsifier molecules followed by the destroying of the interfacial film. Coalescence occurs once the droplets are exposed to each other, as demonstrated in Figure b. As a result, the number of water droplets decreases significantly by the comparison of the microscopic photographs in Figure a,b. The number and size of water droplets varying with demulsification time observed by the microscope in the presence of the demulsifier are shown in our previous work.[1] Unlike the microscopic photographs of Figure b that remain a great number of small water droplets, only a few large droplets are observed in Figure c, reflecting a better aggregation of water droplets of the demulsifier modified with PAPI. Meanwhile, in the presence of the modified demulsifier, the elasticity becomes worse than any other sample. Due to larger molecular weight, higher branching, and stronger hydrogen bonds and π–π interaction with asphaltene, the modified demulsifier can replace more asphaltenes, leading to the damage of the interfacial film faster and more efficiently. Finally, the water droplets coalesce at the fastest speed to achieve the highest dehydration performance as presented in Figure c.

Conclusions

Having many advantages, including low toxicity, easy preservation, solubility, high activity, and multiple benzene rings, PAPI is an ideal modifier to replace TDI and MDI for demulsifier modification. With the addition of the PAPI, the hydrophobicity of the demulsifier is enhanced, showing a decrease in hydroxyl value, RSN value, and cloud point accompanied by an increase in viscosity and molecular weight. Laboratory demulsification shows that the demulsification performance has been dramatically improved by modification, and water removal can be increased by 15.6% in 120 min. Monitoring of water content in demulsification demonstrates a corresponding relationship between water removal and water content of top crude oil. Meanwhile, whether the demulsification performance is strong or not, the water content has little difference finally (only 0.14%), rather than as big as water removal (13.9% at 150 min). Having high interfacial activity especially for DP15, the demulsifier improves the water removal. Consequently, the water removal reaches 95.7% in 60 min on offshore platform demulsification. The result confirms that the DP15 is more suitable for the offshore platform toward WACO emulsion. Therefore, this proportion can be applied to modify the scale-up product and apply it to the pilot production of crude oil on the offshore platform. Demulsification performance is also positively correlated with the reduction in viscoelasticity of crude oil. The larger the slope of elastic modulus decreases, the faster the demulsification speed is. Furthermore, the fewer the plateau modulus, the stronger the demulsification ability. Therefore, the rheometer equipped with a coaxial cylinder and a rotator provides a powerful tool to characterize the demulsification performance by the measurement of viscoelasticity. The pilot-scale demulsification test of WACO emulsions produced in the oilfield shows that the modified demulsifier can reduce the water content from 30% to less than 1%, indicating that the demulsifier can effectively treat the WACO emulsions.

It should be noted that the dynamic distribution of the demulsifier and modified demulsifier with temperature, water content, and time between oil phase and aqueous phase remains to be studied.

Experimental Section

Materials

PAPI (PM200) was purchased from Wanhua Chemical Group (China). Xylene, methanol, and ethanol were purchased from Sino Pharm Chemical Reagent Corporation (China). The DP demulsifier was a scale-up product of 700 kg based on industrial-grade tannic acid according to our previous work.[1] Corresponding to PAPI dosages of 1.0, 1.5, 2.0, and 2.5 wt %, DP10–DP25 were obtained by the reaction of PAPI and DP at 60 °C in xylene (Scheme ).

Scheme 1

Modification Reaction of DP Demulsifier with PAPI

Characterization

Gel permeation chromatography (Shimadzu, Japan) was employed to determine the average molecular weight (Mw) by using polystyrene and chromatographically pure tetrahydrofuran. Viscosity average molecular weight (Mη) was measured at 25 °C in methanol as described in the reference.[15] For poly(propylene oxide), the corresponding Mark–Houwink parameters of K and α were 76.9 × 10–3 and 0.55, respectively.[45]

The hydroxyl number was determined based on the esterification of hydroxyl groups and phthalic anhydride solution in pyridine. The RSN value was measured according to the description of Wu et al.[46] The turbidity of aqueous demulsifier solution was measured by an HACH TL2300 turbidimeter as described in our previous work.[1]

A spinning drop interfacial tensiometer TX-500C (CNG Company, USA) was employed to perform the interfacial tension measurement at 8000 rpm and 70 °C between WACO (30 wt % kerosene solution) and aqueous demulsifier solution.

Demulsification Bottle Test

The WACO emulsion in a graduated cone-shaped tube with a demulsifier at 70 °C was shaken violently 200 times followed by a recording of demulsification efficiency.[23] To observe the water droplets in the oil phase, the oil phase before and after demulsification was examined by a microscope (Chongqing Optec Instrument Co., Ltd., China).

Determination of Water Content of Oil Phase

Water content of crude oil in demulsification was measured by an HYG-809B micromoisture analyzer (Wuhan Huaneng Sunlight Electric Co., China) based on the Karl Fischer method. The solvent was a mixture of xylene and ethanol in a mass ratio of 3:0.8. Equipped with a double platinum electrode, this instrument provided the water quality of the sample. The water content was calculated according to ASTM D1744 (ASTM D1744 standard test method for determination of water in liquid petroleum products by the Karl Fischer reagent). In detail, crude oil (0.15 g) was obtained at 1 cm below the surface with a 1.5 mL centrifuge tube followed by the addition of the above solvent (0.5 g). The crude oil was shaken vigorously until it was completely dissolved. Crude oil solution (0.02 g) was taken out with a 50 μL syringe and injected into the electrolytic cell. The measured water quality of the injected sample (WQm) was recorded. The water content (WC) of crude oil can be calculated as follows:where WQcs is the water quality of crude oil and solvent in the centrifuge tube (μg), WQs is the water quality of the solvent (μg), Qc and Qs correspond to the quality of crude oil and solvent, respectively (μg), Qe is the quality of crude oil solution extracted by a syringe (μg), and WCs is water content of the solvent (wt %).

Viscosity and Rheology Measurements

An HAAKE MARS III rheometer (Thermo Scientific, Germany) with a coaxial cylinder sensor system (CC26 Ti, diameter = 26 mm, gap = 1.90 mm) was employed to perform viscosity measurement of demulsifier xylene solution (50 wt %) at 25 °C and shear rate of 7.34 s–1. To ascertain the linear viscoelastic regime, a stress sweep test of bulk rheology at a fixed frequency of 1 Hz was carried out in the range of 0.1–1000 Pa at 25 °C. The experiment was conducted with parallel plate geometries (C60/1° TiL, diameter = 60 mm, cone angle = 1°, gap = 0.052 mm).

The viscoelasticity measurement was also performed on this rheometer at 70 °C by the oscillation time method at a frequency of 1 Hz and a stress of 1 Pa in the linear viscoelastic regime. A preheated WACO emulsion (40 mL) was taken with a plastic tube followed by the addition of the demulsifier. The plastic tube was shaken vigorously, and this emulsion (22 g) was poured into the cylinder. The rotator was immersed into the WACO emulsion, and its top was just below the surface. To prevent the contact between the rotator (CC20Ti, diameter = 20 mm) and the separated aqueous phase in demulsification, the gap between the bottom of the cylinder and the bottom of the rotator was adjusted to 21 mm as shown in Scheme , and therefore, it could provide the changes of the crude oil phase without the disturbance of the bottom aqueous phase. The concentration of the demulsifier was 100 mg/L, and the demulsification time was 7200 s. A plastic cap was placed over the cylinder to avoid water evaporation. For comparison, a blank emulsion without the demulsifier was employed.

Scheme 2

Schematic Diagram of Demulsification Performance Evaluated by a Rheometer

Simulated Application of the Oilfield

To validate the demulsification performance toward WACO emulsions in production, a pilot test was carried out in the terrestrial terminal treatment plant. These WACO emulsions were collected in the treatment of crude oil. Since WACO emulsions were dynamically produced, their water content was not stable. Its average water content was around 30 vol % by repeating seven times at different times. The experimental equipment with an electric stripping function was provided by Offshore Petroleum Development Engineering Technology Co., Ltd. In this equipment, the WACO emulsion flow was 3 m3·h–1, and the retention time was 40 min. To measure the demulsification performance at different dehydration times, the valve was closed until the specified time. The concentration of the demulsifier was 200 mg·L–1, and the temperature in this equipment was 65 °C.