Preparation and performance evaluation of oil-tolerant and easy-wetting viscoelastic system for enhancing the stability of highly viscous oil-water lubricated flow

Abstract Based on the free sliding phenomenon of the monopterus albus in the cave, an oil-tolerant and easy-wetting viscoelastic system (A3B4C3E5F2) was selected and compounded as a possible lubricant to enable the transportation of heavy crude oil. In the viscoelastic system, the final floating equilibrium velocity of LD1 crude oil droplets with a diameter of about 5 mm is 0.01853 mm/s, and the floating resistance coefficient (C D) is 11067862, which shows favorable oil suspension performance. When the shear rate is approximately zero, the viscosity of the viscoelastic system is about 7000 mPa·s. However, when the shear rate is up to100 1/s, the viscosity is only about 32 mPa·s. A high viscoelastic film is formed at shutdown or low flow rates. Conversely, a low viscosity liquid ring formed at high flow rate. After being wet by easy-wetting viscoelastic system, the contact angle between LD1 crude oil and common pipe materials is wider than 160° at 30 min. The incubation time (t 0) and the wettability composite index (WCI) are shorter than 1800 s and higher than 4800, respectively. Upon stability evaluation, the system can maintain good viscoelasticity and wettability in the range of temperature (5 ∼ 90 °C), oil content (0 ∼ 15 wt.%), and shear rate (0 ∼ 1000 1/s), respectively. Graphical Abstract


Introduction
Over the years, with the ever-increasing consumption of conventional energy, many researches in the petroleum field have focused on the heavy oil and oil sands to improve the crude production rate and to meet the growing demand for fossil energy, especially in China and Canada. [1,2][9] Conventional approaches for transporting heavy oil mainly include heating, [10] dilution, [11] hot water injection, [4] mixing active water, emulsification, [12] catalytic cracking and upgrading, [13] but almost all these methods have their limitations, such as high construction cost, low processing capacity, high energy consumption and high operating costs. [14]At present, flow condition is commonly achieved by adding hot water before it is fed to the pipeline. [9,10]he pipeline capacity for heavy oil is further constrained since the high-viscosity-oil must be mixed with plenty of water (percentage of water is usually more than 90%) to conform to the pipeline' transportation specifications. [11,12]Additionally, the pipeline must be heated and insulated, which significantly increases the capital consumption of investment and operation. [13]Hence, one way to increase the capacity and reduce energy consumption of heavy oil is to bring down the water demand and cancel heating and insulation.Therefore, inspired by the quick sliding of the monopterus albus in the cave, if there is a stable lubricating diaphragm between the oil and the pipe wall, the flow resistance of the heavy oil inside the pipe will be drastically reduced.
The key to reduce the resistance of heavy oil through pipeline is to reduce or eliminate the adhesion of high viscous oil to the inner wall of pipeline. [14]To this end, an eco-friendly technique to form a stable oil-water lubricating layer is proposed.However, there are two important issues that need to be tackled.On the one hand, the stable diaphragm layer can play an effective role in reducing the contact between oil and pipe wall and weakening the upward trend of oil; on the other hand, even if the crude oil collides with the pipe wall, the adhesion work between the oil and the pipe wall can be reduced, thus facilitating the spillage of oil from the pipe wall.
Generally speaking, the inner wall of the pipeline is lipophilic and hydrophilic, with strong lipophilicity, which is easy to be moistened by oil.In oil-water two-phase flow, the adhesion work between oil and pipe wall is larger, which increases the friction resistance of pipeline transportation.Santos [15] found that the polar components of crude oil (asphaltene and naphthenic acid) significantly affect the wettability of crude oil on the surface of the pipe.Due to electrostatic and acid-base interactions between the polar components and the pre-wetted solid surface, the surface of the pipe presents an oil-wettability behavior.As a result, crude oil is easy to adhere to the pipe wall and accumulates gradually, which increases the friction resistance of heavy oil and water two-phase flow.However, adding NaCl and Na 2 SiO 3 to water can change the wettability of pipe surface from oil wetting to water wetting, and Na 2 SiO 3 has a more obvious improvement effect on water wettability of pipe surface than NaCl.Silva [16] found that using KMnO 4 to oxidize the inner surface of the pipeline could improve the hydrophilic and lipophobic properties of the pipe wall and alleviate the adhesion of high viscous oil on the pipe wall.At the same time, increasing the wall roughness can also enhance the hydrophilic characteristics of the wall, but will increase the pressure drop of the oil-water two-phase flow.In addition, keeping the temperature of water solution at a low level, controlling the pH value in a certain range, or adding salts such as sodium metasilicate to water can also improve the water wettability of pipe surface.Changing the wettability of materials has increasingly been concerned in a wide range of engineering and science fields. [17]In the oil industry, wettability alteration, which is the transition of the wetting state of solid-oil-water from oil-wet to more favorable water-wet, has drawn substantial attention, especially in carbonate reservoirs. [18]However, wettability has not yet been applied in heavy oil-water pipeline transportation.Combined with the method, the wetting agent is added to change the wettability of water and weaken the adhesion of oil on the pipe wall, thus facilitating the spillage of oil from the surface of the pipe.
Aqueous solutions of polymers can exhibit viscoelastic characteristics under certain conditions. [19]The relative collision rate between molecular particles prominently affects the conformation of the polymer on the viscoelasticity of polymers. [20]When the collision time between particles is overtly longer than the relaxation time of the polymer, the polymer chains will form bridges. [20,21] At this moment, the viscoelasticity of the polymer is strong.Conversely, when the collision time between the particles is shorter than the relaxation time of the polymer, the adsorbed polymer chains will fit perfectly with the solid surface in random coils.Given the bridges between polymer chains fail to accumulate elastic energy, the elasticity of the network structure is weakened. [20,21]owever, the viscoelasticity of the polymer decreases at this moment.[24][25][26][27][28][29][30][31][32][33][34][35][36][37] On the one hand, the strong viscoelasticity of polymer aqueous solution can inhibit the turbulent kinetic energy in the fluid, so as to weaken the mutual drag force between the oil and water two-phase interface and promote the disappearance of wavy stripes at the two-phase interface, and finally reduce the energy loss caused by turbulent flow energy and two-phase interface drag. [22][30][31][32] On the other hand, when the oil-water two-phase flow forms a central annular flow, the greater the shear force of the viscoelastic liquid ring, the greater the second normal stress difference. [33]When the central oil core rises due to buoyancy caused by density difference, the upper liquid ring becomes thinner and the shear rate increases, while the lower liquid ring becomes thicker and the shear rate decreases.As a result, the second normal stress difference of the upper half of the viscoelastic liquid ring subjected to the central oil core is larger than that of the lower half.Once the oil core is eccentric, the resultant force and buoyancy of the second normal stress difference between the upper and lower viscoelastic fluid rings, and the oil core will reach an equilibrium state, and the oil core will no longer rise.0][31][32][34][35][36][37] Therefore, the present research is designed to use the bridge formed between polymer chains to augment the viscoelasticity of water solution, attenuate the upward trend of crude oil flow, and form a high viscoelastic diaphragm layer, thereby preventing crude oil from contacting with the pipe wall.
Herein, we spared no effort to compound an oil-tolerant and easy-wetting viscoelastic system, with the aim of enhancing the stability of the highly viscous oil-water lubricated flow.Five wetting agents, four polymers, and one auxiliary agent were selected separately.The proportion of different reagents was determined with the help of orthogonal experiment and uniformity experiment.Moreover, an evaluation and analysis were carried out regarding the wettability, suspension oil characteristics, yield characteristics, viscoelasticity, zero shear viscosity, rheology properties, and stability of the prepared hydrogel system.The wetting viscoelastic system prepared in this study can reduce the turbulence intensity of oil-water annular flow, thus reducing the dispersion of oil droplets in water at high flow rates.On the side, the viscoelasticity and wettability of water can be enhanced, which serves the purpose of attenuating the tendency of crude oil to float up and contact with the pipe wall in the flow or shutdown, thereby enhancing the stability of the oil-water lubrication flow.

Materials
In the present research, twelve categories of medicaments were selected as wetting and viscoelastic agents based on the related research, including seven wetting agents, one demulsifier, and four polymers.The foregoing seven wetting agents are all water-soluble, among which there are three nonionic types like alkylphenol polyoxyethylene ether, polyethylene glycol octyl phenyl ether, and polyoxyethylene octylphenol ether, and four anionic types like succinate polymer, sodium diisooctyl succinate sulfonate, sodium polycarboxylate, sodium stearate.The hydrophilic-lipophilic balance (HLB) of the seven wetting agents is all greater than 10, indicating that they all fall within hydrophilic wetting agents.The basic properties and sources can be found in Table S1.Meanwhile, a kind of demulsifier was considered as a wetting agent auxiliary to avert the emulsification of oil and water, and its basic characterizations are listed in Table S2.These four polymers are one nonion and three anions.Some of these polymers have been widely used in oilfield fracturing fluids, and their basic properties and sources of which are listed in Table S3.
Two mineral oils (Mineral oil 1 and Mineral oil 2) and two crude oils (LD1 and LD2) were selected from the Bohai Lvda Oilfield to evaluate the wettability, oil resistance, and suspension properties of the easy-wetting viscoelastic system.The basic physical properties and chemical composition of these four oil samples are available in Table S4 for reference.
The viscosity-temperature characteristics of four oils are listed in Figure S1.It can thus be seen that the viscositytemperature curves of LD1 and Mineral oil 2 are fairly close, and the viscosity has a similar change trend with temperature.Origin software is applied to fit the viscosity-temperature curves corresponding to the four oil samples, and the relationship between viscosity and temperature is concluded, as shown in the equations below.
Mineral oil 1 : (2) Where T is the temperature of oil, � C; and l o is the viscosity of the oil at T, mPa�s.
Secondary distilled water was utilized to prepare synthetic salt with total salinity of 5,000 and 50,000 mg/L, respectively.See Table S5 for its ingredients and proportion.

Wettability
When the oil-water two-phase flow is transported, due to different oil-water densities, the oil in the water floats on the upper part of the pipeline, and there is usually contact between the pipe wall and crude oil as shown in Figure S2a.Therefore, the methods are shown in Figure S2b and c are used to test the contact angle between the pre-wetted pipe wall and the crude oil.The wettability is evaluated pursuant to the change of the contact angle of the oil on the pipe wall with time (Figure S2d).Meanwhile, the wettability composite index (WCI) is introduced to estimate the performance of the wetting agents.
The changing trend of contact angle with time in Figure S2d is divided into incubation period (t 0 ), contact period, and stable period.The incubation period means the time when the oil droplets do not interact with the pipe wall immediately after they float on the pipe wall, and are blocked by the water-soluble wetting agents.During this period, the contact angle is essentially constant.The contact period means the time when the oil droplets begin to interact with the pipe wall, and the oil droplets quickly spread along the pipe wall.The stable period means the time when the tendency of oil droplets to spread is further weakened and is basically leveled off.
By taking 30 min as the evaluation time of WCI, the shaded area surrounded by the curve in Figure S2d reflects the overall performance of the wetting agent.For the purpose of calculation, the trapezoid area is selected to approximate the WCI of the wetting agents, as shown in Equation 5.
Here, t n means the contact time between droplet and pipe wall, min; h n means the apparent contact angle at t n , � .

Viscoelasticity
A Haake Mars � rheometer (Figure S3a and b) is utilized to test the dynamic mechanical behavior of polymer aqueous solution at 20 ± 0.1 � C.An analysis is conducted on the changes of elastic modulus (G') and viscous modulus (G'') with applied stress and oscillation frequency, and an evaluation is made on the viscoelasticity of polymer aqueous solution.To be specific: ‹ Measure the yield stress of different polymers and take 60% of the yield stress as the stress value for the viscoelastic test.› Test the viscoelasticity of different polymers, and calculate the elastic modulus and viscous modulus at the same frequency.
To evaluate the colloidal coupling characteristics of the polymer aqueous solution, each polymer aqueous solution with mass solubility of 0.1 wt.%, 0.2 wt.%, and 0.3 wt.% is prepared, respectively.An ESJ-A electronic balance with an accuracy of 0.001 g is used to measure the downward leakage mass of each 100 g aqueous solution in a 70-mesh (aperture: 0.212 mm) sieve at different times.The schematic diagram is shown in Figure S3c.

Orthogonal experiment
An orthogonal experiment design was carried out with the help of SPSS software to define the optimal wetting agent compounding ratio after adding crude oil influencing factors.Based on a one-factor experiment, X-405, 9485, AP9901, and oil content (LD2 heavy oil) are selected as experimental factors, and the factor levels are listed in Table S8.The orthogonal experiment table of four factors and four levels L 16 (4 4 ) is established. [40]The oil-water mixture with the filter paper is separated, and the pipe wall is wetted with the separated wetting agent aqueous solution to test the contact angle between the oil and the pipe wall.The orthogonal experiment results are shown in Table 1 and Figure 1a, and the wetting agent aqueous solutions separated from each test number can be found in Figure 1b.
As can be observed in Table 1 and Figure 1a, the best wetting effect is achievable when the wetting agent is mixed in the proportions of test numbers 4, 7, 11, 12, and 16, respectively.Figure 1b shows that the separated wetting agent aqueous solution tends to be clearer with AP9901 added.
Based on the results of the range analysis of the orthogonal experiment in Table S9, the effect of each index is sequentially arranged as follows: 9485 > X-405 > AP9901 > oil content.The principal mixing ratio is A 3 B 4 C 3 , which means that the concentrations of X-405, 9485, and AP9901 in the aqueous solution are 0.05 wt.%, 0.07 wt.%, and 0.05 wt.%, respectively.Therein the concentration of 9485 designed in the orthogonal experiment is lower than the optimal concentration.Hence, it exerts more prominent impacts on wettability as the concentration increases.However, the concentration designed for X-405 and AP9901 in the orthogonal experiment is slightly higher than the optimal concentration, so as the concentration increases, its influence on wettability is not obvious.The variance analysis on orthogonal experiment suggests that, F and significance probability (P) of 9485, AP9901, X-405 and oil content are F ¼ 2.132, 7.881, 2.457, 0.359, and P ¼ 0.062, 0.240, 0.275, 0.359, respectively.The results showed that 9485 had a significant effect on the results, while the oil content had a subtle effect.Judging from the value of F, the effect of factors is sequentially arranged as follows: 9485 > AP9901 > X-405 > oil content, which is slightly different from the range analysis result.The multiple comparison results of the Duncan method showed that X-405, 9485, and AP9901 witnessed the best effects when the concentrations were 0.05 wt.%, 0.07 wt.%, and 0.05 wt.%, respectively, while the four levels of oil content were not significantly different.Therefore, A 3 B 4 C 3 is the optimal combination.Oil content is not the main influencing factor, so the optimal oil-water mixing ratio can be determined based on the delivery temperature, oil viscosity, and economy.

Performance analysis of compound wetting agent
The aqueous solution of A 3 B 4 C 3 was mixed with LD2 crude oil at a mass ratio of 3:7 and 6:4, stirred with an electric stirrer for 10 min, and finally filtered with filter paper.Take the upper emulsified oil (A 3 B 4 C 3 :LD2 ¼ 3:7) and record its picture with an XP-5550 polarizing microscope, as shown in Figure S11.Moreover, the viscosity of emulsion oil (A 3 B 4 C 3 :LD2 ¼ 3:7) was measured by a rheometer (Figure S12).Comparing Figure S11 with Figure S8d, we can see that the diameter and number of water droplets wrapped in the emulsion of A 3 B 4 C 3 and oil are slightly inferior to that of water and oil, indicating that A 3 B 4 C 3 can reduce the emulsification of heavy oil.Figure S12 shows that the viscosity of A 3 B 4 C 3 and oil emulsion falls in between the viscosity of oil-water emulsion and dehydrated oil, indicating that A 3 B 4 C 3 can weaken the emulsification degree of heavy oil and reduce the viscosity of emulsified oil.
Additionally, the filtered aqueous solution of A 3 B 4 C 3 was taken as the continuous liquid phase (A 3 B 4 C 3 :LD2 ¼ 6:4), and a test was performed against the contact angle between the PVC pipe, which was wetted by the filtered aqueous solution of A 3 B 4 C 3 , and the LD1 crude oil (Figure 2 and Table S10).The compound wetting agent significantly improved the wettability of the aqueous solution, despite a slightly inadequate wetting effect.t 0 is increased from 3 s to 180 s, and WCI is increased from 1616 to 4138.In allusion to oil-water two-phase flow, this compound wetting agent is sufficient to prevent oil from sticking to the pipe wall.

Uniformity experiment
With a view to escalating the stability of the oil-water annular flow with the least amount and optimal compounding ratio of polymer, the uniformity experiment design was adopted to evaluate the floating time, sinking time, and resistance coefficient of each polymer solution.Thereafter the optimal mixture proportion of the polymer was finalized.Based on the one-factor experiment, PAM and XG were selected as experimental factors, and the levels were shown in Table S13, respectively.A Uniformity experiment table was established for U 11 (11 2 ) of two factors at eleven  levels.Uniformity experiment results were shown in Table 2 and Figure 3.
Figure 3 shows that with the increase of the experiment number, the floating and sinking equilibrium final velocity is on the decline at first and then tends to be stable; while the floating and sinking resistance coefficient C D is crescent at first, and then also tends to be stable.Hence, experiment number 5, on the verge of the inflection point of stable change, is selected as the optimal compound ratio.The concentrations of PAM and XG are 0.125 wt.% and 0.03 wt.%, respectively (simplified as E 5 F 2 ).

Performance analysis of compound polymer aqueous solution system
Based on the E 5 F 2 system, the Haake Mars � rheometer is used to evaluate viscoelasticity, temperature tolerance, shear tolerance, oil tolerance, and salt tolerance.The experimental results are revealed in Figure 4.The oscillation temperature sweep ramp mode is adopted in temperature tolerance, where s 0 is 0.5 Pa, f is 1.0 Hz, and the temperature is in the range of 5 � 105 � C, so as to test the viscoelasticity and complex viscosity of the compound polymer aqueous solution system.The shear tolerance is evaluated by dint of rotation time curve mode.The E 5 F 2 system is tested for 2 consecutive hours at fixed shear rates of 10 1/s, 30 1/s, 100 1/s, 200 1/s, and 400 1/s, 600 1/s, 800 1/s and 1000 1/s, respectively.The thixotropy loop mode is combined to analyze the stability of the E 5 F 2 system.The oil tolerance and salt tolerance are evaluated by dint of the rotation ramp mode.The rheological curves of 5 wt.% and 10 wt.% of LD2 crude oil and 0.5 wt.% and 5.0 wt.% of synthetic salt in the E 5 F 2 system are tested, respectively.
Compared with the PAM at the concentration of 0.1 wt.%, when f ¼ 0.2154 Hz, the G', G" and jg � j of the compound polymer aqueous solution system can increase from 792 mPa�s, 366 mPa�s, 644 mPa�s to 1043 mPa�s, 398 mPa�s, and 825 mPa�s, respectively (Figure 4a and Table S12).The elastic modulus of the compound polymer aqueous solution system increased by 251 mPa, whilst the viscous modulus merely increased by 32 mPa.The elasticity of the compound polymer aqueous solution system is dramatically improved, but the viscosity is not significantly increased, so the compounding effect is achieved overall.According to Figure 4b, when the temperature ranges from 5 � C to 105 � C, the G' of the compound polymer aqueous solution system is larger than the G", and the solution mainly presents the elastic feature.When the temperature is between 5 � C and 90 � C, the G', G" and jg � j change more regularly, meaning that the solution is featured by temperature stability and good temperature tolerance.When the temperature is over 90 � C, the G', G" and jg � j began to change irregularly, indicating that the molecular structure in the solution is at the risk of being damaged.
As shown in Figure 4c, the apparent viscosity is positively related to the shear time, and the solution is characterized by rheopetic.However, when the shear rates are 10 1/s, 30 1/s, 100 1/s, 200 1/s, 400 1/s, the apparent viscosity is negatively correlated to the shear rates.After the shearing is conducted for 2 consecutive hours, the apparent viscosity does not increase observably over time, and the increment is basically maintained within 1 mPa�s.When the shear rates are 600 1/s, 800 1/s, and 1000 1/s, the apparent viscosity increases significantly over time, and it is maximized at 800 1/s.The primary reason is that the conformation of the polymer chain in the solution is somewhat curled, and the curled state cannot be changed at a low shear rate.Therefore, with the increase of the shear rate, the apparent viscosity is not increased obviously.When appropriate, the conformational curled state of the polymer chain will be tackled, and the degree of chain stretch will increase sharply overtime, resulting in a more significant increase in apparent viscosity.However, as the shear rate increases progressively,  the molecular structure of the polymer will be exposed to destruction, and the increment in apparent viscosity will be retard.Simultaneously, Figure 4d shows that the uplink and downlink of the thixotropy test conducted within the range of 600 1/s basically coincide, which indicates that the compound polymer aqueous solution system presents strong shear tolerance when the shear rate is less than 600 1/s. Figure 4e shows that the three curves basically overlap each other, so the oil content in the solution subtly affects the rheological curve of the compound polymer aqueous solution system, which indicates that the compound polymer aqueous solution system has better oil tolerance.However, Figure 4f shows that the increment in viscosity of the compound polymer aqueous solution system is negatively correlated to the content of synthetic salt, which indicates that the salt tolerance of the compound polymer aqueous solution system is relatively weak.

Interaction between wetting agents and compound polymer aqueous solution system
Based on the compound wetting agent concentration, X-405 (A 3 ), 9485 (B 4 ), AP9901 (C 3 ), and A 3 B 4 C 3 are mixed with E 5 F 2 , respectively.A test is performed against the rheological and wetting properties of each wetting viscoelastic system, and an analysis is conducted on the interaction between wetting agents and polymer aqueous solution system (Figure 5a and b). Figure 5a shows that four wetting viscoelastic systems all belong to yielding pseudoplastic fluid, indicating that the apparent viscosity gradually decreases with the increase of the shear rate.The viscosity of four wetting viscoelastic systems is prioritized as follows: A 3 E 5 F 2 >E 5 F 2 >A 3 B 4 C 3 E 5 F 2 , indicating that the apparent viscosity of the E 5 F 2 will vary slightly with the addition of the wetting agent.Figure 5b shows that the viscosity of each wetting viscoelastic system linearly decreases with the increase in temperature.The viscosity of four wetting viscoelastic systems is ordered: It is concluded that A 3 can slightly increase the viscosity of the E 5 F 2 when the shear rate is greater than 40 1/s.While B 4 can reduce the viscosity of the E 5 F 2 , which is in turn increased by C 3 .This is mainly because X-405 (A 3 ) and AP9901 (C 3 ) fall within nonionic surfactants.The hydrophobic bond of the surfactant is associated with the hydrocarbon chain of PAM, and the nonionic surfactant is connected to the polymer skeleton in the form of a "necklace model".Then, the "non-ionic surfactant-polymer" complex comes into being.When the concentration of surfactants is at the critical point, the added surfactants will be present as free micelles without changing the structure of the complex, and the viscosity of the system will be basically stable.However, once the concentration of 9485 (B4) -an anionic surfactant is higher than the critical saturation concentration, the cations ionized by the anionic surfactant will be strongly compressed as the concentration increases progressively.The polymer is tightly bound to an anionic surface-active agent, and spherical flocculation is formed, resulting in a sharp decrease in the viscosity of the system. [38, Nevertheless, the viscosity of A 3 B 4 C 3 E 5 F 2 relative to E 5 F 2 is reduced acceptably.
The contact angles of LD1 crude oil and PVC are tested upon the wetting treatment by the wetting viscoelastic systems.The experimental results can be found in Figure 5c  and d. Figure 5c and d show that A 3 B 4 E 5 F 2 is slightly inferior to A 3 E 5 F 2 , A 3 C 3 E 5 F 2 , and A 3 B 4 C 3 E 5 F 2 that can exert the optimal wetting effect.Wetting agents play a momentous role in ameliorating the wettability of the E 5 F 2 .Based on the rheological evaluation in Figure 5a and b, in case mineral oil is used in pipeline flow experiments, A 3 E 5 F 2 can be conceptually considered as an easy-wetting viscoelastic system that reduces the dosage of wetting agents.In case crude oil is used in pipeline flow experiments, A 3 B 4 C 3 E 5 F 2 can be conceptually considered as an easy-wetting viscoelastic system that weakens oil-water emulsification.

Analysis of influencing factors on the stability
Anti-shear recovery performance.There will also be changes in the rheological properties of an easy-wetting viscoelastic system, depending on external shear.The rheological curves of A 3 B 4 C 3 E 5 F 2 at varying shear rates (0 � 300 1/s) are measured when the temperatures are between 10 � C and 80 � C (Figure 6a).And at 20 � C, a measurement is made for the viscosity curves of continuous shearing at the constant shear rates for 2 h (Figure 6b).When the shear rate falls in between 01/s and 300 1/s, the Hershel-Bulkley model can be applied to well fit the curves of shear stress versus shear rate at various temperatures.Meanwhile and the correlation coefficients R 2 all exceed 0.99 (Figure 6a).As the temperature goes up, the fluidity index n is on the decline which is all less than 1.However, the heavy oil coefficient K n does not change significantly, resulting in a sustained decline in apparent viscosity.The results show that the A 3 B 4 C 3 E 5 F 2 system is characterized by shear thinning, which in turn decreases with the increase of temperature.As delineated in Figure 6b, when the shear rate is lower than 600 1/s, the apparent viscosity is pretty much constant with the extension of shear time, and the molecular structure of the system is not internally damaged.When the shear rate is higher than 600 1/s, the apparent viscosity increases at first and then remains broadly the same with the extension of the shear time, and the molecular structure of the system begins to change internally.When the shear rate is 800 1/s, the maximum increase in apparent viscosity occurs.When the shear rate is increased again, the increment of apparent viscosity becomes slower.The foremost reason is that when the shear rate is lower than 600 1/s, the stretch state of the polymer chain cannot be changed, and the viscosity of the system does not vary markedly as the shear time extents.When the shear rate is higher than 600 1/s, the polymer chain is further stretched due to the external force.In this way, the viscosity gradually increases with the extension of the shear time until it becomes stable.When the shear rate increases progressively, the entanglement state of the polymer chain may be exposed to destruction and the increment in apparent viscosity will be retard.
Two-stage and three-stage thixotropy test methods are adopted to evaluate the structural recovery performance of the A 3 B 4 C 3 E 5 F 2 system (Figure 6c and d).The two-stage thixotropy test results of the system at 20 � C are plotted in Figure 6c.It can thus be seen that at low and medium shear rates, the upward shear line and the downward shear line almost coincide with each other while the thixotropic ring is characterized by a positively small area.It is revealed that the recovery speed of molecular structure is comparable to and even faster than its destruction speed at such shear rate, presenting a favorable recovery performance.However, at high shear rates, the downward shear line is slightly higher than the upward shear line, and the thixotropic ring covers an area of fewer than À 82.6 Pa/s, indicating that energy is essential to recover the polymer molecular structure.Concurrently, the three-stage thixotropy test is also performed, aiming to evaluate the molecular structure recovery performance of the system at low shear rates (Figure 6d).It takes longer for the viscosity to recover at 10 � C, which implies the higher the temperature, the shorter the viscosity recovery time.The viscosity recovery rate (Dl) is introduced to evaluate the recovery performance of the internal molecular structure of the viscoelastic fluid, [ 39 ] as shown in Equation 6.Table S14 reveals that the viscosity recovery rate at 300 s is greater than 90%, namely the easy-wetting viscoelastic system empowers the recovery of the internal molecular structure swiftly with satisfactory thixotropic recovery performance.
Temperature tolerance.On the condition that s 0 is 0.5 Pa, f is 1.0 Hz, and the temperature ranges from 5 � C to 105 � C, the oscillation temperature sweep mode is applied to perform the temperature resistance test of the A 3 B 4 C 3 E 5 F 2 system.The change rules of its viscoelasticity and complex viscosity are depicted in Figure 7a.When the temperature is 5 � 95 � C, the G' of the system is greater than G", and the solution tends to be elastic.G', G" and jg � j all decrease linearly with temperature increasing which indicates that the solution is stable in temperature to some extent.When the temperature is above 90 � C, G', G" and jg � j begin to change irregularly, which indicates that the molecular structure in the solution begins to be jeopardized.Compared with Figure 5a, when the temperature ranges from 5 � C to 90 � C, the viscosity of the A 3 B 4 C 3 E 5 F system also decreases linearly with temperature increase, showing an equivalent change trend to that of viscoelasticity and complex viscosity.The A 3 B 4 C 3 E 5 F 2 system is insulated in a constant temperature water bath for 1 h at 30 � C, 40 � C, 50 � C, 60 � C, 70 � C, 80 � C, and 90 � C, respectively.Following the temperature drops to room temperature, the viscosity of the system heat-treated at 20 � C is measured (Figure 7b).It is observed that at heat treatment temperatures below 50 � C, the viscosity of the system keeps essentially constant as such temperature rises.However, at heat treatment temperatures above 50 � C, the viscosity of the A 3 B 4 C 3 E 5 F 2 system increases mildly as such temperature rises.The phenomenon is induced by water vaporization under high-temperature conditions.The results can also indicate that the stretched state of the polymer chains in the system is basically unchanged, and the system can remain somewhat stable when the temperature is between 5 � C and 90 � C. Simultaneously, an evaluation is made against the wettability of the A 3 B 4 C 3 E 5 F 2 system after being heat-treated at 90 � C, and a test is then performed concerning the contact angle between the LD1 crude oil and the PVC wetted by the system (Figure 8).The contact angle at 30 min of contact is 171.27 � , the t 0 is longer than 1800s, and the WCI is 5010.Therefore, we summarize that the A 3 B 4 C 3 E 5 F 2 system heat-treated at 90 � C can still exhibit preferable wettability, and the temperatures below 90 � Cexert a subtle effect on the wettability of the system.
Salt and oil tolerance.The effect of NaCl concentration on the viscosity and wettability of the A 3 B 4 C 3 E 5 F 2 system is studied to explore the salt resistance of the system The results reveal that as NaCl concentration increases, the viscosity of the system falls abruptly at first and then tends to be balanced (Figure 9a)., 39] At a high salt concentration, the electrostatic shielding will give full play to its role.With the inorganic salt concentration increasing, the structure of the system will show no change, and the viscosity change is gradually leveled off.An evaluation is made against the wettability of the A 3 B 4 C 3 E 5 F 2 system with different NaCl concentrations (0.01 � 0.5 wt.%), and a test is performed against the contact angle between the LD1 crude oil and the PVC pipe wetted by the system.The contact angle at 30 min of contact is all wider than 167.44 � , the t 0 is longer than 1800s, and the minimum WCI is 4981.Therefore, salt faintly affects the wettability of the easy-wetting viscoelastic system.The foremost reason is that the X-405 wetting agent falls with a nonionic wetting agent, which is less susceptible to the anions and cations ionized by NaCl.Consequently, the concentration of inorganic salt has a more significant impact on the viscosity of the system, when compared with the wettability of the system.
LD2 crude oil is added to the A 3 B 4 C 3 E 5 F 2 system so that the crude oil content is up to 1 wt.%, 3 wt.%, 5 wt.%, 7 wt.%, 10 wt.%, and 15 wt.%, respectively.Then, the oil is blended with the HJ-4 magnetic stirrer for 30 min.Allowing to the system stand for 6 h, we test the apparent viscosity of the system at shear rates of 10 1/s, 50 1/s, and 100 1/s, respectively.Synchronously, the easy-wetting viscoelastic system is separated, and the contact angle between the LD1 crude oil and the PVC pipe wetted by the system is then tested.The test results are displayed in Figure 9b.With the increase of oil content, the apparent viscosity of the system is basically unchanged, implying that the oil concentration subtly affects the viscosity of the A 3 B 4 C 3 E 5 F 2 system.The contact angles at 30 min are all wider than 163.37 � , the t 0 is longer than 1800s, and the minimum WCI is 4871.This indicates that in the range of LD2 crude oil concentration tested, the A 3 B 4 C 3 E 5 F 2 system is of better wettability.

Influence of pipe material on wettability.
In the A 3 B 4 C 3 E 5 F 2 system, the contact angles between LD1 crude oil and No.10 carbon steel, 304 stainless steel, X-65 steel, and X-70 steel are tested, in sequence.The pictures after 30 min of contact are shown in Figure 10 below.The contact angles at 30 min of contact are all wider than 171.63 � , the t 0 is longer than 1800s, and the minimum WCI is 4979.Thus, a conclusion can be drawn that the A 3 B 4 C 3 E 5 F 2 system can also preferably wet the pipelines commonly used in oilfields.

Conclusions
The prepared A 3 B 4 C 3 wetting agent is chemically composed of 100 ml water þ 0.05 ml X-405 þ 0.07 ml 9485 þ 0.05 ml AP9901.The contact angle between the pre-wet PVC pipe and LD1 crude oil at 30 min can be increased from 49 � to 133 � , while t 0 and WCI can be increased from 3 s to 180 s, and 1616 to 4138, respectively.This property can prevent oil from sticking to the pipe wall in oil-water two-phase flow.E 5 F 2 is chemically composed of 100 mL water þ0.125 g PAM þ 0.03g XG.The E 5 F 2 falls within a yield pseudoplastic fluid, and its zero-shear viscosity is 7045 mPa�s, while the viscosity is merely 32.92 mPa�s when the shear rate is 100 1/s.A high viscoelastic membrane layer can come into being between crude oil and pipe wall in the event of a shutdown or low flow rate, and low viscosity can accelerate crude oil flow at a high flow rate.At the appropriate temperature, oil content, and shear rate, A 3 B 4 C 3 E 5 F 2 can still exhibit strong wettability and viscoelasticity.A 3 B 4 C 3 E 5 F 2 system can also exhibit good wettability for the pipeline materials commonly used in oilfields.Finally, A 3 B 4 C 3 E 5 F 2 is eventually judged as an oil-tolerance, shear-tolerance, and easy-wetting viscoelastic system.

Figure 2 .
Figure 2. Dynamic contact angle changes of LD1 and PCV in the wettability aqueous solution filtered from the emulsion of A 3 B 4 C 3 and LD2 (A 3 B 4 C 3 :LD2 ¼ 6:4).

Figure 3 .
Figure 3. Equilibrium final velocity and resistance coefficient under each experiment number.

Figure 4 .
Figure 4. Performance of compound polymer aqueous solution system.

Figure 5 .
Figure 5. Interaction between wetting agents and compound polymer aqueous solution system.

Figure 6 .
Figure 6.Effect of shear rate on the stability of A 3 B 4 C 3 E 5 F 2 system.

Figure 7 .
Figure 7. Effect of temperature on the stability of the A 3 B 4 C 3 E 5 F 2 system.

Figure 8 .
Figure 8.The contact angle between LD1 crude oil and PVC pipeline material wetted by the A 3 B 4 C 3 E 5 F 2 system heat-treated at 90 � C.

Figure 9 .
Figure 9.Effect of salt and oil on the stability of A 3 B 4 C 3 E 5 F 2 system.

Figure 10 .
Figure 10.Contact angle between LD1 crude oil and common oilfield pipelines wetted by A 3 B 4 C 3 E 5 F 2 system.

Table 1 .
Results of orthogonal experiment design (LD1 crude oil and PVC pipes).