Adhesion Characteristics of Crude Oil on Non-Metallic Pipelines During Low-Temperature Gathering and Transportation

Abstract

To address the lack of theoretical understanding regarding crude oil wall adhesion in non-metallic pipelines and to compare it with the behavior in metallic pipelines, this study investigates the wall adhesion behavior of crude oil in non-metallic pipelines using a self-developed, simulated stirred tank setup. The main factors influencing crude oil adhesion in non-metallic pipelines were identified, and the differences in adhesion behaviors across different pipeline materials were clarified. Additionally, the reasons behind these behavioral differences were explored through contact angle measurements and the interfacial energy theory. The results indicate that the factors affecting the transportation temperature of crude oil in non-metallic pipelines include the overall water content, shear strength, and wax content of crude oil. On average, the amount of adhered crude oil in the three types of non-metallic pipeline was lower than that in the metallic pipelines. Among them, the flexible, composite non-metallic pipeline showed the greatest reduction in average adhered oil mass by 22.86%. This significant reduction is attributed to the reduced adhesion of crude oil on the non-metallic surfaces. Therefore, using non-metallic pipelines in crude oil transportation networks is advantageous for implementing low-temperature gathering and transportation processes.

1. Introduction

Despite continued oilfield exploitation in China, energy production has gradually declined. As a result, most oilfields require water injection to supplement reservoir pressure and enhance production [1]. This has led to a decrease in the crude oil content of the produced fluid, with the water cut in some oilfield blocks exceeding 90%. Since the specific heat capacity of water is higher than that of oil, the energy required to heat high-water-cut crude oil is significantly high [2,3]. At the same time, the rising water content in the produced fluid deteriorates the gathering and transportation environment, causing issues such as the scaling and corrosion of metal pipelines and secondary pollution [4]. Non-metallic pipelines—such as ceramic pipes, fiberglass pipes, PE pipes, and flexible composite pipes—have gradually been promoted and applied in major oilfields due to their excellent corrosion resistance and good wax deposition properties [5,6]. Currently, some researchers in the major oilfields in China are actively conducting research on low-temperature gathering and transportation processes for crude oil pipelines [7,8]. However, the existing research mainly focuses on metallic pipelines, with a limited number of studies on the wall adhesion behavior of crude oil in non-metallic pipelines during gathering and transportation. Under low-temperature conditions, crude oil in non-metallic pipelines tends to exhibit wall adhesion. Studying the adhesion characteristics and the underlying mechanisms is crucial for enhancing energy efficiency, improving operational safety, and optimizing the performance of oilfield gathering and transportation systems.

The existing studies have found that the main factors influencing the wall adhesion process of high-water-cut, waxy crude oil include the pipeline transportation conditions and the pipeline material [9,10]. The influence of transportation conditions mainly involves three aspects: gathering temperature, the water cut of the produced fluid, and the properties of the produced water [11]. The gathering temperature primarily affects the viscosity of oil–water emulsion. As the transportation temperature decreases, the viscosity of the emulsion increases, leading to stronger adhesion between the oil droplets and between the crude oil and the pipe wall, resulting in an increased amount of oil deposited on the inner pipe surface [12]. When the water cut of the produced fluid is high, its effect on oil wall adhesion inside the pipeline can be summarized in three ways. Firstly, the water phase can pre-wet the inner surface of the pipeline, forming a “water film” that reduces flow resistance [13]. The higher the water cut is, the more significant this effect is, thereby decreasing the amount of oil adhered to the pipe wall [14]. Secondly, the presence of water molecules weakens the internal tension of crude oil, affecting the aggregation of asphaltenes, and thus reducing the amount of oil deposited on the pipe wall [15]. Thirdly, the water phase reduces the oil content in the system, weakening diffusion and cohesion between the oil droplets, making it more difficult for an oil layer to form on the pipe wall. The main properties of the produced water include pH and salinity [4,16]. The pH of the water phase affects oil adhesion primarily through electrostatic interactions; acidic and alkaline environments may impart like charges to the metal surface, resulting in double-layer electrostatic repulsion, which inhibits the adsorption of oil droplets more compared to that of water droplets, thereby suppressing wall adhesion [17]. Some experimental studies have shown that the presence of ions, such as Ca²⁺, Mg²⁺, and Na⁺, in produced water reduces the amount of crude oil adhering to the pipe wall compared to that in neutral conditions [18]. Different pipeline materials, due to variations in thermal insulation and interfacial properties, also affect the transportation process. Research has shown that using fiberglass-reinforced plastic (FRP) pipes in the gathering and transportation of high-water-cut, waxy crude oil can reduce the amount of wax deposited on the pipe wall, confirming the applicability of non-metallic pipelines in gathering systems [19].

Based on the above understanding, this study investigates the influence of flow conditions and crude oil composition on the wall adhesion behavior of crude oil in non-metallic and metallic pipelines through laboratory-scale wall adhesion simulation experiments. The differences in the crude oil adhesion processes between the non-metallic and metallic pipelines are compared. Furthermore, the reasons behind these differences are analyzed based on interfacial property tests of the crude oil and adhesion theory, thereby clarifying the adhesion mechanism of crude oil within non-metallic pipelines.

2. Material and Methods

2.1. Experimental Device

In this study, a simulated, stirred tank device and a high-temperature, high-pressure contact angle tester were used to investigate the wall adhesion characteristics and mechanisms of crude oil in both non-metallic and metallic pipelines. The experimental setups are shown in Figure 1 and Figure 2.

The indoor simulated stirred tank device mainly consists of a magnetic stirring system, a simulated stirred tank, a circulating temperature-controlled water bath, and a control box. The magnetic stirring system uses a four-blade pitched paddle impeller. The inner wall materials of the simulated stirred tank include stainless steel, fiberglass-reinforced plastic (FRP), polyethylene (PE), and non-metallic flexible composite pipe. The tank is 110 mm in height and 69 mm in diameter. The temperature-controlled water bath used is the HAAKE-SC100A28F model (Ningbo Scientz Bi-otechnology Co., Ltd., Ningbo, China). The control box allows for the real-time monitoring of temperature and pressure inside the tank and can adjust the rotation speed of the magnetic stirring system within a range of 0~1000 r/min.

Interfacial property tests were conducted using the data physics OCA25-PMC750 high-temperature, high-pressure contact angle measurement instrument (DataPhysics Instruments, Filderstadt, Germany). This device primarily captures test images using an optical system and a digital camera, and then calculates interfacial tension and contact angle using image analysis software SCA202 (V6.1.19), A detailed introduction has been given in our previous research.

2.2. Experimental Oil Sample

The crude oil samples used in the experiment are waxy crude oils collected from different oilfield blocks across China, labeled as crude oil #1 through #6. To eliminate the influence of thermal and shear history, the dehydrated oil samples were subjected to thermal treatment prior to the testing of their basic physical properties. The test results are presented in

Table 1. Basic physical properties of crude oil.

Oil Sample	Gel Point (°C)	Density (20 °C), kg/m3	Density (50 °C), kg/m3	Wax Appearance Temperature, °C	Peak Temperature Point for Wax Appearance, °C	Wax Content, wt%
#1	38	841	821	54.62	25.88	27.71
#2	30	852	830	51.90	19.89	17.52
#3	37	862	842	54.32	28.71	26.17
#4	34	853	873	43.89	16.21	15.91
#5	33	810	788	49.68	18.49	24.57
#6	30	843	822	59.37	24.30	13.91

.

2.3. Experimental Method

The experiments primarily consisted of indoor wall adhesion tests and crude oil interfacial property tests.

(1)

Prepare oil–water emulsions with water cuts similar to those in field conditions. Mix crude oil and water at a temperature consistent with the wellhead temperature. Set the circulating water bath to 5 °C above the oil’s pour point. Preheat both the emulsion and the test setup in this bath for 1 h to ensure uniform temperature.

(2)

Introduce 200 mL of the prepared emulsion into the test tank. Start the stirring system and set the required rotational speed. The rotational speed is controlled by a frequency converter that regulates the motor speed. Gradually reduce the tank temperature at a rate of 0.5 °C/min. Once the target temperature is reached, continue stirring for 10 min to allow for stable adhesion behavior to occur. Open the tank outlet to drain the emulsion during stirring. Use a scraper to collect the adhered crude oil from the inner wall. Measure the mass of the collected oil.

(3)

Repeat the process at lower temperatures. When a significant increase in adhered oil mass is observed, stop the test. The corresponding temperature is defined as the temperature point where m_n+1/m_n > 50%; here, m_n+1 and m_n represent the adhered oil mass obtained from the (n + 1)th and nth test runs, respectively.

(4)

Set the minimum gathering and transportation temperature above the wall adhesion temperature to prevent oil deposition.

Experimental Steps for Testing Interfacial Properties of Crude Oil.

(1)

Sample Preparation: Cut rectangular plates (2 cm × 1.1 cm) from flexible composite pipe material and stainless steel sheets. Polish both the plates to achieve uniform surface roughness.

(2)

Setup and Preheating: Place the test plate on the sample stage in a solution tank filled with deionized water (water level above plate). Level the stage. Set the oil and water baths to the target test temperature for preheating the system and crude oil sample.

(3)

Contact Angle Measurement: Use a microsyringe (0.52 mm needle) to draw crude oil. Position the needle above the center of the test plate. Start the testing device and adjust brightness for clear visibility. Inject the oil droplet and start recording when it makes contact with the plate. Measure and record the contact angle after 5 min of equilibrium. Replace deionized water and repeat under different conditions as needed.

(4)

Interfacial Tension Measurement: Keep the temperature stable in the chamber. Remove the sample stage and inject a 10 μL oil droplet into the water phase using a microsyringe.

(5)

Measure and record the interfacial tension using the software.

3. Results and Discussion

3.1. Effects of Different Water Cut Contents

The simulated stirred tank wall adhesion experiments were conducted for crude oils from #1 to #6 under different water cut conditions. The wall adhesion temperatures of the six crude oils were tested on both the non-metallic and metallic pipe surfaces at water cuts ranging from 70% to 90% and shear rates from 10 s⁻¹ to 30 s⁻¹. Taking a shear rate of 20 s⁻¹ as an example, the experimental results are shown in Figure 3.

As shown in Figure 3, the influence pattern of the water content is the same for both the non-metallic and metallic pipelines. The wall-sticking temperature of crude oil is negatively correlated with the water content. Moreover, the wall-sticking temperature of crude oil remains consistent across the different pipeline materials. At a shear rate of 10 s⁻¹, when the water content increases from 70% to 90%, the wall-sticking temperature of crude oil #1 decreases from 33 °C to 31 °C; for crude oil #2, it decreases from 24 °C to 22 °C; for crude oil #3, from 33 °C to 31 °C; for crude oil #4, from 29 °C to 27 °C; for crude oil #5, from 27 °C to 25 °C; and for crude oil #6, from 25 °C to 24 °C.

In an oil–water two-phase system, the water phase plays a wetting role, meaning that a thin water film exists between crude oil and the pipe wall, which reduces the adhesion between waxy crude oil and the pipe surface [20]. Additionally, an increase in water content suppresses the aggregation and diffusion of wax particles, thereby decreasing the likelihood of crude oil coming into direct contact with the inner pipe wall [21]. Moreover, a higher water content reduces the distance between particles in the crude oil emulsion, weakening the interaction forces between oil droplets, and thus increasing the difficulty for crude oil to adhere to the pipe wall [22]. As a result, under the same shear rate and other experimental conditions, an increase in water content leads to a decrease in the wall-sticking temperature.

3.2. Effects of Different Shear Intensities

Wall-sticking experiments using a simulated stirred tank were conducted for crude oils #1 through #6 under different shear intensities. The wall-sticking temperatures of the six crude oil samples were measured in both the non-metallic and metallic pipelines at shear rates of 10 s⁻¹, 20 s⁻¹, and 30 s⁻¹ and at water contents of 70%, 80%, and 90%. Taking a water content of 80% as an example, the experimental results are shown in Figure 4.

As shown in Figure 4, the effect of shear intensity on both the non-metallic and metallic pipelines is the same. The wall-sticking temperature of crude oil is generally negatively correlated with shear intensity, and the wall-sticking temperatures are consistent between the non-metallic and metallic pipelines. As the shear rate increases from 10 s⁻¹ to 30 s⁻¹, the adherent wall temperatures of crude oils from #1 to #4 all decrease by 2 °C compared to their original values. The adherent wall temperature of 5# crude oil decreases by 1 °C, while that of #6 crude oil shows a larger decrease of 3 °C. Under the different test conditions, the error of the repeated measurement of the wall surface adhesion temperature does not exceed 1 °C. As the shear rate gradually increases, the shear stress near the pipe wall also increases, enhancing the scouring and stripping effects of waxy crude oil at the wall surface, thereby inhibiting the adhesion of crude oil to the wall. Meanwhile, higher shear intensity leads to an increase in the number of droplets and a reduction in their size within the oil–water emulsion [23]. This results in weaker adhesive forces between droplets, further hindering the wall-sticking process of crude oil.

3.3. Effects of Different Wax Contents in Crude Oil

In this section, six types of crude oil with varying wax contents were analyzed using Differential Scanning Calorimetry (DSC), and laboratory wall-sticking experiments were conducted to determine the wall-sticking temperatures of these crude oils [24]. The tests were performed using two types of pipeline materials, non-metallic flexible composite pipes and stainless steel pipes, under the conditions of a water content of 80% and a shear rate of 20 s⁻¹. The test results are presented in

Table 2. The wax content of crude oil and the wall adhesion temperature.

Oil Sample	Wax Content, wt%	The Adhesion Temperature of Non-Metallic Pipes to the Wall, °C	The Adhesion Temperature of Metallic Pipes to the Wall, °C
#1	27.71	30	30
#2	17.52	21	21
#3	26.17	31	31
#4	15.91	26	26
#5	24.57	24	24
#6	13.91	23	23

.

As shown in Table 2, the wax content of crude oil affects both the non-metallic and metallic pipelines in the same way. Under identical conditions of water content and shear rate, the wall-sticking temperature of crude oil increases progressively with a higher wax content. Furthermore, the wall-sticking temperatures remain consistent between the non-metallic and metallic pipelines. When the wax content increases from 13.91% to 35.09%, the wall-sticking temperature rises from 22 °C to 39 °C.

During the experiment, shear forces disrupted the water film on the surface, allowing for polar compounds in the wax fraction to adhere more easily to the wall, resulting in an increase in the metallic and non-metallic wall-sticking temperatures. Additionally, since the laboratory wall-sticking experiments were conducted at temperatures below the crude oil pour point, the crude oils with a higher wax content precipitated more wax crystals. These crystals form a denser network structure, leading to larger waxy aggregates under the same conditions [25]. This promotes the wall-sticking process of crude oil and causes the wall-sticking temperature to increase accordingly.

3.4. The Quality Difference in Crude Oil Adhering to the Wall Between Non-Metallic and Metallic Pipelines

An investigation and analysis were conducted on the differences in the crude oil adhesion and wall-sticking processes within the non-metallic and metallic pipelines in order to clarify the influence of the pipeline material on the adhesion and wall-sticking behaviors of crude oil during gathering and transportation. The simulated stirred tank wall-sticking experiments were conducted for crude oils from 1# to 6# using different pipeline wall materials, with the water content ranging from 70% to 90% and shear rates from 10 s⁻¹ to 30 s⁻¹. Taking the condition of 80% water content and a shear rate of 20 s⁻¹ as an example, the test results are shown in Figure 5.

As shown in Figure 5, the pipeline material has a noticeable impact on crude oil adhesion and the wall-sticking process. Although the wall-sticking temperature remains consistent across the different pipe materials, there is a significant difference in the amount of crude oil adhered to the wall. The wall adhesion masses shown in this figure are the average values from three repeated experiments, and the maximum relative error among all the test results does not exceed 8%. Among the materials tested, stainless steel exhibited the highest wall-sticking mass, while the non-metallic pipes generally showed lower values, with the flexible composite non-metallic pipe exhibiting the lowest wall-sticking mass. Taking crude oil sample #1 with an 80% water content as an example, the average wall-sticking mass in the fiberglass-reinforced plastic (FRP), PE, and flexible composite non-metallic pipes decreased by 7.48%, 19.85%, and 22.86%, respectively, compared to that of stainless steel. This indicates that the adhesion and wall-sticking behavior of crude oil is weaker in non-metallic pipelines than in metallic ones.

The surface free energy varies across the different pipeline materials [26]. Materials with high surface free energy tend to have a stronger affinity for crude oil deposition. Stainless steel has a higher surface free energy compared to those of the FRP, PE, and flexible composite non-metallic pipes. As detailed in Section 3.6 on the interfacial characteristics of crude oil in non-metallic pipelines, this results in a greater tendency for oil to adhere and accumulate on metallic surfaces. Consequently, the adhesion and wall-sticking strength of crude oil is weaker in non-metallic pipelines, reflected in the reduced wall-sticking mass.

3.5. Modelling of Wall Adhesion Temperature of Crude Oil in Non-Metallic Pipelines

After conducting the experiments and analyses on the factors affecting the wall-sticking process of crude oil in non-metallic pipelines, this section identifies the water content, the wax content, the pour point, and shear intensity as the primary influencing factors for wall-sticking temperature in non-metallic pipelines. Based on the indoor wall-sticking experiment results of crude oils #1, #2, and #3 under various conditions, a predictive model for adhesion temperature in non-heated gathering and transportation systems using non-metallic pipelines was established, as shown in Equation (1).

$$T=a{T}_o^h+b{\varphi }^k{\theta }^m{\gamma }^n$$

(1)

In this model, T is the wall adhesion temperature of crude oil in the non-metallic pipelines, °C; T_o is the gel point of crude oil, °C; ϕ is the water content, %; θ is the wax content of the crude oil, wt%; γ is the average shear rate, s⁻¹; and a, b, h, k, m, and n are the fitting parameters. By fitting the experimental data from crude oils #1, #2, and #3 under various conditions, the values of the parameters a, b, h, k, m, and n were obtained, as listed in

Table 3. Fitting parameters of model.

Parameter	a	b	h	k	m	n
Value	2.945	37.925	0.854	0.211	0.166	0.060

.

This model was then applied to predict the wall adhesion temperature of crude oils #4 through #6. The predicted values were compared with the experimentally measured wall adhesion temperature to evaluate the accuracy of model and determine the prediction error. The prediction results and corresponding comparisons are presented in

Table 4. Comparison results of predicted values and experimental values of #4 crude oil.

Water Content, %	Average Shear Rate, s−1	Predicted Values, °C	Experimental Values, °C	Error, °C
70	10	30.18	29	1.18
	20	28.92	27	1.92
	30	28.16	26	2.16
80	10	29.34	27	2.34
	20	28.04	26	2.04
	30	27.25	25	2.25
90	10	28.57	27	1.57
	20	27.24	26	1.24
	30	26.43	24	2.43

,

Table 5. Comparison results of predicted values and experimental values of #5 crude oil.

Water Content, %	Average Shear Rate, s−1	Predicted Values, °C	Experimental Values, °C	Error, °C
70	10	26.44	27	−0.56
	20	25.09	26	−0.91
	30	24.27	24	0.27
80	10	25.53	25	0.53
	20	24.14	24	0.14
	30	23.29	24	−0.71
90	10	24.70	25	−0.30
	20	23.27	24	−0.73
	30	22.41	23	−0.59

and

Table 6. Comparison results of predicted values and experimental values of #6 crude oil.

Water Content, %	Average Shear Rate, s−1	Predicted Values, °C	Experimental Values, °C	Error, °C
70	10	24.76	25	−0.24
	20	23.53	24	−0.47
	30	22.78	23	−0.22
80	10	23.93	24	−0.07
	20	22.66	23	−0.34
	30	21.90	21	0.90
90	10	23.18	24	−0.82
	20	21.88	22	−0.12
	30	21.09	21	0.09

.

As shown in Table 4, Table 5 and Table 6, the differences between the predicted and experimentally measured wall adhesion temperature for crude oils in the non-metallic pipelines are all within 2.5 °C. This demonstrates the accuracy of the model, indicating that it can be reliably used for predicting the wall-sticking temperatures of crude oil in non-metallic pipeline systems.

3.6. The Wall Adhesion Mechanism of Crude Oil in Non-Metallic and Metallic Pipelines

Interfacial characteristic experiments were conducted using a contact angle tester on crude oils from #1 to #3, as well as the metallic and non-metallic pipeline materials. From the perspective of the microscopic interfacial energy theory, the wall-sticking mechanism of crude oil in non-metallic pipelines was investigated, revealing the underlying reasons for the differences in the wall-sticking behavior between the metallic and non-metallic pipelines. The results of the interfacial characteristic tests are shown in Figure 6, Figure 7 and Figure 8. The contact angles shown in Figure 6 are the average values from three repeated experiments, and the maximum relative error among all the test results does not exceed 6%.

For the crude oil–pipe material contact system, the wetting theory applies. This theory, representing colloid and interface chemistry on a macroscopic scale, is governed by surface free energy interactions. In the wall-sticking process of crude oil in the non-metallic pipelines, the sequence follows a wetting-then-adhesion mechanism; adhesion occurs on the basis of prior wetting. The adhesion and wall-sticking behavior of crude oil inside the pipe is mainly influenced by the contact angle between crude oil and the pipe material, as well as interfacial tension. During the adhesion and wall-sticking process, the interface is subjected to molecular forces from both the sides. The energy required to adhere two phases across the interface is referred to as the work of adhesion. In this section, Young’s equation from the wetting theory is used to explore the quantitative relationship between the crude oil–pipe contact angle and the crude oil’s interfacial tension [27]. Combined with Dupre’s equation for calculating the work of adhesion, these two equations are jointly applied to compute the work of adhesion for crude oil on both the non-metallic and metallic pipe surfaces underwater. The final expression for this calculation is provided in Equation (2).

$$W_{op}={\gamma }_{ow}(1+\cos \theta )$$

(2)

where θ is the contact angle between crude oil and the pipe material, °; W_op is the underwater adhesion work of a crude oil droplet on the pipe surface, mJ/m²; and γ_ow is the interfacial tension of crude oil in water, mN/m.

Using Equation (2), the underwater adhesion strength of crude oils #1 through #3 on the different pipe material surfaces was calculated. The calculation results are presented in

Table 7. Underwater adhesion work of crude oil on different pipe surfaces.

Oil Sample	Test Temperature, °C	Metal Surface Adhesion, mJ/m2	Non-Metallic Surface Adhesion Work, mJ/m2
1#	40	26.25	11.52
	45	27.14	12.22
	50	23.93	11.26
	55	22.08	10.74
	60	18.99	9.40
2#	40	50.97	23.66
	45	45.60	22.16
	50	41.22	20.33
	55	37.84	19.10
	60	32.75	16.66
3#	40	34.40	16.48
	45	32.99	16.72
	50	29.82	15.54
	55	26.24	14.01
	60	23.87	13.04

.

As shown in Table 7, the adhesion work of crude oil on the pipe surfaces does not exhibit significant variation within a certain temperature range. However, the differences in pipeline material and surface roughness have a substantial impact on the adhesion work. Specifically, the adhesion work of crude oil on the pipe surfaces is negatively correlated with surface roughness; more roughness leads to less adhesion. Furthermore, compared to the metallic pipe surfaces, the adhesion strength of crude oil on the non-metallic pipes is significantly reduced, indicating a weaker tendency for crude oil to adhere to non-metallic surfaces.

As the temperature continues to drop, the amount of waxy crude oil adhering to the pipe wall gradually increases. According to the adhesion theory, this phenomenon is influenced by cohesive work. During the wall-sticking process of crude oil in the pipelines, intermolecular forces exist between the oil droplets. The energy required for these oil droplets to interact and coalesce into a unified mass is defined as the cohesive work between the crude oil droplets [28]. In the adhesion process of crude oil on the pipe surfaces, the cohesive value of crude oil is considered to be twice as high as the interfacial tension of crude oil in the water phase. Based on this principle, the formula for calculating the cohesive work of crude oil under different testing temperatures in the aqueous phase is given by Equation (3).

$$W_{ow}=2{\gamma }_{ow}$$

(3)

where W_ow is the cohesive work of crude oil in the aqueous phase, mJ/m²; γ_ow is the interfacial tension of crude oil in water, mN/m. The cohesive work of crude oil in the aqueous phase at different testing temperatures for three types of crude oil was calculated, and the results are presented in

Table 8. Cohesive work of crude oil at different test temperatures underwater.

Test Temperature/°C	#1, mJ/m2	#2, mJ/m2	#3, mJ/m2
40	58.28	73.38	66.94
45	52.98	64.76	62.5
50	46.16	57.32	55.66
55	41.82	51.88	48.12
60	35.04	44.34	43.16

.

As shown in Table 8, the cohesive work between the crude oil droplets is negatively correlated with temperature. This is because as the temperature decreases, the intermolecular forces between the oil droplets increase. Since the cohesive work is solely dependent on the interfacial tension of crude oil in water at a given temperature—and the interfacial tension increases as the temperature drops—the cohesive work of crude oil correspondingly rises with decreasing temperature. Having determined both the underwater cohesive work of crude oil and its adhesion work on different pipe surfaces, the adhesion and wall-sticking processes in the non-metallic and metallic pipelines can be analyzed as follows.

When there is no water film between crude oil and the pipe wall, the oil droplets begin to form a contact angle with the pipe surface, initiating the adhesion behavior. If the adhesive force of the crude oil is greater than the shear stripping force from the flowing oil, the oil droplets will adhere to the pipe wall and form an initial adhesion layer. As the temperature continues to decrease, the increased cohesive work between the oil droplets causes more droplets to adhere to the initial adhesion layer, gradually increasing the mass of crude oil sticking to the pipe wall. Throughout the entire wall-sticking process, the adhesion strength of crude oil on the non-metallic pipe surfaces is significantly lower than that on metallic surfaces. This means that the formation of an initial adhesion layer in the non-metallic pipelines is much more difficult than in the metallic ones. As a result, the wall-sticking tendency of crude oil is weakened in the non-metallic pipes, and any adhered waxy crude is more easily removed under the action of shear forces [5]. This explains why in the laboratory wall-sticking experiments, the amount of crude oil adhering to the non-metallic pipe walls is noticeably lower than that on the metallic pipe walls.

4. Conclusions

This study focuses on investigating the differences in crude oil adhesion and wall-sticking behavior between non-metallic and metallic pipelines, aiming to clarify the mechanisms governing crude oil adhesion and wall-sticking in non-metallic pipes. This research was conducted from both macroscopic and microscopic perspectives. Macroscopically, it identifies the key influencing factors and compares the crude oil wall-sticking behaviors in non-metallic and metallic pipelines, while microscopically, it explores the underlying reasons for these differences.

The overall water content, the wax content in crude oil, and shear strength are the primary factors affecting the wall-sticking temperature. An increase in the overall water content from 70% to 90% or an increase in shear rate from 10 s⁻¹ to 30 s⁻¹ both lead to a decrease in the wall-sticking temperature by approximately 1–2 °C.

Although the wall-sticking temperature remains consistent across the different pipe materials, the wall-sticking mass of crude oil in the non-metallic flexible composite pipes is the lowest, showing a 22.86% reduction compared to that in the metallic pipelines. The microscopic calculations reveal that the reduced adhesion work of crude oil on the non-metallic surfaces weakens the wall-sticking behavior, which explains the significantly reduced crude oil wall-sticking mass in the non-metallic pipes compared to that in the metallic ones.