New Approach for the In Situ Microscopic Observation of Wax Crystals in Waxy Crude Oil during Quiescent and Dynamic Cooling

Abstract

The purpose of this attempt is to present a new investigation approach to achieve the in situ observation of the microscopic structure and morphology of wax crystals under quiescent and shear conditions. The rheo-microscopy simultaneous measurement system of a rheometer is employed to ensure in situ observation. A multi-angle composite light source is created to obtain a high-quality image. It is demonstrated that the new approach can achieve a better identification and distinction of wax crystals, as well as the outstanding wax boundary delineation. Based on this, some new findings related to the microscopic structure and morphology of wax crystals are elaborated. Additionally, the in situ observations of wax crystals under dynamic cooling at different shear rates are performed. It is noticed from the obtained results that wax crystals and their aggregates exhibit significant stereoscopic structural characters, because of growth of wax crystals and their overlap in 3-D space. Shear can change the morphology of single wax crystals, but hardly destroy the structure or growth. The increase of the shear rate can induce the deformation propensity of wax crystals to flow field. The effect of shear on aggregation of wax crystals depends on the chemical composition and inherent structural properties. Normally, a low rate of shear can promote aggregation, and result in a complicated stereo-structure. Upon increasing the shear rate, two opposite effects simultaneously occur including promotion and inhibition of aggregation. As the shear rate further increases, the destructive effect increases and gradually plays a leading role, causing the wax crystal aggregates exhibit a smaller size and a weaker stereo structure.

1. Introduction

In recent years, with decreasing conventional petroleum resources, the high-efficient development, storage, and transportation of waxy crude oil have a significant meaning for stable supply of petroleum energy. The flow assurance challenges imposed by wax during the production and transportation of waxy crude oil draw more and more attention. Issues associated with wax crystallization including increased fluid viscosity, formation of a wax deposition layer, and pipeline restart issues because of waxy gelation become the major challenges. The morphology and structure of wax crystals is one of the most important factors influencing the flow properties and wax deposition of waxy crude oils. The mechanisms of heat treatment and addition of PPD to improve the macrorheological properties of waxy crude oils also primarily alter the morphology and structure of wax crystals or the form and state of aggregates. Therefore, investigation of the microscopic characters of wax crystals is important to meet the challenges during the production and transportation of waxy crude oil.

Different methods were used to study wax crystallization. Differential scanning calorimetry¹⁻⁵ is a method to study the crystallization behavior of wax based on the thermal characteristics during wax precipitation. Based on this method, the wax appearance temperature (WAT) and the wax content of crude oil were measured. Electron microscopy⁶⁻⁸ was commonly used to observe the microstructure and morphology of wax crystals by many scholars. Moreover, rheometry,^9,10 absolute-calibrated small-angle neutron scattering, X-ray scattering,^11,12 dynamic light scattering,¹³ and the nuclear magnetic resonance technique¹⁴ were also effective experimental techniques to investigate the wax crystallization. In recent years, some new techniques both experimentally and theoretically were continued to be developed to investigate the microscopic characteristics of wax crystals.¹⁵⁻¹⁸ However, among those technologies, from the perspective of technology maturity and realization conditions, microscopic observation using a polarized light microscope is still the most conventional and efficient method to study the morphology and structure of wax crystals. Yi¹⁹ performed the microscopic observations of the morphology and structure of wax crystals before and after PPD beneficiation. The fractal dimension of the wax crystal microstructure was determined on the basis of the microscopic images of wax crystals. Bai²⁰ conducted the research on the effect of the carbon number distribution of wax on the yield stress of waxy oil gels. The morphology and structure of the wax crystals was observed using an optical microscope. Norrman²¹ investigated the effect of the amount of coverage of the nanoparticles on the performance of the nanoparticles by visual observation of the formed wax with a polarized microscope. Jung²² investigated the effect of several ethylene copolymers, and small molecules with a long alkyl chain, on wax formation for n-C32H66 in decane and de-aromatized white oil. Li²³ conducted a systematic investigation of how asphaltenes of different polarities affect crystallization and gelation of waxy oils. Microscopic examination was performed using a Nikon OPTIPHOT2-POL polarizing microscope equipped with a Linkam PE60 cooling station and a charge-coupled device (CCD) digital camera. Yang²⁴ studied the effect of the PMSQ microspheres on the flow behavior of two typical waxy crude oils. Li²⁵ studied the crystallization behavior of Shengli waxy crude oil from the sol state to gel state by the microscopic observation and the differential scanning calorimetry (DSC) curve. Moreover, the morphology and structure of wax crystals under different external conditions and the changes in the internal chemical composition were extensively investigated by the optical microscopy method,²⁶⁻³⁰ and some important outcomes were obtained. Because the wax crystal aggregation is important for the formation of a microstructure in waxy crude oil, some investigations were conducted to achieve a detailed description of the morphological and structural information on aggregation. Wang³¹ developed a nucleation kinetics model inspired by a thermal analysis technique. Meanwhile, they provided new insights into the nucleation processes that occur during the formation of a waxy crude oil emulsion gel structure. Singh³² investigated the aging process of wax deposit. In particular, the morphology of wax crystals in deposit was studied in detail.

From the above studies, the related information on the morphology and structure of wax crystals was obtained. However, no consistent cognition has been achieved. This is due in part to the differences in observation instruments, including microscopes and light sources. The susceptibility of wax crystals to external environmental conditions is also a major factor. This results in the higher requirements of experimental and operation conditions. Furthermore, waxy crude oil is always in the dynamic shear flow during production and pipelining. Under shear, the microstructure of wax crystals exhibits more complex nonlinear behaviors. Then, some scholars focused on the effect of shear on the microstructure of wax crystals.

Rønningsen³³ believed that waxy crude oil has weak and strong structures. The weak structure is destroyed under a low shear, and the structural recovery is poor. The recovery of the thixotropic structure in waxy crude oil is higher at a higher shear rate. It shows that in dynamic cooling, the size of wax crystals gradually increases to a critical size with the corresponding shear rate.³⁴ Webber³⁵ considered that the shear mainly affects the overall structure of wax crystals, but has no effect on their morphology and size distribution. Xia and Zhang³⁶ investigated the mechanism of low-speed shear affecting the low-temperature fluidity of the PPD-treated waxy crude oil. They observed the microscopic morphology of wax crystals after applying shear using a polarizing microscope. Using a transmission electron microscope and the sample quick-freezing section method, Kané^37,38 investigated the structure of wax crystals formed under static and dynamic cooling conditions. They found that under the dynamic cold conditions, only the disk-shaped wax crystals were found. And they exist in isolation or in clusters. The size of clusters is determined by the shear rate. Under a low or a high shear speed, the morphology and structure of wax crystal aggregates is quite different. Selomulya³⁹ believed that the gelled waxy crude oil belonged to a diffusion-limited suspension system. Shear may cause large wax crystals or wax crystal aggregates to collide and aggregate. Venkatesan⁴⁰ took pictures of wax crystals through an optical microscope. Under a low speed shear, the flocculation of wax crystal particles is significant. However, the wax crystal structure is destroyed and its flocculation cannot be completed when the shear rate increases. Chen⁴¹ investigated the flow and viscoelastic behaviors around the wax precipitation temperature both after dynamic and static cooling. Microscopy and DSC were used to follow the crystallization of wax. Based on the relationship between the energy-dissipation rate and the shear rate, Zhang⁴² developed approaches for calculation of shear rates during pipelining, shear rates of turbulent pipe flow, and flow through centrifugal pumps and throttle valves. Yi⁴³ examined the influence of shear on both the fractal dimension characterizing the wax crystal morphology and the flow properties of eight waxy crudes treated with the PPD. Chen⁴⁴ studied the characteristic parameters of the wax crystal microstructure during the cooling under the dynamic shear flow. They considered that the shear can destroy the growth of wax crystals. Yi⁴⁵ conducted quantitative research on the mechanism of shear on the wax crystal morphology and structure. They found that the shear and chemical composition of crude oil can significantly affect the morphology and structure of wax crystals. Blake⁴⁶ studied the effect of shear on the microstructure and oil-binding capacity of three wax oleogels. They found that for crystallization under a cooling rate of 1.5 °C/min, shear decreased the crystal size, but increased the box-counting fractal dimension. Furthermore, some scholars have studied the effect of shear on the microstructure of wax crystals by rheological measurements.⁴⁷⁻⁵⁰

The above studies focused on the effect of shear on the microstructure of wax crystals, and a quantitative analysis was also performed. However, some outcomes were indirectly obtained because of rheological behaviors. Most microscopic observations adopted the offline observation method. The sampling operation inevitably brings about the change of the microstructure, because the microstructure of wax crystals is very sensitive to shear and thermal history. In addition, the observation conditions are difficult to replicate the shear because of which waxy crude oil has suffered during the offline observation. As a consequence, the errors in offline observation are hard to be avoided. The in situ observation can achieve more accurate wax crystal microstructure and morphological information, because the shear and microstructure observations are simultaneously performed.

The main purpose of this study is to propose a new approach that can achieve in situ observation of wax crystals under quiescent and shear conditions. The rheo-microscopy simultaneous measurement system of a rheometer (MCR702, Anton Paar) was employed to provide in situ observation conditions. A multi-angle composite light source was created to obtain a high-quality image. In order to examine the new approach, the polarizing microscope observations were also conducted for comparative analysis. Furthermore, using this new approach, the in situ observation of the microscopic structure and morphology of wax crystals during dynamic cooling at different shear rates was performed. In addition, the analysis of shear impact was carried out.

2. Experimental Details

2.1. Basic Physicochemical Properties of Waxy Crude Oils

Two kinds of waxy crude oils herein termed M1 and M2 produced in China have been used as the experimental samples. M1 comes from the Daqing oil field, which is one of the typical waxy crude oil producing areas. M2 comes from the Hulunbeier oil field. To obtain the physicochemical properties of the studied waxy crude oils, the test methods include:

• (1)Test method for the pour point

The test was conducted based on ASTM D5853-11 (standard test method for pour point of crude oils). The sample was heated at 45 °C, and then transferred to a water bath maintained at a temperature of 21 °C. The temperature at which the sample showed no movement was recorded, and added 3 °C as the pour point.

• (2)Analysis method for family composition of crude oil

The test was conducted based on SY/T 5119-2016 (analysis method for the family composition of rock extracts and crude oil), and the column chromatography was used as the method of separation and determination of the family composition of M1 and M2. In the test, n-hexane, dichloromethane, absolute ethyl alcohol, and chloroform were used as the solvents, and a chromatographic silica gel, neutral alumina, and degreasing cotton were used as the experimental materials.

• (3)Test method for density

The densities (20 °C) of two crude oils were determined using GB/T 1884-2000 (crude petroleum and liquid petroleum products-laboratory determination of density-hydrometer method).

Based on the above methods, the data of the physicochemical properties of two crude oils are listed in Table 1.

Table 1. Physicochemical Property Data of Two Waxy Crude Oils.

sample	density at 20 °C (kg/m3)	pour point (°C)	content of saturated hydrocarbon (%)	content of arene (%)	content of resins (%)	content of asphalt (%)
M1	855.4	32.0	68.9	19.1	8.8	3.2
M2	835.6	21.0	65.8	18.0	11.8	4.4

2.2. Thermal Property Parameters Related to Wax Precipitation

The thermal property parameters related to wax precipitation of two samples were investigated by DSC (Q2000, TA instruments). Experimental procedures referred to SY/T 0545-2012 (determination of thermal property parameters of the wax precipitation in crude oil-test method by differential scanning calorimetry) include: the samples were initially heated to 80 °C and kept for at least 10 min, then cooled to −40 °C at a rate of 5 °C/min to achieve the complete wax-precipitation process. The curve of heat flow as a function of temperature was obtained (as shown in Figure 1). Cumulative heat flow during wax precipitation was calculated by integrating the curve. The accumulative content of wax precipitates was calculated by using the wax average crystallization heat of 210 J/g. The obtained thermal property parameters are shown in Table 2.

Figure 1.

(a) Heat flow vs temperature of M1 (b) heat flow vs temperature of M2.

Table 2. Thermal Characteristic Parameters Related to Paraffin Precipitation.

sample	WAT (°C)	cumulative enthalpy (J/g)	wax content (%)
M1	47.85	42.41	20.20
M2	38.51	34.31	16.34

2.3. Carbon Number Distributions of Total Hydrocarbon

The carbon number distributions of total hydrocarbon of M1 and M2 crude oils were measured using a high-temperature gas chromatograph (HTGC) (Agilent 7890B) with a 30 m × 0.5 mm residual oil full range distillate capillary column. A customized software program for crude residual oil high-temperature simulation distillation was used to calculate the carbon number distributions. The full range distillate analysis was carried out according to standard ASTM D7169-18 (standard test method for boiling point distribution of samples with residues such as crude oils and atmospheric and vacuum residues by HTGC). The detailed test procedures are: the FID detector was used, the oven temperature was initiated at 35 °C, and then heated to 425 °C for 10 min at a rate of 15 °C/min. The content of each n-alkane was obtained using nC5–nC100 hydrocarbons to calibrate the standard sample. An external standard method was used to determine carbon number distributions of M1 and M2 crude oils, which are shown in Figure 2. The mass percent of hydrocarbons in different carbon number ranges is listed in Table 3. The summary of simulated distillation results is shown in Table 4.

Figure 2.

(a) Carbon number distributions of M1 (b) carbon number distributions of M2.

Table 3. Mass Percent of Hydrocarbon in Different Carbon Number Ranges.

carbon number
sample	<C37 (%)	C38–C77 (%)	C78–C89 (%)	C90–C93 (%)
M1	53.90	17.45	2.57	0.08
M2	68.91	9.83	0.26

Table 4. Summary of the Simulated Distillation Results.

fraction (%)
sample	gasoline fraction (IBP-180 °C)	diesel fraction (180–350 °C)	kerosene fraction (140–240 °C)	distillate oil fraction (350–500 °C)	heavy oil fraction (>500 °C)	total recovery (%)
M1	6.65	22.94	9.57	23.95	20.76	74.0
M2	14.58	30.80	15.05	23.33	10.89	79.0

2.4. Sample Pretreatment

To eliminate shear and thermal history effects on two samples and to guarantee data reproducibility, consistent sampling and testing procedures were prescribed. First, two crude oils obtained from the oil fields were split into sealed bottles. Then, the samples were placed in water bath to be heated to 80 °C and kept at this temperature for 2 h. Then, the samples were naturally cooled down at room temperature for at least 48 h.

2.5. Rheological Data of Crude Oils

The rheological measurements were conducted using a modularization rheometer MCR 702 (Anton Paar) equipped with concentric cylinder systems (CC 20). The testing temperature was controlled by CTD 180 (for “convection temperature device”), which is a convection chamber that relies on Peltier elements for heating and cooling. The temperature range that can be controlled is from −20 to 180 °C with a control accuracy of 0.1 °C.

The rheometer was first preheated at an initial temperature of 80 °C for the M1 crude oil and 70 °C for the M2 crude oil. The initial preheated temperature is different, because of the different WAT and pour point of two crude oils. Because the WAT and pour point of crude oil M1 are higher, a higher preheated temperature is used to achieve the full dissolution of wax crystals and study the activity of asphaltene and resins. Compared to the M1 crude oil, the WAT and pour point of the M2 crude oil are significantly lower. Then, a preheated temperature of 70 °C was used to achieve the same effect for the M2 crude oil. Before being transferred into the rheometer, the samples were preheated in a thermostat. After being transferred into the rheometer, the sample was kept isothermally for 10 min and then cooled to the desired temperatures (30 °C for M1 and 20 °C for M2) at the same cooling rate of 0.5 °C/min. The cooling procedure is the same as that used for the microscopic observation. During the cooling process, small amplitude oscillatory shear with a stress amplitude of 0.5 Pa and a frequency of 10 rad/s was applied to track the dynamic modulus of the samples. Evolution of storage modulus G′ and loss modulus G″ with temperature was tested and shown in the following figures. As seen in the figures, the gelation temperature of M1 is 35.6 °C, while it is 23.6 °C for M2. The M2 crude oil exhibits a better flow ability and weaker gelation structure than those of M1 at the same temperature (Figure 3).

Figure 3.

(a) Evolution of the storage modulus of M1 (b) evolution of the storage modulus of M2.

2.6. Rheo-Microscopy Simultaneous Measurement System

The rheo-microscopy simultaneous measurement system is provided by MCR 702 TwinDrive modularization rheometry (Anton Paar). This system is constructed using the rheology measurement module and the polarizing microscopy module. For the rheology measurement module, on the one hand, it can test the rheological properties of waxy crude oil. For the rotation module, the min and max torque is 1 nNm and 230 mNm, respectively. The response time of speed and the strain is 5 and 10 ms, respectively. On the other hand, the shear flow field can be established by the rheology measurement module to provide the observation of wax crystal microstructure dynamic transformation and motion behaviors under shear flow. In order to realize the rheo-microscopy simultaneous measurement, a 43 mm transparent quartz parallel plate measurement system is adopted, and the transparent quartz plate is used to carry the sample. Different measurement gaps can be selected for obtaining the best measurement results. Synchronous with the rheological measurement, the microscopic observation is achieved using a polarizing microscope measurement module, which has an objective lens of 20×, a focal length of 30.9 mm, a resolution ratio of 0.7 μm, and a depth of field of 1.6 μm. Also, the CCD camera made by Lumenera Company from Canada is equipped, of which the size is 2/3″ and the visual field is 440 μm × 330 μm. A shadowless cold light source of 150 W is equipped in the apparatus to light on the sample from the bottom. Schematic view of the Rheo-microscopy simultaneous measurement system is shown in Figure 4. The optical pathway diagram is shown in Figure 4a, and the structure of the rheology measurement module is shown in Figure 4b, the structure of the polarizing microscopy module is shown in Figure 4c.

Figure 4.

Schematic view of the rheo-microscopy simultaneous measurement system. (“photograph courtesy of “Anton Paar GmbH”. Copyright 2020.”)

The original measuring system is only equipped with a single light source, which can only provide light in a single direction. To achieve better microscopic observation and to obtain more detailed information on the wax crystal microstructure, we assembled the multi-angle composite light source by adding the external light source so that the sample can be lighted at different angles. Thus, the information related to the microstructure and morphology from different parts of the wax crystal is observed and recorded using this system. Moreover, the field of view can be moved radially along the sample. Thus, a larger range of observation can be achieved. Furthermore, the temperature control module of rheo-microscopy simultaneous measurement system includes: a transparent Peltier temperature control device (P-PTD 200/GL) for controlling temperature of the quartz baseplate; a Peltier protection top cover (H-PTD 200/GL) for covering the sample. Both of the devices can be controlled independently to eliminate the temperature gradient of the samples. The temperature control scope is from −20 to 200 °C. The maximum heating and cooling rate is 30 and 20 °C/min, respectively.

In the experiment, after pretreatment, the sealed sample M1 was preheated in the thermotank at 80 °C, and then it was rapidly transferred and isolated several drops on the transparent quartz plate of the rheometer, which was already set at 80 °C, and kept isothermally for 10 min. For in situ observation of the wax crystal morphology in the static cooling process, the samples were cooled at a cooling rate of 0.5 °C/min to 30 °C, in the meantime, a fixed shear rate of 0.01 s^–1 was imposed on the sample. For sample M2, the initial heating temperature was 70 °C, and the ending temperature after cooling was 20 °C, and the cooling rate was 0.5 °C/min too. During the cooling process, the microscopic observation was made simultaneously. So as to investigate the effect of shear on the microscopic structure of wax crystals, five different shear rates (0.01, 1, 10, 50, and 200 s^–1) were imposed on the samples of M1 during cooling with a rate of 0.5 °C/min, and the evolution of the crystal microstructure was observed in situ. Four different shear rates (0.01, 1, 10, and 50 s^–1) were imposed on the samples of M2 during cooling with a rate of 0.5 °C/min, and the evolution of the crystal microstructure was also observed in situ.

The CCD camera kept recording the evolution of the wax crystal microstructure during the dynamic cooling process. It provides two simultaneous shooting modes including taking the snapshot of the wax crystals at any chosen moment, and recording the video of the whole cooling process. Both modes were used in this investigation. And the images took by snapshots were shown in the following contents.

2.7. Polarizing Microscopic System

The common method to observe the wax crystals using a polarizing microscope was also conducted to compare with the results from the rheo-microscopy simultaneous measurement system. A polarizing microscope (Nikon ECLIPSE LV100NPOL) was employed to conduct the static observation of wax crystals in the M1 and M2 samples. The polarizing microscope is equipped with a 20× advanced plan apochromatic polarized objective, and a 1.625 megapixel full-sized full resolution color digital imaging system. The frame rate can be 45 fpx in 1.6 megapixel mode. The microscope was also equipped with a thermal stage of Linkam T96 to control temperature in the range of −40 to 120 °C with an accuracy of 0.1 °C and the maximum heating rate of 30 °C/min. Without involving shear, the same temperature control procedure as that used in the rheo-microscopy simultaneous observation during the static cooling can be achieved.

3. Effect of the Multi-Angle Composite Light Source

To examine the effect of the multi-angle composite light source, the images of wax crystals in M1 and M2 crude oils under different kinds of light sources are shown in Figures 5 and 6.

Figure 5.

Microscopic images under different light sources obtained using the rheo-microscopy simultaneous measurement system (M1).

Figure 6.

Microscopic images under different light sources obtained using the rheo-microscopy simultaneous measurement system (M2).

As seen in Figures 5a and 6a, when the sole light source is imposed on the sample, the wax crystals can be identified from the image. And their morphology and structure can be observed. However, because the direction of light is sole, only some of the wax crystals or the partial structures of the wax crystals can be illuminated and observed using the microscope because of the anisotropic structural character. Therefore, the boundaries of the wax crystals can’t be well identified, and the structural information is not integrated. This defect makes it more difficult to accurately analyze the changes in the morphology and structure of wax crystals, and to accurately detect the interactions between wax crystals. To overcome this defect, we created and assembled the multi-angle composite light source by adding an external light source so that the sample can be lighted at different angles. After the secondary composite of the light source, nearly all parts of the wax crystals can be illuminated (see Figures 5c and 6c). In particular, the boundaries of the wax crystals can be accurately identified. In addition, more detailed structural information can be obtained. Thus, this new method provides the accurate detection of the structure dynamic transformation and microscopic motions of wax crystals, as well as the interactions between wax crystals.

4. Results and Discussion

4.1. In Situ Observation of the Wax Crystal Microstructure and Morphology during Static Cooling

A rheometer (MCR 702, Anton Paar) equipped with a rheo-microscopy simultaneous measurement system was employed to microscopically examine two different samples at a cooling rate of 0.5 °C/min, under a self-created multi-angle composite light source. The cooling rate of 0.5 °C/min can promote the full growth of the wax crystals, facilitating the observation of the wax crystal structure changes with decreasing temperature. To find a balance between the observation performance and accuracy of the rheological test, the test gap was selected to be 50 μm. The static cooling of waxy crude oil was simulated by applying a shear rate of 0.01 s^–1 on two samples during cooling. The microscopic images of the M1 and M2 samples are shown in Figures 7 and 9, respectively, obtained using the new approach. Meanwhile, the polarizing microscope was also employed to examine the same samples under the same conditions to compare and analyze. The obtained microscopic image sequences are shown in Figures 8 and 10.

Figure 7.

Microscopic image sequences of the M1 sample at a cooling rate of 0.5 °C/min (new method).

Figure 9.

Microscopic image sequences of the M2 sample at a cooling rate of 0.5 °C/min (new method).

Figure 8.

Microscopic image sequences of the M1 sample at a cooling rate of 0.5 °C/min (polarizing microscope).

Figure 10.

Microscopic image sequences of the M2 sample at a cooling rate of 0.5 °C/min (polarizing microscope).

As shown in Figures 7–10, microscopic image sequences of wax crystals during the cooling process can be observed using two different methods, in which the evolution process of the morphology and structure of wax crystals can be observed. Because of the differences in the light sources, microlens, and CCD camera between two observation methods, there is a difference between the color, clarity, and depth of field, but the evolution processes of the microstructure and morphology of the wax crystals observed using different methods are consistent. Comparatively, the images of the wax crystals are clearer and are in higher resolution according to conventional polarized microscope observation. The wax crystals in this kind of image are better distinguished from the background crude oil. This method also has a better dynamic image acquisition speed and is helpful for quantitative identification of the particles and the morphological analysis of wax crystals. However, a more comprehensive field of view can be obtained by using the rheo-microscopy simultaneous measurement system applying multi-angle composite light source on the samples, and thus the microscopic morphology and spatial distribution information on the wax crystals in samples with a certain thickness are more comprehensive and detailed. By comparing the results of the two observation methods shown in Figures 7–10, some new information on the microstructure and morphology of the wax crystals obtained by using the new observation method can be summarized.

• (1)It can be seen by comparing Figures 7–8 that the new observation method better identifies and distinguishes the wax crystals, and more detailed information on the morphology and structure of wax crystals can be obtained. In addition, it has a prominent advantage in the identification of wax crystal aggregation behaviors because of the accurate boundary delineation. Especially, when the temperature is high (e.g. 48 °C, as seen in Figure 5c), using the new method, it can be found that the structures of the wax crystals are already complex and irregular, and the aggregations can be clearly observed. The wax crystals are rod-shaped, which can promote the overlaps and interlocks with each other to form the complex aggregates. Whereas wax crystals observed by the conventional method exhibit a dispersed granular shape (see Figure 8c). Their morphology and structure is relatively single and show a small distinction. The interactions between the wax crystals are not significant too. As a consequence, the identification of the individual wax crystals as part of the aggregates may not be accurate enough, and the wax crystal aggregation may be misunderstood to be a big single wax crystal. Because the new observation method has a better depth of field, the small wax crystals locating on different liquid levels and positions can be lighted up by multi-angle light sources. Additionally, different parts of wax crystals can be better observed. As a result, the identification accuracy of the morphology and structure of the wax crystals is improved. And the interconnections and aggregations between the wax crystals can be better tracked. In order to illustrate this point, magnified images of some wax crystal aggregates are shown in Figure 7c (see Figure 11) and in Figure 7f (see Figure 12). The noise reduction and contrast-brightness adjustments are carried out, so as to make the morphology and structure of the wax crystal aggregates more distinct.

Figure 11.

Magnification of wax crystal aggregates of M1 (48 °C).

Figure 12.

Magnification of wax crystal aggregates of M1 (30 °C).

As seen in Figures 11 and 12, multiple clear contours can be seen in one large wax crystal. Each contour surrounds the individual wax crystal, which forms the aggregate. The aggregation of wax crystals can be accurately identified by these features. Thus, it can be concluded that the aggregations between wax crystals are very common in the M1 crude oil. Furthermore, with decreasing temperature, the wax crystal growth process is shown by the increase of the volume and length. While in the meantime, the morphology of aggregates also changes significantly. Their morphology and structure becomes more and more complex, and the tendency to present an amorphous structure becomes more and more significant. Therefore, it can be concluded that the aggregations between wax crystals increase with decreasing temperature.

When compared with that in M1, the wax crystals in M2 observed using the new method have a more regular morphology and structure. The average roundness value of wax crystals is higher, and the aggregations between the wax crystals are weaker. As a consequence, the distinction of the wax crystals in different crude oils can be better achieved. Furthermore, as seen in Figure 7, according to the images obtained using the new method, the aggregations between the wax crystals play an important role during the entire formation of the microstructure. It is believed that the aggregations even occur at the initial precipitation of wax crystals and have the continuous impacts on the formation of the microstructure. Using the new observation method, these following aggregation behaviors are better identified.

• (2)In the new observation method, the depth of field is better. Based on the SFS (shape from shading) method, the shadow boundary of the crystals in the image contains their contour characteristic information. The brightness and shadow of different crystals in the image reflect the depth of the crystals. As can be seen in Figure 7c–f that there are significant differences in the brightness of the crystals in the image, but the illumination direction and intensity received by different regions in the image are consistent. The presence of dark areas is because of the blocking of light by wax crystals with a certain height. Bright areas correspond to higher heights and reflect light more strongly. Therefore, it is believed that the wax crystals not only have morphology, structure, and distribution behaviors in the observation plane parallel to the camera lens, but also have rather complicated behaviors in the direction perpendicular to the observation plane. This indicates the wax crystals have the 3-D structural characters. As shown in Figure 7c–f, the distribution of the wax crystal size is uneven (also can be indicated that the thickness of the wax crystals is uneven) in the direction perpendicular to the observation plane. The thickness of different parts of one wax crystal is not the same. This results in the appearance of wax crystals and aggregates like tadpole. One reason for the complexity and irregularity of the 3-D structure is that the individual wax crystal has the characteristic of growing in three dimensions. Another reason is that the interconnections and overlaps between wax crystals are not only confined in a two-dimensional plane, but also in three-dimensional space. The appearance of a 3-D structure increases the complexity and the tendency of the physical properties exhibiting anisotropy. It may be related to the complex rheology behaviors of the M1 crude oil at low temperature. Also, the 3-D structure can be found in M2 by the new observation method, while the size distribution of wax crystals is more homogeneous in the direction perpendicular to the observation plane. Thus, the difference in the structure and morphology of the wax crystals is not only observed in the 2-D plane, but also in 3-D space. This difference can be better identified using the new observation method, indicating that the new observation method has the potential advantage of probing the correlative mechanism between the 3-D microstructure of wax crystals and the macroscopic rheology of waxy crude oil. In addition, as seen in Figure 8, the luminance differences are great between different wax crystals according to the conventional observation method in M1. Compared with the images shown in Figure 7 obtained using the new observation method, this luminance difference may be attributed to the different thicknesses of the wax crystals. In M2, the luminance difference is small (see Figure 10), which also corresponds to the even distribution of the wax crystal thickness (see Figure 9). Although the uneven distribution of the wax crystal thickness can be found according to the difference in luminance using the conventional method, it cannot determine whether the difference in luminance is completely caused by different thicknesses of the wax crystals. The intensity of the light source and the distribution of wax crystals in different liquid layers can also result in this condition.
• (3)The new observation method can identify the 3-D structural morphology of the wax crystals, and has more advantages of tracking the crystal aggregation behaviors. Then, the distinction between the wax crystals in different crude oils is more significant, especially at a high temperature at which the wax crystals initially precipitate, as well as at a low temperature at which the integral structure is formed. The new method can not only identify the difference in the wax crystal 3-D structure and morphology as well as the rules of temporal–spatial evolution, but also distinguish the wax crystals according to their aggregation behaviors. This advantage is also of benefit for probing the mechanism of the macroscopic rheology based on the microstructure. For instance, it can be concluded by comparing Figures 7 and 9 that the flocculation and aggregation tendency of the wax crystals in M1 is more significant than that in M2. In order to illustrate this point, magnifications of some wax crystal aggregates are shown in Figure 7d (see Figure 13) and Figure 9f (see Figure 14). The noise reduction and contrast-brightness adjustment were conducted, in order to make the morphology and structure of the wax crystal aggregates more distinct.

Figure 13.

Magnification of the wax crystal aggregates of M1 (42 °C).

Figure 14.

Magnification of the wax crystal aggregates of M2 (20 °C).

As seen in Figure 13, multiple clear contours can be seen in large wax crystals. Each contour surrounds the individual wax crystal, which becomes part of the aggregate. The aggregate form is more complicated in M1 including the overlaps and interlocks between crystals in 3-D space. This promotes the wax crystals to develop a more significant 3-D structure. The interconnections between wax crystals become more compact and stable. Furthermore, this microstructural character causes the poorer flow ability of M1 and the easier gelation with decreasing temperature. In contrast, the morphology of wax crystals is more regular, as well as larger roundness in M2 (see Figure 14). The aggregations and interlocks between wax crystals in M2 are much weaker and simpler even at a lower temperature, and the growth and morphology evolution of wax crystals is more regular. Moreover, the 3-D structural character of the wax crystals in M2 is not so significant according to their small size difference in the direction perpendicular to the observation plane. All these microstructural information indicates a better flow ability and a weaker gelation structure in M2 than those in M1 at the same temperature, which is also consistent with the pour point and rheological testing results. The differences in crystallization in two crude oils are related to the oil composition, which can be demonstrated by two aspects. On the one hand, the contents of resins and asphaltenes are different in two kinds of crude oils (see Table 1). This difference can significantly influence the wax crystallization of different crude oils. On the other hand, the results obtained by HTGC show that the compositions below C37 account for, respectively, 53.90 and 68.91% in M1 and M2 crude oils. The compositions between C38 and C77 account for, respectively, 17.45 and 9.83% in M1 and M2 crude oils. The hydrocarbon with the largest number of carbons can be detected is C93 in M2, while it is C89 in M1. The results indicate that high carbon number hydrocarbons may have certain influences on the complexity of crystallization behaviors.

• (4)The wax crystal images obtained using the conventional observation method can be affected by slide quality, so it is hard to explain the difference between the wax crystals and production defects, scratch, and impurities on the slide. As a result, the WAT may be misjudged, or the statistical results of the wax crystal quantity and morphology may be influenced in the higher temperature range. As shown in Figures 7–10, there are some differences between the WATs obtained using two methods. For M1, the WAT obtained using the traditional observation method is 58 °C, which is higher than that obtained using the new observation method (WAT is 53.8 °C), indicating that there is a deviation of 4.2 °C. For M2, the WAT obtained using the traditional observation method is 48 °C, which is a little higher than that obtained using the new observation method (WAT is 47 °C), indicating that there is a deviation of 1 °C. These deviations may be attributed to two aspects. On the one hand, because the microscopic observation method strongly depends on the resolution of cameras and human factors, these deviations may be caused by the instrumental or human factors. Particularly for the traditional observation method, the effects of these factors are more significant. On the other hand, the existence of the impurities and defects may promote the wax crystallization and growth, speeding up the process of wax precipitation, and thus has an effect on the wax-crystallization behavior. In contrast, the silica glass is used in the rheometer in the new observation method to hold up the crude oil sample, and is also used as the material of the rotor to cover the sample. These accessories are processed elaborately so the influence of the production defects and impurities is decreased.

In addition, there is no guarantee that the sample thickness is consistent as well as the thicknesses of different parts in a sample, in the comparison experiments conducted using the conventional observation method. This may deteriorate the repeatability and contrastive analysis of the observation results. In contrast, the measuring system and sample clamp used in the rheometer for the new observation method can accurately measure and control the sample thickness. As a result, the thickness of different samples is consistent, and the repeatability and comparability of the experiment results can be guaranteed. Also, the thickness of different samples can be accurately controlled in the new observation method. This provides a possibility of investigating the influence of the sample thickness on the wax-crystallization characters, and enriching the understanding of the wax-crystallization behaviors.

4.2. In Situ Observation of the Wax Crystal Microstructure and Morphology during Dynamic Cooling

At present, the off-line observation method is widely used to research on the wax crystal microstructure and morphology under shear. The basic procedures include: ① imposing a certain shear on the crude oil sample; ② taking a sample and making a glass slide quickly; ③ observing the sample using a microscope. It is well known that the structure and morphology of the wax crystals is highly dependent on thermal and shear history. During sampling, the thermodynamic state of the wax crystals is easily changed by the environmental conditions. As a result, the observation results are different from that during shear. In addition, waxy crude oil has thixotropy at certain temperature. The wax crystal structure will recover after shear, which causes the deviation of the observation results. Consequently, for the off-line observation method, the adverse effect caused by the operation is hard to be avoided. In contrast, the rheo-microscopy simultaneous measurement system facilitates the in situ and real-time observation of wax crystals under shear. Together with the multi-angle composite light source, the microstructure and morphology of wax crystals as well as their evolution process under shear can be observed with high precision and can be in situ tracked.

The rheo-microscopy simultaneous measurement system of the rheometer can provide two modes for rheological measurements, which include rotation and oscillation. The shear stress and shear rate can be accurately controlled in the two modes. The rotating mode was selected in this article, and the CSR (controlling shear rate) method was used to simulate the shear flow of pipelining. For crude oil M1, five different shear rates (0.01, 1, 10, 50, and 200 s^–1) were, respectively, imposed on five samples, synchronously the temperature was decreased from 80 to 30 °C with a cooling rate of 0.5 °C/min, and the wax crystal microstructure evolution was in situ observed. For crude oil M2, four different shear rates (0.01, 1, 10, and 50 s^–1) were, respectively, imposed on four samples, synchronously the temperature was decreased from 70 to 20 °C with a cooling rate of 0.5 °C/min, and the wax crystals microstructure evolution was in situ observed. Because the motions of the wax crystals were increased by increasing the shear rate, the structure and morphology of wax crystals cannot be captured clearly. Consequently, during the dynamic cooling process, some temperatures were selected to observe the crystals and to capture images. When the preset temperatures were achieved, the shear rate was suddenly slowed down to 0.5 s^–1 so that the motions of the wax crystals were decreased immediately, and their structure and morphology was nearly frozen. The low shear rate is set to last for 10 s, and after capturing the images, the shear rate was recovered, thus the influence of the low shear rate during the capturing process can be reduced.

For M1, the impact of shear is significant at a cooling rate of 0.5 °C/min, and the morphology and structure of the wax crystals and their aggregates is different at different shear rates. As shown in Figures 15 and 16, compared with the results at a shear rate of 0.01 s^–1, the growth of single wax crystals in M1 is not significantly affected by the increase of the shear rate from 0.01 to 1 s^–1. And the changes in the morphology and structure of single wax crystals are small. However, the aggregation tendency of wax crystals in 3-D space is enhanced sharply. During the entire cooling process, the wax crystal aggregates formed at a shear rate of 1 s^–1 have a more complicated stereo-structure, and the overlaps and interlocks between wax crystals are more significant. As seen in Figure 16 c, when the temperature decreases to 30 °C, lots of wax crystals aggregate into large floccules, which exhibit an amorphous structure and uneven space distribution. Not only the sizes of the wax crystal aggregates in the observation plane increase significantly, the size of aggregates in the direction perpendicular to the observation plane is increased too.

Figure 15.

Morphology and structure evolution of the wax crystals in M1 at 0.5 °C/min, 0.01 s^–1.

Figure 16.

Morphology and structure evolution of the wax crystals in M1 at 0.5 °C/min, 1 s^–1.

When the shear rate is increased in the range of 10–50 s^–1, the influence of shear is more complicated. Comparing the images of Figures 15–18, it can be concluded that when the shear rate increases to a certain range (in this case, from 10 to 50 s^–1), the shear may have two opposite effects at the same time. On the one hand, it may promote the interconnections and overlaps between the waxy crystals and create the compact and large aggregates as it increases the chance of collision between the wax crystals. This can be demonstrated by the increase of the aggregate sizes and amounts of large aggregates. The stereo structural characters of wax crystal aggregates are enhanced with the increase of the shear rate. On the other hand, with the increase of the shear rate, the shear can damage the weak aggregates and the weak interconnections between the aggregates, and cause the large flocs break up into multiple scattered aggregates. This can be supported by comparing the distributions of aggregates shown in Figures 16c, 17c, and 18c. Furthermore, with increasing shear rate, the shear can not only change the aggregation of wax crystals, but also change the morphology of the aggregates. To see more details of the aggregation, magnifications of some wax crystal aggregates are shown in Figure 15c (shear rate of 0.01 s^–1), and in Figure 18c (shear rate of 50 s^–1). In addition, the noise reduction and contrast-brightness adjustments were conducted, in order to make the morphology and structure of the wax crystal aggregates more distinct. These figures are shown in Figure 20.

Figure 18.

Morphology and structure evolution of the wax crystals in M1 at 0.5 °C/min, 50 s^–1.

Figure 17.

Morphology and structure evolution of the wax crystals in M1 at 0.5 °C/min, 10 s^–1.

Figure 20.

(a) Magnifications of aggregates (0.01 s^–1) (b) magnifications of aggregates (50 s^–1).

As seen in Figure 20, with increasing shear rate, in the 2-D plane, most of the wax crystal and aggregate roundness increases sharply. The structure of wax aggregates changes from the irregular amorphous to a regular ellipse. The regularity of the structure and morphology increases sharply, which can facilitate the wax crystals adapt to the shear flow field, and then reduce the flow resistance. This change is mainly because the increased shear promotes single wax crystal itself rotating and shrinking, and then promotes wax aggregates twining and shrinking to form an ellipsoidal structure.

When the shear rate is increased to 200 s^–1, the destructive effect of shear on wax crystal aggregations is more dominant. As seen in Figure 19, the aggregations between wax crystals are largely weakened, and the sizes of the wax crystal aggregates in 3-D are decreased. In contrast, the quantity of single wax crystals increases a lot. The wax crystals and their aggregates are tiny and dispersed, and the distribution is more even. In addition, compared with the results observed at 0.01 s^–1 in Figure 8, the structure of the single wax crystals stretches completely toward the flow field at a shear rate of 200 s^–1. The contractive and twining wax crystals and their aggregate structure are fully extended and untwisted, the morphology turns to be a long chain, and the extending direction of the structure is the same as the flow direction (indicated by the blue arrow in Figure 19c). This indicates that at a high shear rate, wax crystals and aggregates have more significant deformation orientation in the shear direction. However, there is no sufficient evidence that the growth behaviors of singe wax crystals in the M1 crude oil are significantly influenced by the shear in the range of 0.01–200 s^–1.

Figure 19.

Morphology and structure evolution of the wax crystals in M1 at 0.5 °C/min, 200 s^–1.

For crude oil M2, the effect of shear on the morphology and structure of wax crystals is weaker than that for M1. As seen in Figures 21 and 22, when the shear rate is increased from 0.01 to 1 s^–1, the aggregation status of the wax crystals is enhanced during the dynamic cooling process. Especially at a temperature of 38 °C, the aggregation of wax crystals is more significant at a shear rate of 1 s^–1. When the temperature decreases, the relative distance between the wax crystals and wax aggregates is increased obviously, and the homogeneity of distribution is reduced at a shear rate of 1 s^–1. The quantity of the tiny wax crystals in the field of vision is decreased, and the size of the wax crystals increases significantly. However, the shear of 1 s^–1 changes neither the morphology of the single wax crystals, nor their nonisothermal crystallization behaviors. From these figures, it can also be concluded that because of the difference in chemical compositions in two crude oils, the aggregation behaviors and their responses to shear are different in two crude oils. When the shear rate is increased from 1 to 10 s^–1, two opposite effects of shear also simultaneously imposed on the wax crystals in M2. On the one hand, the aggregation of wax crystals is enhanced, so that the aggregate sizes increase and the amounts of large aggregates increase. And the roundness of wax crystals and aggregates increases significantly (especially at 26 °C). On the other hand, shear can also damage the wax crystal aggregates and result in the more dispersed distribution of the wax crystals, thus the connections between the wax crystals are significantly interrupted, and the distances between the wax crystals are increased (especially at 26 and 20 °C). In addition, the effect of shear on the morphology of wax crystals can also be found by comparing Figures 22 and 23. When the shear rate further increases to 50 s^–1, the destructive effect of shear on wax crystal aggregates is more significant (see Figure 24). Not only the sizes of aggregates are decreased, but also the connections between the wax crystals are apparently interrupted. Moreover, it can be seen that the shear of 50 s^–1 can promote the consistency of wax crystal motion orientations, then reduce the viscosity of the system.

Figure 21.

Morphology and structure evolution of the wax crystals in M2 at 0.5 °C/min, 0.01 s^–1.

Figure 22.

Morphology and structure evolution of the wax crystals in M2 at 0.5 °C/min, 1 s^–1.

Figure 23.

Morphology and structure evolution of the wax crystals in M2 at 0.5 °C/min, 10 s^–1.

Figure 24.

Morphology and structure evolution of the wax crystals in M2 at 0.5 °C/min, 50 s^–1.

By comparing the images of M1 and M2, it can be concluded that the effect of shear on the morphology and structure of the wax crystals and aggregates in M2 is not as significant as that in M1. On the one hand, this is because of the relative regular morphology and large roundness of singe wax crystals in M2. There are less effects of shear on these kinds of wax crystals, and it is not easy for them to interlock and overlap with each other. On the other hand, because of the different chemical compositions, the wax crystals in M1 show a significant aggregation tendency in the static state (Figure 7). As it is not easy for crystals in M2 to aggregate with each other in the static state, the effect of shear on aggregation behaviors in M2 is not so significant. In addition, because of the regular morphology and large roundness of single wax crystals in M2, the morphology change affected by shear is not as significant as that in M1.

5. Conclusions

Based on the rheo-microscopy simultaneous measurement system, together with the self-created multi-angle composite light source, a new approach for in situ observation of the wax crystal microstructure and morphology during both static and dynamic cooling with shear was established. According to the observation results of two different kinds of waxy crude oils, it can be found that the new method has a better identification and distinction of the wax crystals, as well as the outstanding wax boundary delineation. It can provide more detailed information on the morphology and structure of wax crystals, and has outstanding advantages for wax crystal aggregation behavior identification. Based on this, we have the following discoveries:

• (1)Wax crystals and aggregates have significant stereoscopic structural characters. The irregularity and complexity are partly because of the characteristics of growth along the 3-D direction of single wax crystals, but also related to the interlocking and overlapping between the crystals in 3-D space. The wax crystal 3-D structure, which is related to complicated rheological properties of waxy crude oil at low temperature, increases not only the structure complexity, but also the tendency of physical properties exhibiting anisotropy. This has a significant meaning for further probing the correlation mechanism between the microstructure of the wax crystals and the rheology of waxy crude oil.
• (2)The differences in the structure and morphology of the wax crystals in different crude oils are reflected not only in the 2-D plane, but also significantly in 3-D space. In addition, the differences in wax crystal aggregation behaviors for different crude oils may be very obvious. Meanwhile, it is believed that the growth of wax crystals and aggregates may relate to the high carbon number hydrocarbons in crude oils considering the HTGC results.
• (3)The new observation method can reduce the influence of defects and impurities during the sample preparation process. Also, the new method can accurately measure and control the sample thickness, which facilitates repeatability and comparability of the experiment results. It is believed that the new method provides the possibility for researching the influence of the sample thickness on the wax-crystallization characters and enriching the knowledge of wax-crystallization behaviors.

For dynamic cooling, the new observation method can achieve the high-precision in situ observation of the wax crystal microstructure and morphology under shear, the obtained findings include:

• (1)For different kinds of crude oils, the effect of shear on the microstructure and morphology of wax crystals is different. The more irregularity of the single wax crystal structure, the smaller the roundness and the more significant aggregation tendency between the wax crystals. As a result, the influence of flow field shear is more significant.
• (2)The wax crystal structure has certain flexibility. Shear can change the morphology of single wax crystals, but hardly destroy the structure or affect the growth of single wax crystals. Specifically, a lower shear rate will promote the irregular wax crystals to deform, make the morphology more like ellipsoidal, increase the roundness, and then reduce the flow resistance. However, when the shear rate is increased to an extent, shear will enhance the deformation propensity of wax crystals to flow field. So that the structure of the chain wax crystals is fully stretched, the sectional area perpendicular to the flow direction is reduced, and the flow resistance is decreased significantly. Consequently, if the structure and morphology of the wax crystals is regular, the roundness is fine, and the effects of shear are little.
• (3)Shear can significantly change not only the aggregation state between the wax crystals, but also the structure and morphology of aggregates. A lower shear rate can promote the aggregation, and make the stereo-structure more complicated and the size bigger. Also, it will promote wax crystal aggregates turning from an irregular branched structure into a regular spheroidal structure. When the shear rate increases to a certain range, the shear may have two opposite effects at the same time. On the one hand, it can promote the interconnections and overlaps between the wax crystals and create compact and large aggregates. On the other hand, the shear can damage the weak aggregates and the weak interconnections between aggregates, and cause the large flocs break up into multiple scattered aggregates. With increasing shear rate, the destructive effect increases and gradually plays a leading role. A high shear rate can make the aggregates appear smaller in size and have a weaker stereo structure, and cause the structure of aggregates stretched completely and turns to be long chain toward the flow field.

The authors declare no competing financial interest.