J. Cent. South Univ. Technol. (2008) 15(s1): 288-292 

DOI: 10.1007/s11771-008-365-4

Some advances in crude oil rheology and its application

ZHANG Jin-jun(张劲军), LIU Xin(柳 歆)

(Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing 102249, China)

Abstract: Waxy crude oil exhibits complex shear-and-thermal-history-dependent non-Newtonian behaviors. In the past 10 years, driven by the petroleum industry, crude oil rheology has been an active field. Studies on crude oil rheology have been passing a way from simply relying on rheological measurements, through quantitative experimental simulation of shear and thermal history effects in pipelining, to recent development of correlation between flow properties and shear and thermal history. Currently, the study is toward quantitative inquiry of relations between the rheological behaviors and micro-structures of wax crystals as well as oil compositions. Advances achieved by the author’ team are summarized, including simulation of the thermal and shear history effects, correlations and computation of flow properties, fractal characterization of morphology and structure of wax crystals, relations of rheological behaviors to fractal dimension and oil compositions, and the most successful example of the application of rheology in crude oil pipelining. Future studies are prospected.

Key words: waxy crude oil; rheology; advances; prospect

1 Introduction

Crude oil can be divided into two kinds according to their flowability. One is the light crude oil with good flowability, and the other is that with high-pour-point or high viscosity, including waxy crude oil and heavy oil. It is reported that waxy crude represents about 20% of world petroleum reserves produced and pipelined^[1], while the heavy crude oil reserve is estimated about equal to that of conventional crude oil. In China, more than 90% crude oil production is waxy or heavy oil.

Among them, rheological behaviors of waxy crude oil are more complicated. Crude oil with a wax content above 5% is usually called waxy crude oil. At temperatures below the wax appearance temperature (WAT), wax precipitates out from the crude oil gradually with decreasing temperature, and the crude oil exhibits non-Newtonian behaviors such as thixotropy, yield stress and viscoelasticity, etc. What’s more, flow behaviors of waxy crude oils are usually strongly dependent on the shear and thermal history. When the precipitated wax amounts to 2%-4% of the total mass of the crude oil, the crude oil will become a gel and thus lose its flowability^[2-3]. Heavy crude oil is characterized by its high viscosity, and because of its low wax content, its rheological behaviors are usually simpler.

Rheological properties of crude oil are fundamental for crude oil pipeline design and operation. China’s study on waxy crude oil rheology began in the early days of exploitation of the Daqing oilfield and has contributed a lot to the pipeline industry. Recently because of declining of the conventional oil reserves and deep water petroleum exploitation, rheology of waxy and heavy crude oil attracts more attentions not only from the petroleum industry, but also from rheologists^[4], chemical engineering scientists^[5] and even mathematicians^[6]. Great progress has been made in crude oil rheology and its applications in the past 10 years. In this paper, advances achieved in the author’s research team are summarized, and discussions are made towards future research on crude oil rheology and its applications.

2.1 Simulation of thermal and shear history effect

Rheological behaviors of waxy crude oil, especially those of the crude oil beneficiated with the pour-point- depressant (PPD), are highly dependent on the thermal and shear history that the oil experienced. In pipelining, crude oils may experience various thermal and shear history, and for different crude oils the thermal and shear history effects are different. Thus, to simulate the thermal and shear history effects accurately is vital.

However, only qualitative shear history simulation could be conducted empirically in past, and due to complexity of the flow field, the overall shear history in pipelining and in the simulating apparatus were not understood well, and so did consequently simulation of the non-uniform shearing that the crude oil experienced during pipelining. Two related Chinese test standards only give a specification of qualitative simulation; and the U.S. patent released in September 1999 also adopted a trial-and-error approach^[7].

After careful studies, ZHANG et al[8] found out that within a certain range of shear rate, a PPD-beneficiated oil exhibits the same flow properties provided that the entropy generation due to viscous flow in the shear process is the same in spite of the rate and time of shear, and this rule is valid to shear conducted both isothermally and non-isothermally. That is, the entropy generation due to viscous flow in the shear process is a proper parameter for simulating the shear history effect. This provides an easily-operated approach to simulation of the shear effect in the non-uniform shear fields.

Besides, some approaches were developed for characterization of shear fields encountered in pipelining as follows: 1) shear rate distribution in turbulent pipe flow^[9-10]; 2) average shear rate, energy dissipation and entropy generation due to viscous flow for shearing in the pump and throttling valves^[11], and in the shear simulation apparatus^[12-13]. We finally established a technique for quantitatively simulating shear and thermal history effect and built up the necessary apparatus. This made accurate assessment of flow properties of PPD-beneficiated oil during pipelining come true^[14]. Field data from several pipelines have verified its applicability. For the most recent advance, please refer to the paper entitled Simulation of pipelining PPD-beneficiated waxy crude oil through China West Crude Oil Pipeline by Dr. LI Hong-ying et al.

2.2 Correlations of crude oil rheological properties

Before 1990’s studies on crude oil rheology was mainly depended and focused on rheological measurements on specific crude oil. Quantitative relation study was seldomly reported except for the shear stress vs. shear rate relation. Development of computer simulation etc for oil pipelines calls for correlations or mathematical models describing the relations between rheological properties and temperature, oil compositions, thermal and shear histories etc. In the past 10 years, remarkable progress has been made in our research team.

Viscosity-temperature relation of waxy crude oils had been determined only by experiments for a long time. By carefully studying mechanism of waxy crude rheology and suspension rheology, LI et al^[15] developed a model for describing viscosity of waxy crude oils as a function of temperature, precipitated wax and shear rate, which may well predict apparent viscosities above the gel point. This model is valid not only to virgin oils, but also to crude oils with various thermal and shear histories and PPD-beneficiated or heat-treated crudes and shows a great advantage compared to PEDERSEN et al’s model^[16].

Gel point of waxy crude oil depends on heating temperature before its measurement. Besides, shear at the temperatures when abundant wax is precipitating out and the oil tends to gel may reduce the gel point as a result of disturbance of the spongy wax crystal structure. An empirical function of gel point versus heating temperature and the ultimate temperature of the dynamic cooling process(UTDC) was found for the Daqing crude oil. Empirical functions were also established for the Daqing crude oil to correlate the power law parameters with the heating temperature. These correlations, combined with LI et al’s model^[15], make it possible to predict viscosities at various temperatures and different heating temperatures usually met in crude oil pipelining, and the approaches to prediction of gel points and viscosities provide a complete tool of flow property calculation for hydraulic simulation of oil pipelines^[17]. The methods will also be applicable to other crude oils.

Correlations for shear effect on gel point of PPD-beneficiated oil were developed by using entropy generation due to viscous flow to represent the imparted shear^[18]. By using these correlations and with the help of only a few easy-to-measure parameters, gel points of beneficiated oils after shear can be estimated quite accurately. Similar method was applied and a correlation of shear effect on viscosity was also established^[19]. Based on these correlations and with the help of viscosity-temperature-shear rate model^[15], a preliminary approach was tried to prediction of flow property variation during pipelining of the PPD-beneficiated oil, with only a few characteristic data of crude oil being required^[20].

In pipelining, different crude oils are usually blended, and the fraction of crude oils in the blend is also varied. It will take plenty of time to measure flow properties of blends, especially in the case of multiple components. More importantly, getting flow properties only from experimental measurements often can not meet requirements of operation simulation. Some models^[21-22] were proposed for predicting gel point of crude oil blends, but lack of verification. Assessment^[23] was made by using collected 259 gel points of 24 crude oil blends with two to five components, and it was confirmed that LI’s model^[21] and LIU’s model^[22] might make accurate prediction from the engineering point of view. Similarly, viscosity data from 22 kinds of binary to nonary blends, totally 1 577 including 682 of Newtonian blends and 895 of non-Newtonian blends, were used to evaluate current 14 viscosity models^[24]. It was concluded that a model modified from Cragoe’s model gave most satisfactory prediction of crude oil blend viscosity with an average deviation of 7.2% for Newtonian blends and 19.1% for non-Newtonian blends, and might well meet engineering requirements. However, the verified models need gel points and viscosities of binary blends with each component equally mixed for non-linear correction, while these particular gel points and viscosities are normally unavailable unless a lot of time and energy is spent to measure them. To solve this problem, some empirical correlations for non-linear correction of the gel points and viscosity models were developed on the basis of experimental data^[25-26]. With these correlations, gel points and viscosities of crude oil blends may be estimated with acceptable accuracy, providing an effective way to engineering uses, and also contributed to the methodology of computation of crude oil properties.

2.3 Rheological behaviors vs microstructure

Wax crystal morphology and structure along with composition of a crude oil play dominant role in rheological properties when the oil temperature is below the WAT. To get quantitatively into the relations between rheological behaviors and wax crystal morphology and structure as well as oil composition will indeed promote understanding of mechanism of rheology.

In recent years, relations between viscosity and wax crystal morphology and structure, or the precipitated wax, or composition of crude oil had been widely dis-  cussed^{[16, 27-31]}. However, due to complexity and irregularity of the wax crystal morphology, independent quantitative characterization of wax crystal morphology and structure was not available, and consequently, establishment of quantitative relations between apparent viscosity and wax crystal morphology, structure and composition of the crude oil could hardly be achieved.

To study the mechanism of waxy crude rheology, GAO et al^[32] developed an image-based fractal characterization of wax crystal morphology and structures. With this approach, the microstructure of the waxy crude oil can be independently, objectively and effectively characterized as opposed to previously- reported investigations where indirect approaches were used to deduce the microscopic properties. It was found that the average fractal dimension generally increased with decreasing oil temperature; PPD-beneficiation resulted in the fractal dimension increased noticeably.

Then, with the multivariable cluster analysis, five representative parameters were picked out from 13 influencing factors of wax crystal morphology and oil composition.  This made characterization of wax crystal morphology and oil composition more effective.

Furthermore, the effects of microstructure and oil composition on the waxy crude viscosity, viscoelasticity, yield stress, and gel point/pour point were analyzed by multivariable stepwise regression analysis^[33].

2.4 Viscoelasticity, yielding and thixotropy

Viscoelasticity of waxy crude oil is a quite active field in recent 10 years. HOU et al^[34] studied the thermal and shear history effects on viscoelastic parameters, such as storage modulus, loss modulus, loss angle and gelation temperature of Daqing crude oil by means of oscillating tests. More recently, PENG^[35] studied the effect of shear on and recovery of the structure of gelled waxy crude oil by measuring moduli of oil specimen sheared at shear rates in the order of 10^-1 to 10³ s^-1.

In the petroleum industry, gel point or pour point has been widely used for evaluating the low temperature flowability of a crude oil. The gelation temperature, at which the storage modulus equals the loss modulus, was studied and compared to the gel point^[36]. We also revealed an eigen-temperature^[37], which may help to understand flow characteristics under flowing conditions.

PENG^[35] studied yielding of gelled crude oils under three stress loading conditions: in stepwise sequence, in a ramp-up way and with a constant value, and found that the strains at yielding under three different conditions fell into a narrow range, and showed little dependence on the stress loading way. Dependence of yielding time on loading rate was found, and a correlation was developed and verified with published data.

Thixotropy in waxy crude oil is peculiar and significant to oil pipelining. ZHAO and CHEN in our team respectively developed thixotropic models^[38]. Compared to the Houska’s model, ZHAO’s model may better describe the shear stress decline under constant shear rate^[38]. More recently, HOU developed another model^[39] and also a method for regression of thixotropic models with the stepwise-increasing of shear rate^[40].

3 Application of rheology in oil pipelining

In the past 10 years, the most typical and successful example of application of rheology in crude oil pipelining is the design and operation of the most complicated crude oil pipeline, technically and operationally, so far in China, the China West Crude Oil Pipeline, 1 837 km long and commissioned in 2007^[41].

Challenges rose from several extra-long distance sections between two neighboring pumping and heating stations, greatly variable physical properties of the crude oils, and the extra-low throughput in winter operation.

After comprehensive study on the rheological properties of the crude oils to be transported, quantitative pipeline simulation and shutdown-restart simulation were conducted to determine the flow characteristics. This suggested the PPD-beneficiation effective and a design scenario was set. As a result, expense for build of four additional heating stations was saved, over 100 million RMB (about 13 million USD).

Field tests were conducted from September to November 2007, to further verify effectiveness of the PPD on field conditions. Oil specimens were taken from the pipeline and flow properties were measured in-situ. Later, crude oil properties were monitored through the whole winter. This ensured the flowability of the transported crude oils be under control, and consequently safe operation of the pipeline during the winter.

4 Prospect of future study

With the increase of both oil price and demand, more problems are presented to the pipeline engineers, such as the further exploitation of waxy crude oil worldwide, flow assurance of waxy crude oils in deep-sea environment, requirements of energy-saving and environmental protection to the pipeline companies. All of these call for more support from rheology study. On the other hand, complicated structures and special non-Newtonian flow behaviors of waxy crude oils will attract more and more rheologists’ attention.

Efforts in quantitatively inquiring relations between rheological behaviors with the wax crystal morphology, structure and composition of crude oil mark the new advances in recent years^{[27-33,42-43]}. However, great efforts must be made to clearly and quantitatively understand relations between macro-behaviors and micro-structures of crude oils because of complexity of wax structures as well as the interactions between compositions.

Rheological behaviors of gelled crude oil have attracted many rheologists in recent years^{[29-31, 44-47]}, such as the linear viscoelastic moduli, creep and yielding, etc. Due to typicality of non-Newtonian behaviors and complex microstructures existed in gelled crude oils, research on this field is academically attractive.

However, gelation of crude oil often means disaster for long-distance-pipeline operators. Gelled oil is not the common occurrence for pipelines under normal operation. For the same reason, computation of non-Newtonian fluid mechanics for the restart process has been carefully studied in recent years^[1,48-52], though all of these studies are restricted in isothermal pipelines, and actually the waxy crude oil pipelines are non-isothermal. Therefore, hydraulic analysis to the restart process must take heat transfer and wax deposition into consideration. 

Models/correlations need further studies for flow property calculation of crude oils including relations with the shear and thermal histories. This is not only requirement from the engineering practice, but also contribution to development of rheology. From the pipeline engineer’s point of view, the most attractive thing is to essentially improve the flowability of crude oils at low temperatures, though this is mainly the task of chemists.

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(Edited by YANG You ping)

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: ZHANG Jin-jun, Professor; Tel: +86-10-89734627; E-mail: zhangjj@cup.edu.cn