Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Basic Fundamentals of Petroleum Rheology and Their Application for the Investigation of Crude Oils of Different Natures Sergey O. Ilyin*,† and Larisa A. Strelets‡ †
A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky prospekt, 119991 Moscow, Russia Institute of Petroleum Chemistry, Siberian Branch of Russian Academy of Sciences, 4 Academichesky prospekt, 634055 Tomsk, Russia
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ABSTRACT: A pair of basic methods for investigating the rheological properties of complex fluids were considered: steady flow and small amplitude oscillatory shear. The methods were applied to study rheological properties of eight petroleum samples belonging to different classes (light, heavy, waxy crude oils, and bitumen). It was shown how the viscosity of a crude oil depends on its nature, temperature, and applied stress. To characterize the viscous behavior of oils, one can use their glass transition temperature: the Williams−Landel−Ferry equation makes it possible to construct a universal temperature dependence of the viscosity. The latter determines the viscosity of any petroleum at temperature of interest from that at some other temperature. Linear and nonlinear viscoelastic properties of oils were demonstrated, and the manifestations of viscoelasticity by various crude oils were shown. It was revealed how the pour point of heavy oil can be calculated from its viscosity.
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INTRODUCTION
to evaluate the viscoelasticity of a sample only by measuring a swell of its jet at the capillary outlet.10 Rotational rheometry includes two basic techniques for studying materials. The first way is to measure the viscosity in the steady-state flow regime when a shear stress is applied to the sample through the rotating surface, and respective shear rate is measured, or vice versa. The viscosity is calculated according to Newton’s law: η = σ/γ̇ For the investigation of non-Newtonian fluids, the calculation of the viscosity will be correct only for the measuring cone−plane pair because the use of a conical surface provides even stress in the sample volume. The use of a plate−plate geometry or a Couette cell (or a capillary) to measure the viscosity of non-Newtonian fluids would require the use of more-complicated equations.11,12 The second basic technique is to study the viscoelasticity of samples in the regime of small amplitude oscillatory shear (SAOS). In this case, one of the measuring surfaces oscillates according to a harmonic law with a certain angular frequency ω and amplitude of strain γ0 (or of shear stress σ0). The amplitude is set so small as to avoid a change in the internal structure of the samples. The amplitude of the sample’s response (σ0 or γ0, depending on what was set) and the value of phase angle δ (the magnitude of the delay between the stress and strain oscillations) are measured. As a result, the dependence on the angular frequency of the dynamic modulus G* = σ0/γ0 is established. The latter consists of two components−the storage (elastic) modulus G′ = G* × cos δ and the loss modulus G′′ = G* × sin δ. This way the linear viscoelasticity of the samples is investigated, while the study of nonlinear viscoelasticity implies a large amplitude oscillatory shear (LAOS) method in which the sample is deformed at a fixed angular frequency with increasing shear amplitude.13,14
To produce and transport crude oils, it is necessary to know their viscosity properties. The viscosity η of simple Newtonian fluids does not depend on the shear rate (or shear stress) and duration of the flow but is determined only by temperature. However, crude oil is a complex multicomponent system,1 and complex systems are characterized by complex behavior. Not only the anomalous viscosity properties but also the manifestations of viscoelasticity by the samples are possible, the latter of which consists in the partial reversibility of their deformation γ after removal of the applied force.2 In this case, the behavior typical for light oil will not be the same for waxy or heavy oil.3 It is important to understand and properly investigate the rheological behavior of crude oils to avoid mistakes in the design and choice of operation modes for their transport systems. A pair of main laboratory devices for the measurement of the viscosity are capillary and rotational viscometers. In both cases, the shear deformation of samples is present. However, the rotational viscometry method, in which the flow of material occurs between fixed and rotating surfaces, has a number of significant advantages. A much smaller volume of samples (less than 1 mL) is required for experiments, which also lead to data that are more accurate. Rotational viscometers make it possible to flexibly and widely vary the deformation mode (the shear rate γ̇ can be set in the range 10−5−104 s−1, the shear stress σ vary in the range 10−3−104 Pa), to study viscoelasticity (viscometry becomes rheometry) and to investigate nonNewtonian fluids in a uniform shear field (using a cone− plane pair). The advantages of the capillary method are the possibilities of modeling the flow in the pipeline4 and of the investigation of crude oils under high-pressure reservoir conditions without contact with compressed gas.5,6 The drawbacks are the need to use corrections that account for the pressure loss at the capillary inlet7−9 and also the possibility © XXXX American Chemical Society
Received: October 10, 2017 Revised: December 15, 2017
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DOI: 10.1021/acs.energyfuels.7b03058 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Density, Viscosity, and Composition of the Crude Oils oil field
ρ20 °C, g/cm3
API gravity
η20 °C, Pa·s
resin, %
asphaltenes, %
paraffin wax, %
sulfur, %
Ashalcha Van-Yegan Karmala Liaohe Chekmagush Usa Ust-Tegus Yarega
0.962 0.906 0.976 0.998 0.883 0.965 0.781 0.942
15.1° 24.1° 13.0° 9.8° 28.1° 14.6° 48.8° 18.2°
4.25 3.32 1.82 145 000 0.0604 54.2 0.054 2.7
23.8 14.63 24.5 31.7 18.0 31.1 11.1 19.8
7.5 0.11 5.4 2.3 5.7 9.9 5.45 2.3
0.2 0.4 1.1 7.5 4.0 1.1 3.31 0.8
3.9 0.9 4.7 0.4 3.1 0.6 G′′. To describe the behavior of complex systems, it may be necessary to use a combination of these basic models. The non-Newtonian behavior of liquids manifests the dependence of their viscosity on shear rate (or shear stress). Solid-like systems that do not flow until some critical stress (yield stress σ Y ) are called viscoplastic (or Bingham plastic).16−19 Liquids whose viscosity decreases or increases with shear rate are called pseudoplastic (or shear-thinning) and dilatant (or shear-thickening), respectively.20−24 In addition, the samples during deformation can exhibit thixotropy or rheopexy, which are, respectively, a decrease or increase in viscosity with time.25,26 Now, having listed the main rheological concepts, let us examine a set of crude oils essentially varying in nature, with the aim to demonstrate similarities and differences in their behavior.
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RESULTS AND DISCUSSION Viscoelasticity of Crude Oils. Viscoelasticity is characteristic for waxy and heavy crude oils.3 The elasticity of waxy oils is due to the percolation structure formed by crystallized paraffin waxes,27 and the elasticity of heavy oils is caused by the large content of high-molecular-weight asphaltenes and resins in their composition.28 The different nature of the two types of crude oils is reflected in their linear viscoelastic properties, which are conveniently characterized by the dependencies of the storage and loss moduli on the angular frequency of the applied deformation (Figure 1).
EXPERIMENTAL SECTION
The characteristics of several crude oils used are listed in Table 1. Most of the samples belong to the class of heavy oils (API gravity between 22.3° and 10°), but light (API gravity >31.1°), medium (>22.3°), and extra-heavy oils (
