Article pubs.acs.org/EF
Thixotropic Rheological Behavior of Maya Crude Oil Sepideh Mortazavi-Manesh and John M. Shaw* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada S Supporting Information *
ABSTRACT: Heavy oil and bitumen exhibit non-Newtonian rheological behaviors at lower temperatures. Thixotropy is one such behavior. Thixotropy affects the efficiency and length scale of mixing during blending operations and flow behaviors in pipes and pipelines following flow disruption, where it affects the pressure required to reinitiate flow. In the present work, thixotropic behaviors of Maya crude oil are explored systematically using a stress-controlled rheometer. Maya crude oil is shown to be a shear-thinning fluid below 313 K. The thixotropic behaviors are identified and explored using transient stress techniques (hysteresis loops, stepwise change in the shear rate, and startup experiments). The magnitude of the thixotropy effect is larger at lower temperatures. Relationships are identified between rest times and other thixotropic parameters, such as hysteresis loop area and stress decay, in startup experiments. Stress growth, which occurs as a result of a step-down in the shear rate, is shown to correlate with the temperature. The results also provide a benchmark data set for validation of rheological models for heavy oil that are immerging in the literature.
1. INTRODUCTION Heavy hydrocarbon resources, such as heavy oil and bitumen, have become essential energy sources globally. Their transport properties in situ and under production, transport, and refining conditions have become key building blocks for decision makers in industry. These resources, which normally contain high asphaltene mass fractions, have been considered to be colloidal in nature for nearly a century.1 However, despite many studies that have been performed on the viscosity of these resources,2−7 uncertainties related to their rheological properties persist. For example, uncertainties arise because reported viscosity data for the same nominal resource are obtained using samples from different locations and depths within formations or reservoirs that are then prepared in different ways.2 Sample shear histories and shear conditions during measurements also vary, and numerous measurement techniques and procedures do not detect non-Newtonian rheological behavior. Consequently, reported viscosity data vary over 2 orders of magnitude at a fixed temperature for nominally similar samples. Non-Newtonian effects, such as thixotropy and viscoelasticity, may account for a significant fraction of this apparent variation in heavy oil and bitumen rheological behavior.2,8 The sample shear history may be responsible for deviations of an order of magnitude for viscosities of heavy oil or bitumen from the same nominal location.7,9−14 While thixotropy and viscoelasticity both reflect memory-dependent properties, they can be distinguished through step-down shear rate experiments, where viscoelastic fluids undergo a monotonic decrease of stress to a new plateau value, while thixotropic fluids undergo an instantaneous drop to a lower value followed by a gradual increase in stress to a new plateau value.15 Thixotropy affects flow in mixers16−18 and flow in pipes and pipelines and is particularly evident during restart operations following flow disruption, arising from gel formation, asphaltene precipitation, or maintenance, among other causes. Impacts are more likely and more severe at low temperatures. During restart, a low-viscosity lubricating layer near the wall © 2014 American Chemical Society
and nonlinear pressure profiles are just some of the features that make the pipeline flow characterization of thixotropic materials complex.19 Problems with respect to pump performance may also arise. While a few studies have accounted for thixotropic behavior of oils in characterizing flow startup in pipelines,20−23 a thixotropic rheological database for heavy oils, which covers lower temperatures found in subsea or northern climates, is still lacking in the literature. Such comprehensive sets of information on thixotropic properties of these materials are essential for accurate and reliable characterization of heavy oil and bitumen flow in pipelines. Thixotropic behavior is characterized by a continuous decrease of viscosity with time when a sample that has been previously at rest is subjected to flow and the subsequent increase of viscosity with time when the flow is terminated.15 This behavior results from the breaking down of particulate networks in a material under shear flow into flocs. These networks rebuild over time if flow is terminated. The building process is typically slower than the network breakage process. In thixotropic material, the microstructural change occurring over time under intermittent flow and rest conditions is competitive.19 The attractive forces among flocs that drive floc association and network regrowth are weak, and consequently, the hydrodynamic forces driving breakup are strong enough to disrupt association.24 Several models have been proposed for thixotropy behavior correlation and prediction.25−31 Among them, models that are based on a structural kinetics approach have attracted much attention. In these models, shear history is related to a temporal structural parameter λ(t). These models are specific to individual fluids and do not predict the thixotropic behavior of others well. Therefore, suitable experimental data sets that represent different aspects of thixotropic properties of specific Received: November 18, 2013 Revised: January 21, 2014 Published: January 21, 2014 972
dx.doi.org/10.1021/ef4022637 | Energy Fuels 2014, 28, 972−979
Energy & Fuels
Article
information about methods used for these analyses are available elsewhere.33
heavy oils and bitumen are necessary to validate and, subsequently, generalize models for these classes of fluids. These data sets include results from startup experiments, hysteresis flow curves, and stepwise changes in the shear rate or shear stress. The results of these experiments serve as a fingerprint for a specific material as well as a fitting or validation tool for thixotropic models for fluids comprising comparable constituents. Care must be taken to avoid artifacts in data sets because the outcomes of thixotropy experiments may vary with the details of the shear history of a sample prior to measurements and sample handling procedures during experiments. It is crucial to establish a well-defined initial condition for experiments to obtain both reproducible and meaningful results. Typical structural kinetics thixotropic rheological behavioral models possess the general form τ(t ) = τyield(λ) + ηλ(λ , γ )̇ γ ̇ + η∞(γ )̇ γ ̇
(1)
d λ (t ) = f (λ , γ )̇ dt
(2)
Table 1. SARA Analysis and Elemental Composition of Maya Crude Oil2 saturates
(3)
dλ = κ1γλ̇ t −β + κ2(1 − λ)t −β + κ3γ 0.5 ̇ (1 − λ)t −β dt
(4)
resins
Mass Fraction 0.425 0.102
0.316 element
C5 asphaltenes 0.157 Maya crude oil
Mass Fraction C H N S O
0.845 0.113 0.003 0.033 0.012 Metal Analysis (mg kg−1)
Al Ba K Ca Cr Fe Mg Mn Mo Na Ni Si Ti V Zn
where γ̇ is the shear rate, τyield is the yield stress, ηλ is the viscosity contribution to the degree of structuring equal to λ, and η∞ is the viscosity when all of the structures present in the fluid are broken down at λ equal to 0. Viscosity measurements at conditions where samples are destructured provide information necessary to obtain the shear rate functionality of η∞ based on a shear-thinning constitutive equation of state. Yield stress experiments provide a basis for modeling the τyield contribution of the shear stress. Results obtained from stepdown and startup experiments define parameters appearing in specific ηλ and dλ/dt models, such as the models proposed by Dullaert and Mewis27
ηλ = λη0
aromatics
1.0
