Dynamic rheological analysis of the gelation behaviour of waxy crude oils

In this work we have characterised the viscoelastic behaviour of paraffin crystals in three different complex crude oils, close to the gelation threshold and after curing the gels under quiescent isothermal conditions, by means of oscillatory shear measurements. An increase in gelation temperature is observed with increasing oils molecular weight. The interactions between wax crystals and the formation of the space-filling network of interlocking wax crystals are thus facilitated by the presence of paraffins with higher molecular weight. The apparent gelation time, obtained from isothermal curing experiments, decreases as the curing temperature was decreased, and it is highly temperature-dependent.The presence and the importance of the ageing of the wax were established under isothermal conditions. It must result from a coarsening of the crystallites presents in the oil and it is, more important, close to the gel point where its full development is very slow taking several days to occur. After ageing the gels, the connective domains or junction zones linking the crystal arrays fail when relatively small strains are applied to the system and the mechanical spectra of the gels reveal an imperfect elastic network, typical rheological characteristics of a particle gel. Despite the compositional differences among the samples, the similarity of their mechanical behaviour is quite remarkable indicating that in all cases the gel-like organisation of the waxy material results from the formation of identical structures in the different oils, which is related not only to the wax content but also to the presence of other material that may reduce the crystallinity of the structure.The low fractal dimensionality obtained indicates elongated substructures. These results, together with the very high elastic modulus obtained at low volume fractions of crystallised material, are indicative of network structures with high degree of porosity: a lattice of wax crystals with large spaces among them filled by the oil and non-precipitated material.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003.     

Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis

A novel technique, based on low intensity ultrasonic wave propagation, has been applied to investigate the gelation of a waxy crude oil, caused by the crystallization of paraffin fractions as the temperature reduces below a threshold value, called WAT (wax appearance temperature). Because this phenomenon significantly affects the rheological behavior of crude oils, the knowledge of the conditions under which it occurs, during oil storage and/or transportation, is a topical issue in the oil industry. In this work, an ultrasonic equipment has been set up, able to propagate longitudinal waves in the MHz range and to display in real time the behavior of ultrasonic velocity and attenuation when the crude oil sample is subjected to heating and cooling cycles. When the ultrasonic probes alternatively rotate as parallel plates of a conventional rheometer, low intensity longitudinal waves (in the megahertz range), and shear oscillations (in the hertz range) are simultaneously applied on the sample, thus widely broadening the frequency range of investigation. On cooling, the crystallization of paraffin fractions and the consequent formation of a network structure in the oil matrix are responsible of the development and growth of the crude oil elastic response, which becomes dominant over the viscous response. This process can be reliably detected by dynamic mechanical analysis and by ultrasonic analysis through the increase of the storage modulus G′ and longitudinal velocity, respectively. The growth and further association between wax crystals causes a dissipation of acoustic energy, which is indicated by the increase of the wave attenuation. The combination of rheological and ultrasonic methods has provided a better insight both on the gel transition of crude oils and the viscoelastic behavior of gelled samples. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol–gel transition in waxy crude oils. Finally, the effect of ultrasonic waves with different intensity on the gel build-up has been also evaluated. A reduction of the gel strength with increasing wave intensity has been observed and the recovery of elastic response after removing ultrasonic irradiation has been monitored. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Application of the Casson model to thixotropic waxy crude oil

The rheological behavior of a waxy crude oil was investigated using a coaxial cylinder viscometer. Experimental flow curves were fitted with the Casson equation. The Casson model was modified to interpret the hysteresis between upward and downward curves obtained in a series of consecutive runs. At the same shear rate, the mean axial ratio of the flow unit related to a down-curve (↓) is smaller than that related to an equilibrium up-curve (↑). This results in a decrease of the Casson yield stress τ_c↓ with respect to τ_c↑. A certain ξ coefficient describing departure from the equilibrium mean axial ratio was introduced into the model. Values of ξ calculated from the Casson yield stresses agreed satisfactorily with the theoretically predicted ones. Deviations from the Casson model at low shear rates were also explained. Received: 28 July 1998 Accepted: 18 February 1999     

Effects of cooling regime on the formation of voids in statically cooled waxy crude oil

Waxy crude oil solidifies when exposed to certain range of low seabed temperature. In this condition, predicting an accurate restart pressure in the pipeline has remained to be a challenge. Recent researches highlighted that the presence of gas voids in the gel has an impact on the gelled crude oil morphology and consequently on the restarting pressure. The gelling due to cooling of the oil is complex and not well understood. This study is aimed at analyzing the effects of cooling regimes on the formation of gas voids along and across the pipeline using a 3 T Magnetic Resonance Imaging (3 T-MRI) system. A flow loop rig simulating offshore oil transportation was designed and developed to produce the gel. It was found that voids were formed within gelled crude oil by 11.3% in volume. The results from the cooling regime under constant start temperature showed that the widest cooling regime resulted in a maximum voids volume in all regions in the pipe. However, reducing the cooling regime did not guarantee a decrease in volume of voids, for which lower cooling regimes occasionally resulted in a higher percentage of voids volume. For the cooling regimes under constant end temperature, in general, an increase in cooling regime resulted in a higher percentage of voids volume in the gel at lower end temperature. On the contrary, the intra-gel voids formation at higher end temperature was found to be different and a higher cooling regime did not result in a higher voids volume near pipe wall. Indeed, a higher cooling regime formed a higher percentage of voids volume around pipe core.     

A new approach to model strain change of gelled waxy crude oil under constant stress

Deformation of gelled waxy crude oil with loaded stress is worthy of research for the flow assurance of pipelining system. A dispersion parameter was introduced to characterize the disruption degree of wax crystal structure in crude oil with shear action. Based on fractional calculus theory, a rheological model incorporating dispersion parameter was proposed to describe creep of gelled waxy crude. A discrete and numerical algorithm was proposed to solve the model. Combining with the experimental results of five kinds of waxy crude oil, the model parameters were regressed and found to change monotonously with test temperature. Multiple creep curves of gelled waxy crude oil at a certain temperature can be described with this model.     

Performance of elastic wheels on yielding cohesive soils

Conventional methods for predicting the performance of wheels on soft soils consider the wheels to be rigid. It is difficult to describe the performance of elastic wheels such as pneumatic rubber tires due to the lack of an adequate expression of their deformation on yielding soils. In this paper a simple method to describe the configurations of elastic wheels moving in deformable soils is proposed. It treats elastic wheels as rigid wheels of larger diameters. Also a theoretical method that can predict the rolling resistances, depth of ruts and driving torques of pneumatic rubber tires on yielding cohesive soils is developed.

Experimental results show that the method is good enough to predict the performance of pneumatic rubber tires for practical use.     

Effects of particle softness on the rheology and yielding of colloidal glasses

We studied the linear and nonlinear rheology of colloidal glasses consisting of hard spheres and soft core-shell particles at several volume fractions to explore the effects of particle softness on the mechanical properties and yielding. Creep and recovery and oscillatory shear measurements were used to determine the shear elastic modulus and the yield strain. Both hard and soft sphere glasses exhibited ‘entropic cage elasticity’ below random close packing, whereas for compressed soft spheres at higher effective volume fractions, the yield strain was determined by shell elasticity. The shear modulus followed a strong increase with volume fraction for hard spheres and a much weaker one for soft particles reflecting their interparticle potential. Nonlinear effects, revealed as strong distortions of the stress signal during yielding, were analyzed via Fourier transform rheology and Lissajous plots. The significant contribution of the nonlinearities was analyzed in terms of strain softening and hardening mechanisms within a cycle of oscillation and discussed in relation to particle softness.     

关于力学系统的守恒量--分析力学札记之四

利用牛顿力学方法,分析力学经典方法与分析力学近代方法都可以求得力学系统的守恒量,中讨论了３种方法的优缺点。     

Effects of physical aging on yielding kinetics of polycarbonate

The purpose of this work was to investigate the effects of physical aging on the kinetics of yielding in polycarbonate. PC samples were annealed over a wide range of aging times and temperatures. Both tensile and compressive tests were performed over various loading rates and temperatures to analyze the effects of aging time and aging temperature on yielding kinetics. Two grades of polycarbonate, Makrolon, of different molecular weights, PC-2608 (low M_w), and PC-3208 (high M_w), supplied by Bayer were analyzed. In unaged condition, PC is hard and tough, but after aging, it becomes more brittle. In terms of molecular movement, the yielding process is a thermally activated process involving inter- and intra-molecular motions. The time–temperature dependence of yielding behavior can be separated into two regions. Aging does not affect localized molecular motions of the β process during yielding. Physical aging in PC results in a slower jump rate of the main segments of macromolecules between two equilibrium positions. It reduces the flexibility of the macromolecules and thus, makes the polymer more brittle. Heat aging also causes a decrease of the entropy (ΔS) in polycarbonate, and this decrease is more important when the molecular weight is reduced. Increasing the annealing time and temperature results in a continuous reduction of ΔS. The rate of aging decreases with decreasing annealing temperature and below about 30 °C, no aging takes place. Annealing also strongly affects the excess of enthalpy in PC. However the effect of physical aging on yielding differs to that on enthalpy excess. The kinetics of yielding and aging processes in polycarbonate are also different. An increase in the strain rate does not have the same effect on the yield stress as an increase in the aging time by a same factor.     

Effects of the strain-hardening exponent on two-parameter characterizations of surface-cracks under large-scale yielding

The influence of strain hardening exponent on two-parameter J-Q near tip opening stress field characterization with modified boundary layer formulation and the corresponding validity limits are explored in detail. Finite element simulations of surface cracked plates under uniaxial tension are implemented for loads exceeding net-section yield. The results from this study provide numerical methodology for limit analysis and demonstrate the strong material dependencies of fracture parameterization under large scale yielding. Sufficient strain hardening is shown to be necessary to maintain J-Q predicted fields when plastic flow progresses through the remaining ligament. Lower strain hardening amplifies constraint loss due to stress redistribution in the plastic zone and increases the ratio of tip deformation to J. The onset of plastic collapse is marked by shape change and/or rapid relaxation of tip fields compared to those predicted by MBL solutions and thus defining the limits of J-Q dominance. A radially independent Q-parameter cannot be evaluated for the low strain hardening material at larger deformations within a range where both cleavage and ductile fracture mechanisms are present. The geometric deformation limit of near tip stress field characterization is shown to be directly proportional to the level of stress the material is capable of carrying within the plastic zone. Accounting for the strain hardening of a material provides a more adjusted and less conservative limit methodology compared to those generalized by the yield strength alone. Results from this study are of relevance to establishing testing standards for surface cracked tensile geometries.     

Crack-tip dynamic isochromatics in the presence of small-scale yielding

A plasticity correction factor for the dynamic stress-intensity factor,K _I ^dyn , associated with a propagating crack tip in the presence of small-scale yielding, is derived from Kanninen's solution for a constant-velocity Yoffe crack with a Dugdale-strip yield zone. Distortions in the otherwise elastic isochromatics surrounding the constant-velocity crack tip are also studied by the use of this model. This plasticity correction factor is then used to evaluateK _I ^dyn from the dynamic isochromatics of a propagating crack in a 3.2-mm-thick polycarbonate wedge-loaded rectangular double-cantilever-beam specimen. The correctedK _I ^dyn is in good agreement with the corresponding values computed by a dynamic, elastic-plastic finite-element code executed in its generation mode.     

The influence of void distribution on the yielding of an elastic-plastic porous solid

In a previous study, the authors proposed an anisotropic yield function for a porous metal. The present investigation extends the previous work by deriving a rate-type constitutive law based on the proposed yield function. A comparison is also made between yield loci derived from the yield function and those determined from an FE model. The FE calculations were performed for a flat plate, with a periodic array of cylindrical holes, under biaxial loading; and also for a porous solid with a periodic array of spherical voids under axial load and superimposed hydrostatic pressure. For the plate, the agreement between the yield function and numerical results was good, less so far the three-dimensional case. Good correspondence can be obtained by introducing a scalar multiplier in the pressure-dependent term in the yield function.     

Experiments on yielding of tension specimens with notches and holes

The yield loads of various ST-37 steel strips with notches and holes, subjected to tension, are determined experimentally. All samples have the same thickness and the same net cross-sectional area. Their width-to-thickness ratio is such that they are assumed to be in a state of plane stress. Essentially, the experiments are performed to find the effect of the angle of the V-notched specimens on the yield load, the most efficient shape of cut-outs and the effect of eccentricity of circular holes on the carrying capacities of the strips. The results are then compared, whenever possible, to the existing theoretical results.     

Numerical simulation of the effect of grain fragmentation on the evolution of microstructure quantities

Meccanica - In this paper the effect of grain fragmentation of a cohesionless granular material on the change of microstructure quantities is investigated using a micropolar continuum model. To...     

Microstructural investigations of the yielding behaviour of bidisperse magnetorheological fluids

Particle level simulations were used to investigate the effects of size bidispersity and particle size ratios on the static and yielding behaviour of magnetorheological fluids (MRF). The MRF were treated as linearly magnetisable, neutrally buoyant particles dispersed in a viscous carrier liquid. In the quiescent mode (static structures), the bidisperse suspensions were found to have a higher tendency to form straight chains than the monodisperse suspensions; this is consistent with previous findings. Under steady shearing, the bidisperse suspensions exhibited higher stress enhancement than the monodisperse systems. The stress enhancement in bidisperse suspensions is likely to be due to the population and orientation of interacting large particles in the bidisperse suspensions.     

Lagrange函数规范变换与Noether守恒量

力学系统Lagrange函数规范变换不改变Noether守恒量,但是Noether等式中规范函数需要调整.