Rheological behavior and structural interpretation of waxy crude oil gels

Rheology of Gels and Yielding Liquids

In this review, today's state of the art in the rheology of gels and transition through the yield stress of yielding liquids is discussed. Gels are understood as soft viscoelastic multicomponent solids that are in the incomplete phase separation state, which, under the action of external mechanical forces, do not transit into a fluid state but rupture like any solid material. Gels can "melt" (again, like any solids) due to a change in temperature or variation in the environment. In contrast to this type of rheology, yielding liquids (sometimes not rigorously referred to as "gels", especially in relation to colloids) can exist in a solid-like (gel-like) state and become fluid above some defined stress and time conditions (yield stress). At low stresses, their behavior is quite similar to that of permanent solid gels, including the frequency-independent storage modulus. The gel-to-sol transition considered in colloid chemistry is treated as a case of yielding. However, in many cases, the yield stress cannot be assumed to be a physical parameter since the solid-to-liquid transition happens in time and is associated with thixotropic effects. In this review, special attention is paid to various time effects. It is also stressed that plasticity is not equivalent to flow since (irreversible) plastic deformations are determined by stress but do not continue over time. We also discuss some typical errors, difficulties, and wrong interpretations of experimental data in studies of yielding liquids.

Infrared Spectral Classification of Natural Bitumens for Their Rheological and Thermophysical Characterization

Natural bitumens consist of many molecules whose chemical composition depends on the oilfield and determines the physicochemical properties of the bitumens as materials. Infrared (IR) spectroscopy is the fastest and least expensive method to assess the chemical structure of organic molecules, which makes it attractive in terms of rapid prediction of the properties of natural bitumens based on their composition evaluated in this way. In this work, IR spectra were measured for ten samples of natural bitumens significantly different in properties and origin. Based on the ratios of certain IR absorption bands, bitumens are proposed to be divided into paraffinic, aromatic, and resinous. In addition, the internal relationship between IR spectral characteristics of bitumens, such as polarity, paraffinicity, branchiness, and aromaticity, is shown. A study of phase transitions in bitumens by differential scanning calorimetry was carried out, and the use of a heat flow differential to find hidden points of bitumens' glass transitions is proposed. Furthermore, the dependences of the total melting enthalpy of crystallizable paraffinic compounds on the aromaticity and branchiness of bitumens are demonstrated. A detailed study of bitumens' rheology in a wide temperature range was carried out, and characteristic features of rheological behavior for different bitumen classes are revealed. Based on the viscous properties of bitumens, their glass transition points were found and compared with the calorimetric glass transition temperatures and nominal solid-liquid transition points obtained from temperature dependences of bitumens' storage and loss moduli. The dependences of viscosity, flow activation energy, and glass transition temperature of bitumens on their IR spectral characteristics are shown, which can be used to predict the rheological properties of bitumens.

Investigation on Wax Deposition Reduction Using Natural Plant-Based Additives for Sustainable Energy Production from Penara Oilfield Malaysia Basin

In empowering the United Nations (UN) Sustainable Development Goal, the oil industry is inclined toward organic wax inhibitor applications when combatting the wax deposition issue during crude oil production. This is because synthetic chemical inhibitors are costly and have the potential to create environmental problems when oil spillage or seepage occurs during transportation or operation. This study evaluates the impact of low-cost, natural plant-based inhibitors such as Jatropha seed oil (JSO), crude palm oil, and crude palm kernel oil (CPKO) on paraffin inhibition efficiency (PIE, %) and rheological properties of Malaysian waxy crude oil. By using cold finger equipment and a Fann viscometer, the amount of solid wax deposits, apparent viscosity, plastic viscosity, yield value, and gel strength were determined. Commercialized ethylene-co-vinyl acetate and triethanolamine compounds were used for a comparative study. For the wax deposition test, the results revealed that the highest average PIE obtained was 86.30% when 5% JSO was blended with Penara crude oil. Meanwhile, the rheological test proved that 5% JSO and 1% CPKO were acting as highest viscosity-reducing agents at 60 °C below the wax appearance temperature (WAT). The discovery of palm-based and Jatropha-based inhibitors from Malaysia's palm oil plantation and Malaysian JSO as a wax inhibitor was found to be beneficial for the application of wax deposition and rheological studies in crude oil production with a less harmful environment for sustainable energy production.

Application of Rheo-optic In Situ Measurement Technology to Study Waxy Crude Oil Rheology

The micromechanism of waxy crude oil gelling is the interaction between wax crystals to form a certain intensity flocculation structure, which significantly increases the cost of production and transmission. In this paper, rheo-optic in situ measurement technology is applied to the rheological study of waxy crude oil for the first time and also to the rheological response of typical waxy crude oil to thermal history, the micromechanism of shear-thinning, and the dynamic behavior of wax crystal. Through the new experimental technique and analysis method, it is found that two types of wax crystals can be formed under certain thermal historical conditions, which have opposite performances in microscopic morphology, mechanic properties, and flocculation tendency, and the change of its proportion in crude oil is the root cause of the initial cooling temperature affecting the fluency of waxed crude oil. It was found that the microscopic behavior of waxy crude oil with the increase of shear rate went through the following whole process: the waxy crude oil system changes from static to dynamic, the wax crystal flocculation network undergoes deformation, cracks, and ruptures, and wax crystal aggregates break, small aggregates orient along the flow field, and small aggregates continues to deform and break. When the shear rate is below 5 s^-1, the crack of the flocculation structure plays a leading role. It is only after the shear rate exceeds 5 s^-1 that the deformation of the wax crystal and its flocs begins to function. Furthermore, according to the microscopic images of the wax crystals motion sequence, the micromorphology of different types of flocs and the dynamic behaviors under shearing are systematically analyzed by dynamic micro-object capture technology.

Extra-Heavy Crude Oil Viscosity Reduction Using and Reusing Magnetic Copper Ferrite Nanospheres

The main objective of this study is the synthesis, use, and reuse of magnetic copper ferrite nanospheres (CFNS) for extra-heavy oil viscosity reduction. The CFNS were synthesized using a solvothermal method resulting in mean particle size of 150 nm. Interactions of CFNS with the crude oil were evaluated through asphaltene adsorption isotherms, as well as static and dynamic rheology measurements for two cycles at 25 °C. Adsorption and desorption experiments corroborated that most of the asphaltenes adsorbed can be removed for nanoparticle reuse. During the rheology tests, nanoparticles were evaluated in the first cycle at different concentrations from 300 to 1500 mg/L, leading to the highest degree of viscosity reduction of 18% at 500 mg/L. SiO2 nanoparticles were evaluated for comparison issues, obtaining similar results regarding the viscosity reduction. After measurements, the CFNS were removed with a magnet, washed with toluene, and further dried for the second cycle of viscosity reduction. Rheology tests were performed for a second time at a fixed concentration of 500 mg/L, and slight differences were observed regarding the first cycle. Finally, changes in the extra-heavy oil microstructure upon CFNS addition were observed according to the significant decrease in elastic and viscous moduli.

In situ observation of microscopic motions and the structure dynamic transformation of wax crystals in waxy crude oil subjected to shear

The high-efficient development, storage and transportation of waxy crude oil has a significant meaning for stable supply of petroleum energy.

Effects of Waxes and the Related Chemicals on Asphaltene Aggregation and Deposition Phenomena: Experimental and Modeling Studies

Solid deposition during production, transport, and storage of crude oils leads to significant technical problems and economic losses for the oil and gas industry. The thermodynamic equilibrium between high-molecular-weight components of crude oil, such as asphaltenes, resins, and waxes, is an important parameter for the stability of crude oil. Once the equilibrium is disturbed due to variations in temperature, pressure, and oil composition during production, the solubility of high-molecular-weight waxes decreases. This results in a decrease in the wax appearance temperature (WAT) and the deposit of wax onto solid surfaces. On the other hand, under these conditions, asphaltenes do not interact sufficiently with the resins/waxes and tend to flocculate among themselves and form asphaltene nanoaggregates. The role of waxes during the asphaltene aggregation and deposition has not been appropriately explained yet. The objective of this research study is to describe the interaction of asphaltenes and waxes and subsequently address the specific example of an asphaltenic oil commingled with a wax inhibitor-containing oil during the combination of different oil streams. It is a crucial building block for the development of a suitable and cost-effective strategy for the handling of wax/asphaltene associated flow assurance problems. In this work, the quartz crystal microbalance (QCM) technique has been used for the first time to investigate the effect of waxes and related chemicals, which are used to mitigate wax deposition, on asphaltene aggregation and deposition phenomena. Asphaltene onset point and asphaltene deposition rate have been monitored using QCM at high pressure-high temperature (HPHT) conditions. This study confirms that the different wax inhibitor chemistries result in significant differences in the pour point decrease and viscosity profiles in crude oil. Different wax inhibitors also showed different outcomes regarding the asphaltene deposition tendency. A comprehensive modeling study has also been conducted for mechanistic investigation of experimental results. In this regard, the perturbed chain statistical associating fluid theory equation of state (PC-SAFT EoS) was utilized to model the systems.

Effect of Textural Properties and Surface Chemical Nature of Silica Nanoparticles from Different Silicon Sources on the Viscosity Reduction of Heavy Crude Oil

The main objective of this study is to evaluate the effect of the textural properties and surface chemical nature of silica nanoparticles obtained from different synthesis routes and silicon precursors, on their interactions with asphaltenes and further viscosity reduction of heavy crude oil (HO). Four different SiO₂ nanoparticles were used, namely, commercial fumed silica nanoparticles (CSNs) and three in-house-synthesized nanoparticles (named based on the silicon source) modifying the silicon precursor: sodium silicate (SNSS), tetraethylorthosilicate (TEOS) (SNT), and rice husk (SNRH). The nanomaterials were characterized through dynamic light scattering (DLS), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, N₂ physisorption (S _BET), atomic force microscopy (AFM), and X-ray photoelectron (XP) spectroscopy (XPS). The adsorption of asphaltenes over the different nanoparticles was evaluated at a concentration of 1000 mg·L^-1 in toluene. The asphaltene-nanoparticle interactions are closely related to several textural properties, such as roughness, surface area, and hydrodynamic diameter, as well as the surface chemical nature of the materials. The results in the textural characterization exhibited that the sizes of the nanoparticles from TEM ranged between 6.9 and 11.5 nm. Nevertheless, the standard deviation of the measurements showed that the sizes are statistically similar. Inversely, the hydrodynamic diameter changed, affecting the surface silanol group's availability due to a hindering effect on functional groups as the hydrodynamic size of the material increased. The rheological measurements were performed at a fixed nanoparticle dosage of 1000 mg·L^-1 and showed that the trend of the degree of viscosity reduction (DVR) was CSN > SNT > SNSS > SNRH with the highest value yielding at 30%. The results of DVR are in accordance with the nanoparticles' adsorptive capacity as higher values were obtained with the material that leads to a higher amount of adsorbed asphaltenes. Also, the oxygen amount related to silanol groups, estimated by the XPS analysis, showed a direct relation regarding adsorption capacity and further HO viscosity reduction.

Brittle solid collapse to simple liquid for a waxy suspension

We show that thanks to the existence of a continuous (percolating) network of weak interparticle bonds in a liquid, wax suspensions behave as "soft breakable (brittle) solids". It appears that, under the action of either a large stress over a short time or oscillating low stress (fatigue test), the initially solid network of these materials is broken and dispersed in the liquid, which makes them turn abruptly ("collapse") and irreversibly into a low viscous fluid. This collapse is more dramatic as the concentration increases. These phenomena are related to the evolution of the microstructure directly observed after different flow histories. The interpretation of these results provides new perspectives for understanding the physical origin of the brittleness or plasticity of solid or pasty materials, and suggests such materials might be used as model systems to simulate and explain natural catastrophic events such as landslides and avalanches.

Development of Nanofluids for Perdurability in Viscosity Reduction of Extra-Heavy Oils

The primary objective of this study is the development of nanofluids based on different diluent/dispersant ratios (DDR) for extra-heavy oil (EHO) viscosity reduction and its perdurability over time. Different diluents such as xylene, diesel, n-pentane, and n-heptane were evaluated for the formulation of the carrier fluid. Instability of asphaltenes was assessed for all diluents through colloidal instability index (CII) and Oliensis tests. Rheology measurements and hysteresis loop tests were performed using a rotational rheometer at 30 °C. The CII values for the alkanes type diluents were around 0.57, results that were corroborated with the Oliensis tests as asphaltenes precipitation was observed with the use of these diluents. This data was related to the viscosity reduction degree (VRD) reported for the different diluents. With the use of the alkanes, the VRD does not surpass the 60%, while with the use of xylene a VRD of approximately 85% was achieved. Dimethylformamide was used as a dispersant of the nanoparticles and had a similar VRD than that for xylene (87%). Subsequent experiments were performed varying the DDR (xylene/dimethylformamide) for different dosages up to 7 vol % determining that a DDR = 0.2 and a dosage of 5 vol % was appropriated for enhancing EHO VRD, obtaining a final value of 89%. Different SiO2 nanoparticles were evaluated in the viscosity reduction tests reporting the best results using 9 nm nanoparticles that were then included at 1000 mg·L−1 in the carrier fluid, increasing the VRD up to 4% and enhancing the perdurability based on the rheological hysteresis and the viscosity measurements for 30 days. Results showed a viscosity increase of 20 and 80% for the crude oil with the nanofluid and the carrier fluid after 30 days, respectively. The nanoparticles have a synergistic effect in the viscosity reduction and the inhibition of the viscoelastic network re-organization (perdurability) after treatment application which was also observed in the rheological modeling carried out with Cross and Carreau models as the reported characteristic relaxation time was increased almost a 20%. Moreover, the Vipulanandan rheological model denotes a higher maximum stress value reached by the EHO with the addition of nanofluids which is derived from the EHO internal structure rearrangement caused by the asphaltenes adsorption phenomenon.

Single step and mask-free 3D wax printing of microfluidic paper-based analytical devices for glucose and nitrite assays

Microfluidic paper-based analytical devices (μPADs) have recently emerged as a simple, portable, user-friendly, and affordable alternative to more instrument-intensive analytical approaches for point-of-care testing (POCT), food safety analysis, and environmental monitoring. However, most of the existing methods for the fabrication of μPADs still face a great challenge because of different trade-offs among cost, convenience, and the pattern resolution. In this work, we report a facile one-step approach to prepare a μPAD using an affordable, easy-to-build 3D printer to generate patterns of solid wax on laboratory filter paper. The presented wax printing method did not require the use of predesigned masks and an external heat source to form complete hydrophobic wax barrier through the use of a custom-made extruder. The results revealed a strong linear relationship (R² = 0.985) between the nominal and the printed widths of the wax barriers. The achievable resolution of the wax barrier printed on filter paper was 468 ± 72 µm, which was lower than previously reported minimum barrier feature sizes achieved by wax printing and other wax patterning techniques, such as stamping and screen-printing. The analytical utility of the fabricated μPADs was evaluated for colorimetric nitrite and glucose detection in artificial solutions. It was found that the fabricated μPADs provided adequate accuracy and reproducibility for quantitative determination of nitrite and glucose within concentration ranges relevant to the disease detection in human saliva and urine. The wax printing approach reported here provides a simple, rapid, and cost-effective fabrication method for paper-based microfluidics and may bring benefits to medical diagnostics in the developing world.

Crystal structure of fractionally crystallized waxes isolated from crude oil

A petroleum wax has been extracted from crude oil and fractionated into two parts, depending on its solubility in methyl isobutyl ketone–toluene solvent at temperatures of 298 and 313 K. The wax and its two derivatives have been analyzed for composition and crystalline nature using various techniques including gas chromatography, NMR and X-ray diffraction. It has been observed that the crystalline structure of the fractionally precipitated waxes is significantly different from the structure of the parent wax present in the crude oil.

Characterizing gelatin hydrogel viscoelasticity with diffusing colloidal probe microscopy

In this study, we investigate viscoelasticity in gelatin hydrogels using diffusing colloidal probe microscopy (DCPM) to directly measure the elastic potential energy interaction between colloidal probes and the underlying viscoelastic media. Gelatin samples are prepared in four different concentrations between 0.3wt% and 0.6wt% to examine changes in viscoelasticity with concentration. A force balance describing the interaction between the colloidal probes and the hydrogel as a spring-damper system lead to a simple model for mean square displacement. A histogram of locations sampled by the colloidal probes is directly related to the elastic potential energy and the effective spring constant of the gelatin hydrogels. The effective spring constant is a fixed parameter used in the mean square displacement model to find effective viscosity. These parameters are comparable to viscoelastic parameters obtain by a microrheology analysis of two-dimensional mean square displacements. These results can serve as a guide for assessing hydrogel systems where viscoelastic properties are an important factor in biomaterial design.

Effect of pre-shear on structural behavior and pipeline restart of gelled waxy crude oil

Pre-shear can reduce the structural strength of gelled oil and the minimum pressure difference required for successful pipeline restart.

Rheological profiling of organogels prepared at critical gelling concentrations of natural waxes in a triacylglycerol solvent

The aim of this study was to use a detailed rheological characterization to gain new insights into the gelation behavior of natural waxes. To make a comprehensive case, six natural waxes (differing in the relative proportion of chemical components: hydrocarbons, fatty alcohols, fatty acids, and wax esters) were selected as organogelators to gel high-oleic sunflower oil. Flow and dynamic rheological properties of organogels prepared at critical gelling concentrations (Cg) of waxes were studied and compared using drag (stress ramp and steady flow) and oscillatory shear (stress and frequency sweeps) tests. Although, none of the organogels satisfied the rheological definition of a "strong gel" (G″/G' (ω) ≤ 0.1), on comparing the samples, the strongest gel (highest critical stress and dynamic, apparent, and static yield stresses) was obtained not with wax containing the highest proportion of wax esters alone (sunflower wax, SFW) but with wax containing wax esters along with a higher proportion of fatty alcohols (carnauba wax, CRW) although at a comparatively higher Cg (4%wt for latter compared to 0.5%wt for former). As expected, gel formation by waxes containing a high proportion of lower melting fatty acids (berry, BW, and fruit wax, FW) required a comparatively higher Cg (6 and 7%wt, respectively), and in addition, these gels showed the lowest values for plateau elastic modulus (G'LVR) and a prominent crossover point at higher frequency. The gelation temperatures (TG'=G″) for all the studied gels were lower than room temperature, except for SFW and CRW. The yielding-type behavior of gels was evident, with most gels showing strong shear sensitivity and a weak thixotropic recovery. The rheological behavior was combined with the results of thermal analysis and microstructure studies (optical, polarized, and cryo-scanning electron microscopy) to explain the gelation properties of these waxes.

How polymer additives reduce the pour point of hydrocarbon solvents containing wax crystals

We have investigated how four different pour point depressant (PPD) polymers affect the pour point transition in mixtures of a single pure wax in a solvent. We used either n-eicosane (C20), CH3(CH2)18CH3, n-tetracosane (C24), CH3(CH2)22CH3 or n-hexatriacontane (C36), CH3(CH2)34CH3 as the wax component with either n-heptane or toluene as the solvent component. For all wax-solvent combinations, the measured variation of wax solubility with temperature is well predicted by ideal solution theory. The variation of pour point temperature as a function of the overall wax concentration is quantitatively modelled using the idea that, for each overall wax concentration, the pour point occurs at a temperature at which a critical volume fraction ϕ* of wax crystals has precipitated. Close to the pour point temperature, extraction and examination of the wax crystals show they consist of polydisperse, irregularly-shaped platelets with axial ratios (h/d, where h is the plate thickness and d is the plate long dimension) in the range 0.005-0.05. It is found that the measured ϕ* values corresponding to the pour point transitions are weakly correlated with the wax crystal axial ratios (h/d) for all wax-solvent-PPD polymer combinations. These results indicate that the pour point transition occurs at a volume fraction larger than the value at which the volumes of rotation of the platelet crystals overlap, i.e., 2.5(h/d) < ϕ* < 11(h/d). PPD polymers work, in part, by increasing the wax crystal axial ratio (h/d), thereby increasing ϕ* and reducing the pour point temperature. Since the PPD's ability to modify the wax crystal shape relies on its adsorption to the crystal-solution surface, it is anticipated and observed experimentally that optimum PPD efficacy is correlated with the difference between the wax and the polymer solubility boundary temperatures. This finding and the mechanistic insight gained here provide the basis for a simple and rapid screening test to identify candidate species likely to be effective PPDs for particular wax systems.

A comprehensive constitutive law for waxy crude oil: a thixotropic yield stress fluid

Guided by a series of discriminating rheometric tests, we develop a new constitutive model that can quantitatively predict the key rheological features of waxy crude oils. We first develop a series of model crude oils, which are characterized by a complex thixotropic and yielding behavior that strongly depends on the shear history of the sample. We then outline the development of an appropriate preparation protocol for carrying out rheological measurements, to ensure consistent and reproducible initial conditions. We use RheoPIV measurements of the local kinematics within the fluid under imposed deformations in order to validate the selection of a particular protocol. Velocimetric measurements are also used to document the presence of material instabilities within the model crude oil under conditions of imposed steady shearing. These instabilities are a result of the underlying non-monotonic steady flow curve of the material. Three distinct deformation histories are then used to probe the material's constitutive response. These deformations are steady shear, transient response to startup of steady shear with different aging times, and large amplitude oscillatory shear (LAOS). The material response to these three different flows is used to motivate the development of an appropriate constitutive model. This model (termed the IKH model) is based on a framework adopted from plasticity theory and implements an additive strain decomposition into characteristic reversible (elastic) and irreversible (plastic) contributions, coupled with the physical processes of isotropic and kinematic hardening. Comparisons of experimental to simulated response for all three flows show good quantitative agreement, validating the chosen approach for developing constitutive models for this class of materials.

Heavy crude oils/particle stabilized emulsions

Fluid characterization is a key technology for success in process design for crude oil mixtures in the future offshore. In the present article modern methods have been developed and optimized for crude oil applications. The focus is on destabilization processes in w/o emulsions, such as creaming/sedimentation and flocculation/coalescence. In our work, the separation technology was based on improvement of current devices to promote coalescence of the emulsified systems. Stabilizing properties based on particles was given special attention. A variety of particles like silica nanoparticles (AEROSIL®), asphalthenes, wax (paraffin) were used. The behavior of these particles and corresponding emulsion systems was determined by use of modern analytical equipment, such as SARA fractionation, NIR, electro-coalescers (determine critical electric field), Langmuir technique, pedant drop technique, TG-QCM, AFM.