SOLIDS (WAX OR ASPHALTENE) PRECIPITATION TEMPERATURE MEASUREMENT FOR RESERVOIR FLUIDS
Denis Gorkovenko
Laboratory Engineer | Business Development Manager | Technical sales engineer
Crude oil is a sophisticated system of hundreds of various chemical substances consisting primarily of hydrocarbons known as paraffin. In general, this paraffins align as long straight chain molecules, branched or cyclic structures. It is known that paraffin wax made from crude oil consists commonly of long chains.
The lighter parts of the crude oils hold the heavier elements, wax and asphaltene, in solution. This solubility depends on the pressure, temperature and composition of crude oils.
When the temperature of crude oil drops as it happens in the production tubing of oil wells and pipelines, the solubility of the heavy fractions can be drastically decreased to cause the precipitation of solid particles of wax and asphaltenes.
Paraffin waxes are blends of a variety of high molecular weight alkanes which could crystallize from crude oils or solutions basically because of temperature decrease. They are rather non-polar molecules and their interactions are anticipated to be van der Waals or London dispersion type. Paraffin waxes are made of branched (iso), cyclic and straight chain (normal) alkanes having chain lengths above 17 carbon atoms (C17) and potentially up to and over C100.
The presence of those solid particles causes a change in the flow behavior of crude oil from Newtonian to non-Newtonian.
Determination of the quantity of wax precipitated at different temperatures based on the definition of wax and technique of measurement. Differential scanning calorimetry (DSC), polarized microscopy, low-resolution pulsed nuclear magnetic resonance (NMR), rheometry, densitometry, cold finger, visual, light transmittance and ultrasonic methods.
Some experimental methods mentioned in the literature such as Microscopy and X-ray diffraction are powerful tools to determine the crystal structures however provide limited insight into the crystallization process, while other methods used to get the liquid-solid equilibrium of paraffin have been used, but they are very complicated because they require the establishment of the equilibrium at every temperature of interest and the measurement of the composition of the phases present. Finally, visual methods have been also reported to measure the solubility and phase behavior of paraffin waxes; however, these methods cannot be applied to test dark samples (e.g. black crude oils). Hence, for the study and measurement of the paraffin crystallization process, differential scanning calorimetry (DSC) is an experimental method widely used due to its simplicity, accuracy and fast response to monitoring the phase transitions during cooling and heating that gives related thermodynamic quantities such as heat capacity and enthalpies of transition. DSC has been usually used for the determination of wax appearance and/or dissolution temperatures (WAT or WDT) in petroleum products. The WAT or cloud point is the singularly most important parameter relating to the wax formation and it is the temperature at which waxes first crystallize from solution during a cooling process. Hence, accurate WAT measurements by using reliable methods such as DSC are desirable since it represents a key factor to characterize the wax precipitation phenomena.
The measurement principle of differential scanning calorimetry (DSC) is based on the measurement of the difference in the heat flows to the sample crucible and reference crucible. These heat flows are directly proportional to the temperature difference between the furnace and crucible, but inversely proportional to the thermal resistance of the system.
Fig. 1 is shown the measuring cell, furnace and liquid nitrogen cooling chamber of the Shimadzu DSC-60A differential scanning calorimeter.
Figure 1 – DSC measuring cell and temperature control system
The method to obtain the liquid-solid phase equilibrium properties from DSC experiments is explained below.
A calibration procedure of the DSC equipment should be performed before carrying out the experiments by using Indium or a series of high purity normal paraffin as standard. Each sample (between 10 and 20 mg) is first heated until reaching a temperature higher than the expected crystallization onset temperature (WAT) but without reaching the boiling point of the sample. Then, the sample is held isothermally for 1 min. and then cooled to the desired temperature at a pre-defined rate. The cooling/heating rate can be variable; in general, low heating/cooling rates would be desirable from an equilibrium point of view. However, by using low cooling rates higher WAT is obtained with a loss sensitivity to identify the DSC peak onsets, whereas high cooling rates depress measured WAT due to supercooling effects (Fig.2. and 3). The crystallization onset temperature (WAT) is determined as the onset of the exothermal peak during the cooling process corresponding to the liquid-solid transition.
Under heating conditions, the melting temperature is recorded as the onset of the endothermal peak, whereas the wax disappearance temperature (WDT), the temperature at which the last precipitated paraffin re-dissolves in the oil or solution, can be recorded as the end of the solid-liquid endotherm.
Figure 2 – Determination of WAT at a cooling rate of 1 °C/min
Figure 3 - Determination of WAT at a cooling rate of 0.2 °C/min
Comparison of experimental methods for measurement of wax precipitation in crude oils has been reported in the literature, where wax detection limits vary depending on the measurement technique, oil composition, thermal history, time of measurement and fluid properties related to crystal nucleation and growth.
For further analysis, we will investigate three methods: DSC, rheometry and densitometry.
Using DSC as a reference lets us evaluate rheometry and densitometry methods since they can be reproduced in the SRLF lab by Anton Paar densitometer and Viscolab PVT which is based on stoke law, in which a piston goes up and down due to magnetic forces induced by two magnetized coils inside a stainless steel body. At the same time, the device takes the timing of each stroke.
Anton Paar is the oscillation type densitometer. The advantages of using these instruments are their response time, use simplicity and the small volume of the sample needed.
The working principle of an oscillation-type density meter is based on the law of harmonic oscillation, in which a U tube is filled with the sample to be analyzed and subjected to an electromagnetic force. The measurement of the frequency and duration of vibration of the tube filled with the sample allows the determination of the density value of the sample. This measuring principle is based on the Mass-Spring Model.
The measuring cell consists of an oscillator formed by a hollow U-shaped borosilicate glass tube that comprises about 0,7 ml of the sample. This tube has double walls and the space between them is filled with a gas with a high coefficient of thermal conductivity. In that space is also placed a platinum resistance thermometer that allows the temperature measurement of the fluid during the density measurement. The remaining instrumentation consists of a system of electronic excitement and electrical components that provide a signal transmission of the period for the processor unit, free of interferences (Fig.4).
Figure 4 - Measurement cell of an oscillation-type density meters
The U tube oscillates at its fundamental frequency, which is a function of the system mass. If we assume that the sample volume inside the cell is constant, it can be seen that the oscillation frequency is therefore a function of the sample density.
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Authors [9] analyzed Mexican crude oils of the southern region labeled as RDO1, RDO2 and J32 are studied in this work. Crude oils RDO1 and RDO2 present wax precipitation and deposition problems during their production and transportation, whereas crude oil J32 presents severe asphaltene precipitation and deposition problem along the well during primary production.
The thermograms of crude oils RDO1 and RDO2 show one well-defined exothermal peak from which the WAT can be easily determined at 19.2 and 18.5°C respectively. In contrast, the thermogram of crude oil J32 presents two exothermal peaks, the first one well defined, a liquid-solid phase transition whose onset corresponds to a WAT of 28°C and a second one broad not well defined, with an onset around 20.8°C (Fig.5).
Figure 5 – Exothermic peaks from DSC Thermograms of crude oils during cooling
Rheometric WAT measurements were carried out on the basis that petroleum fluids exhibit non-Newtonian behavior below WAT and Newtonian behavior above the WAT which follows the Arrhenius temperature dependence:
where μ is the Newtonian dynamic viscosity, A is the Arrhenius pre-exponential factor, Ea is the activation energy of viscous flow, R is the universal gas constant and T is the absolute temperature. The formation and growth of solid wax crystals dispersed in the crude oil medium cause viscosity to increase during a cooling process. In this way, from viscosity-temperature curves, WAT is recorded as the temperature of the deviation of the Arrhenius law as it is shown in Fig. 6 (b).
Finally, the WAT determination by using the densitometry method is carried out by identifying the temperature at which a sharp change in the slope of density – temperature curve obtained during a cooling process that becomes evident at the onset of wax crystallization as can be observed in Fig. 6 (c).
Figure 6 – WAT determination by using different experimental methods:
b) Rheometric WAT measurement for crude oil RDO2 from Arrhenius plot
c) Density-Temperature profile for crude oil RDO1
These plots show that at high-temperature viscosity is a linear function of temperature and crude oil flow behavior is Newtonian. At lower temperatures, the viscosity versus temperature plot deviates from linearity. This deviation is caused by the precipitation of solid particles of wax and hence a change of flow behavior to non-Newtonian. The intersection of the straight line portion to the viscosity vs temperature plot curve is used to define the WAT.
It was found that rheometry and densitometry overestimate WAT for crude oils RDO1 and RDO2.
Obviously, the two methods above need that a critical amount of solid wax come out of the solution to produce a detectable change in the rheological properties or density of the crude oil sample and thus identify WAT.
Hence, the accuracy of determining WAT from viscosity-temperature and density-temperature plots depends greatly on how distinctive is the intersection of the straight lines.
As a result, the effectiveness of these methods highly depends on the equipment used.
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