2. LITERATURE REVIEW
2.3 Rheological Parameters for Waxy Crude Oils Properties
A number of critical properties are important in describing the complex behaviour of waxy crude oil and its yielding process. These are the wax appearance temperature (WAT), the wax disappearance temperature (WDT), the gelation temperature (Tgel), the pour point (PPT), viscosity, gel strength and thixotropy.
Gel strength and yielding process of waxy crude oil are particularly important for this research and will be reviewed in details in subsection 2.6.
2.3.1 Wax Appearance Temperature (WAT)
The WAT, also called the cloud point temperature (CPT) indicates wax precipitation as it is the temperature at which the first wax crystals appear in a clear solution. No wax precipitation or deposition will occur as long as the crude oil
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temperature is above the WAT, but once the temperature drops below the WAT, wax molecules will begin to crystallize out of solution and wax deposition can occur. However, there is a clear distinction between the wax appearance temperature (WAT) and the wax disappearance temperature (WDT). Wax disappearance temperature is the temperature at which the last wax crystals are in equilibrium with the wax liquid phase [16]. Typically, WAT values for high paraffinic crudes range from 38oC to 66oC [17].
2.3.1.1 WAT Measurement Techniques
The WAT of waxy crude oils can be measured using a number of techniques including Differential Scanning Calorimetry (DSC), Cross-Polar Microscopy (CPM), Viscometry, Filter Plugging and Nuclear Magnetic Resonance (NMR). The first two techniques used by Ronningsen et al. [18], Hamouda et al.
[19], Pan et al. [20], Elsharkawy et al. [21], Cazaux et al. [22], Thomason [17], Calange et al. [23], Davidson et al. [14], Alboudwarej et al. [16], and Bordalo et al.
[24], both techniques will be described below.
2.3.1.1.1 Cross Polar Microscopy (CPM)
The CPM is considered as one of the most accurate methods [18] and is based on the theory that all crystalline materials rotate the plane of polarization of transmitted light while liquid hydrocarbons do not. This method requires a light source, an infrared filter, a polarizer, a temperature controller and a microscope.
The sample is enclosed in glass cover slides that are placed on the variable thermal microscope stage and viewed through the crossed polarizer. The microscope is equipped with a video digital camera and connected to a computer where the growth of wax crystals during cooling and disappearance during heating can be monitored directly on screen. When a waxy crude sample is cooled down on the temperature controlled slide under the microscope, wax crystals form and rotate the polarization plan of light i.e. light will be transmitted as paraffin crystals form. These crystals appear as white dots on the dark background. The first white
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dot, i.e. the first wax crystals appear when the temperature reaches the wax appearance temperature [25, 26, 27], as shown in Figure 2.3.1.
Figure 2.3.1: An image of wax crystals obtained at wax appearance temperature (WAT) of BPO crude oil from CPM.
2.3.1.1.2 Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimeter (DSC) measures the difference in the heat released between a reference and a test sample during crystallisation. It is thus a fundamentally precise method. As with the CPM method, only a small quantity of sample is required. The test sample is heated to a certain temperature, usually 30oC above WAT of waxy crude oil, and cooled at a given cooling rate. The reference sample has known properties and it is held in static thermal state over the temperature range used for the measurement. At the WAT, the test sample begins to cool slower than the reference sample due to the release of heat of crystallisation. This is controlled by an analyser which calculates the difference in heat inputs required to maintain the temperature of both samples equal. The temperature at which a melting peak occurs in the heat flow-temperature curve (thermo-gram) is taken to be the WAT [21], (see Figure 2.3.2). This point is seen as a deviation from the straight line trend above the WAT measured on the thermo-gram.
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Figure 2.3.2: DSC thermo-gram of a Middle East Crude (Elsharkaway et al.) [21].
This technique will be used in our research to measure WAT and the concentration of crystalline wax as a function of temperatures and cooling rates.
The enthalpy of fusion (Qoil) of a sample can be obtained from the DSC (see Figure 2.3.3), and the weight fraction of crystallized wax can be calculated according to the model introduced by J. Chen et al., 2003 [28] as expressed in Equation 2.6.
This solid weight fraction as a function of temperature defines the solubility curve for the given sample [26]:
(2.6) where Cwax (wt.%) is the wax content of crude oil, Qoil (J/g) is the total heat used between WAT and −20◦C.
Figure 2.3.3: A sample of DSC curve of crude oil (J. Chen et al., 2003).
16 2.3.2 Pour Point Temperature (PPT)
The pour point is the temperature at which the flow of a waxy crude oil stops and the crude oil becomes a frozen solid. The PPT indicates that an interlocking gel structure is completely formed as a result of crystallization of wax crystals from the oil under static conditions, causes the viscosity and flow properties of the oil to change dramatically. This property of waxy crude oil increases with increasing wax content [29, 30].
The pour point temperature can be measured using ASTM D5853-95 and D-97 procedures. It can also be determined by cooling a sample in steps of 1°C and determining the lowest temperature at which the liquid sample is able to move [31, 32].
2.3.3 Viscometry
From most literature, waxy crude oil is observed to behave as a Newtonian fluid when its temperature is above WAT and non-Newtonian below this temperature. This behaviour is essentially tracked by the key rheological parameter called viscosity. When the temperature of crude oils drops to its gel state, its viscosity increases towards very high values (infinity). In general, viscosity can be Newtonian, apparent or plastic depending on the flow behaviour of the waxy crude at a given temperature. In all cases it is the ratio of shear stress/shear rate:
̇
(2.7) capillary type. The description, merits and limitations of these techniques have17
been presented in detail by Wardhaugh and Boger [8]. For the purposes of this work, a Controlled Stress Rheometer, Anton Paar MCR 301, fitted with cone-plate or plate-plate geometries was employed. This rheometer will be described in detail in the Experimental Methods chapter.