2. PVT Characterisation Experiments: A. Waxes i. Waxy crude characterisation experiments:
- Wax Content: HTGC (High Temperature Gas Chromatography)
- Wax Appearance Temp. (WAT): High Pressure Cross Polar Microscopy (HP-CPM)
- Upper and Lower Pour Point: Live oil pour point apparatus (ASTM D5853)
- Gel Strength: Model Pipeline Test (MPT)
- Rheology and Viscosity: Controlled Stress Rheometry
- Wax Solubility: Bulk Filtration
When assessing a waxy crude production or transport situation, relying solely on conventional dead crude wax tests, including wax content and wax appearance temperature measurements, can be misleading. Stock-tank oil tests are insufficiently representative of field situations because reservoir pressure and solution gas have a strong influence on wax solubility. Laboratory-scale tests must account for the actual thermophysical situation in the field if they are to be applicable. The wax appearance temperature (WAT) is the temperature below which a solid wax phase forms within a hydrocarbon fluid at a given pressure. Below the WAT, significant viscosity increases, deposition, and gelling are possible. The pour point is the temperature, at a given pressure, below which the static fluid may form a gel. For a system cooled below its pour point, restarting flow may be difficult or impossible. ii. Wax Inhibitors: Basically, three groups of wax inhibitor chemicals are used.
- Wax crystal modifiers
- Detergents
- Dispersants
The last two groups are surface-active agents as, for example, polyesters and amine ethoxylates. These act by keeping the crystals dispersed as separate particles, thereby reducing their tendency to interact and adhere to solid surfaces.
Crystal modifiers are substances capable of building into wax crystals and altering the growth and surface characteristics of the crystal. The crystal modifiers will lower the pour point as well as the viscosity. The name pour point depressant is also used for this class of chemicals. The acetate group (CH3COO—) contained in the inhibitor is very unlike the paraffinic branches and will disturb further structuring of the paraffinic molecules.
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iii. Viscosity of Wax – Oil Suspensions:
Oil containing solid wax particles may exhibit non-Newtonian flow behavior. This means that the viscosity varies with shear rate (dvx/dy). At temperatures above the WAT, the oil behaves in a Newtonian manner (viscosity independent of shear rate). Below the WAT, the viscosity varies depending on shear rate.
The apparent oil viscosity below the WAT may be calculated from (Pedersen and Rønningsen, 2000)
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B. Asphaltenes:
Asphaltenes is a component class that may precipitate from petroleum reservoir fluids as a highly viscous and sticky material that is likely to cause deposition problems in production wells and pipelines. Asphaltenes are defined as the constituents of an oil mixture that, at room temperature, are practically insoluble in n-pentane and n-heptane, but soluble in benzene and toluene (Unlike resins which are soluble in n-pentane and insoluble in liquid butane or propane). Because a major part of reservoir fluids consists of paraffins, asphaltene precipitation problems are quite frequent. Unlike wax precipitation, asphaltene precipitation is not limited to low temperatures. Precipitation may occur in the reservoir, in the production well, during pipeline transportation, and in process plants. Gas is often injected into an oil reservoir to obtain an enhanced oil recovery. Because natural gas essentially consists of paraffins, gas injection will tend to worsen precipitation problems. Also there are different opinions about the solubility properties of already precipitated asphaltenes. Just a few years back, it was the general opinion that already precipitated asphaltenes would never go back into solution again. Supporters of this idea saw asphaltenes dissolved in an oil mixture as aggregates, only staying in solution because of an outer protective layer consisting of resins. Removal of this protective layer would make the asphaltenes form even larger aggregates that would precipitate and become insoluble, because it would be impossible to regenerate the protective resin layer. Resins form another solubility class. resins are soluble in n-heptane. They can be adsorbed on silica or alumina from an n-heptane solution, from which state they can be extracted using a methanol–benzene solution. The understanding of asphaltene precipitation as a nonreversible process was essentially based on experimental observations of asphaltenes precipitated from stabilized oils by addition of large quantities of either n-pentane or n-heptane. This precipitation technique gives asphaltenes in almost pure form, and the cohesion between the individual asphaltene molecules in this form may be so high that it becomes almost impossible to dissolve the asphaltenes again. For an oil of a fixed composition, asphaltene precipitation is most likely to take place right at the bubble point. At the bubble point, the oil has the highest content of dissolved gas. The paraffinic gas components (C1, C2, etc.) are bad solvents for the asphaltenes; this is what makes asphaltene precipitation likely to take place. If the pressure is lowered, some gas will evaporate, and the gas concentration in the liquid phase will decrease. This makes the asphaltenes more soluble in the liquid. The asphaltene phase will slowly dissolve and possibly disappear. The pressure at which the last asphaltenes go into solution is called the lower asphaltene onset pressure (lower AOP). Increasing the pressure from the bubble point will also make the asphaltene phase dissolve. Though paraffins are generally poor solvents for the asphaltenes, the solubility of asphaltenes in paraffins increases with pressure, and, at a sufficiently high pressure, the upper asphaltene onset pressure (upper AOP), the asphaltene phase will disappear. APO: Asphaltene Precipitation Onset [link Point to another website Only the registered members can access]
i. Experimental Techniques to study Asphaltene Precipitation:
- Quantification of Amount of Asphaltenes:
The n-C5 or n-C7 precipitation technique (e.g., Burke et al., 1990) is used to determine the asphaltene content in a stabilized oil mixture. The n-paraffin is injected in large quantities (e.g., 40/1 n-C5 /oil on a volume basis), which forces the asphaltenes to precipitate. The precipitate is filtered and washed to purify the asphaltenes. Asphaltene content measurement as a heptane insoluble fraction: IP 143 Asphaltene content measurement as a pentane insoluble fraction: ASTM D893
- Detection of Asphaltene Onset Pressures (AOP):
- Gravimetric Technique
- Acoustic Resonance Technique
- Light-Scattering Technique
- Filtration and Other Experimental Techniques
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