Asphaltene and Wax Phase Behavior and Deposition Envelopes
In this section, a brief summary of the review of the asphaltene and wax phase behavior by Leontaritis (1996) is presented.
Accurate measurement of the asphaltene and wax phase behavior is expensive and requires sophisticated techniques for proper handling of the reservoir fluid samples and laboratory testing of the recombined reservoir fluids. Therefore, Leontaritis (1996) suggests that phase diagrams can be more economically and rapidly developed by simulation with a limited number of actual data required for tuning and calibration. Leontaritis (1996) demonstrated this exercise by applying the Thermodynamic-Colloidal model by Leontaritis (1993). Nghiem and Coombe (1997) state: "Above the saturation pressure, the precipitation is solely due to pressure, while below the saturation both pressure and composition affect the precipitation behavior." However, more research is needed in this area.
Leontaritis (1996) points out that wax crystallization and asphaltene flocculation phenomena occur at low and high temperatures, respectively. Then, he hypothesizes that these two phenomena should, therefore, represent the two extreme cases of the phase behavior and there should be continuously varying intermediate phase behavior in between these two extremes depending on the composition of the heavy fractions of the crude oils, as schematically shown in this article by Leontaritis (1996).
Although it sounds reasonable and there is some evidence of support for this hypothesis, more research is obviously needed to verify this hypothesis. As explained by Leontaritis (1996), the phase diagrams of the asphaltenic fluids typically do not have a critical point, because the asphaltenic fluids can only have bubble-point lines and no dew-point lines, as they cannot vaporize. Leontaritis (1996, 1998) refers to the locus of the thermodynamic conditions for asphaltene flocculation as the
Typically, the pressure-temperature phase diagrams of the asphaltenic oils are characterized by several phase quality lines and a saturation (bubblepoint) line in between the upper and lower boundaries of the asphaltene deposition envelope as indicated on this article by Leontaritis (1996) for a South-American oil. Leontaritis (1996) estimated the intersection of the upper ADE with the bubble-point line at around 370°F for this oil.
Leontaritis (1996) are typical simulated charts showing the asphaltene deposition envelope, the asphaltene phase volume versus temperature, and the asphaltene phase volume versus pressure for a North-American oil, respectively. Leontaritis (1996) refers to the locus of the thermodynamic conditions for wax crystallization as the Wax-Deposition-Envelope (WDE). Leontaritis (1996) depict the affect of the light-ends and the pressure-temperature relationship on the onset of wax crystallization (cloud point) of oils. The affect of the pressure on the onset of wax crystallization is demonstrated for
three live-oils in in this article Leontaritis (1996). The typical Wax-Deposition-Envelope of a North American recombined live oil constructed by laboratory measurements is given by Leontaritis (1996). He also developed the Wax-Deposition-Envelope given in this article for a North Sea live oil by using a Wax Model. Using the same Wax Model, Leontaritis (1996) has predicted the affect of temperature on the fraction of the wax crystallized at 200, 50, and 1 atm pressures as shown in this article, and 14-27, respectively, as well as the affect of the pressure on the fraction of the wax crystallized at 280°K Mansoori (1997) mentions that experimental measurement of the pressure-composition phase diagrams involving heavy organic deposition by miscible gas injection, at reservoir temperatures, is very costly. Therefore, he suggested generating these charts by simulation. Mansoori (1997) indicate the electrokinetics affect on asphaltene deposition in pipelines from typical asphaltenic oils dissolving a miscible component at various temperatures. These figures contain two charts. The upper chart shows the static onset of deposition of asphaltene on a
pressure vs. composition relationship. The lower chart shows the dynamic, Q (defined below) versus pressure relationship for asphaltenic oils flowing in wells or pipelines for different miscible component and oil ratio. The Q function is given by (Mansoori, 1997):
in which U is the average oil velocity in the pipe, d is the pipe diameter, and k is the conductivity of the oil. The regions above and below these curves express the flow conditions leading to deposition and no deposition of asphaltenes, respectively. Hence, these charts help determine the operating conditions of pipes to avoid precipitation.
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