Pressure and Composition Effect on Wax Precipitation: Experimental Data and Model Results
- Huanquan Pan (Reservoir Engineering Research Institute) | Abbas Firoozabadi (Reservoir Engineering Research Institute) | Per Fotland (Norsk Hydro Production, A.S.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1997
- Document Type
- Journal Paper
- 250 - 258
- 1997. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 4.3.3 Aspaltenes, 4.2 Pipelines, Flowlines and Risers, 4.2.5 Offshore Pipelines, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment, 4.1.9 Tanks and storage systems, 4.1.2 Separation and Treating
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Wax precipitation is often studied using the stock tank oil. However, precipitation may be very different in well tubing and production facilities due to the effects of pressure and composition. As an example, the cloudpoint temperature may decrease as much as 15 K from atmospheric pressure to the saturation pressure of 100 bar mostly due to the dissolution of light gases into the oil (i.e. due to composition changes). It is also often assumed that the addition of solvents such as C5 and C6 decreases the cloudpoint temperature. On the contrary, from our modeling results, we have found that the mixing of a crude with a solvent increases the cloudpoint temperature (i.e., enhances the wax precipitation).
In this study, the cloudpoint temperature at live oil conditions and the amount of the precipitated wax at stock tank oil conditions are provided for three crudes. A modified multisolid wax precipitation model is used to study the effects of pressure and composition on wax precipitation. The modeling results reveal that an increase in methane and CO2 concentration decreases the cloudpoint temperature while an increase in C5 concentration increases the cloud point temperature.
Wax formation is a major problem in well tubing and subsea pipelines. The current methods which include the interruption of production to scrub the deposited solid are very costly. A thermodynamic predictive model is important in solving the problem.
Dorest studied the solid precipitated from binary normal alkane mixtures using calorimetry and microscopy. He found that the precipitation is unstable and segregates into two solid phases when the chain-length difference between the two alkanes exceeds a fixed value. He also found the segregated phases consist predominantly of pure components. Recently, Snyder et al. studied the kinetics of the segregation using spectroscopy, calorimetry and electron diffraction. They observed that the rate of the segregation is very sensitive to the chain-length difference. Hansen et al. observed phase transitions of the precipitated wax from the North Sea crudes. Based on these observations, Lira-Galeana, Firoozabadi, and Prausnitz developed a thermodynamic multisolid wax model. The calculated results from this model were in agreement with data of stock tank fluids from the North Sea given by W.B. Pedersen et al. The model of Lira-Galeana, et al. lumps all the various chemical species for a given carbon number and avoids the use of paraffins (P), naphthenes (N) and aromatics (A) by assigning average properties such as the melting point temperature, heat of fusion, and critical properties. However, K.S. Pedersen, et al. and Ronningsen et al. have shown that the wax precipitated from a petroleum fluid consists primarily of normal paraffins, iso-paraffins, and naphthenes. The aromatics do not precipitate as the wax. The use of PNA analysis avoids assigning average properties to the carbon numbers. We have recently observed that certain light oils and condensates with an API gravity of 45 may have a cloudpoint temperature as high as 333 K. Such a high cloudpoint temperature might be attributed to a very high concentration of paraffins and naphthenes. Accordingly, a predictive model should consider the PNA analysis to account for physical properties of various hydrocarbon species.
Most of the literature data on wax precipitation are measured at stock tank oil (dead oil) conditions. Since the concentration of light components at high pressures has a significant effect on cloudpoint temperature, a proper predictive model and experimental data for reservoir fluids will be very useful in investigating the wax formation problems at well tubing, production facility, and pipeline conditions.
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