A generalized predictive model which is based on thermodynamic principle for solid-liquid phase equilibrium has been developed for organic solid precipitation. The model takes into account the effects of temperature, composition, and activity coefficient on the solubility of wax and asphaltenes in organic solutions. The solid-liquid equilibrium K-value is expressed as a function of the heat of melting, melting point temperature, solubility parameter, and the molar volume of each component in the solution. All these parameters have been correlated with molecular weight. Thus the model can be applied to crude oil systems. The model has been tested with experimental data for wax formation and asphaltene precipitation. The predicted wax appearance temperature is very close to the measured temperature. The model not only can match the measured asphaltene solubility data but also can be used to predict the solubility of asphaltene in organic solvents or crude oils. The model assumes that asphaltenes are dissolved in oil in a true liquid state, not in colloidal suspension, and precipitation-dissolution process is reversible by changing thermodynamic conditions. The model is thermodynamically consistent and has no ambiguous assumptions.
Organic deposition is a common problem in oil production and processing. The products of organic deposition are complicated and may contain asphaltenes, asphaltic resins, wax, and solid fines. Deposition may occur in reservoir formations, wellbores, production tubing, submersible pumps, surface equipment, and transportation pipelines. Organic deposition in tubing and production facilities will cause operational difficulties and necessitate costly treatments to remove the deposits. This problem is particularly serious in offshore oil production. In miscible gas flooding, asphaltene deposition often occurs in the wellbore region after gas breakthrough and causes pore plugging. The organic deposition problem has confronted the petroleum industry for many decades. It is important to producers that potential organic deposition problems can be predicted so that a production strategy can be designed to prevent, if possible, or mitigate this problem.
Organic deposition is caused mainly by the change of temperature and pressure conditions or oil composition. Electrical and other factors may promote peptization of the asphaltic material and induce asphaltene precipitation. The solubility of heavy hydrocarbons in crude oil depends very strongly on temperature and the lighter components in crude oil. When the temperature of the oil is decreased, these heavy components may precipitate in the form of solid wax crystals. Asphaltenes and other hydrocarbons may also co-precipitate with the wax crystals. Asphaltene solubility in oil is susceptible to the compositional change of the oil. Adding light alkanes (e.g., C, nC, nC, etc.) to crude oil can reduce the solubility of asphaltenes in the crude oil and cause asphaltene precipitation. Laboratory studies have indicated that carbon dioxide can also cause precipitation of asphaltene from crude oil. Organic deposition is a complicated problem, especially for asphaltenes. Because of the limitation of analytical instruments, the nature of asphaltene in crude oils is still not well understood. The light scattering technique is not applicable in the case of asphaltene solution because of the presence of an intense fluorescence signal and optical absorptions Other techniques such as small-angle X-ray scattering, small-angle neutron scattering, and gel permeation chromatography have been applied for asphaltene structure study. There is still no conclusive answer to the structure of asphaltene. Published data for the molecular weight of petroleum asphaltenes vary with analytical methods. The difficulty for asphaltene characterization has obstructed modeling work.