Thermodynamic Micellization Model for Asphaltene Precipitation From Reservoir Crudes at High Pressures and Temperatures

Abstract

Summary Asphaltene precipitation from reservoir crudes has been modeled using liquid-liquid equilibrium; the precipitated heavy phase is assumed to be in the liquid state and to consist of only asphaltene and resin. The light liquid phase is assumed to consist of the monomers of all the components and the micelles. A thermodynamic micellization model and the Peng-Robinson equation of state (EOS) description of all monomers fully describe the thermodynamic equilibrium state. From the direct minimization of the Gibbs free energy of the liquid-liquid system, the composition and the amount of each of the phases are calculated. The above model is used to calculate precipitation from three different reservoir crudes. Once vapor-liquid equilibrium parameters are obtained, there is no further adjustment of the parameters of the EOS for precipitation calculations. The predictions from the thermodynamic micellization model are in good agreement with the data. The effect of pressure, temperature, and composition on precipitation is properly predicted by the model. Introduction The modeling of asphaltene precipitation from crudes has been a challenge. Part of the challenge is due to the unique features of asphaltene precipitation in comparison to other types of precipitation such as wax precipitation. For example, an increase in pressure increases the wax appearance temperature in crudes (i.e., enhances wax precipitation), while pressure increase may inhibit asphaltene precipitation.¹ Wax precipitation is strongly dependent on temperature, asphaltene precipitation may not be affected by temperature, and temperature may weakly enhance asphaltene precipitation or it may inhibit precipitation.² The composition effect is also very different for wax and for asphaltene precipitation. At high temperatures, an increase in concentration of light hydrocarbons such as C₃ and C₄ and nonhydrocarbons such as CO₂ decreases the wax appearance temperature;¹ on the other hand, an increase in the amount of these components can significantly enhance asphaltene precipitation.² The precipitated wax phase does not contain asphaltenes and resins;¹ the amount of paraffins may be very small in the precipitated asphaltene phase. Another distinct feature of wax and asphaltene precipitation is the state of the precipitated phase. The precipitated wax phase is in the solid state while at high reservoir temperatures, as we will discuss later in this paper, the precipitated asphaltene phase may be in the liquid state. The objective of this paper is to propose a thermodynamic model that can predict all the features of asphaltene precipitation including pressure, temperature, and composition effects. Prior to the description of the thermodynamic model, we will first discuss the reversibility of asphaltene precipitation in light of new measurements on reversibility, and then review the literature on the state of the precipitated phase. Reversibility. Reversibility of asphaltene precipitation is often an issue. Hirschberg et al.³ observed reversibility of asphaltene precipitation with pressure at 367.15 K. The pressure reversibility addressed by Hirschberg et al.³ at high temperatures seems to be accepted by others. Reversibility with respect to composition at low temperatures is still unresolved. Rassamdana et al.⁴ performed experiments at room temperature to study the reversibility of asphaltene precipitation with respect to composition. Normal hexane was mixed and then stripped from the crude. They observed that part of the precipitated asphaltene redissolved into solution and concluded that the asphaltene precipitation process is partially reversible. Chung et al.⁵ studied reversibility using n -pentane. They found only 23% of the precipitate redissolved back in the crude. Recently, Ramos et al.⁶ experimentally verified that the asphaltene precipitation-dissolution process in liquid titration is reversible. Using a method similar to Ref. 4, they precipitated asphaltenes by n-heptane and n-decane, then reduced the precipitant/crude ratio by either evaporating the diluent or adding fresh oil. Full redissolution could not be observed at room temperature after continuous stirring for 24 hours. However, when ultrasound was used for the mixing, complete redissolution was observed immediately, demonstrating that the process is reversible. Ramos et al.⁶ also observed the redissolution of the precipitated asphaltene in a crude/n-heptane mixture by adding toluene. The dissolution of the precipitated asphaltene at room temperature is a kinetically slow process and, therefore, the reversibility may require a very long time. Cimino et al.⁷ and Hirschberg et al.³ also comment that the titration experiments are reversible. At high temperatures (reservoir temperatures), several experimental observations^1,8,9 have shown the reversibility of asphaltene precipitation from the change of pressure or composition. Using the reversible thermodynamics of the asphaltene precipitation, Hirschberg and Hermans¹⁰ introduced an aggregation model for asphaltenes and resins to describe the asphaltene precipitation in crude. The model can describe asphaltene precipitation at high dilution ratios when a crude is diluted with normal alkanes (nC₅,nC₇ , and nC₁₀) at 298.15 K. Based on the evidences above, we postulate that the asphaltene precipitation process can be thermodynamically reversible, especially at reservoir temperatures; the application of an equilibrium thermodynamic model, therefore, is justified. Next, we discuss the issue of the state of the precipitated phase. State of the...