Measurement and Modeling of Asphaltene Precipitation (includes associated paper 23831 )

Abstract

Summary. Experimental asphaltene precipitation data on several live-oil/solvent mixtures at reservoir conditions were measured to study the effects of temperature, pressure, and composition on precipitate formation and the relationships between critical properties, PVT phase behavior, and precipitate formation. Data generated by the model can be used to identify operating conditions conducive to precipitate formation. Introduction Asphaltene precipitation in the reservoir has proved to be a difficult problem to define and study. Field conditions conducive to precipitation include normal depletion, acid stimulation, gas-lift operations, and miscible flooding. Asphaltene precipitation is generally believed to be an irreversible process, which is the main reason it can have a profound impact on production operations. Investigations into asphaltene precipitation have been impeded by a shortage of experimental data and information on precipitation mechanisms. Our investigation included performance of experimental tests to induce and measure precipitation, selection of a thermodynamic model to describe and predict precipitate formation, and performance testing of the model with a variety of reservoir fluids. This paper summarizes the experimental testing, the theory behind the model, and applications of the model to field operations. Relationships observed between the reservoir-fluid phase behavior and the performance of the model are also discussed. Theoretical Basis Several theories have been proposed to explain why asphaltenes precipitate. For our work, we defined asphaltenes as the portion of the crude oil that is insoluble in heptane yet soluble in benzene or toluene. We selected a model that describes the precipitation mechanism by polymer solution theory. The large asphaltene molecules are similar in structure and behavior to polymer molecules. The remaining components in the rude oil act as a solvent in which the asphaltenes are dissolved or suspended. Hirschberg et al. used this approach in studies conducted by Koninklijke Shell E and P. The overall model depends on two types of fluid equilibria: a vapor/liquid equilibriim (VLE) of the total reservoir fluid and liquid/liquid equilibrium(LLE) between he liquid oil and pseudoliquid asphaltene phases. The VLE of the reservoir fluid is modeled first to obtain the composition and fluid properties of the liquid phase. In the LLE segment of the model, the oil is considered to consist of two liquid phases-an oil-rich phase that acts as the solvent and an asphaltene-rich phase that behaves as the polymer. The term "asphaltenes" actually defines a solubility class of components within the crude oil. Precipitates from a crude oil are classified by the solvent used to induce precipitate formation. For this work, n-heptane was used as the precipitating solvent for asphaltene measurements performed on stock-tank oils.