The continuous increase of petroleum production under adverse subsea conditions and the preeminent need for adequate operational conditions and efficient use of additives to warrant flow assurance makes it interesting to set up experimental procedures to carry out n-alkane precipitation studies under high-pressure (p) and high-temperature (T) conditions. In this contribution, some preliminary experimental studies developed to characterize asphaltene precipitation in model systems consisting of asphaltene solutions in toluene or mixtures of hydrocarbons by the addition of propane, n-heptane, or other alkanes at various pressures and temperatures, using a commercial compact equipment, are reported. In general terms, it was established that these tests follow the same pattern described at ambient p and T conditions and the one single study reported in the literature for a stock tank oil sample at 3000 psi and room T. Four crude oils of different characteristics were tested, using diluted or undiluted samples, and it was possible to detect the asphaltene precipitation onset. However, these results cannot be used to infer the stability of the crude oils because results correlating onset and stability at high p and T are not yet available. The effect of pressure at high pressures was not entirely resolved because argon, used as an assumed inert gas to pressurize the system, dissolves in the hydrocarbons and displaces the precipitation onset toward lower values. The need to develop compact equipment to assess the effect of solvents, inhibitors, and other additives on the phase behavior of crude oil at high pressure and temperature and in the presence of CO2 and other gases, representing a valuable contribution to the petroleum industry in the area of flow assurance, still persists.

The conditions of petroleum extraction may allow asphaltenes to precipitate, causing deposition that clogs wells, pipes and equipment, consequently reducing productivity. In this work, the solubility parameters and precipitation onset of polar fractions of heavy crudes from Brazilian fields were estimated using a simplified system of n-heptane/toluene mixtures. Asphaltenes were extracted by two different methods with regard to pressure and temperature. The samples were physically and chemically characterized, and both density (1053-1159 kg m-3) and molecular weight (1176-5316 g mol-1) were estimated based on the density of diluted asphaltenes in toluene solutions. The solubility of those fractions was studied as well as their solubility parameter (ca. 19-23 MPa0.5) based on regular solution theory, Flory-Hüggins theory and empirical correlation. The influence of asphaltene concentration (between 0.5 and 5.0 g L-1) on the solubility parameter and precipitation onset was studied, and a strong linear correlation between them was not found.

Crude oil can be seen as consisting of asphaltenes dispersed in a mixture of solvents, called maltenes. Information on such solubility is very useful, but the literature does not contain a detailed evaluation of asphaltenes solubility as a function of the fraction studied. In this work, we assessed the asphaltenes solubility in maltenes using different asphaltenes fractions, C3I, C5I, C6I, and C7I, extracted from a crude oil using propane, n-pentane, n-hexane, and n-heptane as flocculants. The solubility was tested at different temperatures, homogenization procedures, and asphaltenes concentrations. The solvents used were as follows: maltenes fraction extracted from the same oil sample and a solvent mixture that satisfactorily reproduces the maltenes behavior of the same crude oil (n-decane:toluene:cyclohexane 15:68:17 with solubility parameter = 17.60 MPa1/2). Asphaltenes fraction C3I was completely soluble in the maltenes. The other fractions were partially soluble, under all the conditions evaluated, increasing in the following order: C7I < C6I < C5I. The partial solubility of all these fractions decreased with the increasing concentration of asphaltenes within the range evaluated (from 1 to 10 wt/v%) and increased with higher temperature, longer time, and greater stirring intensity during dissolution. The behavior was the same for the model system and maltenes extracted from the crude oil, corroborating the applicability of the selected model-solvent to evaluate asphaltenes solubility in maltenes.

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