Precipitation and characterization of asphaltenes

Asphaltene precipitation continues to be one of the major problems in the production, transportation and processing of crude oils. Supercritical carbon dioxide is injected into oil reservoirs to enhance oil recovery in the tertiary phase of production. Development of multiple-contact miscibility is important in the success of a carbon dioxide flood. Precipitation of solids believed to be asphaltenes is a common problem during carbon dioxide flooding. Even when the original oil contains low amounts of asphaltenes, major problems due to solids precipitation are reported. Previous studies on carbon dioxide induced precipitation contained conflicting information.; The primary objectives of this research effort are to establish conditions at which solids are formed in carbon dioxide-oil systems, and to compare and characterize the solids so formed by a variety of analytical techniques. The role of the development of multiple-contact miscibility is also examined. A high-pressure thermodynamic system is designed and fabricated, and a number of thermodynamic experiments are performed with dead oils (oils with no initially dissolved gas) and with live oils (oils with a predetermined quantity of dissolved gas). Multiple contact experiments are first performed by adding a “light-intermediate” cut to the crude oil and later by creating actual multiple contacts.; Thermodynamic experiments with Rangely oil are performed at different conditions with CO₂. About 20–25% CO₂ molar concentration is necessary to induce measurable precipitation in crude oils containing originally low quantities of asphaltenes. Live oils yielded significantly higher amounts of precipitates than equivalent dead oils. A hypothesis that multiple contact mixtures yielded significantly higher amounts of precipitates is conclusively proven based on results from simulated multiple contact (addition of an intermediate cut) and true multiple contact experiments. Two types of thermodynamic models are tested on the data generated; the pure dense-phase model and the homogeneous molecular thermodynamic model. The latter provided a reasonable match with the experimental data for first contact and for the simulated multiple contact experiments.; The solids formed by CO₂ contact are compared to asphaltenes generated by conventional means (precipitation using the normal alkane solvents). The solids are analyzed by a, variety of methods; Saturates Aromatics Resisns and Asphaltenes (SARA) fractionation, Fourier Transform Infrared Spectroscopy (FTIR), chemical ionization mass spectrometry, time of flight mass spectrometry, etc. A consistent picture of the nature of solids emerged from these analyses. Measurements showed that CO ₂ induced precipitates contained shorter alkylated chains than asphaltenes defined as pentane or heptane insolubles. The maximum molecular weight on solid precipitates is about 800 amu.