Characterization of Carbon-Dioxide-Induced Asphaltene Precipitation

Asphaltene precipitation continues to be one of the major problems during the carbon dioxide enhanced oil recovery process. 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. In this paper, we establish conditions at which solids are formed in carbon dioxide–oil systems and characterize these solids by a variety of analytical techniques. Crude oils containing low and medium amounts of heptane-insoluble asphaltenes were used in the study. A high-pressure thermodynamic system was designed and fabricated, and a number of thermodynamic experiments were performed with dead and live oils. Multiple-contact experiments were first performed by adding a “light–intermediate” cut to the crude oil and later by creating actual multiple contacts. Precipitation onset for dead oils was obtained between 20 and 30 mol % CO2. CO2-induced solids consisted of about 30 wt % pentane insolubles (asphaltenes) and 7 wt % resins. Concentrations of pentane insolubles (asphaltenes) in the liquid fractions obtained after thermodynamic experiments decreased with an increase in the CO2 concentration used in the experiment. Thermodynamic experiments with live oils showed that the precipitate amounts were 3–4 times higher compared to equivalent dead oil solid deposits. For concentrations higher than or equal to 28 mol % CO2, the precipitates had similar characteristics, with about 34 wt % pentane insolubles and 7 wt % resins. Live-oil experiments also showed that concentrations of pentane insolubles (asphaltenes) in the liquid fractions obtained after thermodynamic experiments decreased with an increase in CO2 concentrations. These values were even lower than the values for dead oils. Solids analyses revealed that the CO2-induced precipitates contained shorter alkylated chains than asphaltenes defined as pentane or heptane insolubles. The functional groups of compounds in the solids obtained using different paraffinic solvents and CO2-induced precipitation were similar. The field deposits in general were heavier in comparison to all of the laboratory-generated samples. Scanning electron microscopy (SEM) images quantified using energy-dispersive spectroscopy (EDS) showed a gradual increase in sulfur moieties as the molecular weight of the solvent increased with CO2-induced precipitates falling between C6 and C7. The field de