Application and revision of the indirect method for determination of asphaltene precipitation onsets for light and medium oils in Argentina

Dilution onsets are an attractive and convenient way to study the tendency of asphaltenes to precipitate from a given crude, since they do not require high-pressure equipment and dead oil samples utilized are quite easy to get in comparison to bottom hole samples. In addition, the so-called indirect method developed and proposed in the last decade presents some advantages over previous direct methods, mainly that precipitation of smaller particles can be detected without complications.

This technique was implemented in this work as a first study of onsets for Argentinian crudes, considering two light and two medium viscous oils, with n-pentane and n-heptane as precipitating agents. Several aspects of the method implementation are discussed, and special attention is paid to the dilution of the supernatant after centrifugation. It was observed that dilution of the supernatant in toluene should be adjusted according to oil density. Onsets were detected not only for the two heavier fluids with important asphaltene contents, but also for one of the light oils studied. In agreement with previous publications, it is seen that a shorter and more volatile alkane, in this case n-pentane, anticipates the onset in comparison to n-heptane.

The use of the measured onset for tuning a compositional model for the reservoir fluid is illustrated for a light oil produced from the Vaca Muerta Formation. Nevertheless, it is shown that the type of compositional information used as input for characterizing the heavier fractions can have an important impact on the prediction of the asphaltene precipitation behavior at high pressures.