Asphaltene Instability Trends to Predict Asphaltene Precipitation Onset Pressure: Constrained for Light and Heavy Crude Oils

The study investigates asphaltene instability trends (ASISTs), originally developed by New Mexico Tech, for crude oils with different degrees API gravities and asphaltene contents. Different ratios of precipitants/solvents have been used to construct the ASISTs at ambient conditions. Results were extended to reservoir conditions using routine PVT data. The ASIST method was used to predict the asphaltene precipitation onset pressure at reservoir conditions. Due to the kinetic effect on asphaltene precipitation using the titration method, pressure depletion induced asphaltene precipitation experiments were carried out for the heavy oil sample. Results, using the modified Miller-Flory-Huggins model, were thermodynamically modeled to validate the ASIST observations and for finding an appropriate lag time that can correspond to reservoir conditions. Similar asphaltene precipitation and thermodynamic modeling were adapted from the literature for the light oil sample. Results suggest that, for the light oil sample, 5 hours is appropriate lag time to perform ASIST, which is in agreement with the reported trends; for the heavy oil sample, however, results suggest that even a 5 hours lag time is not short enough for the titration experiment so that it can be kinetically extended to asphaltene precipitation in the reservoir. We suggest a 1 hour lag time for the samples with low degrees API gravities and high asphaltene contents. More case studies from different crude oils from different geological settings are required to draw any rigid kinetic framework for the ASIST.