Verification of Asphaltene-Instability-Trend (ASIST) Predictions for Low-Molecular-Weight Alkanes

Anticipating when and where asphaltenes may flocculate during oil production is a key step in successfully preventing or mitigating asphaltene problems in the field. Because there will be no deposition without precipitation, mapping of asphaltene stability over a wide range of temperature, pressure, and composition is required. The ASIST allows the determination of the onset of asphaltene instability to be established with a series of liquid n-alkanes. These data are used to predict asphaltene stability of live fluids by extrapolating the onset condition from the base data to reservoir conditions by use of a linear extrapolation of the onset solubility parameter vs. square root of the partial molar volume of the precipitant. This extrapolation has been demonstrated previously to be accurate for methane and a model oil. The present work verifies that such an extrapolation is valid for predicting the asphaltene instability for mixtures of methane, ethane, and propane with a representative stock-tank oil (STO). The STO was combined with known amounts of methane, ethane, or propane. The asphaltene onset pressure was determined by a combination of near-infrared (NIR) light scattering and microscopic observation. The onset conditions at ambient pressures were examined for flocculation periods ranging from 20 minutes to 24 hours. Onset pressures calculated with the 5-hour ASIST trends compared well with measured onset pressures.