Prediction of Asphaltene Instability under Gas Injection with the PC-SAFT Equation of State

Abstract

Proper prediction of the potential for asphaltene precipitation in deepwater reservoirs represents a challenge for the flow assurance area due to both high intervention costs in case of asphaltene problems and extreme conditions of pressure, temperature, and composition. The probability of asphaltene precipitation increases when light-oil gases are considered for miscible gas injection processes. In this work, the applicability of simple and recombined live oil models using the PC-SAFT equation of state to predict the onset of asphaltene precipitation is demonstrated by studying pressure depletion and gas injection (carbon dioxide (CO2), nitrogen (N2), methane, and ethane) in oil reservoirs. The recombined model input includes the SARA characterization, the oil compositional analysis, and the gas−oil ratio. The pure component PC-SAFT parameters are fitted to the saturated liquid density and vapor pressure data, and the oil/gas pseudocomponent parameters are estimated from molecular-weight-based correlations. The EOS asphaltene parameters are fitted to precipitation data from oil titration with n-alkanes at ambient conditions. When asphaltene experimental data are not available, the asphaltene parameters are tuned to data at reservoir conditions, and then the model is run in a predictive mode. The PC-SAFT model was found to be in qualitative agreement with experimental data for the precipitant/crude oil systems in this study. In the prediction of the asphaltene onset using the model live oil, the results indicate that the ethane and CO2 mass fraction concentrations required to produce asphaltene precipitation is greater than for methane at the same pressure. Simulation results using the recombined model show that N2 addition produces a high asphaltene precipitation effect even at low concentrations. The simulated phase envelope for liquid−liquid asphaltene phase separation shows asphaltene stability bounded by an upper critical solution temperature and a lower critical solution temperature. The approach demonstrates that only molecular size and van der Waals interactions can explain laboratory and field observations of asphaltene phase behavior.