An improved vapor-liquid-asphaltene three-phase equilibrium computation algorithm

Abstract

Asphaltene precipitation is one of the major flow-assurance problems in petroleum engineering. Asphaltene precipitation in the reservoirs can lead to formation damage, while asphaltene precipitation in the production tubing can reduce the production rate and even plug the production tubing. It may occur to the naturally produced oil experiencing pressure/temperature changes or during gas injection processes (e.g., CO2 flooding) for enhanced oil recovery. It is a prerequisite to accurately predicting under what conditions asphaltene precipitation will occur and how much precipitates if it occurs in order to better predict and control flow assurance problems due to asphaltene precipitation. In this work, a robust three-phase vapor-liquid-asphaltene precipitation algorithm is developed. To make the algorithm more robust and effective, the new algorithm is built based upon two previously proven works: the asphaltene precipitation model proposed by Kohse et al. (1993) which considers the effects of pressure, temperature and gas injection on asphaltene precipitation, and the initialization method and algorithm developed by Li and Li (2019) which does not consider the effect of temperature on asphaltene precipitation. The algorithm is validated using the asphaltene precipitation data measured by Jamaluddin et al. (2000 and 2002) and Gonzalez et al. (2004). As for the first two oil samples (Jamaluddin et al., 2000 and 2002), both the onset conditions and the amounts of asphaltene precipitation are computed with the consideration of varied pressure-temperature conditions and the injection of different gases. The pressure-temperature (PT) phase diagrams and the pressure-composition (Px) phase diagrams are drawn to illustrate the calculation results. Our algorithm is shown to be converged at every test point involved in the three-phase equilibrium calculations for constructing the PT and Px phase diagrams. After being calibrated using the measured asphaltene onset data for the second and third oil samples, the three-phase VLS equilibrium calculation algorithm is shown to be capable of making reliable predictions of the PT phase diagrams with gas injection and the precipitated asphaltene amounts versus injectant concentration.