Impact of the injection of different gases under the influence of various aromatic solvents on asphaltene precipitation envelope

Abstract

In this work, an effective modeling strategy is applied to predict asphaltene precipitation during gas lift operation at the Zhanazhol oilfield of Kazakhstan. The asphaltene precipitation envelopes (APE) are developed using the popular Peng–Robinson equation of state. Firstly, the APE are generated for crude oil samples at different injected moles percentages using the oilfield gas composition provided. To minimize the asphaltene precipitation, the impact on APE is also determined by adding different aromatic solvents which include xylene, benzene and toluene, as inhibitors, in various proportions in field gas. Alternately, asphaltene precipitation modeling under the influence of other most frequently used gases such as N₂ and CO₂ is carried out. Sensitivity analysis was also performed to determine the effect of input variables on APE. The final results suggest that the injection of field gas and CO₂ gas poses a risk of asphaltene precipitation particularly at temperatures below 60 °C at all gas mole percentages. The asphaltene precipitation risk is also observed at elevated temperatures (above 60 °C) when the injected gas mole fraction is relatively lower. In both cases of CO₂ and field gas, a crossover point is observed at a temperature below which gases act as precipitants, while beyond crossover temperature gases behave as inhibitors. The outcomes of this study also revealed that the addition of aromatic solvents in field gas causes the APE window (distance between upper asphaltene onset pressure and lower asphaltene onset pressure) to reduce which indicates that the extent of asphaltene risk is decreased. Comparatively, toluene performed little better than other solvents. Finally, the results also suggest that N₂ gas is the best option for this case because it causes the shifting of APE over the operational conditions. This means that injection of N₂ gas will prevent asphaltene precipitation. The approach applied in this research work proves to be an effective, novel and reliable strategy toward the selection of a particular gas for enhancing oil production considering the operational condition limits, behavior of gas at different operational conditions and impact of gas composition changes or addition of solvents in gas on APE.