Prediction of asphaltene precipitation during CO2 injection

Aimed at the possible phenomenon of asphaltene precipitation during CO2 injection, the equation of state established by Anderko was introduced to describe the phase behavior of precipitated asphaltene because of its strong polarity. The fugacity of the precipitated asphaltene component was derived and a calculation model of gas-liquid-asphaltene equilibrium was constructed. After matching of PVT experimental data from a reservoir, the model was used to calculate the volume of precipitated asphaltene during CO2 injection. The precipitation model suggests that for a constant injection pressure, the asphaltene precipitation first increases and then decreases as CO2 increases with the asphaltene precipitation reaching a maximum when the gas phase occurs in the system. When the CO2 mole percentage of the CO2-oil system is constant, the precipitated asphaltene reaches a maximum at the saturation pressure.

摘 要注入CO2提高原油采收率过程中可能出现沥青质固相沉淀。鉴于沉淀沥青质的强极性，采用Anderko建立的缔合混合物状态方程描述沥青质的相行为，并由此推导沉淀沥青质组分的逸度计算公式，建立注气过程中气-液-沥青质三相相平衡数值计算模型。以某油田实际原油为例，利用模型计算了CO2注入过程中沥青质沉淀量，结果与实验数据相近，表明沥青质沉淀预测模型具有一定的准确性。在此基础上，预测了注气过程中沥青质沉淀变化规律：注入压力一定的情况下，沥青质沉淀量随着注入CO2量增加呈现先增加后减小的趋势，当CO2-原油体系中出现气相时，沥青质沉淀量达到最大；当CO2-原油体系中CO2物质的量分数一定时，在泡点压力附近沥青质沉淀量达到最大。