Abstract: During IOR by gas injection, asphtene precipitation will commonly block the effect of IOR. In order to make the factor be taken into full consideration and to prevent from being loss when designing production and gas injection schemes, it is necessary to study the method for prediction of asphtene precipitation as to minimize or avoid the harm by the precipitation. On
the bases of knowledge to asphtene precipitation, the paper presents technique for predicting asphtene precipitation through equation of state. Three phase flash equation and three phase flash algorithm which takes asphtene precipitation into account are used, which can not only simulate asphtene precipitation quantitatively, but calculate three phase equilibrium.

Key words: equation of state, asphtene precipitation, phase equilibrium, saturation pressure