In a real black oil reservoir, especially in a heavy oil reservoir, the density, viscosity and other fluid properties of the oil vary both horizontally and vertically from point to point in the formation. For example, formation oil viscosity may vary several times and even more from the top of the formation to the water-oil contact. This phenomenon is due to the variation of crude oil composition. In a heavy oil reservoir, due to the gravity segregation and the difference of the oxidation of water to oil, the asphaltine and gel content of the oil varies with depth and the distance to the water-oil contact. The above mentioned phenomenon and the behaviour of that type of reservoir can not be realistically simulated by the conventional black oil model with multipool approach.

This paper suggests a black oil model with four components, which uses a two-pseudo-component-mixture concept to solve the above mentioned simulation problem. In this model, oil is considered as a mixture of two pseudo oil components with the different asphaltine and gel contents. In this case, the properties of the mixture will be dependent on, in addition to the pressure and temperature, the mass fraction of each pseudo component in the mixture.

The paper uses the 'flash' approach in paper(1) to solve this problem. In the model, the pressure and the four components (water, gas and two pseudo oil components) masses per unit formation volume are considered as basic unknowns to be solved. Though more complicated considerations are included, the model still uses the PVT data of conventional black oil, and the computer time would not be increased too much, as the Sequential Implicit method is applied for the numerical solution.

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