An Integrated Partially Implicit Compositional Simulator for Naturally Fractured Reservoirs

Abstract This paper describes an efficient method for solving the equations corresponding to an equation-of-state compositional model. The method is developed for a partially implicit compositional simulator in which the thermodynamic equilibrium equations and flow difference equations are solved separately and sequentially. The reduced system of linear equations consists of three equations with three unknowns per grid. So the equation number to be solved simultaneously is much less than that of the fully implicit method. Only pressure, oil saturation and gas saturation are solved implicitly in this partially implicit method while the total mole fraction of components is computed explicitly. If the increment of pressure as well as water saturation, gas saturation or total mole fraction of components of some grids is greater than a small given value, a implicit method can be applied easily to one individual grid to modify the calculation results. An efficient integrated compositional simulator is obtained by combination of the simulator mentioned above with reservoir description model. The simulator can generate grid description input data and reservoir property input data conveniently according to the results of geological analysis, core analysis, well testing and log information. It can also help us view the simulation results and modify the geological model easily if necessary. The partially implicit compositional simulator has been tested with several practical gas condensate reservoirs. Its simulation results are reasonably similar to those computed by fully implicit approach.