This paper examines fluid flow mechanism in an infinite rectilinear reservoir with bottom water and/or gas cap. The water-oil contact (WOC) is treated as a no-flow moving boundary while the initial WOC is kept at a constant pressure. The potential distribution around the partially penetrating wellbore is a solution of Laplace or difiusivity equation with uniform potential inner boundary condition (1BC). A new dimensionless variable is introduced. Dimensionless Density, as a function of reservoir geometry, reservoir anisotropy, fluid properties, production rate and the difference of the densities of two fluids. This paper reports the existence of a Critical Dimensionless Density, CDD. above which cone is stable. The results of this study show that for large v alues of Dimensionless Density the cone height is very small and the well pressure response is almost identical to that of the reservoirs without bottom water or a gas cap. Therefore for reservoirs with a high Dimensionless Density a solution of no-flow boundary at the initial WOC level seems more accurate with respect to solution with constant potential boundary. CDD indicates that 1) in anisotropic reservoirs, smaller the vertical permeability the larger is the critical production rate or time of breakthrough. 2) Heavy oils, at equivalent conditions, have smaller critical production rate or time of the breakthrough due to higher viscosity. Thus thermal processes such as steam stimulation reduce the cone height and consequently increase the time of the breakthrough.

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