Compositional Modeling of Molecular Diffusion in Natural Gas Injection of Tight Gas and Gas-Condensate Fractured Reservoirs-Single Block Approach

Regarding molecular diffusion drive simultaneously with the pressure gradient drive in the porous media is usually referred to as multi-mechanistic flow in the reservoir. Molecular diffusion occurs in the reservoir simultaneously with the pressure gradient drive. Since the pressure gradient drive is usually the dominant drive mechanism, molecular diffusion is usually ignored in the simulation. However, in some cases, ignoring molecular diffusion effect in the simulation results in a prediction that is drastically different from the real reservoir behavior. Gas condensate fractured reservoirs are one of the situations in which molecular diffusion plays an important role in the production mechanism. This paper is the first to investigate the molecular diffusion in gas injection for a gas condensate fractured reservoir. Single block approach is used in this work. This single block is firstly saturated with methane followed by methane and ethane and lastly, methane and n-pentane. Pressure at the left side of this matrix block is kept constant by injection while production is done at the constant pressure at the right side. Fully implicit Newton-Raphson technique is applied to solve highly non-linear equations in this simulation model. In order to speed up the simulation technique, Broyden updating approach is used to update Jacobian matrix. At the end, it was concluded that in gas injection in naturally fractured gas condensate reservoirs, diffusion phenomenon is considerable in matrix blocks with the permeability of lower than 0.01 md in the case of single phase flow at the reservoir condition, while this permeability increases to 0.1 md when two phase flow occurs at the reservoir conditions.