Japan National Oil Corporation (JNOC) is conducting a survey on underground gas storage (UGS) using inert gas as cushion gas, which makes good use of assets of depleted gas or gas condensate reservoirs. Since the Japanese regulation for a heating value of city gas is extremely strict, a heating value and hence composition of gas withdrawn from those reservoirs must be carefully predicted to properly design UGS projects.

JNOC has developed an UGS simulator, which can rigorously evaluate a heating value of withdrawn gas taking into consideration all the major phenomena that possibly affect composition of withdrawn gas.

A conventional cubic equation of state (EOS) based 3-D compositional simulator and a PVT simulator were improved to construct the UGS simulator equipped with the functions for calculating physical phenomena: a) molecular diffusion and velocity dependent dispersion, b) three-phase flash including dissolution of gaseous components into water and vaporization of water, c) adsorption of gaseous components onto a rock surface and d) turbulence and Klinkenberg effects. In addition, new functions of MPFA (Multi-Point Flux Approximation) and TVD (Total Variation Diminishing) scheme with local grid refinement in a corner point geometry were incorporated into the simulator to rigorously calculate a fluid flux with permeability tensors and to reduce numerical dispersion, respectively. Furthermore, functions to calculate temperature distributions both in a reservoir and wellbore were added to examine the effect of cooling caused by gas injection and adiabatic expansion on fluid flow and gas composition.

Each function newly developed for the UGS simulator was then validated through test runs using laboratory test data. Field scale simulations for hypothetical reservoirs were also conducted to confirm the simulator's performance as well as to examine the effect of cushion gas volume, working gas volume, reservoir heterogeneity and in situ gas composition on withdrawn gas composition.

This paper describes the development and validation of the UGS simulator followed by the results of field scale simulation runs.

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