Abstract

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Japanese regulation for a heating value of city gas is extremely strict, a heating value and hence composition of gas withdrawn from those reservoirs must be carefully predicted to properly design UGS projects.

JNOC has developed an UGS simulator, which can rigorously evaluate a heating value of withdrawn gas taking into consideration all the major phenomena that possibly affect composition of withdrawn gas.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 simulatorand a PVT simulator were improved to construct the UGS simulator equipped withthe functions for calculating physical phenomena:

  • molecular diffusion andvelocity dependent dispersion,

  • three-phase flash including dissolution of gaseous components into water and vaporization of water,

  • adsorption of gaseous components onto a rock surface and

  • 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 acorner 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 effectof 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 wellas to examine the effect of cushion gas volume, working gas volume, reservoir heterogeneity and in situ gas composition on withdrawn gas composition.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.

Introduction

Since natural gas was first stored underground in 1915, an increasing number of fields have been utilized for UGS for the purpose of peak shaving and stableenergy supply. According to 1991/1992 statistics, there are 550 UGS fields inthe world, 425 of which utilize depleted gas or oil reservoirs [1].

A conventional black oil type simulator or modified black oil type simulator that enables distinction between injection and in situ gas phases has been most commonly used for evaluation/prediction of UGS reservoir performances [2, 3,4]. A black oil type simulator, however, is not satisfactory to rigorously calculate composition of withdrawn gas and hence a heating value of it. Major phenomena that possibly affect composition of withdrawn gas are:

  • *

    mixing between in situ fluids and injection gas,

  • *

    diffusion/dispersion of cushion gas when inert gas is used as cushiongas,

  • *

    dissolution of gaseous components into water phase and water vaporizationinto gas phase,

  • *

    adsorption/desorption of gaseous components onto a reservoir rocksurface,

  • *

    turbulent flow caused by high rate withdrawal/injection and Klinkenberg effect,

  • *

    hysteresis of gas-liquid capillary pressure and relative permeability, and

  • *

    reservoir temperature change that results from cooling by injecting vaporized LNG and adiabatic expansion of gas.

In addition, numerical errors such as numerical dispersion and discretization errors should be minimized for accurately reproducing these physical phenomena.

This content is only available via PDF.
You can access this article if you purchase or spend a download.