Abstract

A numerical two-phase modal is developed to simulate the production performance of the Mobara-type water-dissolved natural gas reservoir in Japan. By using this model, the behavior of producing gas-water ratio and reservoir pressure is matched successfully. This paper also presents some calculation examples by the matched model which describe the effect of permeability, relative permeability and initial gas saturation on the above production performance.

Introduction

Water-dissolved natural gas is the natural gas dissolved in subsurface brine. It is the main Japanese hydrocarbon resource. Because the primary composition is methane, containing almost no hydrogen sulfide, it is demanded considerably both as a town gas or domestic fuel and as feed-stock for chemical industry. About 80% of the water-dissolved gas production comes from the Southern Kanto region, just east of Tokyo, and Mobara gas field is one of the main fields in this region (see Fig.1). The characteristic of the Mobara gas field is that producing gas-water ratio increases rapidly with the drainage of water (see Fig.2). The mechanism was discussed qualitatively by Ueno, et al., Kanto Natural Gas Development Co. Ltd., and Marsden, et al. However this paper presents a two-phase simplified model to simulate the production performance numerically. History matching of the producing gas-water ratio and reservoir pressure by this model shows very good agreement with the actual data. On the other hand, it has been of great interest how the production performance is affected by the reservoir permeabilities, the relative permeability and initial gas saturation after the gas field is put into operation. This paper presents some calculation examples to show these effects by the history matched model.

MOBARA GAS FIELD

Kanto Natural Gas Development Co. Ltd. and Marsden, et al. discussed the technology of methane and iodine production in the Mobara gas field. Some pertinent reservoir description information from these papers is repeated for convenience. Reservoirs in the Mobara gas field are chiefly composed of unconsolidated fine sand within the alternate facies of the Kazusa group, which is composed of thick marine sediments of Pliocene age extending over the Southern Kanto district (see Fig.1). The Kazusa group dips gently northwestwards with monoclinical structure.

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