INTRODUCTION

In Japan, because there is not enough supply of oil, natural gas is regarded as a very precious energy. The natural gas reserves in Japan are estimated to amount to about 400 – 500 billion m3, regular development of natural gas began in the 1930's. At present, it is being tapped in different places, like Hokkaido, Chiba, Niigata, Miyazaki, Okinawa etc.. The extraction of water containing natural gas caused the surface subsidence, since the development of it began. Of recent, levelling surveys show a stop of surface subsidence, and a beginning of probable ground rebound, as a result of control of discharge rate.

The analytical examples in previous works based on the well-known Terzaghi's consolidation theory were almost all in the cases of relatively shallow or unconsolidated strata, such as soft clay. But, for relatively deep or compact strata, we must take account of the ground elastic property. In other words, we must consider two different types of subsidence, the one is plastic, which is not recoverable, the other, elastic, which is partially recoverable. This will lead to an understanding of ground rebound due to the control of discharge rate. Kamata et al.1 analysed the surface subsidence in Funabashi gas field with the vertical two dimensional multi-aquifer model. Their procedure is based on the finite differential method and Hook's law, with which only the subsidence profile of a typical section in the gas field can be obtained, except the plan contour map of subsidence.

The purpose of this report is to show the characteristic behavior of subsidence and to establish a prediction method for it in natural gas fields, especially for deep and relatively compact strata. Our prediction method is applied to a certain gas field, and calculated plan contour map of subsidence is compared with in situ measurements.

MECHANISM OF SURFACE SUBSIDENCE

The surface subsidence in natural gas fields depends on the decrease in piezometric head in the gas seam, due to the extraction of water containing natural gas.

Fig. 1 shows schematically the piezometric head around a well. When the water is pump

(Figure in full paper)

Fig. 1 Schematic diagram showing variation in piezometric head due to the extraction of water containing natural gasout from a well, the piezometric surface, depending on the permeability of gas seam and discharge rate, is lowered. Under three dimensional system the piezometric surface shows a funnel shape, whose outer limit defines the area of influence of well.

The relation between total pressure a and neutral pressure an on solid substance is shown by Terzaghi as;

where σ',: effective pressure Eq. (1) means that the decrease in piezometric head (neutral pressure) is equal to the increase in effective pressure, while the total pressure on solid substance of the ground remains constant. The surface subsidence is caused by this effective pressure. The amount of subsidence is proportional to the increase in effective pressure, or the decrease in piezometric head in the gas seam.

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