Abstract

The Southern Kanto gas field, the largest field of natural gas dissolved in groundwater in Japan, is located in the Chiba Prefecture, eastern part of Japanese main island. In this gas field, 8 companies produce 460*106 m3/y of natural gas. In addition, the iodine is extracted from the brine. Iodine is industrially useful and essential for the human body. About 30% of world production is produced in this area in recent years.

On the other hand, land subsidence due to the gas-brine extraction from the strata has become a big issue to b solved since 1965. Natural gas and iodine producers in this area have made "The Land Subsidence Prevention Agreement" with the local government and have made an effort to prevent and minimize the land subsidence. Although the pumped up brine is inferred as the main cause of the subsidence from the early time, the geological mechanism of the subsidence, which includes the quantitative relationship between the volume change and extracted gas-brine's volume, has yet well known. As the measurement of the actual volume change of each stratum has become an important technological issue to reveal the mechanism, a new monitoring system has been developed jointly by them to identify the volume change of the target stratum.

1. Introduction

"The Southern Kanto gas field", the largest field of natural gas dissolved in groundwater in Japan, is located in Chiba Prefecture, eastern part of Japanese Main Island. In this gas field, eight companies produce 460*106m3/year of natural gas. In addition, the iodine extracted from the brine is industrially used, about 30% of world production is produced in this area in recent years.

On the other hand, land subsidence has become a big concern due to the gas-brine extraction from the strata since 1965. Natural gas and iodine producers in this area have made "The Land Subsidence Prevention Agreement" with the local government and have made an effort to prevent and minimize the land subsidence. These eight producers organized "Environment Committee of the Japan Natural Gas Association Keiyo Natural Gas Association (hereinafter the Environment Committee) " to address environmental concerns including land subsidence.

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