Water-dissolved gas is a natural gas resource dissolved in groundwater. The gas mainly consists of methane gas and has been produced in many parts of Japan since the 1930’s. Although it is considered as an unconventional resource on a global standard, about 20 % of gas production in Japan comes from the water-dissolved gas fields. The resource is considered important in Japan since the R/P ratio of the water-dissolved gas is larger than one hundred years. In addition, iodine is commonly produced with the groundwater as a byproduct in a water-dissolved gas fields, which makes Japan as one of the largest iodine producers in the world.
Although water-dissolved gas is an attractive resource, its development often causes significant land subsidence due to the need of pumping large amount of groundwater. In Nishikambara water-dissolved gas field in Niigata, Japan, the target field of this research, a maximum 200 mm/year of land subsidence was observed at the early stage of the development in the 1950’s. As a countermeasure, all produced water has been re-injected into each reservoir since the early 1970’s, which enabled the gas production without any land subsidence for many decades. In this research, numerical models of water-dissolved gas reservoirs and the method to calculate the magnitude of the land subsidence were developed to investigate the mechanisms of gas production and land subsidence performance for the further development of the field. The numerical reservoir simulation models were developed using a black oil simulator, ECLIPSE100, on the basis of the field data. The land subsidence performance was calculated using a linear equation based on Hooke’s law. The reservoir pressure calculated by the numerical reservoir simulation models were used to calculate the magnitude of the land subsidence. Through the history matching, good agreement between the observed and calculated land subsidence was obtained.