Abstract
This article explains about adsorbed gas, which is a unique state of natural gas stored in unconventional natural gas reservoirs, and also shows possibilities surveying it using geophysical techniques of exploration. The main state of coalbed methane in coal seams is the adsorbed gas and nearly a half of shale gas is stored as the same way. Our measurements of the amount of adsorbed gas in rock samples in the laboratory indicate that a siliceous shale rock of Onnagawa formation has the same performance to adsorb gas as shale rocks in the North America and a mud rock of the natural gas field in the South Kanto region of Japan has also enough performance to adsorb gas. The total amount of adsorbed gas is huge in the world and it must be a natural gas resource with high potential. As development of the adsorbed gas requires continuous production for longer periods than free gas, the technique to estimate the variation of gas production rate becomes important. Evaluation techniques of gas reserves and productivity using geophysical exploration methods can increase the amount of natural gas reserves. The distribution of adsorbed gas in the field must be acquired to develop it strategically. Any survey techniques have not been proposed to directly estimate the possible amount of adsorbing gas in reservoir. However, various ideas have been discussed to indirectly estimate the amount of adsorbed gas using logging tools.
