Natural Gas Dissolved In Brine - A Major Energy Resource Of Japan
Authors: Marsden Sullivan S. 
Published: December 2007

Abstract
One of the major hydrocarbon resources of Japan is a natural gas found dissolved in brine produced from relatively shallow aquifers. These reservoirs are sandstones and siltstones usually found at depths of 500 to 1000 m and in some cases up to a maximum of 2000 m. They are not like and should not be confused with the much deeper, abnormally geopressured reservoirs of the Gulf Coast area.
The gas consists mainly of CH4 with some CO2 even smaller amounts of N2 and essentially no H2S. It is used both for domestic purposes and for chemical manufacture. The brine in which it is dissolved contains a good deal of I which is recovered to provide the major source of I in the world.
The origin of, drilling for and production of this natural gas is described along with the reservoir engineering aspects of the fields. Because of the low solubility of gas in brine at the hydrostatic pressures of these reservoirs, large quantities of pressures of these reservoirs, large quantities of brine must be lifted and handled. In the past this has led to significant geologic subsidence in the two major areas of production. The design, operation and results of successful brine reinjection tests to arrest and prevent this subsidence are described.
Because this kind of natural gas undoubtedly exists elsewhere in the world, we should seek this kind of energy resource elsewhere, particularly in countries which lack conventional natural gas and crude oil.

Introduction
For many years now we have thought of natural gas as coming from two kinds of sources, namely associated or solution gas found with crude oil and free gas or sometimes condensate not associated with oil. With our growing need for hydrocarbon resources, however, we are now looking at other sources such as that dissolved in geopressured brine or in tight Devonian shales or in coal beds. But there is still another source of natural gas of major importance in Japan, minor importance in a few other places and unknown or ignored so far in most parts of the world. This is the natural gas dissolved in brine contained in relatively shallow and easily accessible reservoir rocks. It is not the same as gas found dissolved in brine in deep geopressured aquifers.
Because the geologic conditions under which this gas was formed and has accumulated are not unique to Japan, there is good reason to believe that it exists elsewhere. Either it has been passed up in our drilling operations when we have sought deeper, more productive reservoirs or else it has not been recognized productive reservoirs or else it has not been recognized as a valuable and economic hydrocarbon resource. It is the purpose of this review to acquaint the reader with the historical development and geology of the gas reservoirs, the drilling and production practices followed to produce them economically, the two distinct kinds of reservoirs and the order of magnitude of the Japanese reserves of this kind of gas. Hopefully this will be useful to geologists and engineers in other countries which are deficient in hydrocarbon resources and where such natural gas may exist.

HISTORICAL DEVELOPMENT AND RESERVES
Gas from shallow wells in the Niigata areas of northwestern Honshu, the largest island of Japan (Fig. 1), has been used for domestic purposes since the last century. But in 1926 a well was drilled to 324 m for water and instead it produced brine containing this kind of gas in significant quantities. Up until World War II additional, slightly deeper gas-bearing aquifers were found in this area but the economics did not favor development then. When the post-war fuel shortage came about, production was undertaken and increased to 160,000 m3/d (5.65 MMcf/d) in 1952. A gas-based chemical complex was constructed and production expanded rapidly to 700,000 m3/d (24.7 MMcf/d). By 1962 the productive capacity was estimated at between 645,000 productive capacity was estimated at between 645,000 and 900,000 m3/d (22.8 and 31.8 MMcf/d) but it was limited to 350,000 m3/d (12.4 MMcf/d) because of land subsidence.
In the Southern Kanto gas region of Chiba Prefecture, which underlies part of Tokyo and extends Prefecture, which underlies part of Tokyo and extends eastward to the Pacific coast, production started in 1931.