Natural gas behavior in shallow geologic layers as natural analogues of discharge of CO2

Abstract

Abstract In order to obtain public agreement on carbon dioxide (CO2) geological sto rage, it is required to assess the impact of leaked CO2 to our environment and dispel the anxieties as well as to d emonstrate technically the long-term stability of the geological CO2 storage system. As an essential issue to the assessment of the impact w e should understand CO2 behavior in sh allow geologic layers. From this point of view the authors carried out an inv estigation on natural gas behavior in shallow geologic layers in the vicinity of a natural gas reservoir as one of natural analogues of the behavior of leaked CO2 arriving from the deep storage layer. In this report the authors introduce the scheme of the investigation and the currently obtained results of the natural analogue study. The scheme of the investigation aims proposing the procedure to predict the movement of CO2 in shallow geologic layers, that is to say, geological surveying, numerical modeling, computing and estimating the impact of the CO2 to our living environment. The surveyed area is located in the Mobara gas field near Tokyo, Japan. It is said that the natural gas is dissolved in the brine in the turbidite sandstone layers of Pliocene-Pleistocene at the depth more than 200 meters below sea level. The land area is utilized as rice field and we can observe the spotted natural gas gushing when the rice field is filled with water. In this area the authors performed the exploratory investigation, borehole logging, electrical prospecting, underground radio detection, isotope analysis as well as natural gas flux measurement. They also studied the gas movement in the shallow layers by means of numerical analysis. The results of these efforts are briefly described as follows: The natural gas is not biologically originated near the surface but comes from rather deeper zone. The i mpermeable clayey layer traps the natural gas in gaseous phase and the geological structure such as faults and formations of the sedimentary layers restricts the footprint of the discharge of natural gas at the surface.