Method of Monitoring Underground Diffusion of Carbon Dioxide

An object is to provide a monitoring method for monitoring underground diffusion of carbon dioxide, which is used when injecting carbon dioxide into an underground coal seam to cause it to be adsorbed to the coal seam, and collecting hydrocarbon gases that have been displaced by the carbon dioxide and released from the coal seam, and with which the behavior of carbon dioxide injected into the ground can be continuously measured over a long period of time at a low cost, using a relatively simple device. A plurality of wells including an injection well 3 and a production well 4 extending to underground coal seams (main layer 1 and lower layer 2) are formed. When injecting carbon dioxide into the injection well under pressure, and collecting hydrocarbon gases displaced by the carbon dioxide stored in the coal of the coal seams from the production well 4, high-precision tiltmeters are placed on the bottoms of observation holes drilled in the ground at a plurality of points (A, B, C, D, E and F) between the injection well 3 and the production well 4, and by checking chronological changes in the inclination angles at these points with the tiltmeters, diffusion of underground carbon dioxide gas is monitored.