Natural recovery from Fe-oxyhydroxide clogging of a geothermal well in Osaka, Japan

Causes of clogging and the following unique recovering process of an ATES (Aquifer Thermal Energy Storage) system in Maishima (Osaka, Japan) are documented based on the geochemical analyses of groundwater and stagnant water in the system. Fe-oxyhydroxides precipitation clogged screens in an ATES geothermal well during cooling operation. Chemical analyses of waters in the aquifers and plumbing pipes found out that oxidation of dissolved Fe occurred in association with intrusion of ambient air, which was leaked through inadvertently opened air bent valve, into the well. Dual heat-extraction system was installed in the two aquifers of the same wells separated by packers, and the closing occurred in the plumbing pipe installed in the shallow aquifer of one of the thermal wells. This aquifer could not be used as the ATES until when the Fe-oxyhydroxides were naturally dissolved in about a half year after the clogging. Then, the ATES system recovered to be useful. Increasing dissolved Fe with increasing NH₄⁺ and decreasing oxidation–reduction potential indicated that the Fe-oxyhydroxides were dissolved by microbially induced reduction reactions. This case suggests that some clogs can be mitigated without chemical and/or physical treatment, and that monitoring of groundwater chemistry is essential for diagnosing and treating clogs.