Geothermal Well Seals Subjected to Cyclic Heating and Cooling

摘要

This paper presents the results of a study of the effects of cyclic heating and cooling on the thermal and hydraulic conductivity properties of a geothermal well seal. In the US, bentonite is widely used in the annulus between the fluid circulating pipes and the formation for geothermal well systems and is subjected to many cycles of heating and cooling throughout the design life. In particular, the seal is expected to maintain contact with the circulating pipes and the formation in order to effect efficient transfer of heat into and out of the earth. Secondly, since many of these wells are 100 m deep and more, the seal must maintain a low hydraulic conductivity in order to isolate various aquifers over the well depth from cross-contamination. In the study, after 23 cycles of heating and cooling, the thermal conductivity remained constant but was different for cycles of heating and cooling; 0.173 W/m.K (0.1 Btu/hr ft ℉) for cooling and 0.865 W/m.K (0.5 Btu/hr ft ℉) for heating. A replicate trial yielded similar results. After 12 cycles of hydraulic conductivity testing, there appears to be some degradation in the hydraulic conductivity and intrinsic permeability.