Effect of moisture migration on the thermal conductivity of a geothermal well seal

摘要

Bentonite is typically used as the seal between the fluid circulating pipes and the adjacentrnsoil/rock formation for geothermal well systems. Over the design life of a geothermal well, the seal is subjectedrnto many cycles of heating and cooling. After many cycles of heating and cooling throughout the design life ofrnthe geothermal system, the bentonite is expected to still function as a competent seal, by maintaining contactrnwith the fluid circulating pipes and the adjacent soil/rock formation surrounding the circulating pipes. Moisturernmigration can occur from the adjacent soil/rock formation to the seal or from the seal to the adjacent soil/rockrnformation. If moisture migration out of the bentonite seal is excessive, defects in the seal such as separation ofrnthe seal from either the soil formation or the circulating pipes can occur. Such defects could therefore potentiallyrnaffect the thermal conductivity. This research presents the results of a study of the effect of moisture migrationrnon thermal conductivity. A model well, seal and sand formation were created in the laboratory and the systemrnwas subjected to 30 cycles of heating and cooling. The average thermal conductivity for the heating cycles wasrn0.69W/k.m and that for the cooling cycles was 0.49 W/k.m. In previous tests using a closed system withoutrnallowing moisture migration, the average thermal conductivity for the heating cycles was at 0.79 W/k.m, andrnfor the cooling cycles was 0.19 W/k.m. The study results are encouraging in that after thirty cycles of heatingrnand cooling with the bentonite seal in an open system free to gain or lose moisture, no degradation in thermalrnconductivitywas observed. However, moisture content analysis of bentonite seal samples from the model showedrnmoisture migration had indeed occurred illustrating the need for a long-term field study of the performance ofrnbentonite geothermal well seals.