Thermal Measurement, Analysis Interpretation of a Pilot Standing Column Well

摘要

Thermal testing is an objective design measure for geothermal wells. Thermal testing is performed by heating water pumped from a standing column well and returning it to the annular space of the well, while monitoring the rise in water T over the duration of the test. The average water T is plotted against the log of time to derive three key thermal characteristics of well bore formation; effective thermal conductivity, thermal diffusivity, and thermal decay time for thermal relaxation. The effective thermal conductivity is the most important thermal measure since it allows one to specify a well field design to meet a specified heating and cooling loading.In Spring of 2009 a 535' pilot standing column well (SCW) was tested to determine thermal characteristics of the well bore and its surrounding gneiss formation. While gneiss typically offers a thermal conductivity below 3.5 W/m°K (2 BTUH/ft°F), the "effective" thermal conductivity was shown to be considerably higher, viz. 5.35 W/m°K (3.1 BTUH/ft°F). At 0.18 m~2/day (2.1 sqf/day), the thermal diffusivity, a, was also twice as high as expected. The interpretation of these results and the impact on geothermal HVAC design is discussed herein. Keywords: geothermal well, standing column well, thermal well modeling & simulation