Inferring hydro-geothermal parameters from advectively perturbed thermal logs

We present techniques to infer thermohydraulic parameters from the analysis of underground temperature data from boreholes. Two different cases are analyzed: thermal data from artesian wells and from boreholes drilled in unconfined aquifers. The former allow the assessment of the formation temperature gradient and the flow volume; the latter give estimates of the Darcy velocity and, consequently, allow the inference of the aquifer hydraulic conductivity. The analysis procedure consists in matching thermal logs with analytical models incorporating both heat and mass transfer. The thermohydraulic parameters are obtained from the coefficients of the advective models by means of the least-square fitting method. Examples of applications are given for a set of temperature logs from a geothermal area of northwestern Italy. Preliminarily, thermal gradients and the correction for the recent climate change were evaluated from thermal data of boreholes with no evidence of advection. The analysis of artesian boreholes gave high formation thermal gradients, in agreement with those obtained from boreholes with dominant conductive thermal regime. In boreholes drilled in permeable formations, the models predicting the temperature versus depth distribution resulted generally less sensitive to geological noise and fitted the field data better than the models based on the gradient analysis. If the shape of the thermal profile is taken into account as a qualitative descriptor of the type of flow, reliable estimates of the flow components can be obtained.