Estimation of groundwater velocity in localized fracture zones from well temperature profiles

In fractured rocks, well temperature logs often exhibit abrupt temperature changes over a small distance, typically surging or dropping several degrees over several tens of meters. The abrupt temperature changes usually occur in localized fracture zones or small faults. Away from these zones, temperature quickly restores itself to the background linear conductive profile. A theoretical model is presented herein to interpret the pattern of such abrupt temperature changes. I illustrate that this type of temperature profile originates from a different physical process than the physical processes found in some other typical temperature patterns in large scales. I also demonstrate that, using the field data, the theoretical model can be effectively used to estimate fluid flow velocity in fracture zones or local faults. Using the temperature profiles from two boreholes, fluid flow velocity in fracture zones is estimated to be as high as 1.1×10−6 m/s. Although fracture flow is a highly localized feature, it can reach three orders of magnitude higher than fluid flow in the hosting rock matrices.