The detection of groundwater flow by precise temperature measurements in boreholes

Abstract

The flow of water is a very effective means for the transfer of heat, and one method of detecting such flow is to make precise temperature measurements at closely spaced intervals in a borehole that intersects a flow zone. Water can flow through permeable formations; within a borehole it can flow between two aquifers or fracture systems; it can flow into a fracture system during the drilling of a borehole; and it can flow up or down narrow, dipping fracture zones. Each of these phenomena produces a characteristic thermal signature on a borehole temperature log that can be modelled mathematically. Analysis of such thermal anomalies permits, therefore, a quantitative estimate to be made of the amount and rate of fluid flow. In principle, very small flow rates can be detected from their thermal effects, but in practice other factors, such as thermal conductivity variations, can cause variations in thermal gradients that limit the detectability. Anomalies that persist over large depth ranges compared with the diameter of the borehole can generally be interpreted unambiguously. Examples of each type of flow are given.