Detecting Vertical Movement of Fluid in Wells and Aquifers with High Resolution and Over Long Times Using Fiber Optics

There are few methods by which high-resolution, in-situ characterization of flow in aquifers is possible.  We present a new method which has the potential to quantify both lateral and vertical components of flow in wells and nearby aquifers with 2-meter resolution. The method is based on a novel heating system combined with temperature measurements using a fiber optic distributed temperature sensor (DTS) that provides temperatures each 0.25 m and the ability to detect relative differences of about 0.01 °C on a sub-hour time scale. The method has three phases. In phase one, the background temperature profiles are recorded from which the vertical profile of thermal diffusivity may be obtained. In the second phase the borehole is heated uniformly over the depths of interest.  Based on differential in cooling, this provides data to estimate lateral flux with depth. A key innovation is the third phase in which we apply energy to localized short sections (0.5m) of the cable, or heated “dots,” that are equally spaced (2m) along the well.  The vertical migration of these warmed zones relative to the heated “dotted line” provides data from which the vertical fluid flow can be estimated.  The concept is demonstrated in a 600 m borehole which had 300 heated patches.  The method allows for high-resolution detection over an entire well’s depth and can remain in place and operable for many years.  Numerical simulations are presented which further illustrate expected behaviors.