Tracer testing of geothermal reservoirs
A new model is presented to explain the results of tracer tests of reservoirs. The flow field model is two-dimensional and can include multiple wells, background flow and a high permeability fracture of finite length or a barrier of finite length - likely features of a reservoir in fractured rock. The flow field is represented by a complex potential. This enables the accurate calculation of the time for tracer to travel along the streamline linking the release position and the observation well.
The shape of the concentration curve measured at the observation well is governed by dispersion. Two types of dispersion are considered – small scale dispersion and large scale dispersion. Small scale dispersion is due to interaction with the rock matrix and to small scale variations in velocity (not included in the flow field model), in the neighbourhood of a streamline. Large scale dispersion is due to the spread of tracer over streamlines which have different travel times to the observation well and may go to different observation wells.
In the literature, small scale dispersion models are used to match the concentration curves for individual wells. Here it is shown that models with differing small scale mechanisms give equally good matches to Wairakei data and that response curves for small scale dispersion and large scale dispersion are similar. This means that the shape of a concentration response curve gives little information about reservoir structure.
The important results from a tracer test are whether tracer is detected at each observation well and the travel times to the wells that respond, in conjunction with the locations of the release and production wells. This new approach to analysing tracer tests is used together with the flow field model to deduce the permeability structure and flow field of a simple hypothetical reservoir and of the Wairakei (New Zealand) geothermal reservoir.