The Science and Practice of Oscillatory Flow Testing

Abstract:

Hydraulic testing is a fundamental strategy for characterizing the properties of an aquifer (permeability and storage coefficients) that exert major control over flow and transport in the subsurface. In common practice, hydraulic testing takes place through either an instantaneous change in head – a.k.a. a slug test – or through a continuous injection or extraction of water – a.k.a. a constant rate pumping test. Both of these strategies are easily implemented and time-tested in the field. The meanings of the aquifer parameters obtained through such testing have been shown to be quite different, however. In the former case, slug testing is known to have a small “sensed” volume, and is thus very sensitive to near-well properties, which can present issues if wellbore skin is present. In the latter case, constant-rate pumping tests have been shown to produce parameter estimates that are more representative of the geometric mean over a large averaging volume.

Though less commonly practiced, oscillating flow tests represent another hydraulic testing strategy that provides information about aquifer flow properties. One key advantage of using this strategy is that the chosen frequency of the oscillating flow represents a way to control the volume sensed. Thus, the use of oscillating flow tests at multiple frequencies holds the promise of connecting disparate scales of “effective” aquifer hydraulic properties. Additionally, oscillating flow tests can often be represented using fast-running "steady periodic" numerical models (see abstract image). In this presentation, I review recent research on both the science and the practice of oscillatory flow testing. In particular, I discuss: 1) different implementation strategies for oscillatory testing in the field, along with their particular benefits and drawbacks for interpretation; and 2) the interpretation of these tests in difficult environments such as unconfined aquifers and fractured rock aquifers.