A generalised solution for step-drawdown tests including flow dimension and elasticity

Abstract

Step-drawdown and multi-rate tests present convenient tools for the estimation of the long-term yield of boreholes. However, th e analytical methods commonly employed for the analysis of such tests are all based on the assumption that the drawdown in a borehole is a linear function of the discharge rate. Numerous constant rate tests, of which a few are discussed in this paper, has shown that this is not necessarily the case with boreholes drilled in the Karoo formations of South Africa. The drawdowns in these bo reholes are not only influenced by the peculiar geometry of the aquifers, but also the non-linear deformation of the aquifers during th e pumping of a borehole. The two new non-linear models for the analysis of step-drawdown and multi-rate tests introduced here, tr ies to account for these factors; in particular the deformation of the aquifer, flow dimension and dewatering of discrete fractures . Although the model proposed for multi-rate tests is still based on constant time steps, the one for step-drawdown tests allows the user to use arbitrary time steps, when performing the test in the field. Non-linearities in drawdown curves should always be treated with caution, especially when used to assign sustainable yields fo r boreholes. However, the example of a step-drawdown test performed at the Campus Test Site of the University of the Free State, shows that non-linearities can be addressed with an appropriate model.