An investigation into the distance dependence of storativity in dual porosity aquifers

Abstract

Parameterisation of hydraulic systems becomes increasingly difficult in fractured aquifers. Storativity is one of the main parameters used to quantify the available water in aquifers. This dimensionless value is known to be spatially variable in fractured aquifers and is especially difficult to determine with analytical methods. Dual porosity, a feature in some fractured aquifers, involves two distinct types of porosity influencing flow. It can be identified through diagnostic plots comparing drawdown and derivative curves. In this study, approximately 60% of the drawdown data from fractured aquifers exhibited dual-porosity characteristics. The data were analysed using various analytical methods and a two-dimensional numerical model, RPTsolv. The study aimed to evaluate the accuracy of analytical methods compared to RPTsolv and establish correlations between storativity and the distance from the pumping borehole, enhancing our understanding of storativity's spatial variability. This process involved steps like data acquisition, diagnostic analysis, analytical methods, and the use of RPTsolv. The study revealed limitations of analytical methods in calculating storativity. Comparative analyses between analytical methods and RPTsolv suggested that all analytical methods face challenges in consistently determining storativity values at varying distances from the pumping borehole. In the Karoo Supergroup, storativity values derived analytically tended to increase or decrease with distance from the borehole depending on the method used, whereas most analytical methods tested, produced a decreasing trend with distance in the Cape Supergroup. RPTsolv, in contrast, demonstrated consistent storativity determination regardless of distance, maintaining the parameter within the same order of magnitude. These findings led to the development of an effective radius correlation methodology. Applying this methodology to the Cooper-Jacob analytical method enables more consistent storativity values, even without RPTsolv. The dissertation concludes that relying solely on analytical methods for analysing pumping test data in fractured or dual porosity aquifers is unreliable due to their inconsistency. While the numerical model RPTsolv provides more consistent results, its outdated nature of the software is a drawback. In scenarios, where a numerical model isn't feasible due to resource constraints, the developed method is recommended to achieve acceptable ranges of storativity and transmissivity values.