A NUMERICAL ANALYSIS TO ILLUSTRATE THE USEFULNESS OF DRAWDOWN LOG-DERIVATIVE DIAGNOSTIC PLOTS IN CHARACTERIZING THE HETEROGENEITY IN NON-THEIS AQUIFERS

Diagnostic plots have been long used for reservoir characterization in the oil & gas industry, yet they remain scarcely used by hydrogeologists; hydraulic tests are still interpreted assuming Theis-like conditions, which are in many cases unrealistic and lead to qualitatively and quantitatively erroneous interpretations. For more realistic aquifers characterization, the authors suggest a consistent use of diagnostic plots interpreted with Barker’s Generalized Radial Flow (GRF) theory instead of Theis or Cooper-Jacob methods. This approach uses bilogarithmic plots of drawdown log-derivative signals ds/d(log t) vs. time t, providing a major gain in sensitivity and diagnostic potential in comparison with conventional semi-logarithmic s vs. t plots. Numerical modeling is conducted to illustrate the advantage of such approach in several heterogeneous flow configurations occurring into realistically complex aquifers that are intrinsically non Theis-like: aquifers with inclined substratum (thickness-variable), heterogeneous vertically and horizontally layered aquifers, granular continuous aquifers that are punctually connected to underlying fractured aquifers, and faulted aquifers with various fault dispositions. In all cases, numerical analysis reveal that log-derivative response allow to detect and localize the aquifers heterogeneities by visualizing flow regime changes, with varying flow dimension values. Several original diagnostic plots are proposed, associated to the numerically investigated conceptual models. For instance, the presence of a conductive fault directly or indirectly providing water-supply to the well can be confidently identified as it produces characteristic log-derivative signatures and flow dimension sequences. It is also demonstrated that homogeneous aquifers of variable thicknesses and aquifers punctually connected to an underlying fractured aquifer generate transient responses identical to a recharge barrier on conventional semi-log plots. Conventional methods hence lead to erroneous interpretations that may be avoided using the methods suggested in this study.