A numerical analysis to illustrate the usefulness of drawdown log-derivative diagnostic plots in characterizing the heterogeneity of non-Theis aquifer

Silvain Rafini

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Regional Aquifer Characterization II

ABSTRACT: Diagnostic plots for hydraulic testing and reservoir characterization have been used for several decades in the oil & gas industry. However, they are still not commonly used in the hydrogeologist community; most hydraulic tests are still interpreted assuming unrealistic Theis-like conditions. For the characterization of the properties of aquifers, the authors suggest a consistent use of diagnostic plots interpreted with Barker™s Generalized Radial Flow (GRF) theory instead of the Theis and/or Cooper-Jacob methods. This approach uses bilogarithmic plots of the drawdown log-derivative signals ds/d(log t) vs. time t, rather than conventional semi-logarithmic s vs. t plots, providing a major gain in sensitivity and diagnostic potential. Numerical modeling is conducted to illustrates the usefulness of such diagnostic plots compared with Theis approximation for: 1-Detecting heterogeneities and geometrical variations in granular aquifers when interpreting the transient phase of pumping tests; 2-Characterizing with greater accuracy the hydraulic properties of aquifers by proposing a formula to determine the apparent hydraulic conductivity associated with the flow dimension based on Barker™s GRF theory. Several idealized configurations are numerically investigated including thickness-variable homogeneous confined aquifers, heterogeneous vertically and horizontally layered confined aquifers, granular continuous confined aquifers that are intermittently connected to underlying fractured aquifers, and faulted confined aquifers. In all cases, the numerical results reveal that log-derivative diagnostic plots allow not only to detect and localize the heterogeneities of aquifers by visualizing flow dimension changes, but also to calculate the apparent hydraulic conductivity of aquifers using Barker™s theory. The presence of conductive faults that cross-connect with aquifers can be confidently identified, because it produces characteristic signatures on diagnostic plots, associated with specific values of the flow dimension. It is also demonstrated that homogeneous confined aquifers of variable thicknesses and aquifers intermittently connected to an underlying fractured aquifer generate transient responses identical to, respectively, the impermeable and recharge barriers on conventional semi-log plots. Conventional methods hence lead to erroneous interpretations that may be avoided using the methods suggested in this study.

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