Modeling the effect of clay drapes on pumping test response in a cross-bedded aquifer using multiple-point geostatistics

This study investigates whether small-scale clay d rapes can cause an anisotropic groundwater pumping test response at a much larger scale. The measured drawdown values from a pumping test in the cross-bedded Brussels Sands aquifer (Belgium) reveal an elliptical-shaped pumping cone. The major axis of the pumping ellipse is parallel with the strike of small-scale clay drapes that are observed and measured in several outcrops. This stu dy investigates (1) whether this large-scale anisotropy can be the result of sm all-scale clay drapes and (2) whether application of multiple-point geostatistics can improve the analysis of pumping tests. This study uses the technique of "di rect multiple-point geostatistical simulation of edge properties" which enables simulating thin irregularly-shaped surfaces with a smaller CPU and RAM demand than the conventional multiple-point statistical methods. Th e proposed method uses model cell edge properties for indicating the presence of thin irregularly-shaped surfaces. Instead of pixel values, model cell edge properties such as edge transmissibility indicating the presence of irregularly-shaped surfa ces are simulated using the multiple-point geostatistical algorithm SNESIM. The modelling strategy of this study consists of the following steps. First, a tra ining image displaying clay drape occurence is constructed. Secondly, this small grid cell size training image is converted into an upscaled edge training image whic h is used as input training image to perform SNESIM simulations. The resulting simulations indicate at which cell edges horizontal or vertical clay drapes are present. This information is incorporated in a local 3D groundwater model of the pumping test site by locally adapting vertical leakance values and by locally in serting horizontal flow barriers. All hydraulic parameters including the clay drapes properties are calibrated using the measured drawdown time series in six observatio n wells. Results show that the anisotropic pumping cone can be attributed to the p resence of the clay drapes.

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