CHARACTERIZATION OF A HIGH-TRANSMISSIVITY ZONE BY WELL TEST ANALYSIS - STEADY-STATE CASE

A method is developed to analyze steady horizontal flow to a well pump ed from a confined aquifer composed of two homogeneous zones with cont rasting transmissivities. Zone 1 is laterally unbounded and encloses z one 2, which is elliptical in shape and is several orders of magnitude more transmissive than zone 1. The solution for head is obtained by t he boundary integral equation method. Nonlinear least squares regressi on is used to estimate the model parameters, which include the transmi ssivity of zone 1, and the location, size, and orientation of zone 2. The method is applied to a hypothetical aquifer where zone 2 is a long and narrow zone of vertical fractures. Synthetic data are generated f rom three different well patterns, representing different areal covera ge and proximity to the fracture zone. When zone 1 of the hypothetical aquifer is homogeneous, the method correctly estimates all model para meters. When zone 1 is a randomly heterogeneous transmissivity field, some parameter estimates, especially the length of zone 2, become high ly uncertain. To reduce uncertainty, the pumped well should be close t o the fracture zone, and surrounding observation wells should cover an area similar in dimension to the length of the fracture zone. Some pr ior knowledge of the fracture zone, such as that gained from a surface geophysical survey, would greatly aid in designing the well test.