Overdamped Slug Tests in Aquifers: The Three Diagnostic Graphs for a User-Independent Interpretation

Abstract

An overdamped slug test may be analyzed using several methods, which are known to yield different results for the hydraulic conductivity. The methods belong to three groups: group 1 neglects the influence of the solid matrix strain, group 2 is for tests in aquitards with delayed strain caused by consolidation, and group 3 tries to take into account the elastic and instant solid matrix strain. This paper deals with slug tests in aquifers and, thus, considers the groups 1 and 3. In practice, users select a single theory, which avoids having divergent values. However, this means that something is wrong with the theories. This paper explains what is wrong, and why. First, an approach is proposed to detect inconsistencies. The test data are presented in two semi-log graphs and a third, derivative graph along with theoretical predictions: these are the three diagnostic graphs. The shapes of the semi-log plots are shown to be altered markedly by a small inaccuracy in assumed piezometric level. The derivative plot does not depend upon this assumed piezometric level, but can verify its correctness. Most often, the assumed piezometric level cannot be defined with enough accuracy to avoid systematic bias in the test data. If this is ignored, the results of the first two plots cannot fit a theory. The derivative plot has been used for thousands of slug tests in confined and unconfined aquifers. It appears that all test data follow only one theory, that of group 1. An examination of equations and assumptions concerning solid matrix deformation shows that the group-3 theory, unfortunately, mistreats the links between fluid mechanics and solid mechanics. Therefore, the proposed three-diagnostic-graphs approach unifies the two theories, helps us to find and to be aware of what is wrong in the group-3 theory, and, most important, it yields a user-independent final result.