Quantitative Evaluation of the Effects of Hydrogeological Boundary and Initial Conditions on Slug Tests

Abstract

In analytical models for the slug test, it is assumed that the hydraulic head distribution within a formation is initially at equilibrium and that the hydrogeological boundaries can be defined deterministically. In practice, however, the hydraulic head distribution may be varied, e.g., by fluid injection during the drilling process before a test or by repeated testing, and it may not be possible to ascertain the hydrogeological boundary conditions. The discrepancies between model assumptions and actual conditions may affect the reliability of the data derived analytically from a slug test. The present study examines these problems using a numerical approach. The results showed that: (1) variations in the background hydraulic head caused by drilling or repeated testing may affect estimation of hydraulic properties, especially the value of specific storage; and that (2) discrepancies between the actual hydrogeological boundary conditions and analytical assumptions may cause significant errors in specific storage estimates, of up to two orders of magnitude if the specific storage of the formation is very small.