Summary

Slug tests in fissured low-permeability rocks using small-diameter tubing and straddle packers were analysed according to the conventional homogeneous porous medium analysis of Cooper et al. Because the results cast doubt on the applicability of this conventional slug test analysis, a new fissured porous medium analysis has been devised. Conventional analysis forms a special case of the new analysis. In practice, the new four-parameter (compared to the previous two) analysis is unwieldy and it is considered likely that conventional analysis will continue to be misapplied to fissured rocks.

The new analysis has been used to estimate the errors resulting from the misapplication of the conventional analysis. It is concluded that errors in the values of derived transmissivity are unlikely to exceed a factor of two. In contrast errors in the value of derived specific storage vary from underestimates of 1 × 10-6 to overestimates of 1 × 106. The new analysis has also been used to reinterpret the original data from tests on fissured low-permeability rocks and a more self-consistent set of results was obtained. A facet of tests in fissured rocks is the common occurrence of interpretations yielding exceptionally low values of specific storage. A table of values to construct type curves to match this sort of test involving very low values of α, a function of groundwater storage, is given as an Appendix.

It is concluded that in slug test analysis it is essential to know what values to expect, especially of specific storage, in order to make correct interpretations.

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