AMBIGUITY IN MEASURING MATRIX DIFFUSION WITH SINGLE-WELL INJECTION RECOVERY TRACER TESTS/

Single-well injection/recovery tracer tests are considered for use in characterizing and quantifying matrix diffusion in dual-porosity aquif ers. Numerical modeling indicates that neither regional drift in homog eneous aquifers, nor heterogeneity in aquifers having no regional drif t, nor hydrodynamic dispersion significantly affects these tests. Howe ver, when drift is coupled simultaneously with heterogeneity, they can have significant confounding effects on tracer return. This synergist ic effect of drift and heterogeneity may help explain irreversible flo w and inconsistent results sometimes encountered in previous single-we ll injection/recovery tracer tests. Numerical results indicate that in a hypothetical single-well injection/recovery tracer test designed to demonstrate and measure dual-porosity characteristics in a fractured dolomite, the simultaneous effects of drift and heterogeneity sometime s yields responses similar to those anticipated in a homogeneous dual- porosity formation. In these cases, tracer recovery could provide a fa lse indication of the occurrence of matrix diffusion. Shortening the s hut-in period between injection and recovery periods may make the test less sensitive to drift. Using multiple tracers having different diff usion characteristics, multiple tests having different pumping schedul es, and testing the formation at more than one location would decrease the ambiguity in the interpretation of test data.