USE OF SLUG TEST THEORY TO ANALYZE TEST-WELL RECOVERY DATA

Identification of hydraulic properties of low-permeability materials using test-well flow and drawdown data, even when enhanced by use of down-hole flow meter measurements, is often complicated by the effects of wellbore storage and by those due to clogging at the well-aquifer interface (well skin). Additional information on hydraulic properties may be derived, under appropriate conditions, from slug-test analysis of recovery data following well development and/or testing. The suitability of such an analysis is dependent on whether drawdown behavior in the test well is still dominated by effects of wellbore storage at the time pumping ends, as indicated by the dimensionless parameter t_dp=Tt_pe^-2sk/r_c² being less than 0.8, where t_p is duration of pumping, T is transmissivity, sk is dimensionless well skin (commonly ranging from -5 or so to 20 or 30), and r_c is well casing radius. Synthetic water-level recovery data sets for several values of t_dp were generated using the computer program WTAQ, a code for computing aquifer response to well production. These data sets were analyzed as slug tests by using the method of Cooper, Bredehoeft, and Papadopulos. Results indicated that, for t_dp=0.8, the computed T was within 10%, and for t_dp=0.001, within 5% of the true value. Thus, slug-test analysis should be appropriate to evaluate water level recovery data, so long as t_dp (computed from the solved for T, the known r_c and t_p, and estimated sk) is sufficiently small. Application of the approach to analyze recovery data following intermittent development of a well tapping a deep coal aquifer at Wainwright, Alaska resulted in estimates of permeability, k, of 30 to 100 mD (milliDarcies). The large range resulted from uncertainty regarding the magnitude of sk. These data were useful in the design of a multi-well aquifer test at the site, the analysis of which resulted in the determination of k ranging from 70-100 mD. Thus, slug-test analysis provided useful hydraulic information on the coal bed that would not otherwise have been available.