New Insight Into The Step-Drawdown Test In Fractured-Rock Aquifers

Abstract

Review of the historical development of the step-drawdown pumping test and interpretations of well test data using the step-drawdown method indicates considerable differences in the importance attributed to head losses in the artificial environment (e.g., the gravel pack and screen) in the immediate vicinity of the wellbore and the losses resulting from non-laminar flow in the aquifer itself. These differences appear to be greatest when attempts have been made to apply the step-drawdown analysis to data from wells in fractured rock aquifers. The empirical flow-head loss relationship for a single, hydraulically-rough, open fracture have been incorporated into a mathematical expression for two-regime, radial, convergent flow. The resulting equation estimates the laminar and non-laminar head losses resulting from flow to a well over a range of flow rates. Combining these head loss components yields a non-linear relationship between production, Q, and drawdown, s, that is similar to those observed for actual step-drawdown tests in fractured rock aquifers. These test data are well approximated by:

s = BQ + CQⁿ

where both B and C are assumed to be constants and n is typically in the range of 2.0 to 3.0.

Q

et le rabattement s, qui est semblable à celle que l'on observe dans les essais réels de rabattement par étapes dans des aquifères en roches fracturées. Ces résultats d'essais sont bien approchés par la relation:

s = BQ + CQⁿ

oùB et C sont tous deux supposés constants et oùn prend une valeur typiquement comprise entre 2.0 et 3.0.