Interpreting Step-Drawdown Pumping Test Undergoing Confined-Unconfined Conversion with Well Loss

Area of Science:

• Hydrogeology
• Well Hydraulics
• Aquifer Testing

Background:

• Step-drawdown pumping tests commonly exhibit a transition from confined to unconfined conditions as pumping rates increase.
• Existing well hydraulics models often fail to accurately interpret this confined-unconfined conversion phenomenon.

Purpose of the Study:

• To develop an analytical solution for step-drawdown pumping tests that accurately accounts for the transition from confined to unconfined aquifer conditions.
• To validate the proposed solution through a field test and compare its performance against traditional models.

Main Methods:

• Developed a novel analytical solution based on Girinskii's potential and superposition theory.
• Conducted a field step-drawdown pumping test to gather data on confined-unconfined conversion.
• Utilized the particle swarm optimization algorithm for simultaneous parameter estimation.

Main Results:

• The proposed analytical solution demonstrated a superior fit to observed drawdown data compared to previous models.
• Accurate estimation of hydrogeological parameters (K, S), well loss coefficient (B), and critical conversion time (tc).
• Neglecting conversion leads to underestimation of drawdown and overestimation of aquifer thickness; the well loss coefficient decreased by 88% post-conversion.

Conclusions:

• The developed analytical solution effectively interprets confined-unconfined conversion in step-drawdown tests.
• Adjusting the well loss coefficient is crucial when modeling this conversion to avoid inaccuracies.
• The study provides a validated method for more precise parameter interpretation in pumping tests.