Controls of wellbore flow regimes on pump effluent composition

Area of Science:

• Environmental Science
• Hydrogeology
• Water Resource Management

Background:

• Well water and formation water compositional differences cause pump effluent variability.
• Understanding wellbore flow and mixing is crucial for accurate water sampling.
• Pumping dynamics significantly influence the composition of extracted water.

Purpose of the Study:

• To quantify the impact of purging and sampling methods on well water composition variability.
• To develop models simulating wellbore flow, mixing, and transport during pumping.
• To determine optimal purging volumes for representative water sampling.

Main Methods:

• Developed analytical models simulating Poiseuille flow for flow paths and travel times.
• Utilized finite element modeling to incorporate mixing dynamics.
• Simulated parabolic velocity distributions and annulus contraction during pumping.

Main Results:

• Pump effluent is initially predominantly pre-pumping well water, showing rapid compositional changes.
• Dispersive mixing creates a diffuse formation water annulus, increasing its contribution to effluent.
• After two screen volumes of pumping, effluent comprises 94% formation water.

Conclusions:

• Sampling during early pumping is unreliable when formation and well water compositions differ.
• Significant purging is necessary to obtain representative formation water samples in heterogeneous conditions.
• Passive sampling or early purging may suffice only when water compositions are similar.