In this communication we develop a methodology to simulate head loss and flow characteristics through water well screens, comparing different screen types and well construction parameters (e.g. well diameter, pumping rate, screen slot aperture, screen open area, hydraulic conductivity of the filter/gravel pack, etc.). We use COMSOL Multiphysics to perform coupled simulations of turbulent fluid flow (across the screen slots and within the well) and Darcy flow (in the porous media such as the filter/gravel pack and adjacent aquifer). Solving Randomly Average Navier-Stokes (RANS) equations for turbulent flow requires implementing the theory of Computational Fluid Dynamics (CFD) for an appropriate turbulence model.
Our objective is to set up a methodology to understand how the different elements of a pumping well interact and determine the overall head loss, among other flow characteristics, observed in real pumping wells. At this stage of development, we focus on the hydrodynamic processes taking place at the scale of the screen slot (sub-mm). We formally define the screen head loss as the difference between average hydraulic head of inner- and outer- screen surfaces. We compare the behavior of 4 different screen types available in the market: Louver screen, Wire Wrap screen, Bridge screen and vertical slotted screen. For solving the flow efficiently, we build a cylindrical sector from the well bore to the rock formation that includes a defined “unit cell” for each screen type having appropriate symmetry and/or periodic boundary conditions. The simulations indicate that the screen head loss depends on the screen geometry and slot opening but also depends on the hydraulic conductivity of the filter/gravel pack.
The traditional approach to quantify well head loss consists in fitting an equation of the form BQ+CQ^2 to step drawdown tests. Commonly applied methodologies associate the “screen head loss” with the term CQ^2 through the “orifice law” (as governing equation for flow through the screen slots) while the term BQ is considered a separate entity, subject to Darcy’s law and the hydraulic parameters of the filter/gravel pack and overall aquifer. Our simulations indicate that the screen head losses are intimately related to properties of the filter/gravel pack, and the “orifice law” underestimates the screen head loss more than one order of magnitude.
Acknowledgements: The authors thank Roscoe Moss Company for funding this R&D project and giving permission for publication.