Modern Techniques in Well Design   -   1

Modern Techniques in Well Design

Dennis E. Williams

A direct benefit of efficient wells is reduced pumping costs. Well efficiency is controlled by several factors, including aquifer, drilling damage, and turbulent-flow losses at or near the well screen. Screen entrance velocity is not a critical design factor if values are less than 2-4 fps (0.6-1.2 m/s). For most wells, the required open areas of a screen need only be 3-5 percent. A new method of designing minimum length of well screen based on laminar flow-turbulent flow considerations is presented, along with a simple field test to calculate the efficiency of an existing well or to provide a criterion for termination of development of a newly constructed well.

Efficient well construction depends on a comprehensive understanding of the factors that influence the hydraulics of groundwater flow to wells. Considerable field and experimental study has been devoted to the subject. Many investigators have proposed solutions for the flow of groundwater to wells. These solutions, for the most part, have been confirmed to be sufficiently accurate to allow engineers to properly design and manage groundwater resources. In some cases, however, design parameters not fully understood were oversimplified or grossly approximated-- resulting in inefficient wells.

This project simulated field conditions of groundwater flowing from aquifer sands through the filter zone and into the well screen. This was done with a large sand tank model capable of producing radial flow in a one-sixth section of a well and aquifer. Commercially available 10-in. (250 mm) well screens were used, and a system of reservoirs and pumps allowed flows up to 300 gpm (19 L/s). A constant-head reservoir connected to a chamber along the back of the model provided a line drive, allowing up to 60 ft (18 m) of available drawdown at the well (Figure 1).

Figure 1

Measurement of pressure head and flow throughout the model was achieved with an array of 47 small piezometer tubes. These piezometers were connected to a sensitive transducer,* and values were obtained with a microcomputer. Particles of sand in the well discharge ranging from 20 to 1000 mm in diameter were counted with a particle counter. †

Two aquifer materials were used in these experiments. Six commercially available screens were selected for the tests with open areas ranging from <1 to 36 percent. Well efficiencies were calculated for the six screens based on differences in water levels inside and outside the screen. Testing procedures involved varying the discharge rates and measuring water levels in the aquifer, filter zone, and well.

Entrance velocities and sand content were measured for each of the tests. A complete description is given elsewhere.¹ Much of the information presented in this article is based on results obtained from five years of testing and analysis using this model. Emphasis is placed on those aspects of well design that pertain to wasteful losses of energy and the methods used to control or minimize such losses.

* SCANIVALVE, San Diego, Calif.
† Model PC320, HIAC/ Royco Instrument Div., Menlo Park, Calif.

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