The Well / Aquifer Model (Initial Test Results) - 4
The Well / Aquifer Model
Initial Test Results
Dennis E. Williams, Ph.D.
1.0 INTRODUCTION
1.1 General Background Leading to Development of the Well / Aquifer
Model
Today, more than ever, energy plays a vital role in the planning
and operation of any water resources development project. Providing safe,
reliable ground-water supplies at the lowest possible cost requires careful design
of all factors affecting the pumping and transportation water, from its
source in the aquifer to its final destination and use.
Minimizing energy consumption can result in huge operational
savings over the lifetime of any water resources project. One example of
energy waste is the head loss associated with the entrance of ground water into
the well. Minimizing this head loss requires careful consideration
of the relationships between aquifer and gravel pack materials, well screen
characteristics and location, and pumping rates.
Understanding the laws and principles governing these
interrelationships is the subject of extensive research currently being conducted
by Roscoe Moss Company of Los Angeles. The need for a study of this type and
magnitude has long been overdue in the ground-water profession. Too
often, critical factors affecting the design and construction of water
wells are being obtained from unverified general assumptions, outmoded
techniques, or application of methods and values which clearly do not apply.
Consequently, many wells continue to be improperly designed, with results
ranging from marginal to complete well failure.
The purpose of this investigation is to study interrelationships between
well screens, gravel packs, and aquifers, using both theoretical and experimental
techniques, and to deduce the basic laws governing these relationships.
The original objectives are summarized as follows:
- Determine the physical hydraulic relationships between screen entrance velocity,
sand transportation, and gravel pack design. In conjunction with this, test
the validity of the "opinion" by Bennison (page 26) that entrance velocities must
be between 0.1 and 0.25 ft/ sec.
- Determine the effect of gravel pack design criteria on stabilization of aquifer
materials and well development (e.g., void ratio, grain size and shape, uniformity
coefficient, and percentage passing for various screen openings).
- Verification of Peterson’s basic design criteria (CL/D>60 for minimum frictional
head losses (page 29) on a larger scale than his original test apparatus and with
consideration to aquifer materials. This will also determine the effects of
partial penetration and the resulting converging flow field.
- Determine velocity distribution along the well screen length and check the
validity of Peterson’s statement (page 29) that most flow takes place at the
discharging end of the screen through a length such that CL/D>6.
- Demonstration of the principle of increased drawdown (and higher pumping lifts
due to partial penetration effects even though CL/D>6.
- Determine the importance of screen open area and geometry in development of the
gravel pack and surrounding aquifer material.
- Investigate the effects of different types and density distribution of screen
openings on stabilization of gravel pack and well efficiency.
- Develop well loss criteria for a practical range of commercially available well
screens and compare well efficiencies of these screens for different gravel pack /
aquifer ratios.
- Investigate the structure of the gravel pack immediately adjacent to the well
screen for various types of screens(louver, wire wound, etc.), and determine
whether screen geometry affects entrance velocities and well efficiency.
- Investigate the build-up and methods for removal of incrustations with various
types of commercially available well screens.
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