A Well Design Revisited

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Well Design Revisited
A
Take a look once again at well design as it can be key to efficient wells.
s the costs of drilling a well go
up, the ground water industry
needs to provide the best value
to the well owner. And the best
value is rarely the cheapest solution, but
the one that has fewer problems over the
service life of the well and is generally
less expensive to own and operate.
With this in mind, water well professionals need to understand the important
elements of well design and be able to
convey these to the well owner so they
can make informed decisions about
what to pay for and what contractor to
select to drill the well.
Thomas M. Hanna, RPG, has
more than 25 years experience
as a hydrogeologist. He lives in
Evergreen, Colorado, and works
for Johnson Screens in the
areas of well design, construction, and development. He can be reached at
[email protected]
NGWA.org
By Thomas M. Hanna, RPG
Good well design may not be rocket
science but is based on science, sound
practices, and common sense. Well
design begins with determining the diameter, depth, and anticipated pumping
rate from the completed well.
However, more recently the profession has moved away from some of
the basic principles of well design and
development. Look at the recent history
of well design and most of the studies
on well efficiencies that were conducted
prior to the 1980s as the cost of pumping was a critical factor in design and
operation. This made the focus of most
studies and literature how to optimize
well design.
But as the 1980s brought attention to
the protection of water resources and
energy became cheap, the focus of the
ground water industry shifted to ground
water contamination. These trends are
evident in the volume of literature devoted to different subjects through time,
while less attention recently has been
given to well design and development.
This has resulted in an ambivalence to
the importance in design of efficient
wells, including well development
which to be effective requires the largest
possible open area in the well screen or
intake.
When designing a well, the diameter
of the well is determined by the diameter of the pump the casing will have to
accommodate. Table 1 presents casing
diameters recommended for various
pumping rates.
Depth and maximum production
rates will be determined by the hydrogeology at the well site. Some of this
information might be available from
existing well logs. Hydrogeologic data
for a well site can also be gathered from
drilling a pilot or test hole to collect the
information, but often the critical hydrogeologic data is collected during the
drilling of the production well.
DESIGN/continues on page 42
Water Well Journal July 2009 41/
Table 1. Well diameters recommended for various pumping rates
(after Sterrett 2007).
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Figure 1. Slot openings of 0.050 in.
(1.2 mm) is selected from this grainsize distribution curve for a naturally
developed well (after Sterrett 2007).
DESIGN/from page 41
If the aquifer materials are accurately
described, as is outlined in the NGWA
Press book Guide for Using the Hydrogeologic Classification System for Logging Water Well Boreholes, preliminary
estimates of production can be made.
An explanation of a simple way to
describe cuttings from a water well
borehole is provided by Hanna (2006)
and Sterrett (2007). Quite often the only
information available from a well site is
from the production borehole, so accurate descriptions and collection of representative samples is important for good
well design.
For many domestic wells, screen slot
sizes can be determined in the field
using small sieves or a field card that is
used in the Guide for Using the Hydrogeologic Classification System for Logging Water Well Boreholes. The field
42/ July 2009 Water Well Journal
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Figure 2. Selection of artificial filter
pack (after Sterrett 2007).
guide provides a qualitative analysis for
slot size and filter pack selection.
Selection of slot size for unconsolidated formations is important in the
construction of an efficient water well
that is a good value for the owner. The
industry standard is to have an entrance
velocity of 3 cm/sec (0.1 ft/sec) to maintain laminar flow. When wells are not
efficient, there is a loss of energy as
the water flow changes from laminar to
turbulent.
One of the more recent studies conducted (Wendling et al. 1997) looked at
the relationship to entrance velocities,
slot size, and development. The study
shows that with proper development the
entrance velocities could be maintained
below 0.1 ft/sec, and the corresponding
Reynolds numbers in the formation
(adjacent to the well screen) were close
to 5, indicating laminar flow in the formation and into the well. The Reynolds
number is directly proportional to
the entrance velocity, assuming the
hydraulic radius and the porosity are
unchanged.
Therefore, an entrance velocity of
30 cm/sec (1ft/sec) would increase the
Reynolds number to approximately 50,
making the flow turbulent near the
screen. With such values of the Reynolds
number, additional head loss would
occur and the well would be less efficient. Data collected during this research
confirmed that a maximum entrance velocity of 3 to 6 cm/sec (0.1 to 0.2 ft/sec)
is required to maintain laminar flow in a
well. This corresponds to the maximum
entrance velocities commonly used by
water well professionals.
Design of Wells Using Natural
Development Completion
In completion of wells using natural
development, the formation is used as a
filter pack. The finer fraction of material
during development is removed from
the aquifer and pulled into the well to
increase the hydraulic conductivity and
maintain laminar flow conditions adjacent to the well screen. This is accomplished by removing 50% to 60%
of the finer fraction of the formation
adjacent to the well screen during well
development.
For non-homogeneous sediments,
when the ground water is not particularly corrosive and sample reliability is
good, the typical approach is to select a
slot which passes 60% of the material
and retains 40%. More-corrosive water
or poor sample quality requires retaining 50% of the formation.
Figure 1 provides grain size plot of a
formation that will be completed using
natural development. For this aquifer a
slot size of 0.050 inch is selected, corresponding to 40% retained. Coarse sand
and gravel allow greater latitude in
selecting the slot openings. A slot-size
increase of a few thousandths of an inch
allows only a small amount of additional material to pass through the well
screen during development.
Larger slot size permits extending
the permeable zone around the screen,
which generally increases specific capacity and efficiency, thereby lowering
operating costs. A more detailed explanation of slot selection for natural
developed wells is provided by Sterrett
(2007).
DESIGN/continues on page 44
NGWA.org
Figure 3. Amount of sand collected in
a 5-gallon bucket that represents 10
ppm sand production.
DESIGN/from page 42
Artificial Filter Pack Completion
and Slot Size Selection
An artificial filter pack is used in
many wells. Filter packing can be advantageous in wells where sediments
are uniform, fine grained, highly laminated, and poorly cemented. These
aquifers do not create good natural filter
packs. The well is designed by first sizing a filter pack to match the formation,
then selecting the screen slot size to
match the filter pack.
44/ July 2009 Water Well Journal
After graphs of aquifer samples have
been prepared, the layer composed of
the finest material is used for pack design. Figure 2 shows the grading of two
samples; the finest material lies between
75 feet (22.9 m) and 90 feet (27.4 m).
A specific filter pack size is chosen
to retain most of the formation. A screen
slot size then is selected to retain 90%
or more of the filter pack (after development). Filter pack materials should be
well sorted to assure good porosity and
hydraulic conductivity. Most commercial filter packs have uniformity coefficients of approximately 2.5. The
uniformity coefficient is defined as the
40% retained size divided by the 90%
retained size. The lower the value, the
more uniform the grading of the sample
between these limits.
After plotting the sieve analysis data,
the 70% retained size of the sediment is
multiplied by a factor between 3 and 8
(typically 5). A smaller multiplier is
used if the formation is uniform and the
40% retained size is 0.010 inch (0.25
mm) or smaller. A larger multiplier is
used for semi-consolidated or unconsolidated aquifers when formation sedi-
ment is highly nonuniform and includes
silt or thin clay stringers to aid in complete development. As shown in Figure
2, a commercial filter pack is selected
that is approximately five times bigger
than the finest formation in the aquifer
to be screened.
As a final step, select a screen slot
size that will retain 90% or more of the
filter pack material. In the example
shown in Figure 2, the correct slot size
is 0.018 inch (0.46 mm).
Development
The most important part of well construction is well development, but this
seems to be the part that gets the least
amount of attention these days. Because
well development comes at the end of
the well construction process, it’s where
water well professionals start to realize
where they will be on their budget.
If you go over budget, it’s tempting
to scale down development costs. But if
you want to get top dollar for the job,
you need to convey to the well owner
the importance of well development. No
matter how good the well design or how
expensive the materials, the well will
NGWA.org
not be efficient and can have problems
with sand pumping, scaling, and biofouling if it is not properly developed.
It is important that the well owner
be informed of the importance of well
development so they understand it is
the most important part of the well construction process and is a necessary cost
to avoid unnecessary repairs, cleanings,
and redevelopment or pump replacements over the life of the well.
When developing a well, it is important to get energy into the aquifer and
back into the well so that the fines can
be removed and the filter pack can be
settled around the well screen. A well is
fully developed when it has reached its
maximum specific capacity for the intended operational pumping rate without
pumping sand. Typically, less than 5
ppm is an acceptable sand pumping rate.
However, a properly designed and constructed well should pump virtually
sand-free. As a field test to quickly
measure sand pumping, a 5-gallon
bucket can be used. If the amount of
sand collected in the bucket covers a
dime (Figure 3), that is approximately
10 ppm, and further development is
needed for sand-free pumping. Typi
NGWA.org
cally, a well should pump 1 ppm of sand
or less to prevent damage to pumps.
Summary
Good well construction cannot be
accomplished using an approach of one
size fits all. By properly designing, constructing, and developing a well, the
well owner will have a well that is a
good value to own. This is rarely the
cheapest well to drill. But with a good
understanding of the well completion
process, water well professionals can
explain to the well owner what all is
involved in completing a well they
would want to own. WWJ
References
Hanna, T.M., 2006. Guide for Using the
Hydrogeologic Classification System
for Logging Water Well Boreholes.
Westerville, Ohio: NGWA Press.
Sterrett, R.J. 2007. Groundwater and
Wells, 3rd edition. New Brighton,
Minnesota: Johnson Screens.
Wendling, G., R.P. Chapuis, and D.E.
Gill. 1997. Quantifying the effects of
well development in unconsolidated
materials. Ground Water 35, no. 3:
387-393.
Get the Logging Water Well
Boreholes Guide
The Guide for Using the Hydrogeologic
Classification System for Logging Water Well
Boreholes by Thomas M. Hanna, RPG is one
of the best-selling titles by NGWA Press. This
spiral-bound book offers
detailed direction on
how to utilize the
hydrogeologic classification system for
logging water well
boreholes. The system, which uses a
weather-resistant,
two-sided card,
provides structure
and uniformity to logging wells.
The front of the card provides essential gauges
for measuring size, color, and aquifer characteristics. The back contains an easy-to-follow
flowchart of how to use the classification system. Hanna created the system and his text is
easy to follow and accompanied by pictures.
The book will enable ground water professionals to provide better correlation and interpretation of borehole logs. The cost is $25 for
NGWA members and $30 for nonmembers.
Go to www.ngwa.org or call 800 551.7379
for more information.
Water Well Journal July 2009 45/
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