*TM 5-813-1/AFM-88-10, Vol. 1
second method of providing a highly porous material
around the screen. This involves placement of a
specially graded gravel in the annular space between the
where:
screen and the wall of the excavation. Development
HS = suction lift; vertical distance from
work is required if maximum capacity is to be attained.
the waterline at drawdown under
(3)
Development is necessary because
full capacity, to the pump center-
many drilling methods cause densification of the
line
formation around the hole. Methods utilizing drilling
HD = discharge head; vertical distance
fluids tend to form a mud cake. Good development will
from the pump centerline to the
eliminate this "skin effect" and loosen up the sand
pressure level of the discharge pipe
around a screen. Removal of fines leaves a zone of high
system
porosity and high permeability around the well. Water
can then move through this zone with negligible head
lines and fittings
loss.
2
V = velocity head; head necessary to
(4)
Methods
of
development
in
2g
maintain flow
unconsolidated formations include the following:
The brake horsepower of the motor used to drive the
(a) Mechanical surging is the
pump may be calculated from the following equation:
vigorous operation of a plunger up and down in the well,
like a piston in a cylinder. This causes rapid movement
of water which loosen the fines around the well and they
can be removed by pumping.
This may be
unsatisfactory where the aquifer contains clay streaks or
where:
balls. The plunger should only be operated when a free
P
= brake horsepower required
flow of water has been established so that the tool runs
H
= total dynamic head in feet
freely.
Q
= volume of water in gpm
(b) Air surging involves injecting air
e
= combined efficiency of pump and motor
into a well under high pressure. Air is pumped into a well
below the water level causing water to flow out. The flow
5-7. Development and disinfection
is continued until it is free of sand. The air flow is
After the structure of the well is installed, there remain
stopped and pressure in an air tank builds to 100 to 150
two very important operations to be performed before the
psi. Then the air is released into the well causing water
well can be put into service. Well development is the
to surge outward through the screen openings.
process of removing the finer material from the aquifer
(c) Overpumping is simply pumping
around the well screen, thereby cleaning out and opening
at a higher rate than design. This seldom brings best
up passages in the formation so that water can enter the
results when used alone. It may leave sand grains
well more freely. Disinfection is the process of cleaning
bridged in the formation and requires high capacity
and decontaminating the well of bacteria that may be
equipment.
present due to the drilling action.
(d) Backwashing involves reversal
a.
Development. Three beneficial aspects of
of flow. Water is pumped up in the well and then is
well development are to correct any damage or clogging
allowed to flow back into the aquifer. This usually does
of the water bearing formation which occurred as a side
not supply the vigorous action which can be obtained
effect of drilling, to increase the permeability of the
through mechanical surging.
formation in the vicinity of the well and to stabilize the
(e) High velocity jetting utilizes
formation around a screened well so that the well will
nozzles to direct a stream of high pressure water
yield sand-free water.
outward through the screen openings to rearrange the
(1)
A naturally developed well relies on
sand and gravel surrounding the screen. The jetting tool
the development process to generate a highly permeable
is slowly rotated and raised and lowered to get the action
zone around the well screen or open rock face. This
to all parts of the screen. This method works better on
process depends upon pulling out the finer materials
continuous slot well screens better than perforated types
from the formation, bringing them into the well, and
of screens.
pumping them out of the well. Development work should
(5)
Development in rock wells can be
continue until the movement of fine material from the
accomplished by one of the surging methods listed
aquifer ceases and the formation is stabilized.
above or by one of the following methods.
(2)
Artificial gravel packing provides a
5-19
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