This document discusses well development and efficiency. It covers well drilling methods like augers, cable tool, and rotary mud drilling. It also discusses well completion in unconsolidated and consolidated formations using well screens and gravel packs. The document outlines methods for well development including measuring well drawdown, losses, specific capacity using step drawdown tests, and determining well efficiency.

1. Well Development and
Efficiency
Groundwater Hydraulics
Daene C. McKinney

2. Introduction
• Well Drilling
– Augers
– Cable Tool
– Rotary
– Mud
• Well Completion
– Unconsolidated formations
– Consolidated Formations
– Well Screens
– Gravel Packs
• Well Development
– Well Drawdown
– Well Losses
– Specific Capacity
– Step Drawdown Test
– Well Efficiency

3. Domestic Hand Pumped Well
Domestic dug
well with rock
curb, concrete
seal, and hand
pump
~20 m depth
> 1 m diameter
< 500 m3/day
Hand dug well in Trets, France
Hand dug well in Beirut, Lebanon

4. Augers
Hand-driven augers
~15 m depth
> 20 cm diameter
Power-driven augers
~30 m depth
> 1 m diameter

5. Power Auger
• Auger drilling is done
with a helical screw
driven into the ground
with rotation; cuttings
are lifted up the
borehole by the screw
~ 30 m depth
< 15-90 cm diameter
< 500 m3/day

6. Drilled Well - Cable Tool
• Traditional way of
drilling large diameter
water supply wells.
• The Rig raises and
drops the drill string
with a heavy carbide
tipped drill bit that
chisels through the
rock and pulverizes the
materials.
• 8 – 60 cm
• 600 m

7. Mud/Air Rotary
• Rotary drilling relies on
continuous circular motion
of the bit to break rock at
the bottom of the hole.
• Cuttings are removed as
drilling fluids circulate
through the bit and up the
wellbore to the surface.

8. Drilling Mud Circulation
• Lift cuttings from the
borehole and carry to pit;
• Cuttings drop out in the pit;
• Length of drill pipe is added;
• Film on the borehole wall
prevents caving;
• Seals borehole wall to
reduce fluid loss;
• Cools and cleans bit; and
• Lubricates bit, bearings,
mud pump and drill pipe .

9. Well Completion
• After drilling, must
“complete” the well
– Placement of casing
– Placement of well screen
– Placement of gravel
packing
– Open hole

10. Well Construction
• Well casing
– Lining to maintain
open hole
– Seals out other
water (surface,
formations)
– Structural support
against cave-in

11. Well in Limestone
• Surface casing
– From ground
surface through
unconsolidated
upper material

12. Well in Unconsolidated Aquifers
• Pump
chamber
casing
– Casing
within which
pump is set

13. Well in Consolidated Aquifer
• Cementing
– Prevent entrance
of poor quality
water
– Protect casing
against corrosion
– Stabilize
formation

14. Placing the Pack

15. Well Design, Completion and Development
• Gravel Pack
– Installed between screen
and borehole wall
– Allows larger screen slot
sizes
– Reduces fine grained
sediment entering
• Development
– Washing fines out of the
aquifer near the well
– Cleaning the well with
water
– Air-lifting, surging,
pumping, or backwashing

16. Well Screens
• Head loss through perforated well section
– Percentage of open area (minimum 15%)
– Diameter depends on well yield and aquifer
thickness
– Entrance velocities must be limited
• Vs = entrance velocity
• Q = pumping rate
• c = clogging cefficient
• Ds = screen diameter
• Ls = screen length
• P = Percent open area

17. Well Screens
• May or may not be required
• Proper screen improves yield
• Slot size
– Related to grain-size
• Other considerations
– Mineral content of water,
presence of bacteria, and
strength requirements
– Excess convergence of flow
Groundwater and Wells, Driscoll, 1986

18. Well Development
• After completion, wells are
developed to increase specific
capacity and improve economic
life.
• Remove finer materials from the
formation.
• Pumping
• Surging
• Compressed air

19. Pumps
• Shallow Wells
– Hand-operated
– Turbine
– Centrifugal (shallow, high
volume)
• Deep Wells
– turbine, submersible
turbine submersible
Motor
Motor

20. Well Diameter vs Pumping Rate
(max 5 ft/sec in casing)
Well Casing Well Yield
(in. ID) (gpm)
6 100
8 175
10 300
12 700
14 1000
16 1800
20 3000
24 3800
30 6000
Groundwater and Wells, Driscoll, 1986

21. Drawdown in a Well
• Drawdown in a pumped
well consists of two
components:
• Aquifer losses
– Head losses that occur in
the aquifer where the
flow is laminar
– Tme-dependent
– Vary linearly with the
well discharge
• Well losses
– Aquifer damage during
drilling and completion
– Turbulent friction losses
adjacent to well, in the
well and pipe

22. Well Losses
• Excess drawdown due to well
design, well construction, or
the nature of the aquifer
sw =
Q
2pT
ln
r0
rw
æ
è
ç
ö
ø
÷+ CQn
= BQ+ CQn
B =
ln r0
rw
æ
è
ç ö
ø
÷
2pT
Note UNITS!

23. Specific Capacity
• Specific capacity = Q/sw
– Yield per unit of drawdown
– gpm/ft, or m3/hr/m
• Drawdown in the well
• Specific capacity - linear function
of Q
• Observing change in sw as Q is
increased – select optimum
pumping rate
sw
Q
= B+CQ
sw = BQ+CQ2

24. Step Drawdown Test
• To evaluate well losses
• Pump a well at a low rate
until drawdown stabilizes
• Increase pumping rate
• Pump until drawdown
stabilizes again
• Repeat at least three times

25. Step-Drawdown Test
Q (m3/day) S (m)
500 1
1000 2.6
2000 8.9
2500 14.0
2750 18.6

26. Step Drawdown Test
• Plot sw/Q vs Q
• Fit straight line
• Slope = a1 = C
• Intercept = a0 = B
sw
Q
= B+ CQ
y = a0 + a1x

27. Step-Drawdown Test (Example)
Q (m3/day) S (m)
500 1.14
1000 2.66
1500 5.57
2000 8.82
2500 13.54
3000 18.79
3500 23.67
y = 1.597E-06x + 1.307E-03
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0 1000 2000 3000 4000
s
w
/Q
(day/m
2
)
Well Discharge, Q (m3/day)
C = 1.6x10-6 day2/m5
= 3.32 min2/m5
Severe deterioration or clogging

28. Losses: Formation, Well, Total

29. Well Efficiency
• Specific capacity = Q/s
– Relationship between drawdown and discharge of a well
• Describes productivity of aquifer and well
• Specific capacity decreases with
– Time
– Increasing Q
• Well efficiency = ratio of aquifer loss to total loss

30. Summary
• Well Drilling
– Augers
– Cable Tool
– Rotary
– Mud
• Well Completion
– Unconsolidated formations
– Consolidated Formations
– Well Screens
– Gravel Packs
• Well Development
– Well Drawdown
– Well Losses
– Specific Capacity
– Step Drawdown Test
– Well Efficiency