Well Development and
Efficiency
Groundwater Hydraulics
Daene C. McKinney
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
Domestic Hand Pumped Well
Domestic dug
well with rock
curb, concrete
seal, and hand
pump
Hand dug well in Beirut, Lebanon
~20 m depth
> 1 m diameter
< 500 m3/day
Hand dug well in Trets, France
Augers
Hand-driven augers
~15 m depth
> 20 cm diameter
Power-driven augers
~30 m depth
> 1 m diameter
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
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
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.
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 .
Well Completion
• After drilling, must
“complete” the well
– Placement of casing
– Placement of well screen
– Placement of gravel
packing
– Open hole
Well Construction
• Well casing
– Lining to maintain
open hole
– Seals out other
water (surface,
formations)
– Structural support
against cave-in
Well in Limestone
• Surface casing
– From ground
surface through
unconsolidated
upper material
Placing the Pack
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
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
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
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
Pumps
Motor
• Shallow Wells
– Hand-operated
– Turbine
– Centrifugal (shallow, high
volume)
• Deep Wells
Motor
– turbine, submersible
turbine
submersible
Well Diameter vs Pumping Rate
(max 5 ft/sec in casing)
Well Casing
(in. ID)
6
8
10
12
14
16
20
24
30
Well Yield
(gpm)
100
175
300
700
1000
1800
3000
3800
6000
Groundwater and Wells, Driscoll, 1986
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
Well Losses
• Excess drawdown due to well
design, well construction, or
the nature of the aquifer
Q æ r0 ö
sw =
lnç ÷ + CQ n
2pT è rw ø
= BQ + CQ n
æ
ö
lnç r0 ÷
è rw ø
B=
2 pT
Note UNITS!
Specific Capacity
• Specific capacity = Q/sw
– Yield per unit of drawdown
– gpm/ft, or m3/hr/m
• Drawdown in the well
sw = BQ + CQ 2
•
Specific capacity - linear function
of Q
sw
= B + CQ
Q
•
Observing change in sw as Q is
increased – select optimum
pumping rate
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
Step-Drawdown Test
Q (m3/day)
S (m)
500
1
1000
2.6
2000
8.9
2500
14.0
2750
18.6
Step Drawdown Test
• Plot sw/Q vs Q
• Fit straight line
sw
= B + CQ
Q
y = a0 + a1 x
• Slope = a1 = C
• Intercept = a0 = B
Step-Drawdown Test (Example)
0.008
Q (m3/day)
S (m)
0.007
500
1.14
1000
2.66
1500
5.57
2000
8.82
2500
13.54
0.002
3000
18.79
0.001
3500
23.67
0
C = 1.6x10-6 day2/m5
= 3.32 min2/m5
Severe deterioration or clogging
y = 1.597E-06x + 1.307E-03
sw/Q (day/m2)
0.006
0.005
0.004
0.003
0
1000
2000
3000
Well Discharge, Q (m3/day)
4000
Losses: Formation, Well, Total
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
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