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

~20 m depth> 1 m diameter< 500 m3/day

Hand dug well in Trets, France

Hand dug well in Beirut, Lebanon

AugersHand-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

Well in Unconsolidated Aquifers• Pump

chamber casing– Casing

within which pump is set

Well in Consolidated Aquifer• Cementing

– Prevent entrance of poor quality water

– Protect casing against corrosion

– Stabilize formation

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• 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 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 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

• Shallow Wells– Hand-operated– Turbine– Centrifugal (shallow, high

volume)• Deep Wells– turbine, submersible

turbine submersible

Motor

Motor

Well Diameter vs Pumping Rate(max 5 ft/sec in casing)

Well Casing Well Yield(in. ID) (gpm)

6 1008 175

10 30012 70014 100016 180020 300024 380030 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

Note UNITS!

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

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

• Slope = a1 = C• Intercept = a0 = B

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 0 500 10001500200025003000350040000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

f(x) = 1.59699319727891E-06 x + 0.00130680272108844

Well Discharge, Q (m3/day)

sw/Q

(day

/m2)

C = 1.6x10-6 day2/m5

= 3.32 min2/m5

Severe deterioration or clogging

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