aq

1. Well Design for Optimal Efficiency by
Understanding and Minimizing Well
Losses at Source
Tanna DeRuyter, EIT and Michael Kenrick, PE, LHG
GeoEngineers, Inc. Redmond WA

2. Outline
 Current basis for Well Design
 Components of Well Losses
– Laminar Losses
– Turbulent Losses
 Step-Drawdown Tests
 Calculating Well Losses
 Optimizing Well Design

3. Well Design and Limiting Well Losses
 Design paradigms (filter-packed wells):
– Filter pack: D30 4 to 6 times aquifer d30
– Uniformity Coefficient: 2.5 or less
– Slot Size: to retain 90% of filter pack
– Screen Length: 80 – 90% of Aquifer Thickness
– Open Area: > 5% … or more?
– Entrance Velocity: < 0.1 ft/s
– Upflow Velocity: < 5 ft/s
– Casing size > Pump size: by 2, 4, 6 inches

4. Best Drilling Practices
 Construction paradigms:
– Generally follow BDPs…
– Best Drilling Practices!!!
– Balance water head to avoid heave
– Flush out fluid / suspended sediment
– Avoid wellbore skin
– Develop,
– Develop,
– Develop

5. Well Losses
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
sw
Q
Ground Surface

6. Well Losses
Ground Surface
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
BQ
CQ2
sw = BQ + CQ2
Jacob (1946)
Q
sw

7. Well Losses
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
BQ
CQ2
BQ: Laminar flow
• head loss ∝ velocity
• Darcy’s Law
• Aquifer Loss
Q
Ground Surface
sw

8. Well Losses
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
BQ
CQ2
CQ2: Turbulent flow
• head loss ∝ velocity2
• Forchheimer
• Well Loss
Q
Ground Surface
sw

9. Step-Drawdown Tests

10. Step-Drawdown Tests
Step Test Analysis
Calculate sw/Q
Plot against Q
Intercept = B
Slope = C
Natapoc Example:
B = 0.0665 min/ft²
C = 0.000136 min²/ft5

11. Laminar Well Losses
 Occurs in: Conventional screened wells with filter pack (or naturally developed)
 Calculated by: Aquifer loss/drawdown equation (Thiem, for distance-drawdown)

12. Laminar Well Losses
Source: Houben (2015)
1. Partial Penetration
• Flow path convergence
(blue)
• Non-uniform screen inflow
rate (red)

13. Laminar Well Losses
No
Skin Factor
2. Wellbore skin (skin effect)

14. Laminar Well Losses
No
Skin Factor
Positive
Skin Factor
Positive skin
• Formation damage, clogging
• Increases drawdown

15. Laminar Well Losses
Source: Houben (2015)
Filter Pack
Aquifer Material
Positive Skin

16. Laminar Well Losses
No
Skin Factor
Negative
Skin Factor
Negative skin
• Filter pack development
• Reduces drawdown

17. Well Screen Losses
3. Slot Convergence (Laminar)
• Flow path convergence at
the well screen slots

18. Turbulent Well Losses
 Occurs in: Conventional screened wells with filter pack (or naturally
developed)
 Calculated by: empirical equations

19. Turbulent Well Losses
1. Turbulent flow caused by
the filter pack

20. Turbulent Well Losses
2. Turbulent flow caused by
the screen slots

21. Turbulent Well Losses
3. Turbulent flow caused by
• Upflow inside the well screen
• 90-degree momentum change

22. Turbulent Well Losses
4. Turbulent flow caused by
upflow inside the well casing

23. Turbulent Well Losses
5. Turbulent flow caused by
flow around the pump motor
• Not yet modelled

24. Turbulent Well Losses
6. Turbulent flow caused by swirling
turbulence at the pump intake
• Not yet modelled

25. Does the Well Diameter make a difference?
“doubling the diameter of a 6-inch well creates an approximate 10%
increase in yield (tripling size increases yield by about 17%)”
—Sterret (2007); Groundwater and Wells
• But this is based on the Theim equation
• And it completely ignores well losses
• Turbulent well losses are strongly related to well
and screen diameter

26. Quantifying Aquifer and Well Losses
Houben’s 2015 WellDesigner Spreadsheet Model:
 Flow system broken down into the above series of stages
 Equations provided for the identified well losses
 Losses are grouped as both laminar and turbulent
 Reynolds Number used to differentiate turbulent flow
 All losses account for predicted or observed well drawdown
 Approach verified by comparison with step-drawdown tests

27. Converting WellDesigner to GoldSim

28. GoldSim Dashboard

29. GoldSim Simulated Step Test
Output Matrix

30. Costing out Efficiency Benefits
 Fixed Variables
– Design Flow Rate
– Total Dynamic Head
– Aquifer Depth
– Pump Size
 Sensitivity analysis
– Screen open area / number of slots
– Length of screen
– Diameter of screen
– Diameter of casing
– Pump setting depth

31. Sensitivity Analysis
Smaller diameter
borehole
Larger diameter
borehole
Longer Screen Shorter Screen
Less Expensive More Expensive

32. Capital Cost versus Operating Cost
 Larger wells are:
– more efficient
– more expensive to construct
– less costly to operate
 Use WellDesigner in GoldSim to optimize design
 Include estimated uncertainty in key parameters:
– Aquifer Transmissivity
– Wellbore Skin
– Filter Pack Conductivity

33. Future Improvements
 Transient Flow
– Theis rather than Theim
– More complex aquifer hydraulics
 Unconfined Aquifers
– Reduced Aquifer Thickness
– Development of a Seepage Face
 Heterogeneous Aquifers
– Layered formations
– Vertical Anisotropy
 Wells with excessive well
losses: sw = BQ + CQP
 Fractured Rock Aquifers
– Non-laminar flow in fissures
– Turbulent entry losses
– Upflow in open wellbore
– (Atkinson, Gale & Dudgeon 1994)
 Well deterioration and
rehabilitation
– Fines migration
– Clogging of the filter pack
 Encrustation
 Precipitation
 Biofilm

34. Thank you