well design for optimal efficiency by understanding and ...well design for optimal efficiency by...
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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
Outline Current basis for Well Design Components of Well Losses
– Laminar Losses – Turbulent Losses
Step-Drawdown Tests Calculating Well Losses Optimizing Well Design
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
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
Well Losses
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
sw
Q
Ground Surface
Well Losses Ground Surface
Original Piezometric Surface
Water Level in Well
Confined Aquifer Flow
BQ
CQ2
sw = BQ + CQ2
Jacob (1946)
Q
sw
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
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
Step-Drawdown Tests
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
Laminar Well Losses
Occurs in: Conventional screened wells with filter pack (or naturally developed)
Calculated by: Aquifer loss/drawdown equation (Thiem, for distance-drawdown)
Laminar Well Losses
Source: Houben (2015)
1. Partial Penetration • Flow path convergence
(blue) • Non-uniform screen inflow
rate (red)
Laminar Well Losses
No Skin Factor
2. Wellbore skin (skin effect)
Laminar Well Losses
No Skin Factor
Positive Skin Factor
Positive skin • Formation damage, clogging • Increases drawdown
Laminar Well Losses
Source: Houben (2015)
Filter Pack
Aquifer Material
Positive Skin
Laminar Well Losses
No Skin Factor
Negative Skin Factor
Negative skin • Filter pack development • Reduces drawdown
Well Screen Losses
3. Slot Convergence (Laminar) • Flow path convergence at
the well screen slots
Turbulent Well Losses Occurs in: Conventional screened wells with filter pack (or naturally
developed) Calculated by: empirical equations
Turbulent Well Losses
1. Turbulent flow caused by the filter pack
Turbulent Well Losses
2. Turbulent flow caused by the screen slots
Turbulent Well Losses
3. Turbulent flow caused by • Upflow inside the well screen • 90-degree momentum change
Turbulent Well Losses
4. Turbulent flow caused by upflow inside the well casing
Turbulent Well Losses
5. Turbulent flow caused by flow around the pump motor • Not yet modelled
Turbulent Well Losses
6. Turbulent flow caused by swirling turbulence at the pump intake • Not yet modelled
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
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
Converting WellDesigner to GoldSim
GoldSim Dashboard
GoldSim Simulated Step Test Output Matrix
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
Sensitivity Analysis
Smaller diameter borehole
Larger diameter borehole
Longer Screen Shorter Screen
Less Expensive More Expensive
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
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
Thank you