INVERSE STEP-TESTING...INVERSE STEP-TESTING METHODOLOGY Test goal: Exhaust (upper) bedrock fractures...

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INVERSE STEP-TESTING Short-Term Testing to Predict Safe Yield in Non-Uniform Wells Bruce Fowler, C.G. Andrew Gobeil, C.G.

Transcript of INVERSE STEP-TESTING...INVERSE STEP-TESTING METHODOLOGY Test goal: Exhaust (upper) bedrock fractures...

Page 1: INVERSE STEP-TESTING...INVERSE STEP-TESTING METHODOLOGY Test goal: Exhaust (upper) bedrock fractures of stored water ! Test starts at highest possible rate – exceeds well capacity

INVERSE STEP-TESTING

Short-Term Testing to Predict Safe Yield in Non-Uniform Wells

Bruce Fowler, C.G. Andrew Gobeil, C.G.

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� Safe Yield: Dependable yield under constant long-term pumping conditions (Define only one safe yield value)

� Yield Collapse: Rapid and significant reduction in yield

SAFE YIELD AND YIELD COLLAPSE

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SCENARIO #1 BEDROCK YIELD COLLAPSE � Shallow bedrock well drilled to capture contaminant plume

at industrial site � Driller’s one-hour suction test predicts yield at 20 GPM � Standard step-test with predicted yield of 16 GPM � After six days of constant pumping – Safe Yield = 1.3 GPM

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SCENARIO #2 BEDROCK YIELD COLLAPSE � Manufacturer has three 12-in. bedrock wells drilled to

supply 300 GPM � Each well tested at 150 GPM for short term (hours) � Predicted yield 450 gpm. � After three months total wellfield Safe Yield = 45 GPM

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SCENARIO #3 SPRING (WELL)YIELD COLLAPSE � Shallow gravel well drilled to capture spring water � Standard step-test completed with predicted yield up to

125 GPM

� Six days of constant pumping – Safe Yield = <10 GPM

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IS THERE A MORE ACCURATE SHORT TERM TESTING METHODOLOGY TO PREDICT SAFE YIELD ?

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IMPACT OF YIELD COLLAPSE

� Time and money q Lost volume recovery q Reduction in well capture-zone area q Reduction in open-loop geo-thermal heat capacity

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TRADITIONAL FACTORS INVOLVED IN WELL YIELD REDUCTION

Regional Factors � Watershed (recharge) area � Negative boundary conditions � Recharge reductions � Local area pumping impacts Local wellbore factor � Yield collapse occurs in close proximity to well and over

relatively short pumping intervals

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PREDICTING WELL SAFE YIELD: CRITICAL FACTORS

1.  Hydrogeologic setting (regional geology and hydrogeology) 2.  Well bore structure (driller’s log and notes) 3.  Non-uniform yield conditions (well testing)

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STEP 2: WELL LOG

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STEP 2 (CONT’D): REMOTE SENSING

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STEP 3: SELECT APPROPRIATE WELL TESTING METHOD

Traditional method(s) 1.  Rig-side air-lift testing (short-term) 2.  Step-drawdown pumping test (requires setting pump) 3.  Constant discharge pumping test (requires setting pump) And now – wait for it!

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STEP 3: SELECT APPROPRIATE WELL TESTING METHOD Empirical Method: Inverse Step Testing

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WHY STANDARD RIG-SIDE AND STEP-TESTS ARE NOT ALWAYS RELIABLE PREDICTORS OF SAFE YIELD IN NON-UNIFORM WELLS

1.  Fracture yields vary significantly within well bore (non-uniform and anisotropic)

2.  Accordingly, equations governing step-test analysis do not apply: Sw= BQ +CQN (Jacob 1947)

3.  Local fracture storage can mask short-term pumping effects

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SHALLOW FRACTURE STORAGE CAN MASK SHORT-TERM PUMPING EFFECTS Back to Scenario #1

� Rig side yield=20gpm � Step test yield=16gpm � COT safe yield=1.3gpm

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PUMPING TEST VOLUME SUMMARY

0

1000

2000

3000

4000

5000

6000

Rig-Side Test 20 GPM

Step Test 4-16 GPM

CDT 8 GPM

GA

LLO

NS

PUM

PED

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TRADITIONAL STEP-TESTING METHODOLOGY Test goal: Assess yield in range of interest and well efficiency

� Test starts at lowest rate of yield � Test rates step up incrementally over three to four intervals � Step durations are equal � Step end-points determine well yield and efficiency

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INVERSE STEP-TESTING METHODOLOGY

Test goal: Exhaust (upper) bedrock fractures of stored water � Test starts at highest possible rate – exceeds well capacity � Test rates decrease incrementally after yield collapse occurs � Step durations are typically not equal � Steps can be iterative to identify discrete fracture yields � Step end-points have little mathematical significance � Step end-points determine safe yield

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EXAMPLES OF INVERSE STEP-TEST METHODOLOGY

� 50 GPM Inverse Step-Test predicts Safe Yield of 18 GPM

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RESULTING CONSTANT DISCHARGE TEST

� 6 Day constant discharge test confirms Safe-Yield of 18 gpm

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INVERSE STEP-TEST TO TARGET CRITICAL FRACTURE ELEVATION

� Estimated 30 gpm well reveals shallow factors affecting yield collapse

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BENEFITS OF INVERSE STEP-TESTING METHODOLOGY

� Eliminates storage variable in predicting constant discharge test and/or safe yield

� Allows for determination of critical fracture levels which provide set-points for pumping rate tipping points

� Reduces likelihood of negative test outcomes, extended tests, etc.

� Protects client budget and schedule associated with inaccurately predicted well yields

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LIMITATIONS OF INVERSE STEP-TESTING METHODOLOGY

� Pump capacity for given well bore � ~70 GPM for 4-in. wells � ~200 GPM for 6-in. wells � Some wells have large storage capacities

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BEST APPLICATIONS FOR INVERSE STEP-TESTING METHODOLOGY � Non-uniform wells, typically bedrock with relatively shallow

water-bearing fractures � Very low-yield wells with large drawdowns (deep fractures)

� Wells operating under long-term constant pumping conditions: q Water supply wells

q Geo-thermal q Groundwater recovery q  Irrigation

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IN SUMMARY

� Determine testing methods and identify factors regarding well setting (local and regional) that impact safe yield

� Inverse Step-Testing useful in non-uniform wells for: q Predicting safe yields, confirmed with constant discharge

testing

q Identifying discrete fractures to avoid well yield collapse � Not all non-uniform wells require Inverse Step-Testing to

predict Safe Yield