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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.
� 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
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
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
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
IS THERE A MORE ACCURATE SHORT TERM TESTING METHODOLOGY TO PREDICT SAFE YIELD ?
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
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
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)
STEP 2: WELL LOG
STEP 2 (CONT’D): REMOTE SENSING
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!
STEP 3: SELECT APPROPRIATE WELL TESTING METHOD Empirical Method: Inverse Step Testing
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
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
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
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
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
EXAMPLES OF INVERSE STEP-TEST METHODOLOGY
� 50 GPM Inverse Step-Test predicts Safe Yield of 18 GPM
RESULTING CONSTANT DISCHARGE TEST
� 6 Day constant discharge test confirms Safe-Yield of 18 gpm
INVERSE STEP-TEST TO TARGET CRITICAL FRACTURE ELEVATION
� Estimated 30 gpm well reveals shallow factors affecting yield collapse
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
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
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
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