Well Test Design Dr. John P. Spivey Phoenix Reservoir Engineering Copyright 2006-2008, Phoenix Reservoir Engineering. All rights reserved.
Objectives After completing this chapter, you will be able to: Design a pressure transient test Specify test objective Estimate expected flow rate Estimate expected pressure response Select appropriate pressure gauge Estimate permeability from single point flow test data using the one point method 2
Well Test Design General Considerations Type and Status of Well Reservoir Properties Safety and Environmental Concerns Test Design Procedure Pre-test Estimates of Reservoir Properties Permeability Skin Factor Flow Regime Prediction, Radius of Investigation Gauge Selection Test Design Simulation 3
Exploration vs. Development Exploration Well Focus on Entire Reservoir Objectives Fluid Sampling Initial Pressure Productivity Reservoir Limits Reserves Estimates Typical Test Long Test Time Expensive Results Influence Major Investment Decisions Development Well Focus on Individual Well Objectives Average Drainage Area Pressure Stimulation Effectiveness Wellbore Damage Permeability Typical Test Short Test Time Inexpensive Results Influence Minor Investment Decisions 4
Production vs. Injection Production Well Types Of Tests Drawdown Buildup Produced Fluid Oil Gas Water Disposal of Produced Fluid Injection Well Types of Tests Injection Falloff Injection Fluid Water Natural Gas CO 2 N 2 Source of Injected Fluid Sensitivity to Injected Fluid Water compatibility Clay sensitivity 5
Shallow vs. Deep Shallow Well Lower pressure Lower temperature Lower risk Smaller wellbore volume, WBS coefficient Offset by lower pressure => higher compressibility Deep Well Higher pressure Higher temperature Equipment rating Higher risk Lubricant failure Electronics failure Battery failure Stuck tools Larger wellbore volume, WBS coefficient Offset by higher pressure => lower compressibility 6
Unstimulated vs. Stimulated Unstimulated Well Higher permeability Shorter test times Stimulated Well Lower permeability Longer test times To reach given radius of investigation To reach radial or pseudoradial flow regime 7
High Permeability vs. Low Permeability High Permeability Well Shorter test times to: End of WBS Reach radius of investigation See reservoir boundaries Smaller pressure response High resolution gauge Low Permeability Well Longer test times to: End of WBS Reach radius of investigation See reservoir boundaries Larger pressure response Lower resolution gauge 8
H 2 S CO 2 Environmental Concerns Major Personnel Health Hazard Safety Equipment Training Metal Embrittlement Corrosion Disposal of Produced Fluids Flaring Reinjection Storage and Transportation 9
Test Design Procedure Define Test Objectives Estimate Reservoir Properties Estimate Test Duration Time to end of WBS Time to reach desired flow regimes Time to reach desired radius of investigation Estimate Test Rate Estimate Magnitude of Pressure Response Simulate Anticipated Pressure Response Select Gauges 10
Estimating Skin Factor Rules of Thumb Openhole Completion 0 Small to Medium Acid Treatment -1 to -2 Medium to Large Acid Treatment -2 to -3 Small Hydraulic Fracture Treatment -3 to -5 Large Hydraulic Fracture Treatment -4 to -6 Gravelpack +8 to +20 Fracpack 0 to +8 Local experience may suggest different rules of thumb 11
Estimating Permeability Pseudosteady-State Flow p p wf 141.2qBµ r = ln kh r e w 3 4 + s' k qbµ + ( ) re 3 ln s p p r 4 141.2 = ' h wf w 12
6/1/08 13 Well Test Design Estimating Permeability Productivity Index + = = ' 4 3 ln.2 141 s r r B kh p p q J w e wf µ + = ' 4 3 ln 141.2 s r r h JB k w e µ
Estimating Permeability Transient Flow p i p wf = 70.6qBµ kt ln kh 1,688φµ ct r 2 s 2 w ' Transient Drainage Radius r d kt 377 = φµ ct 1 2 k 141.2 = ' h i wf w qbµ + ( ) rd ln 0.75 s p p r 14
One Point Method For Estimating Permeability 1. Calculate the equivalent producing time from t = 24N q p or t = 24G q g p 2. Make an initial guess for k. Normally, 10 md for oil wells or 0.1 md for gas wells will be satisfactory. 15
One Point Method For Estimating Permeability 3. Calculate r d using r d kt 377 = φµ ct 1 2 4. Calculate new value of k from k qbµ + ( ) rd ln 0.75 s p p r 141.2 = ' h i wf w 5. Repeat Steps 3 and 4 until k converges. 16
One Point Method Example φ = 0.118 h = 6 ft r w = 0.365 ft p wf = 400 psia p i = 3,200 psia s' = -1.0 B g = 1.5 bbl/mscf µ g = 0.015 cp c t = 2.0 10 4 psi 1 q g = 110 Mscf/D G p = 110 Mscf 1. Calculate the equivalent producing time: t = 24G q g p = ( 24)( 110) ( 110) = 24 hr 2. Assume k = 0.1 md. 17
One Point Method Example, continued 3. Calculate r d : r d kt = 377φµ gct = = 134 ft 1 2 ( 0.1)( 24) 4 ( 377)( 0.118)( 0.015)( 2.0 10 ) 1 2 18
One Point Method Example, continued 4. Calculate k: k = 141.2q h B ( p p ) i g µ g r ln r g wf d w 0.75 + s' = = ( 141.2)( 110)( 1.5)( 0.015) ()( 6 3200 400) 0.0864 md ln 134 0.365 0.75 1.0 19
One Point Method Example, continued 5. Calculate new value of r d : r d kt = 377φµ gct = 1 2 ( 0.0864)( 24) 4 ( 377)( 0.118)( 0.015)( 2.0 10 ) 1 2 = 125 ft 20
One Point Method Example, continued 6. Calculate new value of k: k = 141.2q h B ( p p ) i g µ g r ln r g wf d w 0.75 + s' = = ( 141.2)( 110)( 1.5)( 0.015) ()( 6 3200 400) 0.0850 md ln 125 0.365 0.75 1.0 21
Exercise 11-1 Estimate Permeability Using the One-Point Method 22
Field Problem 11-1 Estimate Permeability Using the One-Point Method 23
Estimate Time to End of WBS ( 200,000 + 12,000s) Cµ t ewbs = kh or 200C t ewbs = JB 24
Time to Reach Radius of Investigatoin t = 948φµ ctri k 2 Evaluate near-wellbore region: Alternative: Confirm presence of boundary: Sample entire reservoir: r i > 200 ft r i > 5r s r i > 4L r i > r e 25
Flow Rate Considerations Oil Well Keep pressure above bubble point pressure to maintain single-phase flow Gas Condensate Well Keep pressure above dew point pressure to avoid liquid dropout Gas Well Minimum pressure will be determined by line pressure or separator pressure 26
Flow Rate Considerations Operators often want to flow exploration wells at maximum possible rate to attract investors Rate cannot be sustained Pressure may fall below bubble point pressure, causing multiphase flow Pressure transient testing purposes Constant rate preferred; Slowly changing rate OK, but keep good records! Do NOT change choke size in attempt to maintain a constant rate! If at all possible, conduct transient testing before flowing at maximum rate 27
Flow Rate Considerations Estimate Sustainable Flow Rate ( p p ) kh i wf q = kt 162.6Bµ log 3.23 + 0.869s 2 φµ ctrw Estimate Total Volume To Be Produced During Test Storage Vessels Required? Flaring Permits? 28
Estimate Magnitude of Pressure Response Infinite-acting radial flow m = 162.6qBµ kh Pseudosteady state flow m pss = ( 1 S ) q 24Nc W t 29
Exercise 11-2 Design Reservoir Limits Test 30
Pressure Gauge Terminology Maximum Range, Operating Range Pressure Temperature Accuracy Maximum total error from all sources Calibration, Hysteresis, Repeatability, Thermal Sensitivity Repeatability Max difference between two consecutive measurements at a given pressure across two full scale pressure cycles Resolution Minimum pressure change detectable above noise level From Veneruso et al: Pressure Gauge Specification Considerations in Practical Well Testing, SPE 22752 presented at the 1991 ATCE 31
Pressure Gauge Terminology Drift Change in measured pressure over time not caused by actual pressure change Hysteresis Maximum difference in pressure response at a given pressure between increasing and decreasing pressure excursions over full scale pressure range Stabilization Time Time required for gauge response to read within a specified accuracy following a step change in pressure or temperature From Veneruso et al: Pressure Gauge Specification Considerations in Practical Well Testing, SPE 22752 presented at the 1991 ATCE 32
Pressure Gauge Types Mechanical Gauges Strain Gauges Conventional Sapphire Capacitance Gauges Quartz Gauges Standard Compensated From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 33
Mechanical Gauge Pros rugged, reliable, simple, cheap Cons low resolution, poor accuracy and stability Maximum Range Pressure 20,000 psi Temperature 200 C (372 F) [300 F, 500 F, 700 F ] Time 3-480 hrs (300 F, 500 F), 3-24 hrs (700 F) Resolution 0.05% of full scale Accuracy 40 psi [0.2% FS] Drift, 1,000 psi step ~10 psi/week Stabilization Time 5,000 psi step 10 min 10 C (18 F) step 10 min From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 34
Conventional Strain Gauge Pros good resolution, fast response, rugged, small Cons medium stability, resolution, accuracy Maximum Range Pressure 20,000 psi Temperature 175 C (347 F) Resolution 0.2 psi Accuracy 15 psi Drift, 1,000 psi step < 1.5 psi/week Stabilization Time 5,000 psi step 30 sec 10 C (18 F) step 10 min From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 35
Sapphire Strain Gauge Pros accurate, low hysteresis, reliable, rugged Cons medium stability, temperature sensitive Maximum Range Pressure 17,000 psi [30,000 psi/2,000 bar] Temperature 175 C (347 F) Resolution 0.1 psi [0.01 psi] Accuracy 6 psi [1% FS] Drift, 1,000 psi step < 1.4 psi/week Stabilization Time 5,000 psi step 20 sec 10 C (18 F) step 10 min From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 36
Capacitance Gauge Pros high resolution, low power requirement Cons slow sampling, sensitive, hysteresis Maximum Range Pressure 15,000 psi Temperature 175 C (347 F) Resolution 0.01 psi Accuracy >12 psi Drift, 1,000 psi step ±1.4 psi/week Stabilization Time 5,000 psi step 8 min 10 C (18 F) step 40 min From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 37
Standard Quartz Gauge Pros high resolution, stability, accuracy Cons sensitive to temp, limited pres range Maximum Range Pressure 11,000 psi [30,000 psi or 2,000 bar] Temperature 175 C (347 F) [200 C or 392 F] Resolution 0.001 psi Accuracy ±[0.025% of reading + 0.5 psi] Drift, 1,000 psi step < 0.1 psi/week [0.02% FS/yr] Stabilization Time 5,000 psi step 6 min 10 C (18 F) step 25 min From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 38
Compensated Quartz Gauge Pros dynamic response, stability, range Cons more electronics Maximum Range Pressure 15,000 psi Temperature 175 C (347 F) Resolution 0.003 psi Accuracy ±[0.01% of reading + 1 psi] Drift, 1,000 psi step < 0.1 psi/week Stabilization Time 5,000 psi step Always within 1 psi 10 C (18 F) step 25 sec From Vella et al: The Nuts and Bolts of Well Testing, Oilfield Review (April 1992) 14-27 39
Number of Gauges Multiple gauges for redundancy Different gauge types with different operating characteristics, different failure modes Battery life High temperature reduces battery life, causes premature failure Surface readout Non-volatile memory 40
Pressure Measurement Options Memory gauges Surface readout Permanent gauges Surface pressure measurement Gas wells with moderate permeability No liquids in wellbore Electronic flow measurement (EFM), SCADA Acoustic fluid level measurement Allows testing wells on rod-pump Measurement of both pressure and afterflow rate 41
Chapter Summary Test Design Procedure Define Test Objectives Estimate Reservoir Properties Estimate Test Duration Estimate Test Rate Estimate Magnitude of Pressure Response Simulate Anticipated Pressure Response Select Gauges 42