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UNCONVENTIONAL GAS-WELL STIMULATION TECHNOLOGY
N. R. WarpinskiSandia National Labs
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Resource Characteristics– Low Permeability: Some Form Of Stimulation Required
For Economic Production Of Unconventional Resources• Fracturing• Acidizing• Cavity Completion• Explosives & Propellants• Injecting CO2 In Coal Seams• Other
– Complexity: Stimulation Effectiveness Dominated By Reservoir Characteristics
• Natural Fractures• Compartmentalization• Reservoir Anisotropy • Fluid Compatibility• Multiple Zones• Others
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Elements Of Formation Stimulation– Pre-Stimulation Evaluation
• Formation Evaluation (The Reservoir?)• Fracture/Acid Design Parameters (The Rock?)
– Well Construction & Completion– Stimulation Modeling & Design– Pre-Treatment Diagnostics– Treatment Materials
• Fluids, Proppants, Additives– Stimulation Operations– Stimulation Evaluation
• Production, Pressure Analysis, & Diagnostics– Production Operations– Refracturing
STIMULATING UNCONVENTIONAL GAS RESERVOIRS• Pre-Stimulation Evaluation
– Reservoir Assessment• Well Testing• Geophysics• Logs & Core• Reservoir Size (boundaries or infill distances)
– Rock Mechanics• In Situ Stress, Modulus• Fracture Azimuth, Height, & Complexity
– Stimulation Fluid Compatibility• Rock And Reservoir Fluids
– Target Intervals• Number, Separation, Characteristics
RESERVOIR EVALUATIONWhat Is The Reservoir?
– Size & Shape– Water Saturation– Porosity– Permeability– Vertical/Horizontal
Permeability– Anisotropy– Dual Porosity
• Fracture Sets, Spacing, Continuity
Evaluation Technology– Well Testing– Logs– Core– Geophysics (Seismic)
• Surface & Downhole– Outcrop & Surface Studies
Fractures
IsolatedZones
Anisotropy
PoorVerticalPerm
Tight Gas Sands
Amalgamated Lateral AccretionPoint Bars
Meander Belt System
RESERVOIR EVALUATION• Permeability
– Matrix• Extreme Water
Saturation Effects• Highly Stress
Sensitive• Clay Swelling &
Mobilization• High Capillary
Pressures– Fractures
• Stress Sensitive• Highly Conductive
During Injection– Inject Gels– Difficult To
Clean Up
0.001
0.01
0.1
1
10
100
0 1000 2000 3000 4000
Net Stress (psi)
Perm
eabi
lity
(Mic
roD
arcy
s)
Sw=0%Sw=15%Sw=30%Sw=40%Sw=50%
`
0.001
0.01
0.1
1
10
100
0 2000 4000 6000 8000
PORE PRESSURE (psi)
PER
MEA
BIL
ITY
(nor
mal
ized
)
Injection
ProductionInitial
Matrix
Fractures
Rock MechanicsImportant Parameters
– Stress Field• Fracture Height• Azimuth & Re-Orientation• Proppant
– Crushing & Embedment• Deviated Initiation
– Elastic Parameters• E, ν
– ToughnessMeasurement Technology
– Stress• Direct Measurement• From Sonic Velocity
– Elastic Calculation
– Elastic Parameters• Core Analysis• From Sonic Velocity
– Usually x2 Greater
7600
7700
7800
7900
8000
8100
6000 7000 8000 9000
Stress (psi)
Dep
th (f
t)
0 100 200
Gamma Ray
2,000 psiStress
Contrasts
0 100 200
Gamma Ray
4600
4700
4800
4900
5000
5100
3000 4000 5000
Stress (psi)
Dep
th (f
t)
500 psiStress
Contrasts
Rock Mechanics
• Dynamic vs Static Young’s Modulus– Dynamic > Static– Highly Variable
• Poisson’s Ratio– No Correlation
• Fracture Toughness– Lab Measurement– Scale Question
TARGET INTERVALS
• How Do You Stimulate– 20-40 Intervals Over
3000 ft– A Series Of Thin Coals
4500
5000
5500
6000
6500
7000
7500
0 100 200
Gamma Ray
Dept
h (ft
)
7000
7100
7200
7300
7400
7500
0 100 200
Gamma Ray
Dept
h (ft)
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Well Construction And Completion Strategy– Well Placement
• Infill Location & Spacing• Relationship To Fracture Azimuth• Will Depletion Change Factors
– Well Configuration• Vertical Well• Deviated, Horizontal Or Multi-Lateral
– Liner or Open Hole, Cemented– Perforation Placement Strategy
• Limited Entry Or Other Diversion Strategy• Full Zonal Coverage Or Point Perforating• Phase Angle Coverage
IntersectingNatural FracturesIs Critical
PERFORATION STRATEGY
• Single Point Perforating– Minimize Entrance Problems
• “Tortuosity”• Multiple Fractures
– Is Water An Issue?• Limited Entry
– Can This Really Be Designed• Is There Enough Stress Information• Are Moduli Known• Are Other Complexities Accounted
For• Zero Degree Phasing
– Can Zero Phasing Reduce Entrance Problems
• Extreme Overbalanced Perforating
How Does The Water Get Out If The Perforations Are Not At The Fracture Bottom?
Do Multiple Perforation Sites Induce Multiple Fractures?
PERFORATION INITIATION
MULTIPLE FRACTURES
AT PERFS
Several Distinct Fracture Planes
NEVADA TEST SITEMINEBACK
STIMULATING UNCONVENTIONAL GAS RESERVOIRS• Fracture Modeling & Design
– Type Of Fracture Or Acidization Model• Buttons & Knobs• Calibrated Model• Input Data Requirements Most Significant Problem
– Type Of Fracture• Gel Moderate to High Prop Load• Foam Low to Moderate Prop Load• WaterFrac Low Prop Load• CO2 Or Nitrogen Minimal Prop
– Design Requirements• Required Effective Length• Height Issues (e.g., Aquifers, Depleted Zones)• Conductivity• Clean-Up/De-Watering• Diversion (Limited Entry, Ball Sealers, Other)
FRACTURE MODELS• Design Mode
– Model Differ Greatly• Height Growth• Tip Effects• Coupling
• Pressure Matching– Similar Result
• Same Basic Input– Modulus– Viscosity
• Must Confine Fracture Similarly
– Stress– Tip Effects– Other
• Similar Efficiency
0
1000
2000
3000
4000
5000
0 1 2 3 4 5 6
Layers
Frac
ture
Win
g Le
ngth
(ft)
Model Comparison Study
Constant Height Models
• Multiple Fractures– Usually Observed In
Core-Through Tests– Largely Due To
Process Zone Effects• Similar Behavior In
Volcanic Dikes– Several Strands May
Have Proppant• These Have A
Significant Effect– Large Delta P– Higher Leakoff– Increased Storage
– How Do We Predict Or Account For These
Fractures4678
4675
MWX (7100 ft)
M-SITE (4500 ft)FRACTURE COMPLEXITY
FRACTURE HEIGHT
• Fracture Height– Stress Contrasts Are Major Controlling Factor– Diagnostic Imaging Often Indicates Less Height Growth
Than Stress Contrasts Would Predict– Are There Other Mechanisms Restricting Height Growth?
FRACTURE OFSTRENGTH MEMBER
ORTHOGONALSPLITTING
YIELDING OFSOFT MATRIX
RESULT: DISSIPATION OF ENERGY ANDMULTIPLESTRANDS
OFFSETSAT LAYERS
DISCONTINUOUSFRACTURES
Composite Height Growth Mechanism
FISSURE OPENING(ACCELERATED LEAKOFF)
• Natural Fractures Are A Key Element Of Most Unconventional Gas Reservoirs– Intersect – Produce Through– Avoid Damaging
• Fissure Opening Is A Major Issue– Inject Gel While Open– Clean Up While Closed– Can Leakoff Be Controlled
• 100 Mesh Sand• Is This A Key Element Of
WaterFracs?
Clean Up:Fissures Closed
Injection:Fissures Opened By High Pressure
Frac
ture
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Pre-Fracture Diagnostics– Breakdown Procedure– Diagnostic Tests
• Stress Tests• Pore Pressure Evaluation• Calibration Tests
– Step Rate– Pump-In/Flow Back– Step Down– Permeability Injection Test
• Minifrac
Closure StressBasis For All ModelingDifficult Meaurement
Leakoff (Efficiency)Length/Width/HeightProp PlacementScreenOut
Entrance Conditions
Why Bother With Calibration Tests
PORE PRESSURE CONSIDERATIONS
• Infill Drilling– Lenticular Sands
• Depleted Zones– Lower Stress– Readily Fracture
• Virgin Zones– Higher Stress– Less Fracturing
– Shales• Depleted Zones
– Reorientation Possible
5500
5600
5700
5800
5900
6000
0 100 200Gamma Ray
Dep
th (f
t)
Stress
Depeleted
Virgin
Virgin
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Treatment Materials– Fluids
• Type (Gel, Foam, Water, Gas, CO2)• Reactive, Energized, Stability• Crosslinkers, Stabilizers, Gel Breaker, Bactericides
– Proppant• Type (Sand, Ceramic, Bauxite)• Resin Coatings, Fibers• Conductivity, Crushing, Embedment
– Additives• Leakoff Control (Including Fissure Opening)• Clay Stabilization• Surfactants
STIMULATION MATERIALS
• Fluids – Only Modest Gains Are Likely From Improved Fluids– Low Residue Gels, Foams, CO2, N2, Water, Acid Systems– Breakers, Stabilizers, Inhibitors, Clay Stabilizers– There Is A Fluid System For Most Applications
• Proppants – Adequate Variety– Sand, Resin, Ceramics, Bauxite– Possible Improvements in Resins & Fibers
• However, Often A Mismatch In Fluid Systems & Desired Prop Load– Prop Transport
• Economics Is Probably The Key Driver
• Treatment Operations– Wellbore & Casing Considerations– Fracturing Equipment– Quality Control– Diagnostic Equipment
• Pressure, Tracers, Production Logs Imaging Technology
– Flow Back• Objective• Method
– Clean Up• Fracture• Formation
– Effect Of Liquids In Low Permeability Reservoirs
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
3000
3500
4000
4500
5000
14 16 18 20 22 24Time (hrs)
BH
P (p
si)
0
20
40
60
80
100
Rat
e (b
pm)
OVERBURDEN STRESS
100
1000
10000
0.01 0.1 1 10Time (hr)
Net
Pre
ssur
e (p
si)
CHANGING CHARACTERISTICS
Bottom-Hole Pressure Provides Critical Information
FLOW-BACK PROCEDURES
• Fracture Stimulation– Timing & Rate– Goal?
• Proppant Stabilization• Fluid Cleanup
– Varieties• Delayed, Stable Rate• Immediate, Slow Rate• Forced Closure
• Matrix– Minimize Precipitates
• Get Rid Of Spent Acid As Soon As Possible
Forced Closure
Pres
sure
Length
FRACTURE & RESERVOIR CLEAN UP
• Hydraulic Fracture– Gel Break– Sufficient Energy
• Energized Systems– Fracture Conductivity
• Matrix– Capillary Pressure– Relative Permeability– Reservoir Pressure– Gel In Fissures– Fluid Damage
• Clay Swelling
High Speed Centrifuge Test
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100 120
Water Saturation (%)
Pres
sure
(psi
)
Por=5.2%; DryPerm=0.01md
Por=7.2%; DryPerm=0.04 md
Por=6.9%; DryPerm=0.02 md
Mesaverde Sandstones, CO
LOW PERMEABILITY CARBONATES• Many Reservoirs Deep
– High Temperature• Horizontal/Multi-Lateral Wells Common
• Operational Difficulties– Effective High Temperature Acid System– Technology For Accessing Multi-Laterals
• Workover, Re-Stimulation– Optimum Stimulation Methodology
• Acid, Fracturing, Other, Hybrid
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Stimulation Evaluation– Pressure Behavior
• Nolte Approach & Derivatives• Pressure History Matching (Model)
– Fracture Diagnostics• Wellbore
– Tracers, Temperature Logs, Production Logs• Far Field
– Microseismic, Surface & Downhole Tiltmeters– Well Testing
• Operational Considerations (e.g., Commingled Production)• Pre-Frac Reservoir Understanding
– Production• Ultimate Assessment
FRACTURE DIAGNOSTICS - MICROSEISMIC
• WaterFracs– Activate
Natural Fractures
– Large Amount Of Shear Movement
– Water Dispersed Throughout Fracture System
– Prop?-3000
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
-1000 -500 0 500 1000 1500 2000 2500 3000
West-East (ft)
Sout
h-N
orth
(ft)
Well A
Well B
Well C
Well D
Well E
MonitorWell
FracWell
Courtesy of Devon
WATERFRACS IN SHALES
• Effective Method Of Connecting Fracture System– Shear Displacement
• Microseismic Activity• Tiltmeter Residual
Deformation– Elimination Of Gel In Fissures
• Elimination Of Fissure-Opening Clean-Up Problem
– Penetrating Fluid• Water Penetration• Opening of Nearly Sealed
Natural Fractures• Wide Dispersal
0
1
2
3
4
5
6
5 10 15 20 25 30 35 40
TIME (days)
TILT
(mic
rora
dian
s)
M-SITEAPRIL KCl
INJECTIONS(1B - 4B)
1B2B
3B
4BRESIDUAL DEFORMATIONAFTER UNPROPPEDINJECTIONS AS MEASURED BY DOWNHOLE TILTMETERS
20%
Shear Movement Within The Reservoir
Microseisms
FRACTURE CONDUCTIVITY DEGRADATION
• Limited Data On Shale Fracture Behavior – Cyclic Loading Of
Unpropped Fractures• Reduced
Conductivity• Permanent Damage
– Minimize Pressure Variations
1
10
100
0 5 10 15
Time (days)
Con
duct
ivity
(mic
roda
rcy-
ft)
Shale Unpropped Fracture
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1000 2000 3000 4000 5000
Net Stress (psi)
Frac
ture
Per
mea
bilit
y(D
arcy
s)
Austin Chalk Fracture
FRACTURE DIAGNOSTICS - MICROSEISMIC• Multi-Zone
Stimulation– Unequal
Fracturing Activity
– Some Sands Unstimulated
– Depleted Sands May Fracture Most
7100
7200
7300
7400
7500
7600
7700-400 -300 -200 -100 0 100 200 300 400
Distance Along Fracture (ft)
Dep
th (f
t)
Unequal Distribution of Fracturing Activity
Side View
Gamma Log
Lenticular Sandstone Case
EFFECTIVE FRACTURE GEOMETRY
• Fracture Diagnostics Provide– Fracture Length & Height– Fracture Azimuth– Fracture Complexity
• Secondary or T Fractures
• Need Effective Fracture Length– Propped Length– Cleaned-Up Area
Fracture LengthProppedLength
EffectiveLength
Poor Clean UP
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Production Operations– Dealing With Water
• Natural Lift• Artificial Lift• Wellbore Configuration• De-Watering Of Coal Seams
– Optimizing Production• Pressure Threshold For Damage• Rate Threshold For Damage
Water
Gas
CrushEmbed
Proppant Natural Fractures
Fines
STIMULATING UNCONVENTIONAL GAS RESERVOIRS
• Refracturing– When Is It Advantageous– How Should It Be Done– Why Is It Necessary– What Actually Happens
• Extend Old Fracture– Better Height
Containment In Depleted Zones
• Start New Fracture– Reorientation Of
Fracture Direction• Intercept New
CompartmentsSurface Tiltmeter Measurement
UNCONVENTIONAL GAS RESERVOIR STIMULATION• Summary Of Research Needs
– Reservoir Size & Shape• Compartmentalization
– Permeability• Matrix, Fractures, Anisotropy
– Natural Fractures• Sets, Azimuth, Spacing, Properties
– In Situ Stresses– Elastic Parameters– Completion Method For Multi-Zones
• Large Interval• Large Number Of Reservoirs
– Effects Of Depletion– Perforation Strategy
• Spacing, Phasing, Limited Entry– Closure Stress– Pore Pressure
FracturesFracturesFractures
FracturesFracturesFractures
UNCONVENTIONAL GAS RESERVOIR STIMULATION• Summary Of Research Needs (Continued)
– Model Improvements• Complexity• Height• Effect Of Natural Fractures (Fissure Opening)• Accurate Input Data
– Bottom-Hole Pressure During Treatments– Waterfracs
• Mechanism That Make This Approach Effective– Understanding Of Flow Back Process– Improved Fracture Imaging Capabilities
• Less Expensive• Longer Distances• Effective Fracture Length
– Improved/Enhanced Artificial Lift– Improved Production Strategy– Refracturing
• Mechanisms That Make Refracturing Effective
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