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

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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

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Contrasts

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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

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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

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ture

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ngth

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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

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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

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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

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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

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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

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INJECTIONS(1B - 4B)

1B2B

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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

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Austin Chalk Fracture

FRACTURE DIAGNOSTICS - MICROSEISMIC• Multi-Zone

Stimulation– Unequal

Fracturing Activity

– Some Sands Unstimulated

– Depleted Sands May Fracture Most

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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