Post on 05-Feb-2018
Targeting horizontal stresses and optimal hydraulic fracturing locations through seismic data
David GraySenior Research Advisor CGGVeritas
What matters in Hydraulic Fracturing?
FracturesWill they open?Will they stay in zone?How wide?What type?What direction?Will they stay open?
What do we need?
Rock StrengthYoung’s modulus, Poisson’s ratio, Unconfinedcompressional strength
StressHorizontal and Vertical stresses, σHmax, σhmin, σz
Closure stress = σhmin
Hoop stress = σθθ and Fracture Initiation PressureStress state
What can we get from seismic?
Rock StrengthYoung’s modulus, Poisson’s ratio
StressVertical and horizontal stressesClosure stressHoop stressStress state
How can we get these from seismic?
Rock StrengthYoung’s modulus, Poisson’s ratio
AVO/LMR (Amplitude Versus Offset / Lamé’s Moduli)Multicomponent Seismic Analysis
StressAzimuthal AVO, Azimuthal VelocitiesMulticomponent Seismic Fracture Analysis
When can we get these from seismic?
Rock Strength – NOW!AVO (1982-present, 27 years)LMR (1997-present, 12 years)Joint and Simultaneous Inversion (~2004–present, ~5 years)Multicomponent Analysis(~1980-present, ~30 years)
Stress – NOW!Azimuthal AVO (1996-present, 13 years)Multicomponent Fracture Analysis (~1986-present, ~23 years)Horizontal and Vertical Stresses (Now)
We’ll show you later
Rock Strength
Generating Rock Strength Values from Simultaneous or Joint Inversion of Seismic DataExample from SE of Red Deer, AB
Rock Strength
http://www.kgs.ku.edu/Geophysics/OFR/2006/OFR06_01/index.html
Brinell Hardness Test Hammer & Plate Seismic
Density – Simultaneous Inversion
Colorado Shale2WS
Shear Modulus (μ=ρ∗Vs2)
Important according to Goodway (CSEG lunch, Feb. 09), it can serve as a proxy for differential stress
Colorado Shale2WS
MuGPa
Lame’s Modulus (λ=ρ∗Vp2−2μ)
Colorado Shale2WS
LambdaGPa
Poisson’s Ratio (ν)
( )μλλν+
=2
Colorado Shale2WS
PoissonRatio
Young’s Modulus (E)
( )νμ += 12E Brittleness
Colorado Shale2WS
YoungGPa
Rock Strength Conclusions
Rock Strength derived from seismic since LMR in 1997Translate to values Engineering want
Young’s modulusPoisson’s ratio
Seismic measures “dynamic” rock strengthCalibrate to static values
Usually done with a scalar (e.g. 0.65)Static closer to dynamic in older, harder rocks like these
Stress
Vertical, Maximum and Minimum Horizontal Stresses from SeismicExample from SE of Red Deer, AB
How do we get stress from seismic?
Hooke’s LawStrain = Compliance * Stress
Isotropic Stress
We know σy > σx, in generalAnisotropic stresses
⇒ Anisotropic Compliance
Hooke’s LawStrain = Seismic * StressFor a Horizontally Transverse Isotropic (HTI) medium
Linear Slip Theory
How do we get stress from seismic?
ZNZN
ZT
How? - Horizontal Stresses and Seismic AzAVO
Seismic AzAVO TermsE – Young’ s Modulusν – Poisson’s RatioZN – Normal Compliance
ZNZN
Hooke’s LawStrain = Seismic * StressMovement = Seismic * ForceΔMovement = Seismic * ΔForceFracture = Seismic * ΔPressure
How do we get stress from seismic?
Assumptions
Some assumptions have been made in order to produce these results. Many are due to the preliminary nature of this work and will be addressed in future work. The assumptions are:1. Shear-wave background velocities for the inversions are
estimated from the “Mudrock Line” of Castagna et al (1985).
2. Calibration of estimated stresses, assumed the WCSB Atlas stress measurements are valid for this area.
3. MZN ≈ μZT
4. Azimuthal anisotropy in the seismic is due to differential horizontal stresses.
5. Linear Slip theory and all its assumptions are valid for these rocks.
6. The rocks behave elastically.
Vertical Stress Gradient from g*ρ
Colorado Shale2WS
GradientMPa/m
Vertical Stress
σz ≈ Σi[g*ρ(i) *z(i)]
Colorado Shale2WS
StressMPa
Calibrated Minimum Horizontal Stress
σhmin = σx = Κx∗σz
Colorado Shale2WS
StressMPa
Calibrated Maximum Horizontal Stress
σHmax = σy = Κy∗σz
Colorado Shale2WS
StressMPa
σx (= Closure Stress) in 2WS
Note range varies from ~20 MPa - ~26 MPa over this 10 km2 area.
Differential Horizontal Stress Ratio DHSR=(σH- σh)/σH
Colorado Shale2WS
Estimates require only seismically derivable parameters as σz cancels.
DHSRRatio
Stress State
What do these stress measurements tell us?Example from SE of Red Deer, AB
Stress variations with Depth (after Zoback)
σH > σz > σh
σz > σH > σh
σH > σh > σz
Directio
n o
f M
ovemen
t
Norm
al to
Fractures
Thrust = Horizontal fractures that move horizontally w/o much overburden allows them to go up
Shear = Vertical fractures that move horizontally
Normal = vertical fractures that move vertically
Stress and Fractures
If σHmax ≈ σhminTensile cracks any direction
|| rock weaknessFracture network
If σHmax >> σhmin (>5%)Fractures || σHmax
Shear FracturesTensile Fractures
Connect to existing fracture network for production σHmax
σHmax
Pressure
σhmin = Closure Stress
σhmin
DHSR with Orientation on Young’s Modulus
Young’s Modulus
DHSR
Cross-plot DHSR vs. E
E > 17 GPa, DHSR < 6%
Hydraulic fracture swarms where rocks are brittle (E > 17 GPa) and stress ratio is small (DHSR < 6%)
Aligned FracturesE > 17 GPa, DHSR > 6%Ductile
E < 12.5 GPa
Good and bad areas for Hydraulic Fracs in the Colorado
Colorado Shale
Areas in green indicate zones where hydraulic fractures will be optimal. In yellow, where aligned fractures are more likely to occur. In red, where hydraulic fracturing won’t work because the rock is too ductile.
2WS
Differential Horizontal Stress Ratio (σH- σh)/σH
Colorado Shale2WS
DHSRRatio
Good and bad areas for Hydraulic Fracs in the Colorado
Colorado Shale
Areas in green indicate zones where hydraulic fractures will be optimal. In yellow, where aligned fractures are more likely to occur. In red, where hydraulic fracturing won’t work because the rock is too ductile.
2WS
Young’s Modulus (E)
Colorado Shale2WS
YoungGPa
Probable zones of better Hydraulic Fractures in the 2WS Median
Map of 2nd White Speckled Shale showing zones highlighted in seismic attribute crossplot. Green is where fracture swarms will form, red is where the rocks are more ductile and yellow is where aligned fractures will occur. Note that only about ¼ of this reservoir is optimal for hydraulic fracturing.
Differential Horizontal Stress Ratio Slice
Probable zones of better Hydraulic Fractures in the 2WS Median
Map of 2nd White Speckled Shale showing zones highlighted in seismic attribute crossplot. Green is where fracture swarms will form, red is where the rocks are more ductile and yellow is where aligned fractures will occur. Note that only about ¼ of this reservoir is optimal for hydraulic fracturing.
Young’s Modulus in 2WS
Fracture Initiation Pressure Vertical Borehole
PFIS ≈ 3σhmin - σHmax
Colorado Shale2WS
Fracture Initiation Pressure Horizontal Borehole Side
PFIS ≈ 3σHmax - σv
Colorado Shale2WS
PressureMPa
Fracture Initiation Pressure Horizontal Borehole Top
PFIT ≈ 3σv - σHmax
Colorado Shale2WS
PressureMPa
Conclusions
What can we get from seismic?Rock Strength
Young’s modulus, Poisson’s ratioDynamic needs to be calibrated to static
StressVertical and Horizontal stresses, σz, σHmax, σhmin
Closure stress = σhmin
Hoop Stress, Fracture Initiation PressureStress state
Better places for hydraulic fracturesFracture type – vertical or horizontalFracture motion – vertical or horizontal
σH > σv > σh
σv > σH > σh
σH > σh > σv
Thrust = Horizontal fractures that move horizontally w/o much overburden allows them to go upShear = Vertical fractures that move horizontally
Normal = vertical fractures that move vertically
Acknowledgements
CGGVeritas, EnCana, Apache, Talisman, Magnitude, FairborneJonathan Miller, Paul Anderson, Bill Goodway, John Varsek, Louis Chabot, Doug Anderson, Ron Larson, John Logel, Eric Andersen, Jared Atkinson, Chris Irvine, Darren Schmidt, Franck Delbecq, Ron Schmid, Mark Houston, Christophe Maison, Dragana Todorovic-Marinic, Christian Abaco, Lee Hunt, Scott Reynolds
Targeting horizontal stresses and optimal hydraulic fracturing locations through seismic data
David GraySenior Research Advisor CGGVeritas