Post on 22-Sep-2020
The Microseismic Event Cloud Surrounding Hydraulic Fractures
Mark Zoback
Stanford University
The Microseismic Event Cloud Surrounding Hydraulic FracturesWhat The Events Are?
(and are not)
Why They Occur?
What Attribute Contributes Most to Enhanced Production?
Mark Zoback
Stanford University
SHmax
Shmin
a
b c d
HF and Microseismicity 101
What The Microseismic Events Are
Small Earthquakes That Result from Slip on Pre-Existing Planar Discontinuities
• Seismic Wave Radiation and Focal Plane Mechanisms
• Source Dimension ~1m (for Magnitude About -2)
• Slip Controlled by State of Stress and Fluid Pressure Perturbation During HF
Gale et al., 2014)
• Vertical Fractures
Often Mineralized
Varied Orientation
• Small Faults
Varied Orientation
• Bedding Planes
Usually sub-horizontal
Pre-Existing Discontinuities
What Microseismic Events Are Not
(Except in Special Cases)
• Do Not Result from Slip on Bedding Planes
• Microseismic Events Are Not Opening Mode Fractures
Mapping Stress Orientation and Relative Magnitude
Lund Snee and Zoback (in prep.)
Lund Snee and Zoback (2016, 2018)
Wellbore Failure for Stress Orientation
Earthquake FM’s for Relative Stress Magnitude
~300 New Data Points in Texas, NM
~80 New Data Points in Oklahoma
Well B, n = 1462.6˚ mean, 3.1˚sd
Well A, n = 1060.5˚ mean, 3.7˚ sd
Mapping Stress Orientation With Microseismic Lineations
Courtesy XTO and Microseismic Inc.
Mapping Stress Orientation and Relative Magnitude
Lund Snee and Zoback (2016, 2018)
Mapping Stress Orientation and Relative Magnitude
Lund Snee and Zoback (The Leading Edge, Feb. 2018)
Delaware Basin
Normal Faulting – low horizontal
stress anisotropy
Midland Basin
SS/Normal Faulting – high
horizontal stress anisotropy
What The Microseismic Events Are
Small Earthquakes That Result from Slip on Pre-Existing Planar Discontinuities
• Seismic Wave Radiation and Focal Plane Mechanisms
Typical Slickwater Multi-Stage Hydraulic Fracturing
Barnett Shale
P-waves S-waves
S3
S1
S2
5 Independent FM Inversions
Each Yielded The Same Stress Orientation
and Relative Magnitudes
A= ~ 0.9 (Normal/SS Faulting)
Confirmed by HF and Wellbore Failures
Pre-Existing Fractures Are Often Assumed to be
Aligned with Current Stress State
Some Normal to Shmin ( || to HF)
Some Normal to SHmax (⊥ to
HF’s)
Pre-Existing Fractures Are Often Assumed to be
Aligned with Current Stress State
Why Does Hydraulic Fracturing Cause MS Events to Occur?
Slip on Pre-Existing Planar Discontinuities
• Often Mineralized
• Highly Variable Orientations
At the Highly Elevated Pore Pressure Transmitted to Pre-Existing Discontinuities
During Hydraulic Fracturing (Thanks to Slickwater), it is Possible to Induce Slip on
Old, Dead, Poorly-Oriented Planes
1
6
Sij
Shmin
Hypothetical Strike-Slip/Normal Faulting Stress State
Applicable to Central Oklahoma, Midland Basin
Fault plane orientations from focal mechanisms
in Kuang et al. (2017) Barnett shale, Stage 5
The Crust is Critically Stressed…But Few Preexisting Fractures are Well-Oriented To Slip in the Absence of Stimulation
1
7
Sij
Shmin
Increasing ΔPP Stimulates Slip on Some Faults
1
8
Sij
Shmin
Increasing ΔPP Closer to the Frac Gradient Stimulates on Most Faults
1
9
Sij
Shmin
Even Increasing ΔPP to the Frac Gradient, Some Faults Will Not Slip (Those Approximately Normal to SHmax or SV)
What Microseismic Events Are Not
(Except in Special Cases)
20
• Do Not Result from Slip on Bedding Planes
• Microseismic Events Are Not Opening Mode Fractures
Slip on Horizontal Bedding Planes?
Does Slip Occur on Horizontal Bedding Plane
or Near Vertical Fracture?
(or Horizontal slip)
Gale et al., 2014)
2
3
Sij
Shmin
Bedding Plane Slip Seems Possible Only When
SV ≈ S3 (Compressive Stress Environments Where
Horizontal HF May Occur
Does Slip Occur on Horizontal Bedding Plane
or Near Vertical Fracture?
2
4
Sij
Shmin
Can Opening Accompany Shear Slip?
Yes, But Only on Planes Sub-Parallel to HF’s if the
Net Pressure is Sufficiently High
What Attribute Contributes to Enhanced Production
• How Much of the Reservoir is Contacted
Fault Patch Size (m)
Major: can cause serious damage over large areas.
Moderate: can cause damage to poorly constructed buildings
Minor: felt but does not cause damage
Noticeable shaking but damage is unlikely
Strong: can be destructive in populated areas
Fault Dimension and Earthquake Magnitude
EQ stress drop
slip on fault
8
7
6
5
4
3
2
1
0
-1
-2
-3 100 101 102 103 104
105
106
108
1010
1012
1014
1016
1018
1020
Ea
rth
qu
ake
Ma
gn
itu
de
Ea
rth
qu
ake
Mo
me
nt (N
m)
Creating A DFN from Microseismic Data
in the Barnett Shale
Fracture Radius = 𝑓(𝑀𝑜, 𝝉)
Direct calculation of stress drop is difficult and uncertain
Stress drops in a population of earthquakes span two orders of magnitude
Upper Bound
𝑟 =7𝜋
16
𝑀𝑜
𝜏
13
Source DimensionsObserved Stress Drops
• Linear flow into all HF’s
from matrix within ~10 m
would recover
approximately 3% of gas
in place
• Linear flow into all HF’s
and the stimulated
fracture network
accesses ~25% of the
gas in place
Acknowledgements
Alex Hakso, Wenhuan Kuang, Jens Erik Lund Snee
Data From
Apache Corporation, ConocoPhillips, Devon Energy,
MicroSeismic Inc., Pioneer Natural Resources, Total and XTO
Thank you