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