Ahmad Ghassemi

Rock Failure, Stimulated Volume & 

Permeability Enhancement in Gas 

Shale HF
HF, Multiple Fractures

 Multiple horizontal are drilled to 

increase more contact area 

between the well and the 

reservoir

 Wells are stimulated in multiple 

stages (1-10 per well, depending 

on need)

 Total frac lengths 1000-1500 ft

Sh

Sv

SH
Multiple Well Stimulation

 In a typical stage, water and sand were pumped at a rate of 50-70 

bbls/min

 Large volumes of pumped water per stage (100000-1,000,000 

gal)

Horizontal Wells 

Stage 1

Stage 2

Stage 3

S-Frac Z-Frac

Stage 1

Stage 2

Stage 3

heel

toe
Mechanical Fracture Interaction

 HFs propagate perpendicular to the least principal 

in-situ stress unless:

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Out of zone growth encouraged

Sh

Sv

SH
In Addition to Stress, Fracture Growth is 

Controlled by

 Discontinuities

 Frac fluid density 

 Permeability

 Local fracture growth 

controlled by rock fabric

 Global fracture growth 

controlled by total stress 

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Bons (2000)

Courtesy Nemcok
H-Fracturing Perturbs Pore Pressure & Stress in Rock

6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300

Δ

σz/net pressure

Distance from Fracture Surface (ft)

x=0

x=50

x=100

x=150

x=200

x=250

x=300

x=350

x=390

Analytical model for induced stress perpendicular to penny HF (stress 

shadow)-used for FRV
Stress, and Failure Analysis for Multiple HF-Numerical 

(BEM) in Barnett Shale 

3 elliptical transverse fractures of 150 m length each (50 m spacing)

Fracture aperture distribution (in cm) after 3 hours of pumping.

43

39 MPa

56

28

H

h

V

p














Pore Pressure Distribution

8

Distribution of pore pressures (MPa) in the formation after 3 hrs of injection 
In-Situ Stress (Syy) Increases

9

Sxx ; previously Max. 

Syy

43

39 MPa

56

28

H

h

V

p














Poroelastic stress component for (Syy) 

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Effective Stress is Reduced, Promoting Rock 

Failure 

Distribution of the maximum and minimum (iii) principal effective stresses

in the reservoir (MPa).
Shear/Tensile Failure

 Failure is caused by high 

stresses near the tip of the 

fractures, & by increased pore 

pressure elsewhere

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Estimating Stimulated Volume: Use MEQ 

 MEQs are believed to be caused by pore pressure induced 

shear slip around HF 

 Accepting this, the SRV (volume of “failed” rock ) is 

assessed using the areal extent of the MEQ cloud 

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s3

s1

This is based on the assumption that energy 

release is exclusively related to fluid penetration, 

which may not always hold true
Issues

 It is assumed Mode I fracture propagation 

dominates, but MEQ monitoring shows shear 

dominates

 Event location problems

 Need to incorporate Mode II (slip and shear 

dilation) into models

 History matching without considering the 

above mechanisms can be inadequate

 Feedback between failed rock and HF? 

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Rock Failure Enhances Permeability 

(Brittle, Ductile)

15

De Paola et al. 2009
Prediction of Enhanced Permeability

 The usual methodology for predicting the permeability 

in the failed region is a trial and error procedure:

(i) Guess a value for permeability

(ii) Predict the failed rock volume (FRV) using the 

stress analysis for a selected net fracture pressure

(iii) Vary perm until the FRV matches the extent 

shown by induced seismicity for the given net 

fracturing pressures

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Issues

 This method is based on the equivalent 

permeability for the failed rock, details of fracture 

network not considered 

 Rock heterogeneity and time dependent behavior 

of rock and fractures not considered –Symmetric 

Frac envisioned

 It is assumed that the MEQ’s are related to local 

pore pressure perturbations 

 Aseismic deformation with significant slip and 

perm enhancement 

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Remarks

 The interaction of the multiple hydraulic fracture 

stimulation on dynamics of fluid flow on a larger 

scale is not clearly understood

 The large volume of failed rock tends to redistribute 

the stresses in the nearby region and can modify the 

nearby rock permeability

 In this context, the presence of faults (active and 

inactive) need be considered

 HF/Fault interaction become important

 Others....

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