## 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 S h S v S H

# 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:

4 Out of zone growth encouraged

S h S v S H

# 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

5 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 σ z/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 .

# Pore Pressure Distribution

8

Distribution of pore pressures (MPa) in the formation after 3 hrs of injection

# In-Situ Stress (Syy

# ) Increases

9 S xx ; previously Max. 

### Syy

# Poroelastic stress component for (Syy

# )

10

# 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

12

# 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

13 s 3 s 1 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? 14

# 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

16

# 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 17

# 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.... 18