Hydraulic Fracturing in Naturally Fractured Rock

1. Effects of Natural Fracture Reactivation during Hydraulic Fracturing of the Barnett Shale, Fort Worth Basin TXSeth BusettiOctober 2010ConocoPhillips Subsurface Technology

2. Hydraulic Fracturing in Naturally Fractured Rock Barnett Shale (ConocoPhillips) • Geomechanical Processes: • Near-wellbore propagation • Fracture Reactivation • Shear-slip • Shear + dilation • Local propagation • 3D interactions • Macroscopic flow enhancement • Residual effects: proppant, damage, pressure perturbation Busetti, 2010 Murphy et al., 1988

3. Models for Fracture Network Stimulation Non-interaction Isolated Interaction *Full Interaction* Sv Sv SH Sh SH Sh Modified after Baer et al. (1994) Tectonic stress, injection qualities, fluid flow, propagation, full scale 3D… Present Analysis: Partial Interaction Tectonic stress, driving pressure, aperture, volume change Busetti, 2009 Busetti, 2008

4. 3D Fracture Reactivation Model (Non-Interaction) Analytical or graphical solution for reactivated fracture planes: Busetti, 2009 (poles to planes) Model Prediction: Stimulated Volume (MEQ cloud) ≈ Fracture Network Dilation Potential σ1 ~2θL ~90-2θ3 σ3 ~2θw σ2 High θL High θW Low θL High θ3 SHmax >> Shmin Tectonic Stress Ratio SHmax ≈ Shmin Low Net P Fracture Fluid Pressure High Net P

5. 3D Non-Interaction Model: Barnett Shale Wells Fracture dilation potential predicts field response reasonably well… …but ignores any interactions (*note no Pp) Well 5 --θ3-- --θW-- 4 --θL-- --θW-- 3 Modified after Daniels et al. (2007) 1 2 After Busetti and Reches (2008) → Use Numerical Method to Solve Problem → Use Properties Appropriate for Barnett Shale

6. Finite Element 3D Model Configuration σ1 =Sv Top View Set 1: 135° Set 2: 45° μ = 0.6 σ2 =SHmax σ3 =Shmin 2m x 2m x 2m Elastic-plastic Layers: E = 30GPa; ν = 0.32 ≈semi-brittle siliceous mudstone Limited plastic propagation Fractures pressurized equally Pf increases linearly 0 – 10 MPa No leak-off

7. Finite Element 3D Model Results

8. FE 3D Model Results: Nonlinear Deformation σ3 isosurfaces Shmax>>Shmin High Frac-Gradient σ3 =34 MPa Low Frac-Gradient σ3 =30 MPa Pf Volumetric Strain εV Non-linear volume increase at R > 0.05 R= (Pf-σ3)/(σ1-σ3) Ф= (σ2-σ3)/(σ1-σ3) Fracture Pressure Shmax >> Shmin Volume expansion is non-linear dεV increases with: (1) Internal Fracture Pressure (2) Lower Minimum Stress, σ3 (3) Tectonic stress ratio (differential stresses)

9. FE 3D Model Results: Non-Linear Deformation Case 10: Ф = 0.167 σ3 = 30 MPa Low Frac Gradient: Compilation of 5 Simulations εY εZ Strain εX Fracture Pressure σ1 Strain ellipses Shmax >> Shmin σ3 εmin σ2 dεY= σ3 ≈ Shminwidening – Set1 fracture dilation dεZ= σ1 ≈ SV subsidence – dip-slip (normal faulting) dεX= σ2 ≈ SHmaxobliquity – Set 2 dilation and slip εmid εmax

10. FE 3D Model Results: Natural Fracture Reactivation vs. Propagation R Volumetric Strain σ3 = 34 natural fracture propagation no natural fracture propagation 3 σ3 = 30 1 4 2 Ф FE simulations terminate after the onset of fracture propagation at low R. Field stress and pressure data indicates multiple network fractures should propagate.

11. Summary of Key Results • Analytical 3D Non-Interaction Model • 1st order approximation for stimulation shape • Matches Barnett Shale field data w/o Pp • FE 3D Interaction Model for a “Typical” Barnett Shale Configuration • Effects of Internal Fracture Pressure • R > 0.05 directional and volumetric strain evolves non-linearly • R > 0.1 - 0.2 reactivated fractures begin to propagate • Effects of Tectonic State of Stress • In all cases, σ3-parallel dilation dominates (dilation perpendicular to SHmax) • σ1-parallel contraction occurs when fractures are reactivated as small normal faults • σ2-parallel dilation occurs for Ф < 0.3-0.4 (dilation oblique to SHmax) • Interpretation of Field Stimulation Data • Barnett Shale wells indicate likely reactivation and propagation of natural fractures under typical in-situ stress and injection pressure conditions • May explain non-planar, non-uniform MEQ patterns observed in the field