Structural Analysis of Fractured Hydrocarbon Reservoirs:

Role of Rock Rheology

Seth BusettiUniversity of Oklahoma

November 2008

Understanding Rock Deformation

Sanders et al., 2004

Lacazette, 2000

Maerten and Maerten, 2006

Analog Experiments

Kinematic Restoration / Forward Modeling

Linear Elastic Modeling

Mechanical Simulation

Relatively SimpleUses only Geometry

Simple ComputationValid for Small Strain

Deformation of Rock LayersFour main stages of rock deformation

Mt. Scott Granite

(Katz and Reches, 2004)

Crack/pore closure

Linear Elastic Stage

Damage byMicrocracking [Strain Hardening]

Extensive Damage,Crack Coalescence

Macroscopic Fracturing

Mechanical Simulations of StructuresPhysical Observations Mathematical ExpressionStructure

LayeringFoldsFaults/Fractures

Geologic FeaturesRamps, pins, blocksLayer Friction

Stress ConditionsTectonic StressLocal Stress

Rock Mechanics PropertiesElasticityPlasticityFailurePorosity/Permeability

Numerical MethodGeometry

Discretization (nodes/elements)Discontinuities

Boundary ConditionsDegrees of FreedomPenalty Contact

Loading ConditionsSurface PressurePoint/surface Loads

*Rock [Material] Rheology Material Model Parameters σ-ε curve

u1x

u1y

u2x

u2y

u3x

u3y

u4x

u4y

[Ke]

[K]{u}+[M]{a}+[C]{v} = {f}

{f}

Ellenberger LimestoneBarnett Siliceous ShaleBarnett MudstoneBarnett Calcareous MudstoneBerea SandstoneIndiana Limestone

Preliminary Material Modeling: Calibration / Benchmark Testing

Deformation of Rock Layer: 4-Point BeamBerea Sandstone Rheology: Elastic-Plastic with Damage

0 0.001 0.002 0.0030.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

1.0E+07

1.2E+07

1.4E+07

1.6E+07

1.8E+07Differential σ vs. Axial ε

Strain

Diffe

renti

al S

tres

s

Onset of Damage [Plasticity]

Stiffness Degradation

Failure [Fracture]

Piston Down

10 MPa Confining Pressure [Triaxial]

Beam

Loading Piston

Load CellConfining Pressure

Deformation of Rock Layer: 4-Point BeamBerea Sandstone Rheology: Elastic-Plastic with Damage

0.00E+00 1.00E-02 2.00E-02 3.00E-02 4.00E-02 5.00E-02 6.00E-02 7.00E-02 8.00E-020.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20 Fracturing Stage

Coalescence

Microcracking Stage

Elastic Stage

Damage Before Failure

Distance from Center (m)

d2y/

dx2

Large-Scale Deformation ApplicationMohr-Coulomb Rheology

20,000 m

10,000 m

Open Questions:Damaged Shear Zones vs. Fault Planes?Mechanisms for Fault Rotation?Role of Footwall Deformation?

Chimney and Kluth, 2002

Summary

Rheology strongly effects rock deformation

Deformed rocks contain pervasive damage

Damaged layers frequently behave plastically

A Mechanical approach may be necessary to understand many reservoirs, especially where fractures and faults are prevalent

Numerical (i.e., finite element) techniques are a powerful tool for analyzing complex reservoir structures using realistic mechanics