Structural Analysis of Fractured Hydrocarbon Reservoirs Role of

Structural Analysis of Fractured Hydrocarbon Reservoirs: Role of Rock Rheology Seth Busetti University of Oklahoma November 2008

Understanding Rock Deformation Analog Experiments Relatively Simple Uses only Geometry Linear Elastic Modeling Kinematic Restoration / Forward Modeling Sanders et al. , 2004 Maerten and Maerten, 2006 Lacazette, 2000 Simple Computation Valid for Small Strain Mechanical Simulation

Deformation of Rock Layers Four main stages of rock deformation Mt. Scott Granite Macroscopic Fracturing Extensive Damage, Crack Coalescence Damage by Microcracking [Strain Hardening] Linear Elastic Stage Crack/pore closure (Katz and Reches, 2004)

Mechanical Simulations of Structures Physical Observations Mathematical Expression Structure Layering Folds Faults/Fractures Geologic Features Ramps, pins, blocks Layer Friction Stress Conditions Tectonic Stress Local Stress Rock Mechanics Properties Elasticity Plasticity Failure Porosity/Permeability Numerical Method Geometry {f} u 2 x u 2 y [Ke] u 3 x u 3 y u 1 x u 1 y u 4 x u 4 y [K]{u}+[M]{a}+[C]{v} = {f} Discretization (nodes/elements) Discontinuities Boundary Conditions Degrees of Freedom Penalty Contact Loading Conditions Surface Pressure Point/surface Loads *Rock [Material] Rheology Material Model Parameters σ-ε curve Preliminary Material Modeling: Calibration / Benchmark Testing Ellenberger Limestone Barnett Siliceous Shale Barnett Mudstone Barnett Calcareous Mudstone Berea Sandstone Indiana Limestone

Deformation of Rock Layer: 4 -Point Beam Berea Sandstone Rheology: Elastic-Plastic with Damage Differential σ vs. Axial ε Piston Down 1, 8 E+07 1, 6 E+07 Differential Stress 1, 4 E+07 1, 2 E+07 1, 0 E+07 8, 0 E+06 6, 0 E+06 4, 0 E+06 10 MPa Confining Pressure [Triaxial] 2, 0 E+06 0, 0 E+00 0 Onset of Damage [Plasticity] Loading Piston Confining Pressure Load Cell Beam Stiffness Degradation Failure [Fracture] 0, 001 Strain 0, 002 0, 003

Deformation of Rock Layer: 4 -Point Beam Berea Sandstone Rheology: Elastic-Plastic with Damage Fracturing Stage 0, 20 Coalescence 0, 18 Microcracking Stage 0, 16 Elastic Stage d 2 y/dx 2 0, 14 Damage Before Failure 0, 12 0, 10 0, 08 0, 06 0, 04 0, 02 0, 00 E+00 1, 00 E-02 2, 00 E-02 3, 00 E-02 4, 00 E-02 5, 00 E-02 Distance from Center (m) 6, 00 E-02 7, 00 E-02 8, 00 E-02

Large-Scale Deformation Application Mohr-Coulomb Rheology 10, 000 m Open Questions: Damaged Shear Zones vs. Fault Planes? Mechanisms for Fault Rotation? Role of Footwall Deformation? 20, 000 m 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