Colin Sayers

Seismic Geomechanics Short Course

The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, and this leads to many important applications of elastic waves for solving problems in petroleum geomechanics. This course provides an overview of the sensitivity of elastic waves in the earth to the in-situ stress, pore pressure, and anisotropy of the rock fabric resulting from the depositional and stress history of the rock, and introduces some of the applications of this sensitivity. The course provides the basis for applying geophysics and rock physics solutions to geomechanical challenges in exploration, drilling, and production. A variety of applications and real data examples is presented, and particular emphasis is placed on the rock physics basis underlying the use of geophysical data for solving geomechanical problems.

The course uses the 2010 SEG/EAGE Distinguished Instructor Short Course book “Geophysics under stress: Geomechanical applications of seismic and borehole waves" as a basic reference, but goes into more detail and gives more applications and examples than in the book.

The topics to be addressed include the following:

• Introduction to the effects of stress in the earth. Why pore pressure, in-situ stress and geomechanical properties are important. The Mechanical Earth Model (MEM). Data needed to build a Mechanical Earth Model.

• Stress and strain. Scalars, vectors and tensors. Transformation of stress and strain under coordinate rotation.The Mohr's circle construction. Principal axes. Stress invariants. Linear elasticity and Hooke's law. The elastic stiffness and compliance tensors for various material symmetries. The Christoffel equation and elastic wave propagation in anisotropic media.

• Sediment compaction and the state of stress in the earth. Vertical stress, pore pressure and sediment compaction. Poroelasticity and the concept of effective stress. Horizontal stress in a relaxed basin. Estimation of the minimum and the maximum horizontal stress from borehole data. Tectonic strains. Stress regimes in sedimentary basins and the Anderson classification of faults.

• Rock deformation and failure. Compressive and tensile failure. The Mohr-Coulomb criterion. Correlations between geophysical measurements and geomechanical properties. Wellbore breakouts and drilling-induced fractures. Fracture gradient and lost circulation.

• Pore pressure. Overpressure mechanisms. Pore pressure estimation from porosity, velocity and resistivity. Velocity vs. effective stress relations. Clay diagenesis. Loading vs. unloading. Velocity analysis and the need for fit-for-purpose seismic velocities. Combining seismic velocities with well velocities for improved pore pressure estimation. Uncertainty analysis and real-time updating. Dipping layers and lateral pore pressure transfer.

• Stress sensitivity of sandstones. Dependence of elastic wave velocities on porosity in sandstones. The importance of compliant grain boundaries, microcracks and fractures on velocities in sandstones. The use of elastic waves to monitor stress-induced damage. Third-order elasticity theory.

• Wellbore stability and wave velocities near a borehole. Stress redistribution in the vicinity of a borehole. Mechanical behavior of rock in the vicinity of a borehole. Borehole breakouts and drilling-induced fractures. Stress-induced changes in elastic wave velocities in the vicinity of a borehole. Linearized expressions for the change in velocity for small changes in stress.

• Reservoir geomechanics and 4D seismic monitoring. Reservoir stress path. The effect of stress path on rock deformation and failure. Reservoir compaction, surface subsidence, and casing deformation and failure. Sand production. Production-induced faulting. Problems drilling into depleted zones. Stress rotations resulting from production. Monitoring stress changes in the reservoir and surrounding rocks using time-lapse seismic. The difference in reservoir stress path between injection and depletion.

• Fractured reservoirs. Effect of fractures on seismic wave propagation.Treatment of multiple fracture sets. Computation of elastic and permeability anisotropy from discrete fracture networks (DFNs). Amplitude Versus Offset and Azimuth (AVOA). Simplifications for weak anisotropy. Effects of inequality between the normal and shear compliance of fractures. Microstructural models for the ratio of the normal and shear compliance. Scale dependence.

• Resource shales. The seismic anisotropy of shales. The relation of shale anisotropy to microstructure. Effects of low aspect ratio pores and grain contacts on seismic anisotropy. Effect of silt inclusions on shale anisotropy. Effect of kerogen on shale anisotropy. Clay mineral anisotropy. Effect of disorder in the orientation of clay particles. Young's moduli and Poisson's ratios of a transversely isotropic medium. Hydraulic fracturing and fracture containment. Microseismic monitoring of hydraulic fractures. Effects of horizontal stress anisotropy on hydraulic fractures and estimation from AVOA. Propagation of hydraulic fractures in the presence of natural fractures. Rock strength anisotropy.