Geophysics Under Stress: Geomechanical Applications of Seismic and Borehole Acoustic Waves
by Colin Sayers
The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves. This leads to many important applications of elastic waves for solving problems in petroleum geomechanics.
The purpose of this course is to provide 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 to introduce some of the applications of this sensitivity. The course will provide 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 will be presented. Particular emphasis will be placed on the rock physics basis underlying the use of geophysical data for solving geomechanical problems. Integration of seismic inversion with well data to predict pore pressure, in-situ stress and fracture gradient in unconventional reservoirs will be illustrated using examples from North America and the Middle East.
The integrated nature of this course means that it is suitable for individuals from all subsurface disciplines including geophysics, geomechanics, rock physics, petrophysics, geology, geomodeling, and drilling, reservoir, and petroleum engineering. The short-course presentation, limited to three days, will provide an overview of the basic concepts and applications. The course presentation does not require a theoretical background and can be attended by a broad section of working geoscientists and engineers interested in applying geophysical data to the solution of geomechanical problems.
Prerequisites (Knowledge/Experience/Education Required)
Participants should have a basic knowledge of Geology, Geophysics, and Petrophysics. Experience with elastic waves such as ultrasonics in the laboratory, sonic or seismic is beneficial. Useful for geophysicists, geologists, petrophysicists, and engineers wishing to understand how elastic waves respond to pore pressure and stress within the earth, and who are interested in how this knowledge can be used to solve geomechanical problems.
The following topics will be addressed in the course:
- Introduction to the effects of stress in the earth. Why pore pressure, in-situ stress and geomechanical properties are important.
- Sediment compaction and the state of stress in the earth. Vertical stress, pore pressure and sediment compaction. Horizontal stress in a relaxed basin. Estimation of the minimum and the maximum horizontal stress. Tectonic strains.
- Pore pressure. Velocity vs. effective stress relations. Pore pressure estimation from velocity. Clay diagenesis. Unloading. The need for fit-for-purpose seismic velocities. Uncertainty analysis. Combining seismic velocities with well velocities for improved pore pressure estimation. Dipping layers and lateral pore pressure transfer.
- Stress sensitivity of sandstones. Third-order elasticity theory. 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.
- Wellbore stability and wave velocities near a borehole. Stress changes in the vicinity of a borehole. Mechanical behavior of rock in the vicinity of a borehole. Stress dependence of elastic wave velocities. 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. Rock failure. Monitoring reservoir stress changes using time-lapse seismic. The difference in reservoir stress path between injection and depletion.
- Fractured reservoirs. Effects of fractures on seismic waves. Multiple fracture sets. Amplitude Versus Offset and Azimuth (AVOA). Simplifications for weak anisotropy. Effects of inequality between the normal and shear compliance of fractures. Microstructural models of fracture compliance.
- The seismic anisotropy of shales. The relation of shale anisotropy to microstructure. The effect of interparticle regions on seismic anisotropy. Clay mineral anisotropy. Effect of disorder in the orientation of clay particles. The static elastic moduli for a TI medium and the implications for hydraulic fracture containment.
- Applications to characterization of lithology, pore pressure and in-situ stress in unconventional reservoirs.
- Describe the importance of pore pressure, in situ stress and geomechanical properties for geophysical applications.
- Define effective stress and explain its role in determining seismic velocities and sediment compaction.
- Describe how pore pressure can be predicted from seismic velocities.
- Explain how stress varies around a borehole.
- Appraise the impact of near-wellbore stress variations on sonic velocities.
- Distinguish between the effects of saturation and pore pressure changes on 4D seismic.
- Explain how fractures can be characterized using seismic data.
- Describe the effect of shale anisotropy on in situ stress estimation.
- Describe the workflow needed to build a 3D Mechanical Earth Model and explain how it can be used to predict pore pressure, in-situ stress and fracture gradient in unconventional reservoirs.