Geophysics Under Stress: Geomechanical Applications of Seismic and Borehole Acoustic Waves
The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, which 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 and particular emphasis will be placed on the rock physics basis underlying the use of geophysical data for solving geomechanical problems.
Upon completion of the course, participants will have an understanding of the sensitivity of elastic waves in the earth to mineralogy, porosity, pore shapes, pore fluids, pore pressures, stresses and the anisotropy of the rock fabric resulting from the depositional and stress history of the rock and how to use this understanding in quantitative interpretation of seismic data and in the construction of mechanical earth models.
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.
The integrated nature of this course means that it is suitable for individuals from all subsurface disciplines including geophysics, geomechanics, rock physics, petrophysics, geology, geomodelling and drilling and reservoir and petroleum engineering. The short-course presentation, limited to one-day, will provide an overview of the basic concepts and applications and minimizes the use of mathematical developments. As a result, 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. The course book will provide support for the lecture and further extend some of the more technical considerations.
Participants should have a basic knowledge of geology, geophysics and petrophysics.
Geophysics Under Stress: Geomechanical Applications of Seismic and Borehole Acoustic Waves, by Colin M. Sayers.
About the Instructor
Colin Sayers is a Scientific Advisor in the Schlumberger Seismic for Unconventionals Center of Excellence in Houston, providing consultancy in geophysics, rock physics, drilling and reservoir geomechanics and the characterization of fractured reservoirs. He entered the oil industry to join Shell’s Exploration and Production Laboratory in Rijswijk, The Netherlands in 1986, and moved to Schlumberger in 1991.
His technical interests include geophysics, rock physics, drilling and reservoir geomechanics, pore pressure prediction, wellbore stability analysis, analysis of production-induced reservoir stress changes, subsidence, fault reactivation, 3D mechanical earth modelling, sanding, fractured reservoir evaluation, borehole/seismic integration, stress-dependent acoustics, advanced sonic logging, AVAZ, fluid flow in fractured reservoirs.
He is a member of the AGU, EAGE, GSH, HGS, SEG, SPE and SPWLA, and the SEG Research Committee. He has served on the editorial board of Geophysical Prospecting, the International Journal of Rock Mechanics and Mining Science, and The Leading Edge. He has a B.A. in Physics from the University of Lancaster, U.K., a D.I.C. in Mathematical Physics and a Ph.D. in Physics from Imperial College, London, U.K. He has published numerous papers and holds several patents in the areas covered by this course.
He was the 2010 chair of the editorial board of The Leading Edge, and presented the 2010 SEG/EAGE Distinguished Instructor Short Course on “Geophysics under stress: Geomechanical applications of seismic and borehole waves”. In 2013 he was awarded Honorary Membership of the Geophysical Society of Houston “In Recognition and Appreciation of Distinguished Contributions to the Geophysical Profession”. He was awarded 2013 Best Paper in The Leading Edge, Society of Exploration Geophysicists.