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Speaker: Colin Sayers, University of Houston
Unconsolidated sands provide zones of high porosity and permeability important for freshwater aquifers, hydrocarbon production, and CO2 sequestration. An understanding of the acoustics of unconsolidated sands enables the characterization of such formations using ultrasonics, borehole acoustics, and seismic methods. Inversion of ultrasonic compressional and shear velocities measured for unloading as a function of confining pressure for room-dry unconsolidated sands allows information on the mechanical properties of the grain contacts to be obtained using an approach based on the divergence theorem. This allows the effective compliance of sand to be written as the sum of the compliance of the pores and of the grain contacts, and it does not assume that the grains are identical spheres, in contrast to previous approaches. Grain contacts are found to be more compliant under shear than under normal compression, and the ratio of the normal-to-shear compliance decreases with decreasing confining pressure, implying that the shear compliance increases faster with decreasing confining pressure than the normal compliance. This is of importance in understanding the role of shear in the failure of unconsolidated sands, such as occurs in shallow water flow, sanding, and failure around injectors, where the change in stress is a function of the normal-to-shear compliance ratio of the grain contacts.
Speaker Biography: Colin Sayers, University of Houston
Colin Sayers received a B.A. in physics from the University of Lancaster, UK, and a D.I.C. and Ph.D. in physics from Imperial College, London, UK. He has previously worked for Shell in the Netherlands and the UK and for Schlumberger in the UK and USA. He currently holds a position as an Adjunct Professor in the Department of Earth and Atmospheric Sciences, University of Houston. He is a member of AGU, EAGE, GSH, SEG, and SPE, a member of the Research Committee of SEG, and has served on the editorial boards of the International Journal of Rock Mechanics and Mining Science, Geophysical Prospecting, and The Leading Edge. In 2010 he presented the SEG/EAGE Distinguished Instructor Short Course “Geophysics under stress: Geomechanical applications of seismic and borehole acoustic 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 shared an award for best paper in The Leading Edge in 2013.
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