Because of their low permeability, economic production from unconventional reservoirs requires increasing the surface area in contact with the reservoir via hydraulic fracturing. Important to the design of efficient hydraulic fractures is an understanding of pore pressure, the orientation and magnitude of principal stresses, the geomechanical properties of the rock, and the density and orientation of any natural fractures. This can be achieved using AVA (Amplitude Variation with Angle) inversion of properly processed, imaged, and well calibrated surface seismic data to build a predictive 3D Mechanical Earth Model (MEM). This consists of a structural framework, with geological surfaces and faults from seismic interpretation, mechanical stratigraphy, obtained by identifying the various lithoclasses using inverted P-impedance and VP/VS, elastic properties and rock strength parameters obtained from P-impedance and VP/VS using dynamic-to-static transforms calibrated to geomechanical measurements on core, and estimates of pore pressure, vertical stress, minimum and maximum horizontal stress, stress direction, and the density and orientation of natural fractures.
Shales are usually anisotropic, and a neglect of shale anisotropy may lead to an incorrect estimate of geomechanical properties and in-situ stress. As a result, MEMs built using isotropic inversion may fail to describe hydraulic fractures correctly. In addition to the intrinsic anisotropy of shales, anisotropy due to the presence of natural fractures, and due to horizontal stress anisotropy, plays an important role in determining the geometry of hydraulic fractures in unconventional reservoirs. The use of AVO (Amplitude Variation with Offset) for lithoclassification, and the determination of pore pressure is discussed first, followed by use of AVOAz (Amplitude Variation with Offset and Azimuth) inversion to determine horizontal stress anisotropy and fracture density and orientation. The use of seismic AVA inversion for building 3D MEMs is illustrated using examples from the Middle East and North America. Seismic AVA inversion of properly processed wide azimuth seismic enables optimization of well location, borehole trajectory, well spacing, and the design of hydraulic fractures, before the well is drilled.
About the Author(s)
Colin Sayers is a Scientific Advisor in the North America Land Seismic Group in Houston. 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.
He is a member of the AGU, EAGE, GSH, HGS, SEG, and SPE, a member of the Research Committee of the SEG, and has served on the editorial boards of the International Journal of Rock Mechanics and Mining Science, Geophysical Prospecting, and The Leading Edge. He has a B.A. in Physics from the University of Lancaster, U. K. and a Ph.D. in Physics from Imperial College, London, U.K. 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 received an award for best paper in The Leading Edge in 2013.