Introduction to June Focus: Unconventional Resources

Within the unconventional era of hydrocarbon exploitation, a geophysicist is asked the “question” of how to high-grade drilling locations for optimal and efficient reservoir development. He, or she, is asked to interpret subtle changes in elastic property estimated from surface measurements, on rock thousands of meters beneath the earth’s surface. The interpretation of these elastic properties must also be relevant in terms of exploiting an extremely low permeability rock with nano-scale structures exhibiting complex matrix, organic and fluid phase interactions. A geophysicist can answer this question in one of two ways: 1) the problem is too complex and seismic data cannot be used to make more than structural interpretation; and 2) the problem is complex and it will take advanced geophysical techniques, integrated with geological and engineering data to intelligently interpret subtle elastic property variations. To meet the challenge of positively answering the “question”, a geophysicist must keep up with the latest technologies available and ensure that each step within the seismic analysis workflow, acquisition through to processing, and finally interpretation, is of the highest possible quality. With that goal in mind, this issue of the RECORDER introduces some recent advances in seismic processing and interpretation. These are examples of how seismic data is being stretched in an attempt to capture subsurface properties that geophysicists can exploit to assist in unconventional hydrocarbon development.

The first article, by Benjamin Roure, introduces the limaçon to the geophysical community as a novel way to interpret azimuthal Fourier coefficients. This mathematical tool allows for a unique look at the Fourier coefficients leading to a quick inspection of the anisotropic gradient, the anellipticity and their interrelation.

Understanding the physical mechanisms that result in the AVOAz variation would lend this method of data visualization to highlight areas of fracture or stress-induced anisotropy.

The second article, by Bill Goodway, investigates the impact that two different anisotropic symmetries have on estimating closure stress. He shows the impact that anisotropy can have on estimating subsurface stresses, which is useful for assessing completion efforts. The geophysical impact is that the relevant parameters for subsurface closure stress estimation can be uniquely extracted from seismic data. Wide azimuth and long offset 3D data can be used to estimate subsurface matrix symmetries and then infer subsurface stress perturbations.

The third article, by David Cho and coauthors, is a case study application which discusses discrepancies between traveltime based (velocity variation with azimuth, VVAZ) and amplitude based (amplitude variation with azimuth, AVAZ) anisotropy products; the former are interval based processes while the latter methods describe elastic property changes at an interface. To bridge the gap between these methods, they propose an amplitude based method that solves for fast and slow shear wave velocities. The results are promising in that azimuthal variations are consistent with observed structural complexity leading to an interpretation of increased fracture density.