Evaluating VTI Anisotropy in Shale Using New Borehole Sonic Measurements for Improving Seismic Ties

Pluemer, Brian¹, Adam Donald², Tom Bratton² (1) Schlumberger, Denver, CO (2) Schlumberger, Greenwood Village, CO

Anisotropy describes an attribute as a function of the direction of measurement. Quantifying and identifying 3-dimensional anisotropy is critical for surface, borehole and micro-seismic calibration. Shale formations are of importance for conventional and unconventional hydrocarbon recovery. Shale is also anisotropic due to its layering microstructure. It has been well documented that elastic wave propagation through shale is affected by this anisotropy, which can lead to improper correlation between borehole measurements and surface seismic surveys.

The latest sonic technology combined with a 3D anisotropic algorithm transforms the compressional, shear (fast and slow) and Stoneley slownesses to anisotropic elastic moduli. These moduli are then used to classify the formation as to its type and amount of anisotropy. Shear and compressional velocities within shale are often faster in the horizontal plane than the vertical plane due to layering. This formation characteristic can be described as transversely isotropic with a vertical axis of symmetry (VTI). Quantifying the VTI anisotropy parameters in shale is required for AVO analysis, VSP analysis in deviated wellbores and micro-seismic calibration.

A case study will be presented from an unconventional shale-gas play in Arkansas where the formation VTI parameters were computed and analyzed for seismic and micro-seismic calibration.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana