How Do Fractures, Stress, and Rock Fabric Influence Velocity of the Marcellus Shale?
A systematic petrophysical investigation of sonic logs from multiple wells with Marcellus Shale core was undertaken to determine the relative importance of fractures, stress, and rock fabric on influencing sonic velocity. The shale is typically well laminated throughout the entire core, with numerous sub-vertical, calcite-filled veins. Some veins are propped open by bridging cements that maintain porosity in the subsurface. The larger and longer of these features are typically visible on image logs, thus allowing us to determine their orientation. The present-day maximum horizontal stress, as determined from observations of borehole breakouts and induced tensile fractures in image logs, trends approximately ENE-WSW throughout the region. Simultaneous radial and vertical compressional velocities were measured on vertical core plugs from numerous stratigraphic levels within the shale. Horizontal transverse isotropy (HTI) was characterized during isostatic compression tests from velocity measurements on four opposing series of radial transducers, spaced 45 degrees apart. The whole core and associated plugs were oriented with respect to magnetic north, allowing orientation of the azimuthal velocity differences relative to the subsurface and comparison with the in-situ stress orientations characterized from image logs. The samples indicate that vertical velocities are consistently less than horizontal velocities, which we infer to be related to the laminated nature of the shale. Ongoing core analysis focuses on determining the role of bedding planes, mineralogy, and volumes of organic carbon in producing this fabric. Horizontal anisotropy is present in all of the measured samples, with anisotropy increasing as a function of confining stress. HTI magnitudes typically range from 5-10%, with the fast velocity direction predominantly aligned with the maximum horizontal stress. Detailed micro-structural analysis of pore-structures is underway to determine the relative role of porosity and micro-cracks in influencing anisotropy. While the vertical transverse anisotropy (VTI) has similar values relative to the HTI, no clear relation to net confining stress was found. Interestingly, VTI from core and sonic logs are consistent, while the HTI from core is often much higher. In fact, most HTI from sonic logs correlate with washed-out borehole. For this, and possibly other reasons, a relationship between observed veins and HTI anisotropy from sonic logs are unclear.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California