Marcellus Fullbore-Resistivity Image Logs: The Bearing of Regional Structures and Stratigraphy on Steeply-Dipping Fractures
Warner, Travis B.; Jacobi, Robert
Fullbore-resistivity image logs in horizontal and vertical wellbores in SW PA and N WV are utilized to gauge fracture complexity and its effect on horizontal wellbore productivity in the Marcellus shale. A limited number of horizontal wellbore image logs show the majority of Marcellus fractures observed are high-contrast, steeply-dipping, closely-spaced (depending on stratigraphy, averaging from 2.5 - 40 feet) and tightly-oriented (falling within a 20° window). Steeply-dipping calcite-filled fractures in offsetting core confirm the image inferences. The populations of steeply-dipping fractures observed in the horizontal wellbores are statistically different from those observed over the same Marcellus interval in the associated vertical wellbores. This difference is likely due to the sampling bias created by the horizontal tool orientation and increased length-in-formation of horizontal wellbores.
Vertical image logs show that the present day stress field is fairly constant across the study area. Yet, the orientation of steeply-dipping fractures observed in horizontal and vertical wellbores is not consistent. Horizontal wellbore image logs show stratigraphy affects steeply-dipping fracture orientation and spacing. Fracture orientation also appears to be a controlled by structure. For example, in one SW PA well, the steeply-dipping fractures strike NW, in the J2 orientation. This fracture trend is collinear with the strike of a nearby monoclinal warp of the Onondaga observed in a 3-D seismic survey. In a N WV horizontal well, the steeply-dipping fractures generally strike NE, in the J1 orientation. At the heel of the horizontal wellbore the fractures are nearly parallel to present-day SHmax and to the assumed J1 orientation; toeward, with increasing proximity to a nearby regional fault, the fractures rotate into near orthogonality to the fault. Rotation of steeply-dipping J1 fractures in N WV suggests the regional fault was open at the time of J1 propagation. On 2-D seismic the Devonian Elk Formation thickens alongside the fault, suggesting the fault was active during the Acadian and predated J1 propagation.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013