Subsurface Geomechanics, Fracture Breakdown Pressures, and Fracture-tunnels in the Midwest U.S.

Abstract

A great deal of new geotechnical information is available from shale gas wells, carbon dioxide storage research, and brine disposal wells. Together, this data provides a better understanding of subsurface geomechanical conditions, which is an important factor for subsurface resource management. To characterize geomechanical conditions in the Midwest Region, a combination of geophysical image logs, fracture breakdown pressures, and horizontal shale gas well paths were compiled for analysis. Fourteen geophysical image logs were analyzed for breakouts, induced tensile fractures, and natural fractures. These data were processed to determine stress orientation and fracture density. Maximum horizontal stress axis was in an east-northeast to west-southwest orientation, consistent with regional stress records. The western part of the studied area appears to be more fractured than the eastern part, and fractures tend to strike sub-parallel to the axis of SHmax. Over twenty thousand records from well treatment fracture breakdown pressures were compiled for evaluation. Maximum fracture breakdown pressures and instantaneous shut-in pressures were evaluated for different formations to constrain stress magnitudes. While data showed a large amount of variation, instantaneous shut-in pressures averaged 0.85 psi/ft gradient. Data from shale gas wells had higher fracture shut-in pressure gradients due to hoop stress effects and other factors. Finally, well paths for 12,793 horizontal Devonian Marcellus Shale and Ordovician Utica-Point Pleasant formation wells were plotted based on top- and bottom-hole locations. The horizontal wells provide an empirical indication of the regional stress directions, because the wells are drilled along the minimum stress orientation. Geospatially visualizing of stimulated reservoir volumes suggests that horizontal wells with many fracture stages have resulted in swathes of ‘fracture tunnels’ along the horizontal well paths in certain areas. These zones should be noted for other subsurface applications where the fractured zones may be considered caprocks. Data were integrated to provide representation of geomechanical conditions in the Midwest U.S. Together, this information may be used to support carbon dioxide storage projects, brine disposal, and hydrocarbon production in the region.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016