Geomechanical Modeling of Hydraulic Fracturing: Why Mechanical Stratigraphy and Stress State Matter
Smart, Kevin J.; Ofoegbu, Goodluck I.; Morris, Alan P.; Ferrill, David A.; McGinnis, Ronald N.
The increasing interest in unconventional resource plays (e.g., Eagle Ford, Marcellus) in the past several years has been accompanied by a greater need for understanding the effectiveness of multi-stage hydraulic fracturing programs, particularly in long (>5000 ft) sub-horizontal boreholes. Traditional (analytical) analysis techniques for estimating the size and orientation of fractures induced by fluid injection typically result in predictions of relatively long and planar fractures that may not be representative of some (most) natural systems. While these traditional approaches offer the advantage of rapid analysis, the lack of inclusion of key features of the natural system (e.g., realistic mechanical stratigraphy, pre-existing natural faults and fractures, and in situ stress heterogeneity) may render results unrealistic for planning and executing multimillion dollar hydraulic stimulation programs. Numerical geomechanical modeling, although somewhat time consuming, provides a means of including the key natural complexity in the simulation of hydraulic fracturing. In this study, we present results of finite-element-based geomechanical modeling of fluid-injection-induced rock deformation that combines a coupled stress-pore pressure analysis with continuum damage mechanics. The models include both the natural mechanical stratigraphic variability as well as the in situ stress state anisotropy, and permit tracking the temporal and spatial development of shear and tensile permanent strains that develop in response to fluid injection. Our results show that long planar fractures are unlikely to be induced in most mechanically layered natural systems. Analyses that assume this type of fracture geometry may significantly overestimate the reach of hydraulically induced fractures and/or effectively stimulated rock volume.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013