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Mechanical Stratigraphy and Barriers to Hydraulic Fracture Growth: Insights From Geomechanical Models

Abstract

The importance of mechanical stratigraphy in controlling both natural and induced deformation in the subsurface has become well established in recent years, particularly for the exploration and production of unconventional or self-sourced reservoir plays (e.g., Eagle Ford, Wolfcamp, Bone Springs). Although recent drill-through and coring experiments have provided unprecedented observations on the complexity of fractures induced by fluid injection, most conceptual models of hydraulic stimulation are still predicated on the development of long (wide, tall) planar fractures that are interpreted from indirect data such as microseismic events. In this study, we use numerical geomechanical models to investigate the role of mechanical stratigraphy in acting as barriers to vertical growth of hydraulic fractures. The two-dimensional simulations consider starting conditions with a normal-faulting stress state at a depth of approximately 3000 m for both hydrostatic and over pressured conditions. Geologic layers are represented using elastic-plastic-damage constitutive relationships and material properties similar to those of the Eagle Ford Formation, and injection is simulated for a 50-m-long frac-stage length. Model results show that the mechanical stratigraphic configuration controls both the evolution of stress and pore pressure states in response to fluid injection, and the induced deformation pattern (failure orientations, dimensions). Deformation in the simulations includes formation of complex networks of tensile and shear failure surfaces and/or zones. Control configurations with uniform mechanical properties (i.e., not mechanically layered) are characterized by greater vertical fracture growth whereas mechanically layered configurations show upward and/or downward retardation of fracture growth. In general, the sharpest barriers to fracture propagation develop at transitions from relatively weak/ductile to relatively strong/brittle intervals.