Mechanical Unit Control of Hydraulic Fracture Geometry

ABSTRACT This paper presents a quantitative evaluation of the effect of mechanical stratigraphy on induced hydraulic fracture shape and extent. A detailed geomechanical and hydrodynamic model was built for Hydraulic Fracture Test Site 1 (HFTS-1) in the Permian Basin, considering available geomechanical, geological and geophysical characterization data. This model was then implemented using a discrete fracture network (DFN) approach, to facilitate simulation of induced (tensile) hydraulic fractures, and reactivation of pre-existing natural fractures. INTRODUCTION The geometry of natural and induced tension cracks is important to civil, mining, geothermal, and energy applications. According to Griffith Crack theory for failure of brittle materials (Griffith, 1921), natural and induced tension cracks in an infinite, homogeneous, isotropic medium are circular or elliptical. Over 100 years of laboratory studies have provided empirical support for this theory. However, extensive evidence from microseismic monitoring and offset well intersections indicates that the shape of in situ hydraulic fractures vary considerably from the elliptical ideal. GEOMECHANICAL STRATIGRAPHY Mechanical stratigraphy refers to the subdivision of a layered rock mass into distinct units (mechanical units) that reflect internally consistent rock properties and styles of deformation (Laubach et al., 2009). In response to tectonic deformation, elements of mechanical layering often control fracture dimensions, terminations and failure mode (Gross et al., 1995, 1997; Ferrill et al., 2014). This mechanical behavior occurs across many scales, and for an unconventional oil and gas field is manifested by hydraulic fracture lengths considerably greater than their heights (Fu et al., 2022), an indication that anisotropic layering limits the vertical growth of hydraulic fractures (i.e., serve as frac barriers). Observations of natural fractures in outcrops provide insight into the growth and geometry of induced hydraulic fractures in the subsurface (Lacazette and Engelder, 1992; Engelder, 2004; Engelder et al., 2009). Surface morphology preserved on opening-mode fractures ("joints") are manifestations of fracture initiation, propagation and arrest, as demonstrated in Figure 1. In this siltstone bed from the Appalachian Plateau in upstate New York, lineations on the fracture surface converge backwards to the initiation point (yellow dot), about one-third of the way above the bottom of the bed. The lineation pattern shows the joint grew radially in all directions, until it reached the bottom and top of the bed.