51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: The inherent anisotropy in shales due to the presence of fine laminations makes the process of fracture prediction complex. An optimized Hydraulic Fracture design requires a better understanding of anisotropy in rock strength and its impact on fracture propagation. This paper presents a workflow to generate continuous fracture toughness (KIC) profiles (parallel to bedding and perpendicular to bedding) using laboratory generated correlations between fracture toughness and other rock properties like UCS, Young’s Modulus and Tensile Strength. Anisotropic nature of fracture toughness has not been studied in detail previously which this paper seeks to address. A Boundary Element Method model was used to study the effect of anisotropy in fracture toughness and the magnitude on fracture geometry for a field in North America. Fracture simulations show anisotropy in KIC does impact hydraulic fracture propagation especially in cases where stress barriers are absent and neglecting it would lead to inaccurate predictions of fracture geometry.
Understanding fracture growth across layers is critical to accurately predict fracture growth and in generating an optimized Hydraulic Fracture design. Fracture toughness (KIC) is often overlooked and considered unimportant since the in situ stress profile is considered the controlling factor when it comes to fracture growth, containment and geometry (Fisher and Warpinski, 2012.). In laminated formations, such as shales, fracture height predictions based on numerical models grossly overestimate the fracture height when compared with results from fracture diagnostics. (Baree et al., 2010).
Numerous authors published results indicating fracture toughness significantly affects fracture size (Thiercelin et al., 1989, Dudley et al., 2016, Abe et al., 1976, Shoji et al., 1985). Fracture toughness variations across different layers also have a significant effect on fracture geometry as shown by Settari (1988). Correlations of fracture toughness with P-wave velocity, Young’s modulus, Poisson’s ratio, UCS, Tensile strength and other physical properties have been derived from experimental data of different rock types (Barry et al., 1992, Zhixi et al., 1997, Brown and Redish, 1997). Dudley et al. (2016) developed correlations of fracture toughness with density, P-wave velocity and S-wave velocity exclusively for gas shales. None of the above studies considered the direction dependency of KIC. Ghamgosar et al., (2015) found that anisotropy has a strong influence on the magnitude of fracture toughness and crack propagation using the Cracked Chevron Notch Brazilian Disc (CCNBD) specimens of Brisbane sandstone.
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