Numerical evaluation of Failure Assessment Diagram (FAD) for hydraulic fracture propagation in sandstone

• Published in: Engineering fracture mechanics 2022-03, Vol.263, p.108311, Article 108311
• Main Authors: Wei, Shijun, Tomac, Ingrid
• Format: Article
• Language: English
• Subjects: Acoustic emission; Coalescing; Confinement; Crack propagation; Damage; Deformation; Discrete element method; Discrete Element Method (DEM); Elasticity; Elastoplasticity; Evaluation; Failure; Failure Assessment Diagram (FAD); Failure assessment diagrams; Fracture mechanics; Hydraulic fracturing; Hydraulics; Inelastic fracture propagation; J integral; Linear elastic fracture mechanics; Microcracks; Propagation; Sandstone; Stiffness; Stress propagation
• ISSN: 0013-7944; 1873-7315
• DOI: 10.1016/j.engfracmech.2022.108311

•Failure Assessment Diagram (FAD) elastic and inelastic curves are quantified for rock.•Hydraulic fracture propagates mainly inelastically in biaxially confined sandstone.•An inelastic portion of J-integral significantly increases with rock confinement.•Shorter dry tip zones and fracture lengths characterize inelastic fracture growth.•Linear Elastic Fracture Mechanics cannot model hydraulic fracturing well enough. This paper evaluates the degree of inelasticity in hydraulic fracture propagation in weak sandstone by using models obtained by the Discrete Element Method as a basis for quantifying Failure Assessment Diagram (FAD) and J-integral. Although current engineering models use Linear Elastic Fracture Mechanics (LEFM), including improvements like fracture tip damage zone where fracture tip plasticity is small enough to fit into the near-field stress zone, this paper hypothesizes that hydraulic fracture propagates in an inelastic regime in certain conditions typically for underground rock reservoirs. Discrepancies within field or laboratory data, including microcrack clouds identified as acoustic emissions that induce plastic deformations, motivate the investigation of inelasticity and implementation of elastoplastic fracture propagation theories. Discrete Element Method (DEM) has been widely used to approach rock failure problems from the micromechanical level, where explicit modeling of local particle-bond breakage enables insights into propagating fracture branching, damage, microcrack coalescence, stress–strain re-distribution, irreversible deformation, and fracture arrest. Results show that inelastic J-integral increases dramatically with rock confinement, especially its non-elastic portion, and the elastic portion of the total J-integral remains a relatively small part. Higher confinement stresses and higher confinement stress contrast enhance the inelastic fracture propagation. Additionally, shorter fracture lengths and smaller dry tip zones characterize inelasticity. Rock stiffness increase leads to total J-integral increase and decrease of J-elastic, leading to pronounced inelasticity. Therefore, results indicate that LEFM is rarely applicable for describing fracture propagation through rock at higher confinement stresses and stiffness.