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Significance Statement
In a recent article by Li et al. (2016) the authors developed a simulation based on linear fracture mechanics theory in order to study the effect of perforation geometry on the initiation fracture pattern. Their work was published in journal, Rock Mechanics and Rock Engineering, Most previous research conducted on the discussed properties is mainly based on conventional reservoirs, in cases of unconventional reservoirs little or no development has been achieved. Mechanical properties of conventional reservoirs are based on assumption that rock is an elastic isotropic material where rock can be defined two constant material mechanical properties such as Young’s modulus and Poisson’s ratio. It can be further said that shale formations in rocks is defined by five independent mechanical properties such as Young’s modulus in a horizontal and vertical direction Eh and Ev respectively, Poisson’s ratio in horizontal and vertical direction νh and νv and shear modulus G. Moreover, little studies have been conducted on the investigation of fracture initiation by considering the effect of perforation geometry. The stress distribution around a deviated wellbore was considered in the simulation. For the simulation method, theory of linear elastic fracture mechanisms was used to predict the initiation fractures from an individual perforation at the wellbore wall originally prepared by (Abass et al. SPE Hydraulic Fracturing Technology, 2013). The longitudinal fractures were modeled as 2D plane strain fractures and the transverse fractures were modeled as 2D radial fractures both edging from the wellbore well. The authors also utilized a recently developed 2D finite-discrete element method FDEM code to predict and validate the initial fracture pattern evolution in elastic isotropic formations. Wellbore radius (rw) was chosen to be 0.107m. Biots’ poroelastic coefficient is 0.7 which is a typical value for engineering applications fracture toughness was selected to be 1.32MPa√m Young’s moduli and Poisson’s ratios were assumed to be average of their horizontal and vertical directions respectively simulation based on finite element-discrete element method model has average matrix element block size of 0.12mm and perforation size was set to be 0.107m (equal to rw). Prediction of fracture initiation pressures and patterns for elastic isotropic formations showed that change in horizontal (isotropic) stress h does not affect the initiation pressure of longitudinal fractures which results in all pressure curves overlapping for all three longitudinal fracture cases. For initiation of transverse fractures with increasing horizontal stress (σh), initiation pressure for transverse fractures and critical perforation depth increases. Finite -discrete element method simulation model further validated these results. Studying the effect of horizontal stress H on fracture initiation pressures for both fracture geometries showed that change of horizontal stress σH does not affect the initiation pressures of transverse fractures. Initiation of longitudinal fractures under the normal fault regime (V>H>h) with increase of σH, increases initiation pressure of longitudinal fractures while critical perforation depth decreases. It can be inferred that if horizontal stress σH gets smaller, initiation of a longitudinal fractures becomes more likely. It’s also noted that the initiation pressure of a longitudinal fractures will reach a minimum value that’s smaller than horizontal stress at a specific perforation depth (for case with σH/σV`=0.8, io/w≈0.16) then increases slightly and approaches the value of σH for long perforation depths. Finite element-discrete element method simulation showed that small σH/σV ratio case leads to fracture propagation along the wellbore at the beginning and propagates perpendicular to the wellbore afterward while larger σH/σV cases, fracture propagates perpendicular to the wellbore without creating any longitudinal fractures. This results prove that linear elastic fracture mechanism analysis and finite element-discrete element method simulation had similar results. For prediction of fracture initiation pressures and patterns for elastic anisotropic functions showed that ratio of Eh/Ev affects initial pressures of both transverse fractures and longitudinal fractures initial pressures but critical perforation depth slightly decreases. Increase in ratios of νh/νv increases both transverse and longitudinal fractures as critical perforation depth decreases. Prediction of fracture initiation pressures and patterns for deviated wellbore with effect of inclination angle on fracture initiation showed that at azimuth angle set to be 00, fracture initiation pressures for both longitudinal and transverse fractures reach their minimum values when an inclination angle is 900 but as wellbore inclination angle increases from 900, fracture initiation pressures for both fracture increases. For effect of wellbore azimuth angle on fracture initiation pressures at inclination angle set at 900, longitudinal fractures initiation pressure reaches its minimum value at azimuth angle of 00 regardless of how transverse functions initiation pressure reaches its maximum value. As wellbore azimuth angle increases from 00, fracture initiation pressures for longitudinal fractures increases but decreases for transverse fractures. The simulation model in this study provides an effective framework for well completion designs in unconventional reservoirs.
H. Li 1 , Y. S. Zou 2,4, S. Liu 1, G. Q. Liu 1, Y. Z. Jing 3, C. A. Ehlig-Economides 4 . Prediction of Fracture Initiation Pressure and Fracture Geometries in Elastic Isotropic and Anisotropic Formations. Rock Mechanics and Rock Engineering, pp 1–13, 2016. 
Journal Reference
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