Impact of Mechanical Anisotropy on Design of Hydraulic Fracturing in Shales

Abstract

Shale formations have laminated structures that result in directionally dependent mechanical properties. Conventional completion design approaches do not consider the material anisotropy or the laminated nature of shales. This can result in an underestimation of stresses, and lead to incorrect conclusions about the lateral landing points and the perforation intervals. In this paper, the authors demonstrate the importance of considering the anisotropy in the completion design using a case study from the Horn River Basin (HRB), the largest shale gas play in Canada. Shale formations in the HRB are strongly anisotropic with horizontal to vertical Young's modulus ratios varying from 1.2 to 3.5. Field data from the HRB is examined to evaluate the impact of mechanical anisotropy on break down pressure, fracture initiation and fracture containment. Numerical simulation of the completion design was conducted using a planar 3D fracture model.Results of the numerical simulation indicate that the mechanical anisotropy greatly influences the minimum horizontal stress which in turn impacts the fracture containment and fracture geometry. Strong mechanical anisotropy results in lower fracture initiation pressures and lower tortuosity at the wellbore face. Consequently, selecting the landing point in sections with high anisotropy will minimize the fracture initiation problems. The authors conclude that the heterogeneous and anisotropic nature of shales needs to be properly characterized and taken into account when making decisions on lateral landing points and completion design.

Material Constants of Anisotropic Poroelasticity and Its Impacts on Shale Gas Production

“…While the bedding plane orientation has an evident effect on the horizontal stress parallel to the wellbore (in our case, σ xx ), its effect on σ yy is less significant, as shown by Figure 9. This finding is quite consistent with the findings from the work on rock anisotropy, 4,10,17,25 i.e., as opposed to the habitually applied isotropic assumption, the horizontal stress parallel to the wellbore shows a strong dependency on the anisotropic Figure 5. Three different mesh sizes in the mesh sensitivity analysis of the 3D problem.…”

“…6,13−15 Therefore, in this study, we focus on the so-called intact anisotropy, 10 which means that for any material point in the domain, the stiffness and/or permeability vary with the observing direction. 2,11,16 This anisotropy can have a significant impact on the way that fractures propagate through the rock, 4,17 the direction in which gas flows, 2,18 and the shear failure. 19−22 Therefore, coupled hydro-mechanical simulations should take the anisotropic characteristics into account.…”

A multiscale framework for the simulation of the anisotropic mechanical behavior of shale

“…Especially, deformation and strength anisotropy are detected in both ultrasonic pulse velocity measurements [8,9,16], Brazilian tensile [11, 18, 19], uniaxial compression [11, 17], triaxial compression [13, 20], and triaxial creep tests [9,10]. In addition, researchers reported that the heterogeneous and anisotropic nature of shale has an impact on break down pressure, fracture initiation, and fracture containment during hydraulic fracturing processes [21][22][23]. Therefore, a reliable numerical model for the mechanical characterization of shale needs to take into account these properties.As shale at the macroscale and mesoscale, layered rocks are usually modeled by the numerical methods classified into continuum-based and discontinuum-based approaches [24,25].…”

Coupled Geomechanics and Fluid Flow Model for Production Optimization in Naturally Fractured Shale Reservoirs

“…is the height of the fracture (ft), is Poisson's Ratio, is Young's Modulus (Mpsi), and Δ ! "# is the net fracture pressure (psi), or the difference between the closure pressure and the pressure in the fracture (Khan et al, 2012). The PKN model is used in more conventional hydraulic fractures where the fracture is long in length compared to the height.…”