Impact of Anisotropy Induced by Shale Lamination and Natural Fractures on Reservoir Development and Operational Designs

Abstract

Summary The laminated nature of shale rocks leads to different mechanical properties parallel and perpendicular to the bedding plane, which is known as transverse isotropy (TI). If natural fractures (NFs) are present, the additional elastic anisotropy is introduced within the bedding plane, making shale an orthorhombic (OB) medium. Most current geophysical log interpretations ignore such shale anisotropies, resulting in erroneous estimates of the elastic moduli, brittleness, and stress gradient, which consequently causes problematic or suboptimal drilling, completion, and hydraulic-fracturing design. The objective of the current study is to investigate the effects of the two shale anisotropies on geomechanical-property characterizations, and, hence, on operational designs. In this paper, synthetic OB-rock stiffness tensors are built by introducing NF sets in a vertical symmetric-axis (VTI) background. Then, the VTI model, which ignores the NF effect, and the isotropic model, which ignores both anisotropies, are applied to the synthetic OB rock to interpret the stiffness coefficients, elastic moduli, and stresses. The results are compared with the “true” values interpreted from the original OB-rock stiffness tensors. The impacts of the two types of shale anisotropies on shale-rock geomechanical-property characterizations—and, hence, operational designs—are examined. According to the modeling results, the lamination-induced anisotropy is more significant than the NF-induced anisotropy when predicting Young's modulus. Besides, ignoring the two anisotropies can lead to an overestimation of both the minimum-horizontal-stress magnitude and the stress contrast; the overestimation is larger for stiffer zones and less-tectonically-active zones. As a result, ignoring two anisotropies can lead to an overoptimistic design of the mud-weight window, resulting in higher risks of borehole tensile failure and shear failure. Ignoring the two anisotropies will not alter the brittleness-index (BI) trend observed in the OB rock. However, the stress contrast is overestimated, which will lead to shorter stage-spacing design and suboptimal selections of perforation locations. If one is ignoring the anisotropy induced by NFs, the fracture width is underestimated, leading to insufficient proppant size or pumping-amount design and, hence, to suboptimal low fracture conductivity.