Improved Characterization of Anisotropy in Tight Reservoirs Using Stiffness Tensors and Variable Biot's Poroelasticity

Abstract

Abstract Anisotropy measurements in unconventional rocks require fully characterized azimuthal rock properties. Accurate characterization of the minimum in situ horizontal stress plays a vital role in fracture modeling. Underestimation of stresses from applying the assumptions of isotropy leads to poor drilling and completion design. On the other hand, applying a simplified tensors’ assumption and assuming a constant Biot's Poroelastic Coefficient of one overestimates the stresses leading to costly drilling issues and incorrect proppant placement. First step involves obtaining high frequency directional core samples i.e. vertical (0°), inclined (45°) and horizontal (90°) to derive five independent and continuous velocity profiles (including vertical shear and vertical compressional velocities) and mechanical properties for anisotropic models. Importance of core testing is imperative to deriving pseudo velocity profiles and for dynamic to static conversion of Young's Moduli and Poisson's Ratios. Rock physics govern static core based properties (stress-strain measurements) be used as opposed to dynamic properties measured directly from velocities for accurate characterization of stresses. Obtaining an inclined ASTM (American Standard Testing Material) standard core plug for rock testing is one of the biggest challenges the industry is facing due to the fragile nature of shale material. Shorter core plugs lead to end-cap friction and scaling related errors by increasing uncertainty in rock properties. These challenges have forces the industry to make a simplified assumption on tenors (i.e. C12=C13) that are imperative to deriving anisotropic mechanical properties from measured azimuthal velocities. Second challenge is the characterization and validation of Biot's poroelasticity theory. Building pore pressure to measure grain compressibility to calculate Biot's constant in unconventional rock types is time consuming and costly. Therefore, many standard models assume a constant Biot's of one. The tensor and Biot's assumptions were investigated independently and fracture modeling was performed to predict the fracture geometry that was also compared with the available calibration data. It was determined that the both tensor and Biot's of one models overestimated the stresses, resulting in an inaccurate fracture geometry prediction. Finally, a variable Biot's model without the simplified tensor assumption derived using inclined velocity measurements was proposed, that composed of fully integrated anisotropic core based rock properties. The proposed model is a useful tool to accurately characterize the geomechanical properties of unconventional rocks and hence accurately predict the resulting fracture geometry. Good understanding of the stress field around a wellbore allows operators to make informed decision regarding the drilling and completion program of the in-fill wells leading to optimum field development strategies and significant cost savings.