Geomechanical Characterization of the Wufeng and Longmaxi Shales: Mechanical and Velocity Anisotropy, Static and Dynamic Moduli

Abstract

Summary The Wufeng and Longmaxi shales of Sichuan Basin, Southwest China have been the primary targets for shale gas development. Because hydraulic fracturing and seismic interpretation require detailed characterization of formation mechanical properties, a sufficient understanding of anisotropy and elastic behavior in Wufeng and Longmaxi shales is necessary. In this study, we conducted Brazilian and triaxial tests and ultrasonic velocity measurements to characterize tensile and compressive strengths and P- and S-wave velocities, respectively. Shale samples were cored at a range of orientations relative to bedding and tested at multiple confining pressures, which allowed a detailed study of mechanical and velocity anisotropy, static and dynamic moduli. Our experimental work shows that Wufeng and Longmaxi shales possess similar compressive strength and associated anisotropy with other shale formations but apparently weaker tensile strength anisotropy and velocity anisotropy. These two shales also exhibit much lower static moduli than dynamic values, which are interpreted to be caused by compliant pores such as microcracks and fractures. Comparison between Wufeng and Longmaxi shales reveals distinct levels of heterogeneity. Wufeng shale shows more pronounced heterogeneity regarding measured tensile and compressive strengths as well as elastic moduli. These general characteristics of Wufeng and Longmaxi shales provide valuable first-order understanding regarding anisotropy, heterogeneity, and elastic behavior. Utilizing this understanding could help improve hydraulic fracture design and seismic data interpretation.