Anisotropic mechanical properties of Mesoproterozoic shales in Beetaloo Sub-basin, Northern Territory, Australia

Abstract The Mesoproterozoic Velkerri and Kyalla shales in the Beetaloo Sub-basin hold significant energy potential, yet their mechanical properties remain understudied. This study examines the anisotropic mechanical properties of these formations by integrating core and log data. The results of X-ray diffraction analyses were used to understand the mineralogical composition and clay types. Ternary plots show a clay-rich siliceous mudstone to siltstone composition. The results of various imaging techniques reveal that laminated fabric in the studied formations is evident across different scales. Unconfined (uniaxial) compressive strength (UCS) tests and triaxial compression tests results, complemented by ultrasonic axial wave velocity measurements on samples oriented both parallel and perpendicular to the bedding, were used to investigate the static and dynamic rock elastic properties of the studied formations and to derive conversion equations for horizontal and vertical directions. Rock mechanical analysis shows horizontal Young’s modulus and Poisson’s ratio are higher than in the vertical direction. The static Young’s modulus ranges from 15 to 35 GPa in vertical specimens and from 28 to 45 GPa in horizontal specimens. Poisson’s ratio varies between 0.15 and 0.3 in both directions, with samples parallel to the bedding exhibiting higher values. Conversely, the average UCS value for samples parallel to the bedding is lower than that for samples perpendicular to the bedding. Horizontal samples have an average UCS of 74 MPa, while vertical samples reach 129.5 MPa. Similarly, Brazilian test results reveal anisotropic behaviour in tensile strength, with horizontal samples exhibiting higher tensile strength than vertical samples. Values range from 3 to 15 MPa for samples parallel to the bedding and from 7 to 20 MPa for those perpendicular to the bedding. Finally, by integrating limited anisotropy measurements, vertical and horizontal wave velocity measurements on core samples, and dipole sonic log measurements, the stiffness tensor components were reconstructed for the studied wells. This enabled the creation of a continuous profile of anisotropic rock mechanical properties. The results highlights the importance roles of total organic carbon and clay minerals in controlling anisotropy of the Kyalla and Velkerri formations, respectively. This study advances our understanding of the geomechanical behaviour of unconventional energy resources in the Beetaloo Sub-basin, providing essential insights for field development and production planning. Highlights Anisotropic rock mechanical properties of the Velkerri and Kyalla formations in the Beetaloo Sub-basin were investigated. Stiffness tensor components and a continuous profile of anisotropic rock mechanical properties were modelled by integrating core and log data. The impact of rock anisotropy on geomechanical analysis was demonstrated in the stress modelling.