Effect of height-diameter ratio on the mechanical characteristics of shale with different bedding orientations

Geological exploration cores obtained from shale gas wells several kilometers deep often show different height-diameter ratios (H/D) because of complex geological conditions (core disking or developed fractures), which makes further standard specimen preparation for mechanical evaluation of reservoirs difficult. In multi-cluster hydraulic fracturing, shale reservoirs between planes of hydraulic fractures with different lengths could be simplified to have different H/D ratios. Discovering the effect of H/D on the mechanical characteristics of shale specimens with different bedding orientations will support mechanical evaluation tests of reservoirs based on disked geological cores and help to optimize multicluster fracturing programs. In this study, we performed uniaxial compression tests and acoustic emission (AE) monitoring on cylindrical Longmaxi shale specimens under five bedding orientations and four H/D ratios. The experimental results showed that both the H/D-dependent mechanical properties and AE parameters demonstrated significant anisotropy. Increasing H/D did not change the uniaxial compressive strength (UCS) evolution versus bedding orientation, demonstrating a V-shaped relationship, but enhanced the curve shape. The stress level of crack damage for the specimens significantly increased with increasing H/D, excluding the specimens with a bedding orientation of 0 degrees. With increasing H/D, the cumulative AE counts of the specimens with each bedding orientation tended to exhibit a stepped jump against the loading time. The proportion of low-average-frequency AE signals (below 100 kHz) in specimens with bedding orientations of 45 degrees and 60 degrees increased to over 70% by increasing H/D, but it only increased to 40% in specimens with bedding orientations of 0 degrees, 30 degrees, and 90 degrees. Finally, an empirical model that can reveal the effect of H/D on anisotropic UCS of shale reservoir was proposed, the anisotropic proportion of tensile and shear failure cracks in specimens under four H/D ratios was classified based on the AE data, and the effect of H/D on the anisotropic crack growth of specimens was discussed. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).