Impact of Size and Aspect Ratio on Sedimentary Rock Strength in Triaxial Compressive (TXC) Conditions

ABSTRACT: Mechanical properties from triaxial compressive (TXC) tests of sedimentary rocks are essential for geomechanical problems in the energy industry and CO2 storage. TXC strength is typically measured on standard plugs (25.4 or 38.1 mm diameter, 2:1 length-diameter (L/D) ratio) sampled from conventional cores, which are generally time-consuming, expensive, and hard to obtain in challenging drilling zones. In the absence of whole cores, one possible way to quantify rock strength is using "mini" plugs (with suboptimal size and/or aspect ratio) subcored from side-wall cores or cavings, from which standard plugs cannot be produced typically. Past studies indicate that the unconfined compressive strength (UCS) of sedimentary rocks varies with sample size and L/D ratio, but comprehensive studies under TXC conditions are scarce. To better understand the size and L/D ratio effects, TXC tests with varying confining pressures were conducted on rock samples with different diameters and L/D ratios. Results show that local rock heterogeneity can overshadow these effects. Multi-parameter linear regression can help to reveal the true impact of size and aspect ratio. A workflow was proposed to correct these effects in TXC testing of "mini" plugs, aiming to provide high-quality geomechanics characterization and fill data gaps in wells/fields lacking conventional cores.