Effect of specimen size and shape on the mechanical properties measured on plugs subsampled from sidewall cores

Abstract

For several disciplines, the increasing acquisition of rotary sidewall cores (RSWC) in the oil and gas industry gives the opportunity for more extensive experimental rock characterization but, on the other hand, the limited amount of recovered material, their shape and orientation are not optimal for geomechanical experiments in the triaxial cell. These drawbacks can be overcome by performing experiments on properly shaped and oriented small diameter plugs subsampled from the RSWCs. These plugs are smaller than conventional ones, so the effects of both specimen shape (i.e., length over diameter L/D ratio) and size (i.e., diameter) on strength, stiffness (i.e., Young’s modulus), and dilation (i.e., Poisson’s ratio) must be carefully considered. In this work, an experimental study on specimen shape and size effects on rock mechanical properties has been tailored to materials and testing conditions generally encountered in petroleum related geomechanics. Triaxial compression experiments at three confining pressures have been performed on outcrop analogues of typical reservoir rock lithologies, i.e., two sandstones and two limestones, and within a range of shapes and sizes relevant for specimens prepared from RSWCs with emphasis on 10 mm diameter specimens. The strength, stiffness and dilation results were analysed and compared with relevant data and relations available in the literature. Then, analytical equations, useful for the practice of geomechanical experimental studies on RSWCs, have been derived to correlate mechanical properties measured on specimens of any shape or size to the ones obtainable on reference specimens.