Length-to-Diameter Ratio (L/D) of Rotary Sidewall Cores and Its Implications for Estimating Compressive Strengths (UCS or CCS) in Petroleum Geomechanics

The length-to-diameter ratio (L/D) of core samples obtained from rotary sidewall coring does not meet the specifications for strength or elastic properties measurements as described in ASTM or ISRM. However, strength and elastic properties obtained from sidewall core samples have valuable use in drilling, evaluations, completion, and reservoir modeling for conventional and unconventional resources. The suitable correction for unconfined compressive strength (UCS) obtained from core plugs with an L/D ratio less than 2 is well established. The suitable correction for confined compressive strengths (CCS) and thus internal friction angle, and its associated elastic properties under triaxial loading has been attempted by current co-authors and co-workers, but it is not well established due to limited research. This study re-examines the L/D effect on compressive strength based on a large number of published works as well as extensive test results and develops a comprehensive size-correction factor on UCS and CCS. This would apply correction for apparent high strength in UCS and CCS, which are key inputs in geomechanical models.