Formation Specific Size Correction for Strength (UCS) on Rotary Sidewall Cores

Unconfined compressive strength (UCS) is the simplest, least-expensive, most commonly used and understood parameter by professionals in Geomechanics. UCS results obtained from lab testing are used in various petroleum geomechanical workflows for calibrating log-based UCS estimations during drillability, mud-weight window selection, optimum well trajectory selection, and hydraulic fracture design. ASTM or ISRM standards prescribe high-quality, intact right circular cylindrical sample core plugs, with a diameter covering 10 grains or more, and a length (L) to diameter (D) ratio (L/D) of 2.0 to 2.5. However, L/D ratio of the core samples obtained from wireline rotary sidewall coring or shale or fragile samples rarely meet these specifications. The current review work shows that the UCS increased rapidly as soon as L/D<1 due to an absence of a shear plane that causes failure in compression testing. For L/D 1.0-1.5 the decrease in UCS is greatest, for L/D 1.5-2.0 the decrease is moderate. Further, the data analysis shows how L/D correction can be obtained for any formation type in any test plan.