Unconfined compressive strength of Lower Paleozoic shales from the Baltic Basin (northern Poland)

Abstract

Unconfined compressive strength (UCS) is one of the crucial parameters for geomechanical modelling of unconventional reservoirs useful for the design of hydraulic stimulation of hydrocarbon production. In spite of a large amount of UCS data collected from the Lower Silurian and Ordovician shale successions of the Baltic Basin (northern Poland), no comprehensive study on this subject has been published so far. Here, we compile the results of 247 single-stage confined compressive strength tests (CCST) provided by our industrial partner from four exploration boreholes. Based on the integration of these results with geophysical logging data, including dipole sonic logs, we derive empirical equations describing the relationship between UCS and Young’s modulus or sonic wave slowness. Considering the strong anisotropy of elastic properties in shales we have introduced different empirical equations for UCSV (vertical) and UCSh (horizontal), respectively perpendicular and parallel to bedding. The formula for UCSh is determined with less accuracy than for UCSV due to scarce laboratory tests with bed-parallel loading. Based on the empirical formula proposed, we have estimated the VTI-type of anisotropy to be in the range of 12–27%, depending on the lithostratigraphic formation. The results of our UCS estimations are compared with the results of multi-stage CCST from the adjacent borehole. Both confined tests yielded similar results for UCSV, with slightly higher values obtained from the multi-stage tests. In turn, a comparison of our solution with the results of true uniaxial compressive strength tests (UCST) for vertical samples from one of the studied boreholes revealed a significant discrepancy. The mean UCS results for shale formations from UCST are several times lower than those evaluated from the single-stage CCST. The usefulness of the results obtained for borehole breakout analysis is discussed.