Effects of stress cycles on static and dynamic Young's moduli in Castlegate sandstone

Abstract

Abstract

It is well reported in the literature that the static moduli of rock determined from stress-strain testing are characteristically lower than dynamic moduli. We demonstrate that with a consistent definition of the static elastic modulus in terms of small load-unload cycles, the difference between static and dynamic moduli is negligible for Castlegate sandstone. This result is based on a series of stress-cycling measurements on room-dry Castlegate sandstone (UCS = 16 MPa) where we have made simultaneous measurements of both the static stress-strain behavior and the dynamic sound speeds (P and S) on the same sample of rock during unconfined, uniaxial compression. These tests include three “major” load-unload cycles up to 70% of unconfined compressive strength (UCS) and ten minor load-unload cycles of about 6% of UCS. The static minor cycle Young’s modulus is similar to the dynamic Young’s modulus at 10 MPa axial stress and has the value 10 GPa (to within 10%). We work with dry material to focus on the fundamental issues of modulus differences since fluid effects are described in the literature.