Correlations between the static and anisotropic dynamic elastic properties of lacustrine shales under triaxial stress: Examples from the Ordos Basin, China

Abstract

Shales are abundant and are increasingly important for the hydrocarbon industry as source rocks and unconventional reservoirs. The anisotropic dynamic elastic properties of shales are important in the exploration stage of shale reservoirs whereas their static elastic properties are key for the hydraulic fracturing for the more efficient development of shale gas and oil. However, the correlations between the static and anisotropic dynamic elastic properties that could provide a basis for the seismic methods to potentially evaluate the fracturing ability of shales without the need of cored samples from the borehole are still poorly understood. We have demonstrated, through dedicated simultaneous laboratory measurements of the anisotropic velocities and the strains of samples under triaxial stress, how the static and anisotropic dynamic elastic properties are correlated in seven lacustrine shales from the Ordos Basin, one of the major shale gas plays in China. The results show that the static and anisotropic dynamic elastic properties are stress-dependent. More importantly, the anisotropic velocities are found to be approximately linearly correlated with the axial strains of the samples at differential stress (the difference between axial stress and confining stress) greater than 30 MPa, with the slopes of the linear correlations in excellent linear relationship with Young’s moduli determined from the static elastic measurements. The results not only reveal the internal link between the static and anisotropic dynamic elastic properties of lacustrine shales, but they also pave a potential way for the anisotropic seismic explorations to remotely evaluate the fracturing ability of shales.