Anisotropic dynamic-static elasticity correlations in lacustrine shales: Experimental insights for in situ stress estimation

Abstract

Dynamic-static elasticity correlation is of great concern in numerous geo-engineering applications, like in situ stress prediction. However, establishing a precise dynamic-static correlation in unconventional shales is challenging due to their intrinsic anisotropy and the in situ stress field anisotropy. We perform multi-stage deviatoric stress cycling tests together with ultrasonic velocity measurements on 13 pairs of lacustrine shales, intending to establish the anisotropic dynamic-static elasticity correlations considering the in situ horizontal stress. The experimental results reveal that dynamic Young's moduli are greater than their static counterparts, while there are no unified relations for dynamic-static Poisson's ratios. From a microscopic view, the dynamic-static contrasts are attributed to friction-slip-related events across bedding/grain contacts and crack interfaces induced by a stress increment. The bedding-normal dynamic-static Young's modulus correlation is directly established using the measured data at 21 MPa confining pressure, equivalent to the in situ horizontal stress. Additionally, there exists a linear relationship between dynamic and static Young's modulus anisotropy (E₁₁/E₃₃). Hence, the bedding-parallel static Young's modulus is indirectly deduced by combining the bedding-normal dynamic-static correlation with the dynamic-static anisotropy linear relationship. Ultimately, the anisotropic dynamic-static correlations are applied to calculate the stress coupling factor, a key parameter in stress profile prediction. After comparing with the stress coupling factor derived without dynamic-static conversions in well logging, we get an implication that neglecting anisotropic dynamic-static correlations would significantly underestimate in situ horizontal stresses in shale reservoirs.