Pore Pressure Coefficient Anisotropy Measurements for Intrinsic and Induced Anisotropy in Sandstone

Abstract

Abstract In this study, we determine experimentally the effect of inherent and stress-induced anisotropy on stiffness components, elastic moduli, and Biot's pore pressure coefficients (PPCs) for Lyons outcrop Colorado sandstone which exhibits a clear transverse isotropic rock structure. Both dynamic and quasi-static methods were used under a non-hydrostatic state of stress to perform the measurements on dry core samples. Our assumption of apparent transverse anisotropy was initially confirmed with acoustic velocity measurements and at a later stage in the loading with experimental transverse anisotropic failure analysis. The objective of this study is to identify and isolate the effect of stress-induced anisotropy from the inherent transverse anisotropy on the measured stiffness components, elastic moduli, and Biot's PPCs. The effect of stress-induced anisotropy appears to have significant control on measured stiffness components, elastic moduli, and Biot's PPCs as compared with the inherent transverse anisotropy effect. Our work shows that the stiffness components, Mij, thus computed elastic moduli, are highly influenced by the stress-induced anisotropy especially the off-diagonal stiffness components, M12 and M13, where the increase in their magnitudes from the dynamic measurements prior to failure is determined to be 100% and 81%, respectively. The difference in the magnitude between the axial and lateral Biot's PPCs, in line with bedding planes and perpendicular to them, is measured to be 24% and 16% from the quasi-static and dynamic methods, respectively; whereas, the effect of stress-induced anisotropy reduced the dynamic average magnitude of the Biot's PPCs along the bedding planes and transverse to these planes by 63% over a stress range of 145 MPa.