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- Title
Stress‐Induced Anisotropic Poroelasticity in Westerly Granite.
- Authors
Elsigood, Bobby; Brantut, Nicolas; Meredith, Philip G.; Healy, David; Mitchell, Thomas M.; Aben, Frans M.
- Abstract
We measured poroelastic properties of cracked granite under triaxial conditions, at elevated confining pressure and a range of differential stresses. Skempton's coefficients and undrained Young's modulus and Poisson's ratio were determined directly by recording in situ fluid pressure during rapid cycles of axial and radial stress. Drained properties were measured both statically and dynamically at ultrasonic frequencies. At a given confining pressure, increasing differential stress leads to the development of elastically transverse isotropy, with symmetry axis aligned with the compression axis. Skempton's coefficients are also anisotropic, with larger changes in pore pressure in response to radial stress (coefficient Bx) than to axial stress steps (Bz). The anisotropy in the Skempton coefficients increases with increasing differential stress, with Bz decreasing and Bx slightly increasing. The evolution of static moduli and the Skempton coefficients is well approximated by Gassmann's equation using dry moduli obtained from ultrasonic measurements. Simplified predictions of the Skempton coefficients based on crack density tensors inverted from dynamic data also shows acceptable agreement with direct observations. Perfect quantitative agreement is not reached, due to the imprecision of our dynamic measurements, model simplifications, and inherent differences between static moduli obtained using stress steps of several MPa stress and dynamic ultrasonic stress oscillations. Plain Language Summary: Granites are crystalline rocks, almost entirely devoid of holes except for thin fissures (also called micro‐cracks), which are partially open in stress‐free rock and contain a fluid such as water. Cracks tend to close in response to application of external loads in a direction perpendicular to their surface. When the fluid inside the cracks is not free to entirely escape, crack closing also causes the internal water pressure to increase. Here, we carried out experiments wherein we measured certain mechanical properties of cylindrical samples of granite while applying different loads on the flat faces of the samples and different levels of pressure on the side surface. We observed that the rock response was different when measured in the load direction or perpendicular to it. This dependence on direction is called anisotropy. We also found that applying an axial load or a lateral pressure caused different sets of cracks to close and therefore produced different changes of the internal water pressure. Finally, we validated a theoretical model capable of reproducing our experimental observations. Key Points: The transversely isotropic poroelastic parameters were measured in Westerly granite at a range of differential stressesThe Skempton coefficients were determined via in situ pore pressure measurements and compared to models values using crack density tensorsOnly the qualitative relationship and not the quantity agreement between direct and estimated Skempton coefficients was achieved
- Subjects
POISSON'S ratio; DEVIATORIC stress (Engineering); RADIAL stresses; AXIAL stresses; GRANITE; POROELASTICITY
- Publication
Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 11, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB026909