Non-hydrostatic stress field orientation inferred from orthopyroxene (Pbca) to low-clinoenstatite (P21/c) inversion in partially dehydrated serpentinites

Seismic and electromagnetic properties generally are anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, [Formula: see text], and attenuation [Formula: see text]) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20–35 MPa) and pore (2–17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale all affect the elastic and electrical anisotropies. Strong linear correlations ([Formula: see text]) between [Formula: see text], [Formula: see text], and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore-pressure changes (effective stress coefficients [Formula: see text]), whereas P-wave properties provide simultaneous information about the confining (from [Formula: see text], with n slightly less than 1) and pore pressure (from [Formula: see text], with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities.

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Internal uniform hydrostatic stresses in a three-phase non-elliptical inclusion subjected to a nearby concentrated couple

Mathematics and Mechanics of Solids ◽

10.1177/1081286519827059 ◽

2019 ◽

Vol 24 (9) ◽

pp. 2931-2943 ◽

Cited By ~ 1

Author(s):

Xu Wang ◽

Peter Schiavone

Keyword(s):

Stress Field ◽

Conformal Mapping ◽

Hydrostatic Stress ◽

Geometric Parameters ◽

Interphase Layer ◽

Infinite Matrix ◽

Elliptical Inclusion ◽

Three Phase ◽

Uniform Hydrostatic Stress Field ◽

Concentrated Couple

We apply conformal mapping techniques with analytic continuation to study the existence of a uniform hydrostatic stress field inside a non-elliptical inclusion bonded to an infinite matrix via a finite thickness interphase layer when the matrix is simultaneously subjected to a concentrated couple as well as uniform remote in-plane stresses. We show that the desired internal uniform hydrostatic stress field is possible for given material and geometric parameters provided a certain constraint is placed on the remote loading. Subsequently, when the single loading parameter, five material parameters and three geometric parameters are prescribed, all of the unknown complex coefficients appearing in the series representing the corresponding conformal mapping function can be uniquely determined from a set of nonlinear recurrence relations. We find that the internal uniform hydrostatic stress field, the constant mean stress in the interphase layer and the hoop stress along the inner interface on the interphase layer side are all unaffected by the existence of the concentrated couple whereas the non-elliptical shape of the (three-phase) inclusion is attributed solely to the influence of the nearby concentrated couple.

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