Stress Sensitivity of Sandstones
Elastic wave velocities in sandstones vary strongly with stress because of stress-sensitive discontinuities within the rock, such as grain boundaries, microcracks, and fractures. For example, Figure 1 shows P- and S-wave velocities measured by Han (1986) as a function of increasing hydrostatic stress for an air-filled Gulf of Mexico sandstone from a depth of 15,845 ft (4830 m). Those curves are concave down, which is consistent with the expected decrease in grain-boundary compliance that results from increasing contact between opposing faces of the grain boundary as stress increases.
In the presence of a nonhydrostatic stress field, elastic wave velocities in sandstones often show significant stress-induced anisotropy because the response of a discontinuity (such as a grain boundary or microcrack) to stress depends on its orientation relative to the applied stress field. Examples of ultrasonic wave anisotropy in sandstones in the presence of an anisotropic applied stress include the measurements of Holt and Fjaer (1987) for weakly consolidated sandstones, Sammonds et al. (1989) for Darley Dale Sandstone, and Sayers et al. (1990) and Scott et al. (1993) for Berea Sandstone.
This chapter discusses stress dependence of elastic wave velocities in sandstones, initially in the context of third-order elasticity theory. The theory of third-order