Stress around a borehole promotes the deformation of the borehole wall. That deformation records modern day stress state.

Design of a standard mini-hydraulic fracturing experiment.

Use of an mini-hydraulic fracturing experiment to measure the minimum compressive stress.

Estimated uniaxial compressive strength based on correlation (above) and sonic velocity log.

Stress model at Coso Geothermal Field based on mini-hydraulic fracturing experiments, borehole deformation including breakouts, tensile fractures, and petal-centerline fractures, and borehole geophysical logs. The model includes the stress conditions for optimally oriented normal and strike-slip faulting regimes at various estimates of the coefficient of sliding friction. A sensitivity analysis evaluates our uncertainty in  SHmax based on all model parameters including Shmin, breakout width, uniaxial compressive rock strength, and fluid pressure.

Rock failure under stress depends on the strength of the rock. To estimate rock strength at depth, we correlated tri-axial experiments of rock strength with sonic velocity, then applied this correlation using a sonic velocity log.

ROCK STRENGTH

THE MINIMUM COMPRESSIVE STRESS IS MEASURED BY PROPAGATING A MODE I FRACTURE UNDER ELEVATED FLUID PRESSURE

STRESS MODEL

Stress concentration around a borehole coinciding with one of the principal stresses based on the Kirsch equations.

Examples of failure along the walls and floor of a borehole. Several borehole tools are available to image and analyze the physical properties of the rock pierced by the borehole.