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HF Introduction


The in situ state of stress in the earth's crust has been widely recognized as a basic parameter necessary in the engineering design of underground openings. Quantitative evaluation of horizontal in situ stresses in rock at a specific site cannot be made since gravitational forces are practically the only one clearly understood. Therefore, these horizontal stresses require direct measurements in the field. Presently the most common method of measuring in situ stress from near-surface to considerable depths is hydraulic fracturing (or hydrofrac).

Typically hydraulic fracturing is conducted in vertical boreholes. A short segment of the hole is sealed off using an straddle packer. This is followed by the pressurization of the fracture-free segment of the hole by pumping in water. The pressure is raised until the rock surrounding the hole fails in tension at a critical pressure. Following breakdown, the shut-in pressure, the lowest test-interval pressure at which the hydrofrac closes completely under the action of the stress acting normal to the hydrofrac. In a vertical test hole the hydrofrac is expected to be vertical and perpendicular to the minimum horizontal stress.

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Hydraulic Fracturing