The state of stress in the earth’s crust has been widely recognized as a basic parameter necessary in the engineering design of underground openings, in the earthquake studies, and in the extraction of oil and gas. The magnitudes and directions of in situ stresses, obtained by hydraulic fracturing, provide the crucial design parameters – layout (orientation, depth, and cross-sectional shape) of the opening, frictional strength of the fault, and orientation of the fracture propagation. 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 Fracture Technique.
Hydraulic Fracture Technique for Stress Measurement:
Hydraulic fracturing field testing is a two-step procedure. Sequentially, it consists of (a) pressurization of the selected borehole segment until fracturing occurs, and (b) delineation of the induced fracture. Typically, a complete test will consist of two trips down hole to the test interval, each with different equipment assemblies.
The hydraulic fracture technique may be used to determine the in-situ rock stress in a plane perpendicular to borehole. This is done by application of fluid pressure (normally water) in a test section in a borehole isolated by straddle packers until the rock fails in tension. The fluid pressures required to generate, propagate, sustain and reopen tensile fractures in the rock are recorded as function of time, and these may be related to magnitude of the existing stress field. Directions of measured stress are normally achieved by observing and measuring the orientation of the hydraulically induced fracture plane by the use of a so-called impression packer. The induced hydro fracture is oriented parallel with the major secondary principal stress σH in a plane perpendicular to the borehole.
Hydraulic fracture may be carried out in vertical holes drilled from the surface or underground in holes drilled from tunnels (vertical, inclined or horizontal). May be combined with overcoring from tunnels, i.e. hydraulic fracturing is carried out after overcoring in the same hole.
• Maximum borehole length: 250 m
• Borehole diameter: 46 – 76 mm
• Computerised logging equipment May be used in combination with overcoring in holes drilled from tunnels
• Equipment units may be shipped by normal air freight
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