Determination of rock stress- Code of practice, Part 1: Using hydraulic fracturing technique
1994 Edition

For hydraulic fracturing assessments, the recommended setup includes drilling equipment capable of producing a stable borehole to the required depth, with a hole diameter compatible with the packer system. The test interval should be at least 0.9 meters in length, with the test section typically extending to a minimum of five times the borehole diameter. Inflatable packers, preferably a double packer system, are used to isolate the test zone, set hydraulically or by gas expansion. Additional tools such as impression packers and optical or acoustic logging instruments assist in fracture characterization and borehole inspection, ensuring reliable isolation and pressurization of the test interval.

Fracture initiation pressure (Pt) is the fluid pressure inside the borehole when the rock first fractures due to induced tensile stresses reaching the rock's rupture threshold. This is identified by a sudden pressure change during gradual fluid injection. Shut-in pressure (Ps) is recorded immediately after stopping fluid injection once the fracture has propagated approximately three times the borehole diameter, with the system sealed. Reopening pressure (Px) is determined during subsequent pressurization cycles when the previously formed fracture reopens. The procedure involves monitoring pressure variations during controlled injection, shut-in, and repressurization phases to accurately capture these critical pressure points.

Accurate in-situ horizontal stress measurement involves understanding and accounting for factors such as overburden pressure, geological structures, tectonic influences, residual, and thermal stresses. The hydraulic fracturing technique in deep boreholes is the most direct and reliable method, where induced fractures and pressures are measured to derive stress magnitudes and orientations. Complementary methods and numerical or analytical modeling can extrapolate data near excavations to undisturbed zones. This multi-faceted approach ensures precise stress evaluation crucial for underground engineering applications.

Test intervals must be chosen with a minimum fracture length of 0.9 meters and typically a test section length around five times the borehole diameter. The borehole should be drilled slightly beyond the intended depth to accommodate these intervals. Inspection tools such as borehole periscopes, TV cameras, acoustic televiewers, calipers, and orientation devices (magnetic or gyroscopic compasses) are utilized to evaluate borehole conditions, diameter suitability, and alignment. Packers are initially set at pressures below anticipated fracture initiation levels and gradually increased during testing to prevent premature fracturing, ensuring accurate interval selection and reliable testing.

The test report should comprehensively detail the test site location, geological and geotechnical logs including discontinuities and borehole conditions, rock type and core availability, and specifics of the test depth, zone length, and borehole size. It must include graphical presentations of pumping rate and injection pressure over time, tabulated data for depth, various pressures, calculated maximum and minimum horizontal stresses along with their orientations, and fracture tracer records such as photographs or acoustic logs. The methodology for calculating maximum horizontal stress and laboratory data on borehole tensile strength should be described, along with explanations for any significant discrepancies with other datasets. This ensures thorough documentation for rock mass characterization.