Studies on Hydraulic Fracturing Stress Measurement Assisted by Water Jet Boreholes Slotting

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Both laboratory tests and in-situ tests for hydraulic fracturing stress measurement assisted by water jet borehole slotting were carried out.In the laboratory tests, Pairs of deep longitudinal slots were made by abrasive water jet from the inside of boreholes which were drilled in large scale mortar specimens.Confining pressures were applied to the specimens by flatjacks and a loading frame. Hydraulic fracturing tests were carried out in the sections including the slots and the fractures on the plane of prolongation of the slots were produced. Shut-in pressures of the fractures were then measured.It was shown in the tests that the stresses detected were slightly greater than the stress applied.In the in-situ tests, both the longitudinal slots and the circular slots perpendicular to the boreholes were made in the hard rock at the site which is about 700 meters below the surface. Hydraulic fracturing tests were carried out and it was confirmed that the longitudinal and transverse fractures on the plane of prolongation of the longitudinal slots and the circular slots, were produced, respectively. Re-opening pressures were then measured. The test results show that the maximum and the minimum principal stresses detected were nearly equal to the stresses which were measured using the same borehole by the double fracture method.Numerical simulation based on the boundary element method of fracture extension by internal pressurization at the borehole section in an infinite solid was also carried out. The computed results shows that the turning point of fracture propagation curve movesfarther away from the slit tip with an increase of principal stress ratio and decrease of fluid pressure drop rate induced by the crack increment.