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DS Subrahmanyam


Notwithstanding the broad hypothetical work about hydraulic fracturing technique for estimation of stresses in rock mass, the experimental work has so far been restricted, particularly on account of its legitimacy in fractured rocks. Much work has been completed about the induced fractures created by hydraulic fracturing tests, however not on the impact of previously existing fractures and joints and their impact. When a fracture has been originated at the borehole wall, the fluid infiltrates the splitting of the rocks, and the pressure is applied to the walls of the fracture. In this way, the least downhole infusion pressure expected to hold open and expand a crack is somewhat higher than the normal to the plane of the fracture.


To overcome the above issue, a sequence of hydraulic fracturing tests was performed on fractured rock masses inside the tunnel of one of the hydroelectric projects. The applied pressures in these tests were stretched out past the standard reaches (6 to 8 l/min), coming to up to 20 MPa with a flow capacity of up to 16 l/min. It was seen that by expanding or diminishing the pumping pressure for each cycle, the fracture opening pressure declined consequently after specific augmentation. It is deciphered that, since the entire fractured rock mass is exposed to the flow of water; these events are because of the opening of fractures at various spatial positions. When high pressure gradients exist, especially when the permeability increases abruptly as the effective pressure approaches zero, a sharp pressure front develops in the region, moving outward from the borehole. If the permeability upsurges with increasing pore pressure, a steep pressure gradient will tend to grow. After shutoff the pump, rapid shut-in pressure is obtained to get the normal stress across the fracture and to calculate the minimum principal stress magnitude and direction. This is the state-of-the-art technique established by the authors for determination of the stresses in fractured ground environments.



KEYWORDS: Hydraulic fracturing; fractured rocks; high flow rate; normal stress.

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