Despite the extensive theoretical work on the subject of hydraulic fracturing for stress measurements, the amount of experimental work has thus far been limited, especially in the case of its validity in fractured rocks. Much work has been carried out about the induced fractures created by hydraulic fracturing test, but not on the effect of already existing fractures and joints and their influence. Haimson (1968) showed that the pumped flow raises the pore fluid pressure in fractured / porous formations and creates additional stresses and displacements. This in tum lowers the critical pressure required to initiate the fracture and reduces the width of the newly formed fracture. Once a fracture has been initiated at the borehole wall, the fluid penetrates the parting of the rocks and the pressure is applied to the walls of the fracture. Therefore the minimum downhole injection pressure required to hold open and extend a fracture is slightly higher than the original and disturbed regional stress normal to the plane of the fracture.
In an attempt to overcome the above problem, a series of hydraulic fracturing tests were performed on fractured rock masses inside the tunnel of a hydroelectric project. The applied pressures in these tests were extended beyond the usual ranges (6 to 8 l/min), reaching up to 20 MPa with a flow capacity of up to 16 l/min. It was observed that by increasing or decreasing the pumping pressure for each cycle, the fracture opening pressure also declined automatically after certain increment. It is interpreted that, since the whole fractured rock mass is subjected to the flow of water; these occurrences are due to the opening of fractures at different spatial positions. When high pressure gradients exist, especially when the permeability rises sharply as the effective pressure approaches zero, a sharp pressure front develops in the zone, moving outward from the borehole. As long as the permeability increases with increasing pore pressure, a steep pressure gradient will tend to develop. After shutoff the pump, instantaneous shut-in pressure is obtained to get the normal stress across the fracture and also to calculate the minimum principal stress magnitude and direction. This is an innovative technique developed by the authors for determination of the stresses in fractured ground conditions.
