*2.3.1 Methodology adopted*


#### **2.4 High flow rate technique in fractured rocks**

In the literature on hydraulic fracture experiments it is generally assumed that a crack will initiate when the tensile stress at the borehole wall exceeds the tensile strength of the rock. It is possible, however, that in regions under tectonic shear stress, shear failure could be induced in the rock about the borehole at much lower fluid pressures than would be required to produce tension cracks, simply by lowering the effective pressure (confining pressure minus pore pressure) to the point where the shear strength of the rock is exceeded [14].

Haimson [11] showed that the compressional strength of the rock mass depends on effective pressure and differential stress. He suggested that a sample subjected to a given confining pressure and differential stress could be made to fail in shear or tension simply by controlling the pore pressure [21]. One way of testing this hypothesis would be to vary the pore fluid injection rate. At slow injection rates the water or any other fluid which is having low viscosity would have time to be drawnout into the fractured zones and lower the effective pressure, whereas at fast injection rates a steep pore pressure gradient would develop near the borehole. If fluid were pressurized fast enough, even though the shear strength of the rock near the borehole would be surpassed, the load on the area would be supported by the neighbouring rock in which the pore pressure was still low. In this way, shear failure of the sample would not occur and instead, a tension crack would form when the tensile strength of the rock near the borehole was exceeded [7].

In settings with extreme overpressure, pore-water pressure approaches the pressure required for natural hydraulic fracturing. Unlike other fractured seals, hydraulic fractures remain open only if pore pressure exceeds fracture pressure [13].

To test this hypothesis, a series of 24 experiments was conducted at different zones inside the EX-size boreholes (core drilled boreholes of 38 mm diameter) where the rock mass is highly fractured. In all these experiments, the differential stresses were ranging from 10 to 200 bars and the fluid injection rates were varying by 4–16 l/min. It was assumed that the failure mechanisms (shear or tension) observed for different injection rates would be controlled by the pore pressure

*Hydraulic Fracturing in Porous and Fractured Rocks DOI: http://dx.doi.org/10.5772/intechopen.106552*

distribution in each test at the time of failure. The results are validated with normal flow rate of HTPF method in good rock mass zones of the same bore holes. Rock mass quality are characterised using a rating system. The rock mass is categorised into different classes (i.e., very good to very poor), incorporating the combined effects of different geological and geotechnical properties. This enables the comparison of rock mass conditions throughout the site and the delineation of regions of the rock mass ranging from 'very good' to 'very poor', thus providing a map of the boundaries of rock mass quality. The details of the investigations, stress evaluation procedure in fractured rocks and the results are given below.
