**2.1 Hydraulic fracturing in fractured rocks**

In Haimson's thesis [10], about 400 tests on hollow cylindrical and cubical specimens of 5 different fractured and non-fractured rocks were conducted under constant tri-axial external loading and increasing borehole fluid pressure [12]. In all of the samples tested, the induced hydraulic fractures were always found to be tensile and no shear failure was observed. The fractures in all the rock types were either vertical or horizontal depending on the applied stresses. These fractures are observed in pairs, mostly parallel to the nearly vertical wellbore axes, and on diametrically opposite sides of the borehole walls [15].

Haimson and Fairhurst [19]showed that the pumped flow increases the pore fluid pressure in fractured/porous formations and produces additional stresses and displacements (**Figure 2**).

Hence it is difficult to get the breakdown pressure (*P*c) or the peak pressure in the first pressure cycle in normal flow rates in fractured rocks. Before reaching its peak, pressure typically declines even if pumping is continued at the initial flow rate as the pressure required to induce a hydraulic fracture in HF tests, or fracture reopening in hydraulic fracturing tests on preexisting fractures (HTPF) tests is not sufficient enough. It clearly indicates the following:


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

**Figure 2.** *Existing fractures in a borehole.*

4. Shut-in pressure cannot be reached, signifies that maximum fluid is infiltrated in the fractures. (The shut-in pressure denotes at which a hydrofracture pauses generating and closes following pump shut-off. The determination of the shutin pressure *P*si is when a sharp break is detected in the pressure-time curve after the initial fast pressure drop following pump shut-off) [9].

In normal conditions or in good rock mass, the shut-in pressure (*P*si) reaches, after the pump is shut off following breakdown or fracture reopening. But in the present case in fractured rocks, shut-in pressure cannot be achieved even after repeated cycles. The first difficulty is the pressure decay just before shutting off, and the other difficulty will be not getting shut in pressure to calculate the minimum principal stress [17, 20].
