**2.5 Balloon phenomena**

At slow bloating rate it is very difficult to inflate the punctured balloon as the air will be leaked through the hole, but at the heavy bloating rate it is possible to inflate the punctured balloon even though the leakage exists. Hence the solution is the rate of bloating should be much higher than the rate of the leakage through the puncture (**Figure 3**). The same balloon phenomena are applicable in the case of hydraulic fracturing testing in fractured rocks. At slow injection rates the fluid would have time to diffuse into the fractures and lower the effective pressure, whereas at fast injection rates a steep pore pressure gradient would develop near the borehole, i.e., If fluid were injected fast enough, even though the shear strength of the rock near the borehole would be exceeded, the load on the sample would be supported by the surrounding 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 is exceeded (**Figure 4**). Hydraulic fracturing is initiated when the fluid pressure exceeds the minimum principal compressive stress by the tensile strength of the host rock. Typical in-situ tensile strengths of rocks are in the order of 0.5–6 MPa (Haimson & Rummel [22], Amadei & Stephansson [23], Enever & Chopra [24]) [2, 19]. The propagation is made possible by the linking up of discontinuities in the host rock ahead of the hydraulic fracturing tip. Discontinuities are significant mechanical breaks in the rock, normally with low or negligible tensile strengths.
