Effect of pore pressure gradient on fracture initiation in fluid saturated porous media: Rock

There are infinite number of small pores at inside of almost rocks, while the pore’s shape and the interconnectivity among the pores are dependent on the type of rocks. The existence of such pores brings special feature in mechanical behavior of rocks, different to cases of other materials. For the case when the pressure in a drilled borehole is raised by fluid injection into the borehole in hydraulic fracturing carried out for rock stress measurements, the fluid penetrates through the interconnected pores into a rock from the borehole wall, and the fluid penetration develops the change in pore pressure, i.e., fluid pressure in the pore. The developed pore pressure distribution causes a circumferential stress in compression around the borehole, where the mechanics to cause the additional stress is analogous to thermal stress. Furthermore, the pore pressure affects to loose strength in failure following the Terzaghi effective stress law. The point stress criterion can be applied to predict tensile failure of rock occurring in such complicated conditions. Then we should assume that the fracture initiation occurs when the maximum effective stress reaches the tensile strength of a rock at a point that is not on the borehole wall but is inside the rock. This approach allows us to interpret quantitatively strange phenomena associated with fluid permeation such that breakdown pressure, i.e., a critical borehole pressure to cause the tensile failure, increases with the rate of borehole pressurization.