Mechanism of Fracture Initiation and Propagation Using a Tri-axial Hydraulic Fracture Test System on the Cement Blocks

In this study, a series of laboratory tests using a real Tri-axial Hydraulic Fracture System were performed to investigate the mechanism of fracture initiation and propagation on thecement blocks in different reservoirs in normal and tectonic stress regimes. The influences of crustal stress field, confining pressure, and natural fractures on the fracture initiation and propagation were discussed. Experimental results demonstrate that stress concentration around the hole would significantly increase the fracture pressure of the rock. At the same time, natural fractures in the borehole wall would eliminate the stress concentration, which leads to a decrease about two thirds in the fracture initiation pressure. Two interaction types between induced fractures with pre-fracture were observed including crossing and opening the pre-existing fracture. In a normal stress regime, hydraulic fracture crossed the pre-fracture. But in tectonically stressed or shallow reservoirs, due to high interaction between hydraulic and natural fractures, hydraulic fracture was arrested by opening of the pre-fracture.