Effect of water and nitrogen fracturing fluids on initiation and extension of fracture in hydraulic fracturing of porous rock

An improved fracture initiation pressure criterion based on the initiation criterion of the mode I crack is proposed to interpret the different phenomena induced by water and nitrogen fracturing fluids. Hydraulic fracturing experiments on sandstone, coal and shale cores are performed using water and nitrogen to investigate the effects of the rock type and fracturing fluid type on the fracture initiation pressure and to verify the improved initiation pressure criterion. The crack propagation process and extended fracture complexity are also investigated based on hydraulic fracturing experiments. The sensitivity of the key parameter to the crack initiation pressure is explored using the improved initiation pressure criterion. The results show that the improved initiation pressure criterion is reliable. Nitrogen fracturing fluid can effectively reduce the fracture initiation pressure of rock compared with water, especially for low permeability shale. Micro-crack initiation occurs more easily before failure and the crack extension process is more violent in nitrogen fracturing. In water fracturing, a single fracture is induced, but complex fractures including tortuous and multiple fractures are produced in nitrogen fracturing. The extended fracture complexity in nitrogen fracturing is more likely to be affected by the lithologic character of rock compared with water fracturing. From the parameter sensitivity study, there are upper and lower limits on the permeability of shale causing pressurization rate to have a significant impact on the fracture initiation pressure. The upper and lower limits of permeability for nitrogen are four orders of magnitude greater than those for water. In the affected range of permeabilities, whether the fracturing fluid is water or nitrogen, changes in the fracture initiation pressure with the increase of the pressurization rate show an approximately logarithmic increase, linear increase and exponential increase in order of increasing permeability.