Damage evolution characteristics of heterogeneous fractured sandstone reservoir under different fracturing fluids

To investigate the coupling effect of in situ stress and fluid pore pressure and the propagation law of damaged area during hydraulic fracturing and gas fracturing in heterogeneous fractured reservoirs, according to the effective stress principle and the smooth Rankine stress criterion, this paper establishes a coupled fluid-solid-damage mathematical model of heterogeneous rock layers and carries out numerical simulations on the characteristic field changes of rock layers under water and nitrogen fracturing. The results show that the continuous injection of nitrogen or water can cause an increase in pore pressure in natural fractures and rock matrices, leading to rock damage and an increase in porosity. Under the same parameters, the damage area and length of the rock mass during nitrogen fracturing are larger than those of hydraulic fracturing, and the changes in pore pressure, permeability, and porosity are consistent with the damage area. The pore pressure near the connected natural fractures is relatively high, resulting in tensile strain; the pore pressure at isolated fractures is relatively low, and some even produce compressive strain. During nitrogen fracturing, the damage evolution is less affected by the in situ stress and the matrix permeability, while hydraulic fracturing is more affected by them. The damage area is the smallest when the horizontal in situ stress is equal. When the initial permeability of the matrix is low, the damaged area mainly follows the natural fractures and is distributed in a strip shape. As the permeability of the matrix increases, the fracturing fluid can enter the matrix, and the damaged area is in the shape of a block.