A Comparative Study on Fracture Characteristics of the Red Sandstone under Water and Nitrogen Gas Fracturing

Abstract

Because of the disadvantages in fracturing with the water-base fracturing fluids and the development of reservoir reconstruction technology, nonaqueous fracturing fluid plays a more and more important role in the worldwide exploitation of unconventional natural gas. In this paper, the fracturing experiments of the red sandstone by using water and nitrogen gas are firstly carried out, and the breakdown pressures and failure patterns of the red sandstone specimens under different fracturing fluids are compared. Then, based on the governing equations, the fracturing experiments with water and nitrogen gas are modeled by using a finite element method software—COMSOL Multiphysics. The conclusions can be obtained as follows: (1) The breakdown pressure of the nitrogen gas fracturing is 60% that of the water fracturing. The ultralow viscosity property of nitrogen gas is the reason for this phenomenon. (2) Compared with the water fracturing, the nitrogen gas fracturing causes greater volumetric strain and a more complex fracture pattern in terms of the number, length, and width of the cracks. (3) The numerical results are close to the experimental data. It implies that numerical modeling in this study can be used as a tool for predicting the breakdown pressure and rupture time. (4) After a sensitive study based on the numerical modeling, it can be found that the loading rate will influence the seepage range which dominates the pore pressure distribution and affects the breakdown pressure for the water fracturing. However, for the nitrogen gas fracturing, the breakdown pressure almost does not change with the loading rate as the nitrogen gas can easily penetrate the specimen from the radial direction.