Experimental study on the relationship between the matric potential and methane breakthrough pressure of partially water-saturated shale fractures

Abstract

The breakthrough pressures of shale fractures are of great significance for the understanding of shale gas seepage or accumulation, CO2 geological sequestration and radioactive waste storage in shale, and pollution problems caused by gas leakage. In this study, three shale cores taken from the eastern Qaidam Basin in China were used to investigate the breakthrough pressures of partially saturated shale fractures. The fractured shale core had a single longitudinal fracture across the core along the axis. A quantitative description of the relationship between the breakthrough pressure and the water content in the fracture is difficult because of the difficulty in the measurement of the water volume in the fracture. We propose a method to describe this relationship. In this method, the water content in the fracture is described by the matric potential. Cores with different matric potentials were prepared by moistening the cores in different humidity environments. Methane breakthrough experiments were conducted on three core samples under six different matric potentials. The six matric potential levels were −51.38 MPa, −39.08 MPa, −23.45 MPa, −8.55 MPa, −2.33 MPa, and 0 MPa. Based on the experimental results, we apply a conceptual model to qualitatively describe the relationship among the matric potential, water distribution in the fracture, and breakthrough pressure. At a low matric potential, water forms very thin water films on fracture surfaces, and the effect of water on the breakthrough pressure is not notable. As the matric potential increases, the thickness of water film in the fracture increases, and the increase in the breakthrough pressure become notably. We establish a mathematical model to quantitatively describe the relationship between the breakthrough pressure and matric potential. The experimental data show that the increase in breakthrough pressure with the matric potential is a process that proceeds from slow to fast, and there is a critical value of matric potential in this process. When the critical value is reached, the connected gas passages are gradually sealed by water films, and the breakthrough pressure begins to increase significantly.