Experimental Study of Fracture Toughness in Simulated Cores of Natural Gas Hydrate Sediments

Natural gas hydrate reservoirs have low permeability, limited temperature and pressure conductivity, and low development efficiency. Hydrate reservoir fracturing is a potential solution to the above issues. Fracture toughness, as an inherent property of material resistance to tensile fracture, is an essential parameter for evaluating the fracturability of hydrate reservoirs. In this paper, the methods of simulating artificial cores of natural gas hydrate sediments and controlling ice saturation are explored. Based on the semi-circular bend (SCB) test, the effects of notch length and ice saturation on the fracture toughness of artificial cores of natural gas hydrate sediments are investigated, and the deformation characteristics on the surface of the rock specimens are analyzed using the digital image correlation (DIC) method. The results indicate that when the dimensionless notch length β = 0.2−0.5, the peak load and the strain at core damage decrease gradually with the increase of the notch length, and the fracture toughness of the core increases from 0.076 MPa·m1/2 to 0.115 MPa·m1/2, which shows an overall trend of increasing, and the core exhibits obvious brittle damage characteristics. For SCB specimens with dimensionless notch length β = 0.4 and ice saturation in the range of 10%−90%, with the increase of ice saturation, the peak load, strain at core damage and fracture toughness values all decrease slowly at first and then increase rapidly, and the fracture toughness value of the core with ice saturation of about 20% is the lowest, and the fracture toughness value of the core is in the range of 0.132 MPa·m1/2−0.347 MPa·m1/2.