Mechanical properties and strength criterion of clayey sand reservoirs during natural gas hydrate extraction

To study the changes in mechanical properties of hydrate bearing clayey sand reservoirs during the mining process and explain the mechanism from a microscopic perspective, soil samples were artificially configured based on the parameters of SH7B drilling cores in Shenhu area of South China Sea. Natural gas hydrate was formed in the samples, and the mining process was simulated by hydrate decomposition under depressurization. Multistage triaxial tests were carried out, and scanning electron microscopy was conducted on samples after triaxial experiment. The results showed that the shear strengths of samples decreased with the decomposition of hydrate, and the effective cohesion was positively correlated with hydrate saturation degree. An expression for the linear growth of shear strength with effective confining pressure and hydrate saturation degree was established. In addition, hydrate saturation degree was introduced as a parameter into the Mohr-Coulomb criterion to establish a strength criterion applicable to clayey sand reservoirs in the mining process. With the decrease of hydrate saturation degree, the microstructures of samples after shearing were gradually compacted, indicating that the hydrate and soil particles bore the external load together during shear test, which maintained the morphology of pores and improved the shear strength of samples. •Analyze the mechanical properties of the samples during depressurization mining.•A Mohr-Coulomb strength criterion for clayey sand reservoirs is established.•Explain the mechanism of mechanical property changes from a microscopic aspect.