ABSTRACT:

Special rock mechanical characteristics of natural gas hydrate (NGH) sediments lead to the inapplicability of traditional well stability models. In this paper, a new model to calculate the collapse pressure and fracture pressure of a borehole in NGH formation is studied based on the laboratory experiment results in published literature, mainly considering the effect of the saturation of NGH. Amended Mohr-Coulomb failure criterion is applied to calculate the collapse pressure while the fracture pressure is calculated combining Griffith criterion. Case study of this model is carried out and sensitivity analysis is performed to reveal the effects of original rock cohesion, Biot coefficient, frictional angle and pore pressure. The results show that the collapse pressure increases, while the fracture pressure decreases, with the decrease of NGH saturation. The collapse pressure decreases with the increase of the original rock cohesion and the decreases of Biot coefficient and pore pressure at any saturation. however, it decreases with the increase of the frictional angle in low saturation region while shows opposite trend in high saturation region. The fracture pressure increases with the increases of the frictional angle and the original rock cohesion and the decreases of Biot coefficient and pore pressure.

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