Deepwater shallow sediment is less-consolidated, with a rock mechanical behavior similar to saturated soil. It is prone to borehole shrinkage and downhole leakage. Assume the deepwater shallow sediments are homogeneous, isotropic, and ideally elastoplastic materials, and formation around the borehole is divided into elastic and plastic zone. The theories of small deformation and large deformation are, respectively, adopted in the elastic and plastic zone. In the plastic zone, Mohr–Coulomb strength criterion is selected. The stress and deformation distributions in these two zones, and the radius of plastic zone are derived. The collapse pressure calculation formula of deepwater shallow sediments under the control of different shrinkage rates is obtained. With the introduction of excess pore pressure theory in soil mechanics, the distribution rule of excess pore pressure in these two zones is obtained. Combined with hydraulic fracturing theory, the fracture mechanism of shallow sediments is analyzed and the theoretical formula of fracture pressure is given. The calculation results are quite close to the practically measured results. So the reliability of the theory is confirmed.

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