Abstract: The shallow weakly consolidated strata in Bohai Sea area are characterized by new sedimentary age and poor diagenesis, resulting in loose and unformed core samples, and difficulty in conducting conventional rock mechanical tests, which is not conducive to the targeted drilling design in shallow sections. Through comprehensive mineral characterization, physical simulation experiments, and digital core modeling, the mechanical properties and failure mechanisms of shallow strata have systematically been analyzed. By using XRD testing, casting thin section analysis, and nanoindentation technology, the results show that montmorillonite accounts for 48.57% of the clay minerals in the formation, with strong water sensitivity, poor cementation degree, and well developed pores of rock samples. Physical simulation experiments indicate that as the water cut increases from 20% to 30%, the compressive strength of the rock sample decreases from 1.68 MPa to 0.41 MPa, the elastic modulus decreases from 22.1 MPa to 4.81 MPa, the failure mode changes from brittle shear failure to plastic deformation, the number and width of shear zones significantly increase, and the dilatancy effect enhances. The digital core model constructed based on PFC 6.0 is highly consistent with the physical experimental results, which can not only reproduce the dynamic process from elastic deformation to plastic failure of core samples, but also reveal the mechanism of weakening the interparticle bonding force caused by water infiltration through microscopic damage analysis. As a result, a digital core simulation model for shallow weakly consolidated formations in Bohai Sea area has been established for the first time, quantitatively elucidating the degradation law of increasing water cut on rock mechanical properties. The mechanism of water sensitivity induced disasters has been revealed from the dual scale of “macroscopic mechanical response–microscopic damage evolution”, which provides theoretical support for drilling fluid system optimization, wellbore stability control, and sand control process design, as well as new ideas for mechanical study of weakly consolidated formations in sea area.

Key words: Bohai Oilfield; shallow formation; weak consolidation; digital core; rock mechanics