Unconsolidated sandstone and soft mudstone formation are generally shallow buried and have special physical and mechanical properties. Under the influence of in-situ stresses, the borehole in unconsolidated sandstone and soft mudstone will be deformed after drilling, resulting in the drilling pipe stick, increasing the non-production time and affecting the drilling safety. Therefore, it is of great importance to determine the drilling fluid density which can effectively control the borehole deformation to reduce drilling cost and ensure drilling safety. The indoor mechanical experiments were carried out using shallow unconsolidated sandstone and soft mudstone cores from the Bohai oilfield. The results show that both types of cores will produce obvious transient deformation under stress and have strong creep characteristics. The elastic modulus of unconsolidated sandstone is about 0.25 GPa, while the Poisson ratio is 0.31, and the steady-state creep rate is 4.7-9.9×10-8 s-1 under the deviatoric stress of 5-10 MPa. For the soft mudstone, the elastic modulus is 0.3 GPa, and the Poisson ratio is 0.35, and the steady-state creep rate is 3.1-6.3×10-8 s-1 under the deviatoric stress of 5-10 MPa. Analysis shows that the creep of sandstone and mudstone conforms to the Weertman model, and the finite element numerical simulation method was further used to calculate the wellbore deformation during drilling. The results show that when the drilling fluid density is 1.25 g/cm3, the borehole shrinkage rate in the unconsolidated sandstone is about 0.0006 h-1 and in the soft mudstone is about 0.00045 h-1. If the borehole is exposed for 20 days during drilling, the shrinkage will exceed 20%. The mechanical characteristics of unconsolidated sandstone and soft mudstone can be analyzed by the methods above, and the deformation change with time after drilling can be further analyzed, which can determine the safe drilling time under different drilling fluid density conditions and reduce the effect of borehole shrinkage on drilling engineering.

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