Analysis of Drilling Fluid Loss Behavior in Deepwater Shallow Formations.

Deepwater shallow formations are typically weakly consolidated, highly porous, and highly permeable, which results in a narrow safe-density window and makes drilling fluid loss highly sensitive to small wellbore-pressure fluctuations. In this study, a finite-discrete element method (FDEM) model implemented in MultiFracS was used to investigate the transition from permeability-induced loss to fracture-induced loss around a deepwater shallow wellbore. The model couples triangular finite elements, zero-thickness cohesive joint elements, Darcy-type matrix seepage, fracture flow governed by the cubic law, and matrix-fracture fluid exchange. A South China Sea field case was used to validate the predicted loss and flowback volumes. The simulations show that permeability-induced loss is controlled mainly by pressure diffusion in the high-porosity matrix and remains relatively limited, whereas once the wellbore pressure reaches the fracture initiation condition, newly connected fractures provide preferential flow paths and sharply increase cumulative loss. Repeated wellbore-pressure fluctuations further narrow the operational pressure margin by promoting the progressive weakening of the near-wellbore formation. Sensitivity analyses indicate that porosity, permeability, drilling fluid viscosity, yield stress, and pressure fluctuation history jointly control the magnitude of loss and flowback. The results clarify the mechanism by which shallow formation breathing can evolve into severe fracture-induced loss and provide a modeling basis for pressure management and drilling fluid property optimization in deepwater shallow formations.