Constructing a tough shield around the wellbore: Theory and method for lost-circulation control

According to the mechanism analysis of drilling fluid loss in the formation with low pressure-bearing capacity, three basic theories, including stress cage, plugged zone, and strength ring for wellbore strengthening, were summarized and the corresponding models were established. The application conditions for the three theories and their requirements on the material properties were determined based on the models. Fracture propagation, filtration effect of drilling fluid and depleted formation pressure are the major factors that lead to low formation pressure-bearing capacity. The stress cage theory strengthens the wellbore by establishing the balance between wellbore pressure and formation stress. It is applied in the formation with low-development fracture. It demands that the material has enough compressive strength and reasonable size distribution to sustain certain fracture width and low fracture pressure. The plugged zone theory works by establishing the balance between wellbore pressure and formation pressure. It is suitable in pore and fractured formation and demands that the plugging material has the properties of high surface friction coefficient, resilient deformation, volume fraction and strength to create the synergy effect of rigid granule, fiber and resilient particle. The strength ring theory applies to fracture-vug formation and it requires that the material can cement the weak structural plane or form a high strength body to separate the pressure propagation from wellbore to formation. The effect and guiding significance of the three theories were further confirmed by field applications.