The annulus has a
wide and narrow clearance due to casing eccentricity
in the cementing process and due to the eccentricity of casing in
the process of cementing. Because the flow resistance of the drilling
fluid in the wide gap is less than that in the narrow gap, the phenomena
of a delayed flow or even an overall nonflow occurs in the narrow
gap. Based on the existing displacement efficiency calculation model,
this paper establishes the cementing displacement efficiency model
under the condition of oil-based drilling fluid, explores the residual
layer thickness of drilling fluid on the casing side and the sidewall
side, and then links the annular displacement efficiency to the injection
displacement in combination with the circulating mode resistance pressure
drop formula so as to explore the change in the cementing displacement
efficiency under different displacements. Considering the change in
the physical parameters of annulus fluid, the change in annulus displacement
efficiency is obtained. On this basis, the relationship between the
wellhead cement injection flow and the annulus retention layer is
studied; then, the displacement is calculated. The reasonable cementing
displacement is calculated by combining the displacement with annulus
displacement efficiency. The results show that the thickness of the
annular detention layer increases with the increase in the casing
eccentricity in the same well depth, and the growth rate of the detention
layer on the wellbore side is greater than that on the casing side
at the same circumferential angle. The greater the displacement, the
greater the annular circulation pressure drop and circulation equivalent
density, thus increasing the cementing risk. The displacement is reasonably
designed. The research results in this paper have a certain guiding
significance for improving the displacement rate of isolation fluid
under oil-based drilling fluid conditions.