This work discusses the major factors affecting the pressure drop in the cemented annulus which are filtration from the wellbore into permeable layers, shrinkage of the slurry due to hydration, and changes in the rheological properties of the slurry. A model for determining wellbore pressures in cement slurry columns is presented. It accounts for the depth-dependent or placement history-dependent behavior of the gel strength evolution. Experiments conducted in the laboratory demonstrate that static gel strength development is a function of temperature, pressure and shear rate. Results show that large differences in wellbore pressures can be obtained when the depth-dependent characteristics of gel strength are neglected in wellbore pressure loss calculations.