Effects of irregularity on displacement flows in primary cementing of highly deviated wells

Anywhere between 0% and 80% of primary cemented wells have integrity failures. Strong local variability suggests both geological and operational factors. One way geology affects cementing is via irregular wellbores, e.g. washouts. Here we study the effects of washouts on mud removal in strongly inclined wellbores, experimentally and via 2-D computational simulations, with the aim of identifying key control parameters. Experiments were performed with 2 fluids with properties representative of drilling mud and cement (or spacer), displaced at constant flow rate through a 10 m long annular flow loop. One downstream section of annulus had an enlarged outer diameter to mimic a wellbore with a washout. In the uniform horizontal sections, the density difference promotes slumping towards the bottom, while eccentricity promotes flow along the wide side of the annulus. At inclinations less than horizontal the slumping tendency is reduced. This competition between slumping and eccentricity is affected by the washout. In most of the experiments the mud was removed from the washout, but the simulations showed this is not true for higher yield stress muds than those tested experimentally: residual fluid was left in the washout section. The simulations also provide detailed information on the evolution of the fluid-fluid interfaces as they pass through the annulus and washout.