Slim-well construction helps to significantly reduce overall well construction costs. The first step of the successful completion process is achieving effective zonal isolation. Slimhole configurations experience higher shear rates at standard annular pump rates as compared to conventional configurations. The increase in shear rate could exacerbate intermixing of the fluids, diffusion, and fingering in the annulus, thereby leading to incomplete mud and filter-cake removal and poor cement placement. Real-world consequences of these phenomena can include interzonal communication, loss of production, remedial squeeze work, and even well abandonment. Whether a conventional or slim hole well, designing an effective zonal isolation job involves balancing two technical challenges: (1) providing adequate hole cleaning through effective erodibility shear stresses; and (2) reducing channeling of mud or spacer by minimizing intermixing at fluid interfaces, controlling diffusion and minimizing fingering in the annulus. This paper deals with the underlying interactions between fluids and the challenge of controlling the aspects affecting intermixing
Intermixing lengths can commonly be as high as 500 feet for slimhole cementing jobs owing to the large difference between central and peripheral velocities of the annular fluids. In an attempt to minimize this problem, a three-dimensional (3D) numerical fluid-flow simulator was used to compare flow profiles in a low clearance 143/4-in. × 133/8-in. annulus to those in a conventional 16-in. × 133/8-in. annulus. Rheologies of the annular fluids were accurately modeled using the Herschel-Bulkley scheme. Using the same fluids and pump rates, numerical simulations in the slimhole configuration clearly showed worse displacement as compared to the standard configuration. The effects of the three rheological parameters viz., τ0, µ∞, and n of the fluids, pump rate and eccentricity on dynamic velocity profiles, displacement efficiency, intermixing lengths, and top of fluids were then studied to improve drilling-mud removal, and cement slurry placement. Results of various simulations are shared to reveal the properties and parameters that are needed to help achieve a competent cement sheath over the desired interval.