September 2011 SPE Drilling & Completion 341 Dynamic Aspects Governing Cement-Plug Placement in Deepwater Wells P.E. Aranha, C.R. Miranda, J.V.M. Magalhães, G. Campos, and A.L. Martins, Petrobras; and A.B. Ramalho and M.F. Naccache, PUC-Rio Copyright © 2011 Society of Petroleum Engineers This paper (SPE 140144) was accepted for presentation at the SPE/IADC Drilling Conference and Exhibition, Amsterdam, 1–3 March 2011, and revised for publication. Original manuscript received for review 22 November 2010. Revised manuscript received for review 26 April 2011. Paper peer approved 13 June 2011. Summary Plug cementing is still considered to be a critical operation, and cases of failure eventually happen. A large annular gap and eccen- tricity, typical of these operations, are factors that may promote unstable flows, resulting in cement-slurry contamination. Deepwa- ter conditions enhance chances of free fall, and, consequently, low displacement velocities can occur in the annulus. This article presents a parametric study of the role of rheologi- cal properties of fluids (drilling fluid, spacers, and cements slur- ries), string rotation, and flow rates (including free-fall effects) in the displacement quality of cement plugs. Analyses are based on two different simulation tools. Conventional cement-pumping soft- ware defines flow-rate profiles at the annulus entrance, account- ing for free-fall effects, and computational fluid dynamics (CFD) simulates the interface propagation and contamination levels. The main issues addressed by the simulations are • What are the maximum yield stresses that guarantee nonstag- nation regions while circulating the drilling fluid? • How can one optimize density and rheology hierarchy, which minimizes contamination and avoids channeling? • What is the role of string rotation on the displacement effi- ciency? The compilation of simulation results into useful guidelines and procedures for displacing cement plugs in vertical, inclined, and horizontal offshore wells is presented. Introduction The cementing process in an oil well is a crucial operation that needs to be performed properly to guarantee the desired well life because the lifetime of the well is strongly influenced by the cementing operation. As a means of having a successful operation, it is very important that the displacement process end with the cement paste homogeneously distributed at the desired position in the wellbore. From an industrial perspective, a good displacement corre- sponds to a firm bottom, with the ability of supporting weight (normally 15,000 to 20,000 lbm). Specific operations, such as sidetracks, lost-circulation plugs, or abandonment plugs, will pres- ent different requirements. Literature on the subject is scarce and concentrates on both laboratory tests and field cases (Heathman 1996; Salahub and Ripley 1980; Smith 1984). Successful jobs require the full displacement of the drilling fluid, with minimal interface instability and with the interface moving steadily at the mean pumping speed. Moreover, the cement slurry should achieve the desired mechanical properties, such as adherence, compressive strength, and impermeability. To avoid contamination, which would affect the cement properties, one or more spacer fluids are usually pumped between the drilling mud and the cement slurry. Failure to achieve proper zonal isolation can result in significant economic effect in terms of loss of well productivity or can lead to adverse environmental effects. The analysis of the replacement process of one fluid by another fluid with different physical properties can be performed properly only by the simulation of a multiphase flow. The solution of the governing equations aims to represent the evolution of the inter- face shape between each pair of fluids (cement/spacer fluid and spacer fluid/drilling mud) during the displacement process. This is a complex problem, especially because of the non-Newtonian behavior of the involved fluids. The numerical simulation of flows is a powerful tool in the evaluation of different processes in industry. Particularly in the oil industry, an experimental investiga- tion in an actual oil well is an expensive task and sometimes not operationally feasible. Most previous studies about the subject aimed at represen- tation of cementing operations, where complete filling of the annular space with the cement slurry is the optimal condition for zonal isolation. Some work (Haut and Crook 1979, 1982; Sauer 1987; Lockyear and Hibbert 1989) shows that the process of fluid displacement through vertical oil wells is governed mainly by the viscosity ratio between fluids, the eccentricity of the annular space between the column and the casing, the flow rate, and the density ratio. In another work (Jakobsen et al. 1991), the authors experimen- tally analyzed the effects of viscosity ratio, buoyancy force, and turbulence intensity in mud displacement through an eccentric annular region. The results obtained show that displacement is more efficient at the wider region and that turbulence reduces the mud channeling that may happen at the narrower region. The theoretical and experimental study performed by Tehrani et al. (1992) detailed the laminar flow of drilling fluids through eccentric annular spaces. They observed that, as the eccentricity increases, the displacement becomes worse. Tehrani et al. (1993) describe an experimental rig for fluid displacement in an annulus with variable eccentricity and inclination where they observed that pumping large volumes of displacing fluid and maximizing annulus cen- tralization enhance the success of the displacement process. For vertical displacements, it is also shown that the process is more efficient for higher density differences between the displacing (higher-density) and displaced fluids. Two studies, Frigaard et al. (2002) and Frigaard and Pelipenko (2003), present some theoretical results of cement displacement through eccentric annuli, considering a 2D situation, based on the well-known lubrication theory. They show that the displacement front may reach permanent regime for some combinations of physical properties. For these cases, an analytical expression for the interface shape is obtained. This approach, however, neglects curvature effects, resulting in unreliable results for the wide annu- lar gaps that characterize cement-plug displacement. Dutra et al. (2004) numerically analyzed the flow of two adja- cent fluids through annular eccentric pipes. The effects of rheologi- cal parameters and eccentricity were investigated for different flow rates. The results obtained show that the displacement is better when a more-viscous fluid is used to push the other fluid. Also, it was observed that the interface shape is a function of flow regime and viscosity ratio; however, it is insensitive to eccentricity. The effects of density and rheology differences between Newtonian fluids displacing non-Newtonian fluids and vice versa are detailed in Dutra et al. (2005).