Mud/Cement Displacement in Vertical Eccentric Annuli

Abstract

Summary A successful cement placement can provide zonal isolation and environmental safety. Effective design of cement placement and mud removal affects all the stages of the wellbore life, from drilling ahead to production. Accurate predictions of fluid displacement in the wellbore are vital to design fluid properties and plan the cementing job. In this work, an analytical model is developed to simulate the displacement of fluids in eccentric annuli. This paper presents an analytical method for the solution of cement/mud displacement and evaluation of interfluid contamination during displacement in vertical eccentric annuli. This new approach starts by addressing the problem of single-fluid flow in eccentric annuli by analytically solving the governing transport equations for a flow inside an unwrapped annulus. The solution is then extended to a system of two fluids in a vertical annulus by adjusting the boundary conditions for displacement. The model is completed by adding the time-dependent calculation of interface between the two fluids, enabling the accurate determination of the amount of interfluid mixing and displacement efficiency. The analytical method proposed is used to simulate single- and multifluid flows and study the effect of fluid properties of cement, spacer, and drilling mud at different flow rates on displacement efficiency for both concentric and eccentric vertical annuli. Noting that the drilling fluids are non-Newtonian, the concept of apparent viscosity is used, accounting for variable apparent viscosity at different annular gaps. 3D computational-fluid-dynamics (CFD) simulations were performed and the results were compared with the analytical solution. Moreover, instability of the interface in all cases was studied, and the analysis offers an understanding of the role of fluid properties and proposes applicable optimized design to enhance the displacements. The amount of interfluid mixing and contamination that occurs during the displacement was calculated for both methods. The analytical solution and CFD produce results within a 13% difference, which sufficiently validates the analytical model. Evidence was gathered to support that the improper design of fluid properties and flow rate along with a highly eccentric annulus can lead to substantial cement contamination. This can lead to underdesigning the amount of fluids to be pumped to provide a complete mud removal and an efficient cement placement. On the other hand, learnings and models developed allow the optimization of fluid properties that can lead to the best outcomes, even for a highly eccentric annulus. The present work aims to take part in addressing the undeniable importance of a complete cement displacement by means of a semianalytical solution for the fluid displacement coupled with the interface-instability analysis, attempting to provide a realistic prediction of the amount of interfluid mixing and cement contamination, along with qualitative judgements on the quality of the cementing job. This methodology is intended to offer improvement techniques for the displacement and provide enhancements for practical industrial applications.

Cement Placement: An Overview of Fluid Displacement Techniques and Modelling

“…In general, presuming that the turbulent flow can enhance the fluid displacement can be misleading and there remains a need for clarification on whether a turbulent regime improves the displacement process, or the high flow rate will act adversely, trigger the instability of the interface, and increase the amount of inter-fluid mixing. More recently, it was argued by Foroushan et al [13,30] that increasing the flow rate is not always in favor of the displacement process. They showed that a critical value of flow rate can be obtained, below which the interface remains stable and, having proper density and viscosity contrasts, a good displacement can be achieved.…”

“…Howard and Clark [7] were probably the first to suggest that a decrease in viscosity of the drilling fluid would increase displacement efficiency. Later the effect of rheological properties of displacing and displaced fluids was studied in various works, such as Flumerfelt [8], Nguyen et al [9], Tehrani et al [10], Miranda et al [11], Aranha et al [12], and Foroushan et al [13]. There were also other investigations, such as the ones of Lockyear and Hibbert [14], Lockyear et al [15], and Silva et al [16], that discussed the effect of yield stress and gel strength of mud on the displacement process.…”

“…There have been more investigators that found that a displacing fluid with a higher density than that of the displaced one improves the cementing operation and the displacement process. Their works include both experiments, such as those performed by Lockyear et al [15], Tehrani et al [10], Miranda et al [11], and analytical methods, such as those developed by Flumerfelt [8], Nguyen et al [9], and Foroushan et al [13].…”

“…Moran and Savery [24] presented a comprehensive experimental study on the effect of eccentricity and affirmed the importance of centralization. There have been indeed numerous works, such as the ones by McLean et al [23], Couturier et al [29], Jakobsen et al [19], Guillot et al [33], Chen et al [34], Foroushan et al [13], Ytrehus et al [22], Skadsem et al [35], Lund et al [26], where the importance of proper centralization and the adverse influence of eccentricity is emphasized.…”

“…Following the idea of flow in a narrow slot, Foroushan et al [13,39,40] proposed a model for displacement of fluids in narrow eccentric annuli at different inclination angles. Through the conservation of momentum, they solved a Poisson-type of differential equation with an irregular boundary, because of the unwrapped annulus, for each fluid region.…”