Mud/Cement Displacement in Vertical Eccentric Annuli
Authors: Foroushan Hanieh K. , Ozbayoglu Evren M. , Gomes Paulo J. , Yu Mengjiao 
Published: February 2020

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.