Advanced Modeling of Cement Displacement Complexities

Abstract

Cement job success is largely determined by fluid displacement efficiency. Optimum displacement requires understanding of flow patterns, frictional pressure losses and mutual interactions of mud, spacers and cement in annular spaces. Modeling this complex behavior is very difficult, but understanding it is essential to guarantee displacement success. A state-of-the-art cement displacement study was carried out using the very latest in computational fluid dynamics (CFD) modeling techniques, to identify practical guidelines and solutions to cement displacement challenges. A state-of-the-art 3D "3-phase" (i.e. mud-spacer-cement phases) CFD model was created and simulations were carried out, featuring tracking of fluid interfaces during displacement, calculation of frictional pressure drops, and characterization of complex flow profiles. These simulations accounted for the effects of such complexities as non-Newtonian rheological behavior of all fluids involved, eccentric / narrow annuli, and pipe movement / rotation. The integrated study clearly identifies the root cause(s) of cement displacement failures and highlights comprehensive practical solutions, which are proposed for implementation in field operations. There are many causes for cement displacement problems and failures, including poor borehole conditioning, inappropriate displacement flow rates, insufficient casing centralization, viscosity contrast mismatches between mud-spacer-cement leading to interface instabilities, etc. Our high-resolution finite element study quantifies the effects of many of these causes and highlights parameters that can improve displacement, such as avoiding high shear strength in non-Newtonian mud and cement rheology, reducing pipe eccentricity and applying pipe rotation during displacement. The modeling approach is used to identify optimum parameters values, and studies interdependencies between factors, for instance determining optimum rheology, flow rate and pipe rotation speeds when pipe is placed eccentrically in the hole, in order to maximize the probability of displacement success in the field. Particularly revealing are the non-intuitive results obtained while modeling mud, spacer and cement as non-Newtonian yield power law (YPL) fluids, which has never been done before. This paper presents: (1) a new, state-of-the-art 3D CFD model; (2) advanced numerical analysis of cement displacement, taking into account complexities such as non-Newtonian rheology, borehole enlargement, pipe eccentricity, and pipe movement during displacement; (3) practical guidelines derived from the modeling results that can be used for improved cement job pre-planning and field application.

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