In the completion of oil and gas wells, successful cementing operations essentially require the complete removal of the drilling mud and its substitution by the cement slurry. Therefore, the displacement of one fluid by another one is a crucial task that should be designed and optimized properly to ensure the zonal isolation and integrity of the cement sheath. Proper cementing jobs ensure safety, while poor displacements lead to multiple problems, including environmental aspects such as contamination of fresh water bearings. There are a number of factors, such as physical properties of fluids, geometrical specifications of the annulus, flow regime, and flow rate, which can remarkably affect the displacement efficiency. The shape of the interface plays an influential role during the displacement process. For a highly efficient displacement the interface has to be as flat and stable as possible. However, unstable and elongated interfaces are associated with channeling phenomenon, excessive mixing, cement contamination, and consequently unsuccessful cementing operations. Thus, the stability of the interface between the two fluids has major importance in cementing applications. In the present work, a novel method for the prediction of interface instability and displacement efficiency is introduced. Instability analyses of the interface between the two fluids are carried out following the main ideas of the original Rayleigh-Taylor and Kelvin-Helmholtz instabilities. Moreover, using the same analyses, optimized designs for improvement of the displacement process in any specific situation are proposed. The influence of density, rheological properties, surface tension, and flow rate of the fluids on the instability and shape of the interface, and consequently on the displacement efficiency, is studied. Furthermore, an analytical solution of the displacement of fluids in the annular space that enables the calculation of the mixed volumes is developed. Additional time-dependent considerations are made for the calculations of cases with unstable interfaces. For the same cases studied, 3-D Computational Fluid Dynamics (CFD) simulations are performed, using commercially available CFD software. For the purpose of validation of the results, a number of experiments were conducted for fluids with various combinations of physical properties. The results present the effect of physical fluid properties, geometrical configurations, and flow rate on the instability of the interface and displacement efficiency. Reasonably good agreement between the results of all three approaches presented in the paper are observed and they all emphasize the importance of the proper selection of fluid properties and flow rates for any specific sequence, to minimize the degree of contamination and mixing. The discussions and results of this work provide insight into displacement process, invaluable guidelines for industrial applications, and compelling evidence of the importance of correct predictions and appropriate designs of the displacement of fluids in cementing operations.
Cement plugs are used for various reasons in wellbores, including control of lost circulation zones, initiation of deviation or side tracking, wellbore abandonment, and more. Unfortunately, one of the most questionable processes during the drilling and completion stage of a wellbore is the quality of the cementing job. As in conventional cementing operations, cement plug setting requires an efficient fluid displacement process to be considered "successful." Due to the nature of the operation, i.e., displacing one fluid with another inside a tubular, mixing of the fluids during this process is inevitable, because the fluids are in contact via a contacting interface. The stability of this interface depends on many parameters, such as flow rate, pipe size, inclination, displacing and displaced fluid rheological properties, as well as densities, interfacial tension between the fluids, etc. Therefore, an optimization process is required to minimize the mixing and thus to maximize the quality of the cement plug to be set by controlling the physical properties of the displacing fluid. In this study, the displacement process of one fluid with another inside circular pipes is investigated analytically and experimentally. Analytical work includes a simplified mathematical model that can predict the structure of the interface between the displacing and displaced fluid. The model allows determination of the mixed volume during the flow. In addition, CFD (computational fluid dynamics) simulations of the mixed volume are conducted using a commercial software. During the experimental work, different combinations of fluids with specified rheological properties and densities are compared based on the extent of displacement and mixing in circular pipes at different flow rates using The University of Tulsa – Displacement and Mixing Facility. Analytical modeling, experimental tests, and CFD simulations all indicate that significant mixing takes place during a displacement process, and even in a "successful" case, 15% by volume of the cement plug is contaminated by the displacing fluid. If the physical properties of the displacing fluid are not optimized, this contaminated volume becomes more than 50% of the total volume of the cement plug. This level of mixing will result in failure of the set plug's function. The study also shows that rheological properties and density of the displacing fluid significantly influence the displacement efficiency, which can be used as an optimization tool for reducing / minimizing the mixed volume during the displacement process. The quality of the primary cementing and cement plug setting processes directly depends on minimization of the mixed volume. This is not only a safety issue, it can also result in serious environmental impacts, such as contamination of the water table. The information obtained from this study can be used as to establish guidelines for the optimization of displacing fluid properties that result in successful primary cementing and cement plug setting.
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