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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