Stability of foams in pipe and annulus
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2019 Elsevier B.V. Foam fluids present numerous advantages for many oil field applications. Due to their bubble structure and a gaseous component, foams exhibit very high viscosity and low density. Low density makes foam an attractive fluid in underbalanced drilling operation in which light fluid is essential to maintain wellbore pressure below pore pressure. Due to its high viscosity, foam exhibits good hole-cleaning performance even at low annular velocities. Nevertheless, foams are thermodynamically unstable fluids and their structures eventually collapse. Foam decays as a function of time and the decay occurs mainly due to bubble coalescence and the gravity drainage of the liquid phase. Maintaining fluid stability is necessary for foam drilling. For a successful drilling operation, the half-life of foam, which is the time taken by foam to drain 50% of its liquid volume, should exceed circulation time. For other applications, such as hydraulic fracturing or cementing, the half-life of foam should exceed the time taken for fluid to reach target zones. In this study, foam stability experiments were conducted in a concentric annulus (50 mm 75 mm) and 25-mm pipe sections. The pipe section was transparent to observe drained liquid level. Aqueous, polymer-based, and oil-based foams were considered in the investigation. The effect of gas volume fraction (quality) was studied under static condition. All experiments were performed at 400 kPa and ambient temperature (20 2 C). Foam quality was increased from 40% to 80%. However, at high qualities (greater than 70%), the oil-based foam was unstable. Hydrostatic pressure data was collected from the annular section and converted into density curves to determine drained liquid volume. In the pipe section, the drainage was physically measured from the liquid level and recorded periodically. A digital camera with a microscope was used to capture foam image in real-time to examine bubble coarsening and foam decay. For a given quality, the polymer-based foam was the most stable, while oil-based foam was the least. For all three types of foams, stability increased with quality. Foams were more stable in the annular section, as compared to the pipe, possibly due to the impact of container wall on bubbles and drained liquid motion. Foam decay and bubble coarsening were observed by comparing bubble sizes in captured images at various times. On average, larger bubbles were observed in higher quality foams. The rate at which foam bubbles coarsening mostly reduced with time.
