We study experimentally the isoviscous displacement flow of two immiscible Newtonian fluids in an inclined pipe. The less dense displacing fluid is placed above the denser displaced fluid in a density-stable configuration. The displacing and displaced solutions are oil-and water-based, respectively. The former exhibits nonwetting behavior in the vicinity of the pipe wall, whereas the latter is wetting. The pipe has a small diameter-to-length ratio. The mixing and interpenetration of two fluids have been studied over a wide range of controlling parameters, revealing remarkable results. Compared to the previously studied miscible limit, we observe behavior at the interface between the two fluids where the displaced fluid stays "pinned" to the lower wall of the pipe upon pumping the displacing one. This phenomenon, which is observed over the full range of investigated flow rates, tilt angles, and density contrasts, is associated with the wetting characteristic of the displacing liquid and is also present when light and heavy viscosity mineral oils are used as the displacing fluid. Ultrasonic Doppler velocimetry revealed a segmented velocity profile at the interface of the immiscible fluids. Due to pinning, the efficiency of the removal of displaced fluid in the immiscible limit can be lowered by 14% compared to the miscible case due to the combined effects of the density-stable configuration and the immiscibility of the flow. Within the family of immiscible fluids, the maximum efficiency is achieved at close-to-vertical tilt angles, large density contrasts, and counterintuitively low imposed flow rates, which is of great importance in industrial design.
There exist many industrial processes in which it is necessary to remove a gelled material from a duct. Some examples include mud removal in oil and gas well cementing, waxy crude oil pipeline restarts, and Enhanced Oil Recovery (EOR). Here, the dynamics of the removal of a viscoplastic fluid by a Newtonian fluid are investigated experimentally in an inclined pipe. We focus on both miscible and immiscible fluids mimicking wells drilled using water-based as well as oil-based mud. Under the miscible limit two major flow regimes, namely center-type and slump-type, are observed depending on the density difference between fluids. These flows are explored in great details through displacement front speed measurement which is inversely related to the efficiency of removal process. Displacement flows in the immiscible limit are accompanied by interfacial instability, gel fracture, and droplet formation. These flows are quantified through flow visualisation and spatiotemporal diagrams of fluids concentration. The findings of our study can help improve well cementing operations worldwide.
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