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Charabin, Scott

University of British Columbia

This thesis investigates exchange flow dynamics between a dense, viscoplastic fluid and a lighter Newtonian fluid within a vertical pipe, simulating conditions in abandoned oil and gas wells. The study is motivated by the need to improve the decommissioning of old oil/gas wells, where dense cement slurry is pumped into water-filled wellbores to create a hydraulic seal. Due to the density difference between cement and water, the significant buoyancy forces can deform the plug before the cement solidifies, leading to leakage. With the increasing number of abandoned wells in Western Canada, understanding how to enhance cement plug placement is crucial. This experimental study investigates buoyancy-driven exchange flows within a vertical pipe, where the lower fluid has low Newtonian viscosity and the upper dense fluid has a yield stress. A gate valve is opened to allow the fluids to mix. No sustained fluid motions are observed when the ratio Y of the yield stress to the buoyancy stress exceeds a certain threshold, indicating a no-flow regime. However, an exchange flow ensues for smaller Y values, characterized by the upward penetration of the less-dense fluid in a central finger, displacing the upper fluid downwards along the pipe's walls. The observed flow behaviors belong to three distinct regimes: helical finger, disconnected finger, and slug flow. The increasing influence of inertial stresses relative to viscous stresses in counterbalancing buoyancy effects dictates the transition between these regimes. The disconnected finger and slug flow regimes are marked by a yielded fluid at the interface and early onset of instabilities. In contrast, helical fingers are primarily governed by viscous effects and slow evolution until the later stages of the experiments. A lubrication model is developed to quantify the changing behaviour of the flow regimes. The study recommends using lower-density cement to reduce buoyancy forces and employing a spacer between water and cement to delay flow initiation until the cement is sufficiently hydrated to resist movement.

Text

2024-08-27

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/89032

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/