Exchange flows in a vertical pipe : a laboratory study of failure regimes in plug cementing
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".