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King Fahd University of Petroleum and Minerals (2005)

Effect of rate of water table rise on LNAPL entrapment in uniform and well-graded sands

Dalhat, Mu’azu Nuhu

Titre : Effect of rate of water table rise on LNAPL entrapment in uniform and well-graded sands.

Auteur : Dalhat, Mu’azu Nuhu

Université de soutenance : King Fahd University of Petroleum and Minerals. (Saudi Arabia)

Grade : MSc thesis 2005

Résumé
Vertical water table movement is liable to trap part of any light non-aqueous phase liquid (LNAPL) present within the vicinity of capillary fringe and water table in contaminated aquifers. Due to toxicity and low dissolution of LNAPLs in water, the trapped component could be a long-term groundwater contamination source. This present work experimentally investigated and modeled the influence of rate of water table rise on the entrapment of Saudi Arabian light crude oil in natural uniform and blended well-graded sands. Series of laboratory column tests were conducted to simulate a rising water table at several rise rates and the subsequent entrapment of the LNAPL for range of initial water saturations between 0-71% ; from which macroscopic quantitative measurements of trapped LNAPL saturation were performed after attainment of quasi-static equilibrium. The experimental results apparently show that, at high rate of water table rise, in addition to initial water saturation, the entrapment of LNAPL is also significantly influenced by the rate of the water table rise. Increase in the rise rate was found to increase the LNAPL trapped saturation for water-wet initial condition, while it decreases the LNAPL trapped saturatin for the LNAPL-wet condition, suggesting that the initial soil matrix wetting status controls the flow condition during water displacing LNAPL processes. The trapped saturation ranges for the water-wet conditions are 1.5-37.3% and 28.1-37.5%, while for the LNAPL-wet conditions the ranges are 33.6-13.4% and 27-15.2% for the two porous media respectively. Marked deviation from the established linearity of trapped saturation as a function of initial water content was observed to amplify with increase in the rise rate. Predictions from a semi-empirical model formulated in the study to account for the rate of wetting fluid imbibition rate imposed by a rising water table provided much better results compared to predictions from linear trapping models of Land [1968] and Steffy et al [1996] as they profoundly underestimated the trapped specific volume. It is concluded that LNAPL tapping models are bound to be erroneous when rate of imbibition of wetting fluid is disregarded at high rate of water table rise.

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Page publiée le 26 septembre 2010, mise à jour le 6 décembre 2017