Characterisation of the Thermal Regime Through Oil Sands Fluid Fine Tailings in an End Pit Lake

Kathryn Dompierre, Lee Barbour, Geoff Halferdahl

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Mining Geotechnics

ABSTRACT: Thirty End Pit Lakes (EPLs) have been planned for remediation of oil sands mines in the Athabasca Oil Sands region, however their feasibility as a reclamation feature has yet to be fully evaluated. End Pit Lakes are constructed within mined-out pits, and many will incorporate the storage of fluid fine tailings (FFT). These lakes are anticipated to provide passive treatment of mine site waters, and establish sustainable biological communities, while isolating the underlying FFT. The thermal regime within the lake is central to these EPL functions. Significant heat transfer from the FFT to the EPL may occur due to conduction and forced convection (advection) as the tailings are deposited at elevated temperatures and continue to undergo settlement. Defining the thermal regime through the FFT will provide insight on these heat transfer mechanisms, and will offer a better understanding of the thermal conditions developing at the FFT-lake interface. Numerical modelling was undertaken to evaluate the various forms of heat transport through the FFT given seasonal temperature changes at the top boundary (seasonal temperatures in the lake water). These models were compared to seasonal temperature measurements to determine which form of heat transport matched the observed trends. Preliminary results demonstrated the presence of forced convection at rates similar to pore water flow estimates from existing settlement models for FFT in Base Mine Lake. Given the presence of forced convection, a simple equation was developed to evaluate the ratio of energy stored in lake water to energy released in pore water discharge (via forced convection). Given a forced convection rate of 0.004 m/d, as determined by the heat transport models, the energy released by pore water discharge constituted between 15 and 20 percent of energy stored in Base Mine Lake. Therefore, energy released to the lake water from the FFT is significant given a thermal regime dominated by forced convection. Finally, discrepancies between the modelled temperature profiles and field measurements indicate the presence of free convection in the top portion of FFT. R…SUM… Trente lacs de kettle ont ”t” pr”vus comme mesure de correction pour les r”gions des sables bitumineux Athabasca, mais leur faisabilit” en fonction de r”clamation nÕa pas encore ”t” ”valu”e. Les lacs de kettle sont construits dans des foss”s min”s, et plusieurs dÕentre eux vont aussi int”grer les r”sidus de fins liquides (FFT). Il est pr”vu que ces sites vont aussi servir au traitement des eaux des sites miniers et ”tablir des communaut”s biologiques durables; tout en isolant les r”sidus. Il est n”cessaire que chaque lac ait un r”gime thermique. Il est possible que la chaleur soit transf”r”e des r”sidus fins liquides aux eaux par la convection forc”e; ceci est parce que les r”sidus sont d”pos”s ‹ des temp”ratures ”lev”es et continuent ‹ se d”poser. Donner une d”finition au r”gime thermique par la FFT donnera une nouvelle perspective aux m”canismes de transf‘re de chaleur et offrira une meilleure compr”hension des conditions thermiques qui se d”veloppent dans les lacs qui ont des r”sidus des fins liquides. La mod”lisation num”rique a ”t” entreprise pour ”valuer la vari”t” dans les modes de transport de chaleur dans le FFT lorsquÕil y avait des changements de temp”ratures saisonniers ‹ la limite sup”rieure (apr‘s quÕil y a eu des changements de temp”rature avec les saisons dans le lac). Ces mod‘les ont ensuite ”t” compar”s aux mesures de temp”rature saisonni‘re pour d”terminer quelle forme de transport de chaleur correspondait aux observations pr”c”dentes. Les r”sultats pr”liminaires ont d”montr” la pr”sence des taux de convection forc”s qui ”taient similaires aux estimations du mouvement des eaux interstitielles dans des mod‘les existantes du FFT au lac Base Mine. …tant donn” la pr”sence dÕune convection forc”e, un formulaire de calcul a ”t” d”velopp” pour ”valuer le ratio entre lÕ”nergie tenue dans les eaux interstitielle (par convection forc”e). Avec un taux de convection forc”e de 0.004 m/j tel que d”termin” par les mod‘les de transport de chaleur, lÕ”nergie d”gag”e par la d”charge de lÕeau interstitielle repr”sente entre 15 et 20% de lÕ”nergie tenue dans le lac Base Mine. Par cons”quent, lÕ”nergie dispers”e aux eaux du lac de lÕFFT est important ”tant donn” un r”gime thermique domin” par la convection forc”e. Enfin, les ”carts entre les profils de temp”ratures model”s et les mesures prises en plein champ indiquent la pr”sence de la convection libre dans la partie sup”rieure de la FFT.

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