Review of current research on drilling-mud sumps in permafrost terrain, Mackenzie Delta region, NWT, Canada.

Julian C.N. Kanigan, S.V. Kokelj

Dans les comptes rendus d’articles de la conférence: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: T2-E

ABSTRACT: Sumps in the Mackenzie Delta region of the Northwest Territories are intended to encapsulate drilling wastes within frozen ground, but subsidence of sump covers suggests that the permafrost has degraded. Recent research on drilling-mud sumps in permafrost is reviewed to assess key factors related to sump performance. Mean permafrost temperature is significantly associated with sump integrity, and can supersede the effect of sump closure practices. Tall shrubs and deep snow on the sump cap play an important role in the thermal evolution and long-term integrity of drilling-mud sumps. Climate warming is also likely to have significant impacts on drilling-mud sump integrity. These studies suggest the importance of a long-term sump management approach. RÉSUME Des bassins à boues de forage dans la région du delta de Mackenzie des Territoires du nord-ouest sont prévus pour encapsuler des pertes de perçage dans le pergélisol, mais l'affaissement des couverts de bassins à boues de forage suggère que le pergélisol a dégradé. La recherche récente sur des bassins à boues de forage de perçage-boue en pergélisol est évaluer pour des facteurs clé liés à l'exécution de bassin à boues de forage. La température moyenne de pergélisol est associée de manière signicative à l'intégrité de bassin à boues de forage, et peut remplacer l'effet des pratiques en matière de fermeture de bassin à boues de forage. Les grands arbustes et la neige profonde sur le bassin à boues de forage jouent un rôle important dans l'évolution thermique et l'intégrité à long terme des bassins à boues de forage de boue de perçage. Le réchauffement climatique peut également avoir des impacts significatifs sur l'intégrité des bassins à boues de forage de perçage-boue. 1 INTRODUCTION Sumps have been constructed to contain wastes associated with exploratory oil and gas drilling in northern Canada since the 1920's (Nassichuk 1987), and in many northern jurisdictions sumps remain a regulated drilling waste disposal option today. In permafrost terrain, the design intention of a sump is to immobilize saline drilling wastes by encapsulating them within permafrost (Figure 1) (Imperial Oil Resources Ventures 2004). Drilling fluids are deposited in the sump and backfilled with the excavated materials. However, if the drilling fluids are not completely frozen when the sump is being capped, they may be squeezed out of the sides (French 1978a, 1978b, 1980). Capping materials excavated from near-surface permafrost are often ice-rich so that reestablishment of an active layer in the cap can lead to significant thaw subsidence and ponding (Figure 2). The ensuing thermal disturbance may promote further active-layer deepening and the eventual loss of drilling-mud containment (Dyke 2001). Uncontained saline drilling fluids can alter soil chemistry and inhibit vegetation growth (Johnstone and Kokelj 2008). Managing the long-term integrity of drilling-mud sumps is a priority for communities, resource managers and industry who wish to minimize the environmental impacts of hydrocarbon exploration and development. Oil and gas exploration in the Mackenzie Delta region of the Northwest Territories (NWT) since the late 1960's has resulted in over 150 legacy sumps (Figure 3) (AMEC Earth and Environmental 2005). The majority of sumps Figure 1 Configuration of a sump to encapsulate drilling wastes in permafrost (from Jenkins et al. 2008, Fig. 1). (70%) were constructed between 1970 and 1980 (Jenkins et al. 2008). The most recent sump was constructed in 2007; however, due to public concern over the impacts of sumps in the region, many companies have recently opted to transport drilling wastes by truck to disposal facilities outside of the NWT. Community concern regarding sump performance and industry and government interests related to a proposal for a pipeline 147

RÉSUMÉ: of current research on drilling-mud sumps in permafrost terrain, Mackenzie

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