Assessing the Impact of Bentonite-CMC Drill-In Fluids on Slug Tests in High-Permeability Aquifers

Matthias A. Zenner

In the proceedings of: GeoManitoba 2012: 65th Canadian Geotechnical Conference

Session: T2D - Seepage and Hydrogeology

ABSTRACT: Groundwater observation wells are frequently installed by direct-mud rotary drilling employing bentonite-carboxyl-methylcellulose (bentonite-cmc) drill-in fluids. This drilling technique is known to often induce formation damage in high-permeability aquifers. Slug tests conducted in such wells may show reduced curvature of head responses when plotted in a semi-logarithmic Hvorslev-style format. Head responses showing a strictly exponential decay in time are rarely seen when slug testing high-permeability aquifers, though. Whereas such slug test responses are traditionally associated with negligible aquifer storage, this work offers an alternative interpretation attributing these characteristics to spherical flow entry into the tested well across one or more narrowly flushed flow passages on the circumference of a completely plugged up screen. Considering high-permeability aquifers exponential slug test responses in this respect should always be judged for being indicative of severe formation damage.

RÉSUMÉ: nt forés en utilisant la technique de forage rotary avec injection directe de fluides de forage à base de bentonite-carboxyle-methylcellulose (bentonite-cmc). Cette technique de forage a tendance de générer du dommage méabilité. Les données relevées lors des slug tests -logarithmique, montrent souvent des rictement exponentiel sont rarement constatées pour des aquifères à haute perméabilité. Alors que les réponses de slug tests strictement exponentielles sont traditionnellement associées à un volume de stockage en aquifère négligeable, cette contribution prhaute perméabilité, les réponses exponentielles de slug tests devraient toujours être interprétées comme une indication de graves dommages causés à la formation.

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