Geophysical Study of Annular Well Seals |
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| Authors: | Martin C Christman Craig H Benson Tuncer B Edil |
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| Institution: | Martin C. Christman;received a B.S. in civil engineering from the University of Texas at Austin, and an M.S. in civil and environmental engineering from the University of Wisconsin-Madison. He currently is a staff engineer for GeoSyntec Consultants Inc. (3600 Bee Caves Rd., Ste. 101, Austin, Texas;). Craig H. Benson;() received a B.S. in civil engineering from Lehigh University and an M.S.E. and Ph.D. in civil engineering from the University of Texas at Austin. He is a professor of civil and environmental engineering and a professor of geological engineering at the University of Wisconsin-Madison. Tuncer B. Edil;() received a B.S. and M.S. in civil engineering from Robert College in Istanbul, Turkey. He received a Ph.D. in civil engineering (geotechnical emphasis) from Northwestern University. He is a professor of civil and environmental engineering and a professor of geological engineering at the University of Wisconsin-Madison. |
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| Abstract: | Geophysical logging was conducted in 35 in-service water and monitoring wells installed in unconsolidated sediments using a downhole ultrasonic probe to assess how sealant type, construction method, and site geology affect annular seals. Collapsed sand and gravel was frequently detected in the annulus of wells constructed with mud-rotary methods, precluding the placement of sealant and, in some wells, potentially providing a preferential pathway for contamination. High-solids bentonite grout appeared to create high-quality seals when formation materials did not collapse into the annulus. Seals composed of bentonite chips and pellets remained intact when hydrated upon placement; those not hydrated during placement remained dry and porous, posing a risk of surface water infiltration. Seals made with cement-bentonite grout were generally unsaturated and possibly cracked. Analysis of the data suggests that well designers should specify a construction method that minimizes collapse of the formation to ensure that the sealant is placed where intended. When collapse is avoided, high-solids bentonite grout and hydrated bentonite chips and pellets appear to yield intact seals. However, bentonite chips and pellets may not hydrate adequately in the annulus unless hydration water is added during installation. When cement-bentonite grouts are used, the well designer should ensure that the grout will remain intact and plastic after installation. |
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