Strength of Bentonite Water‐Well Annulus Seals in Confined Aquifers

Abstract

The advantages of bentonite clay for sealing applications are well known. Bentonites have extremely low permeability, do not affect formation water chemistry, and have the ability to swell and deform in response to subsurface changes. The major limitation on the applicability of bentonite for water‐well annulus sealing is strength. Strength tests conducted in a physical model of a water well identified the expected magnitude of shear strength for several commercially available bentonite well‐sealing products. The dependence of bentonite strength in the annulus of a water well on both setting time and borehole geometry is discussed. Force balance calculations for seals constructed of both slurry grouts and granular products are presented. Seals constructed of bentonite slurry grouts do not have sufficient shear strength to resist hydrostatic forces in confined aquifer installations. Annulus seals constructed of granular bentonite products possess adequate shear strength to resist push‐out forces in limited instances. Designers of bentonite water‐well annulus seals in confined aquifer systems must be aware of the strength limitations of both granular‐ and slurry‐type bentonite‐clay well‐sealing products.