Sealing characteristics of commonly used water well annular space grouts were investigated using a large‐scale laboratory model. The investigation was carried out in two phases. In the first phase, a bentonite drilling mud, often used as a well sealant, was investigated both in the laboratory well model and in the field. For this purpose, Quik‐Gel® mud with different amounts of formation material (in this case sand) entrained in it was used as a sealant. In the second phase, an evaluation of four grouts: neat cement, bentonite‐cement, powder bentonite (Volclay®), and granular bentonite (Benseal® and bentonite slurry mixed according to the “Ohio” recipe) was undertaken with respect to their effectiveness in sealing the annular space. A well model consisting of a sand‐filled plexiglass container was constructed in this investigation. Four metal well casings were installed in boreholes formed in the sand. The annular space between the well casings and the boreholes formed in the sand were filled with various grouts. The effectiveness of these materials in sealing the annular space was evaluated by observation of their structural stability, infiltration of water placed on top of the sealants, and inspection of the sealants dissected during disassembling of the models. A finite‐element computer program modeling the seepage in these well experiments was used to assist in the interpretation of the experimental results. The results indicate that the final success of a sealant depends on its structural stability as much as its permeability. Benseal®‐bentonite slurry grout behaved best; neat cement and bentonite‐cement grouts also provided good seals. Volclay® and various Quik‐Gel® slurries formed poorer seals compared to these. Among the Quik‐Gel® slurries, best results were obtained in the laboratory when a Marsh Funnel Viscosity of about 70 sec/qt was achieved and a mud weight of about 10 to 11 Ib/gal resulted from mixing of formation soils. However, field observations of drilling mud slurries with entrained cuttings exhibited problems of excessive settling even when the slurries had heavy mud weight, greater than 11 Ib/gal. The extent of settling appears to be positively correlated with the relative permeability of the native formation. This limits the types of well construction where these slurries may be used as annular space seal.

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