Sealing Characteristics of Sodium Bentonite Slurries for Water Wells

Summary: A laboratory investigation of the effectiveness of sodium bentonite slurries (drilling mud type) in sealing the annular space in water wells was undertaken. “Quick-Gel” marketed by N. L. Baroid Company as a drilling fluid was studied in viscosities (based on a Marsh Funnel) of 50, 70, 90 and 170 sec/qt as a sealant. The methods of investigation included both a study of the material properties of these slurries and well model experiments in a sand container. The material properties studies included Marsh funnel viscosity, mud weight, gel strength, filtration, shrinkage and permeability (hydraulic conductivity). The well model experiments included both a small annular space model and a large well model utilizing four wells in a sand filled plexiglass container of 1.5 ft x 6 ft x 6 ft deep. In the well models, infiltration of water placed on top of the annular space sealant was studied along with the visual inspection of the sealants dissected during disassembling of the model at the end of the experiments. Additionally, a finite element computer program modeling seepage in the well experiment was developed and used in the interpretation of the experimental results.

Based on the results of the laboratory experiments and within the inherent limitations of a laboratory study of an essentially field problem, the following conclusions are advanced:

1. Quick-Gel slurries without any entrained formation materials provide varying degrees of sealing in the annular space of a well in a coarse sand formation. The infiltration rate of water through the annular space takes place primarily by exfiltration laterally into the formation through the 7 filter cake and at a rate of 0.5 to 3.0 in/sec.
2. The coefficient of permeability (or hydraulic conductivity) of the gelled Quick-Gel slurries with Marsh funnel viscosities of 50 to 170 sec/qt was in the range of 10⁶ cm/sec.
3. Volume defects, such as cracks, however, result in infiltration rates higher than the rates consistent with the material permeability.
4. Based on the measured infiltration rates, filter cake thicknesses and permeabilities, as well as the observations of the dissected sealants in the well experiment, it has been found that the 70 and the 90 sec/qt slurries behave significantly better than both the 50 and the 170 sec/qt slurries as an annular space sealant. The lowest and the highest viscosity slurries used in the study were more prone to cracking than the intermediate viscosity slurries.