Characterizing Salinity Dependent Double Layer Effects on Clay Properties
Stoke’s Law states that the terminal velocity of a settling spherical particle in a laminar flow environment is proportional to the product of the particle size squared and the difference of densities of the solids and the fluid. We investigate the impact of decoupling the effects of density and particle size by performing a series of experiments to characterize influence of clay mineral double layer effects. These include the impact of salinity changes on bound water, grain density and grain size. Settling experiments were performed in distilled water, and 75,000 ppm NaCl solution for three different mud-rock samples with different CEC values. Nuclear Magnetic Resonance (NMR) spectroscopy, Thermo-Gravimetric Analysis (TGA), and grain density calculations using Boyle’s Law experiments were performed to compare the properties as a function of salinity and grain size.
In the settling column, the larger floccules settle with a higher velocity. Increasing salinity caused the clay particles to bind together more strongly than in distilled water. The flocculation of clay particles removed the smallest particles from the suspension resulting in a sharp break in the cumulative grain size distribution. At low salinities, the flocculated particle size increases, however above 75,000 ppm no further increase in floc size was observed.
When the settling experiments were complete (indicated by constant hydrometer readings), the sample material was divided into three different particle sizes. The volume of associated bound water was determined for each size fraction using TGA and NMR with good agreement between the two results. We found that the volume percent of bound water associated with clay minerals, increases with decreasing particle size. Boyle’s Law experiments were to determine grain density on each size fraction as a function of salinity. The NMR measurements and the grain density were both functions of the bound water volumes.
At a given particle size, clay minerals equilibrated with the 75,000 ppm brine have a lower grain density than those equilibrated with distilled water. These effects are attributed to the salinity dependence of the flocculation. In a given particle size range, at low salinity, the particle sizes are dominated by quartz, feldspar and carbonate. At higher salinity clay floccules are a larger contributor to the large particle size fraction.
These grain density data were also interpreted using the model of Hill, Shirley and Klein to estimate a bound water volume. This comparison allows an independent interpretation of grain size and grain density. This model compares well to our measurements and validates its use for estimating bound water. To summarize we found a significant impact of salinity on the settling experiments performed. The high salinity allowed the clays to flocculate and determine the floc size with only a small dependence on CEC. The cumulative grain size was skewed towards larger grains. In addition, there were significant effects on the grain density due to bound water which varied with salinity.