Characterization and Rheological Study of a Tunisian Bentonite for its Use in the Drilling Fluids

Abstract

The Jebel Shemsi ore contains 81 % of clays; mainly quartz and carbonate
are present as impurities. The dominant phase in the clay fraction
is smectite (80%), it also contains illite (15%) and some kaolinite. The
cationic exchange capacity (CEC) value of the purified clay is 87 meq/100
g of calcined clay; the total specific surface area (S_T) is equal to 711 m²/g.
The dehydration (hygroscopic water) of the raw and purified clay occurs
around 109°C and its dehydroxylation at 502°C for the raw bentonite and
at 468°C after purification. The study of the clay percentage (w/w) effect
on the rheological properties shows that the flow curves have a Newtonian
behaviour for the clay percentage lower than 10%. After this percentage,
the behaviour becomes rheofluidifiant. The effect of pH on the rheological
properties has been studied at the clay percentage of 10% and shows two
states of maximum interaction in acidic medium (pH range 2-4) and basic
medium (pH range 9.5-11). The suspensions corresponding to the first
maximum are gels and exhibit a rheofluidifiant behaviour, whereas those
corresponding to the second basic maximum are gels but flow easily and
exhibit a Newtonian behaviour. The natural pH of the clay (pH = 7.2)
corresponds to the minimum of interactions.