The effect of particle size of clay on the viscosity build up property of mixed metal hydroxides (MMH) in the low solid-drilling mud compositions

Mixed metal hydroxide (MMH) is a purely inorganic viscosifier of montmorillonite suspensions, having environmental and thixotropic advantages over conventional viscosifiers. It was observed that the particle size distribution of the clay plays a crucial role in building up viscosity of clay–MMH suspensions. Two drilling clays, their separated < 2 μm fractions and a pharmaceutical grade bentonite were studied. It was found that in a clay with 80% (w/w) particles below the 2 μm level the yield point (YP) rise of 2.5% (w/w) aqueous clay suspension is not stopped even on 0.3% (w/v) MMH treatment, whereas in a clay with less than 30% (w/v) particles below the 2 μm level the YP reaches a constant level of 8 lb/100 ft2 after 0.1% MMH (w/v) treatment. When the < 2 μm separated fraction is taken from the latter clay and hydrated it was found that YP rises even above the constancy level. On further homo-ionising this separated fraction to Na form and treating with MMH the YP rise was found still higher. When some carefully separated fraction containing a high amount of particles with less than < 1 μm fraction was treated with MMH the YP rise was even higher. With the decrease of the particle size of the clay there is associated an increase of cation exchange capacity (CEC). Therefore, both CEC and particle size play a vital role in viscosity buildup which is due to the formation of an extended gel-network of clay and LDH particles at the prevailing alkaline pH of the system giving rise to high thixotropy. Finally, to reach a bench mark level of YP ≈ 20 lb/100 ft2 with the latter clay one must opt for a base mud concentration of a minimum of 4% (w/v) aqueous system.