Rheological behavior of bentonite-water suspension at various temperatures: Effect of solution salinity

Bentonite suspension have been widely used as drilling fluid in drilling engineerings. In this work, a modified thixotropic loop test was conducted on 6 wt% bentonite suspensions with 4 solution salinities, to investigate the influence of salinity on rheological behaviors of Na-bentonite suspensions. Most of flow curves were well fitted with Herschel-Bulkley model except first upward curves, where stress overshoot appeared in the initial stage. It was found that the yield stress and flow index decreased with increasing salinity. The suspensions present shear thinning behavior in the first upward curves but shear thickening or Bingham behavior in the second loops. Besides, at 20 and 14 degrees C, a notable thixotropic type transition was observed from positive to negative when the second shear loop was applied to the suspensions in case of low salinity. Both the change of flow type and thixotropy mentioned before may be attributed to the further structural evolution resulting from the long-time shearing at low salinities. However, this change was suppressed at 4 g/L due to the effect of high salinity on particle association mode. The test at 7 degrees C showed interesting results that the twice upward flow curves presented the stress overshoot and the twice loops performed a positive thixotropic behavior, not as same as the cases of the other temperatures. It indicates the structural evolution during shear process may be affected at low enough temperatures.