Abstract
This paper describes the rheological behavior of Na+-montmorillonite suspensions in the presence and absence of different electrolytes (NaCl, KCl, LiCl) and a polyethylene oxide type polymer under high pressure and high temperature conditions. Shear stress - shear rate flow curves were obtained for 5% (w/w) Na+-montmorillonite suspensions for temperatures up to 120°C and pressures up to 1000 Bar. The data was fitted to both the Bingham model and Herschel-Bulkley model, from which the corresponding rheological properties were derived. Results indicate that the addition of electrolytes and a rise in temperature leads to an increase of the yield stress, due to flocculation of the clay suspension and thermal induced swelling respectively. However, the gel structure may be broken down upon the application of pressure depending upon the electrolyte. It was found that the addition of KCl to the Na+-montmorillonite suspensions originated a strongly flocculated structure, less sensitive to pressure variations, where swelling was reduced at higher temperatures. An aggregating behaviour at high temperatures was observed for the clay suspensions containing PEO and either NaCl or LiCl, attributed to the theta-temperature of PEO. At a lower temperature, the presence of PEO on the clay particles allowed the clay suspension to recover after the pressure was applied, preventing irreversible aggregation of the suspension. The rheological properties for PEO stabilised Na+-montmorillonite suspensions containing KCl were very slightly affected under HPHT conditions, in contrast to the presence of other electrolytes (NaCl, LiCl). Swelling of the clay particles was strongly inhibited by the combined use of KCl and PEO. Estimation of the rheological properties of Na+-montmorillonite suspensions in the presence of PEO with the different electrolytes led to the conclusion that the extent of swelling occurs in the order KCl < NaCl < LiCl, coinciding with the order of increasing size of the hydrated cation.
