A Comparative Coagulation Study for Concentrated Mixes of Sodium Bentonite Brands Used in the Oil and Gas Industry

The stability of clay dispersions when mixed with dissolved electrolytes is important to assess, especially for use in wellbores and in the subsurface where electrolytes can be present naturally or introduced during operations (e.g., mix water). It is well known that the type and concentration of dissolved electrolytes impact stability; however, most studies are devoted to dispersions with relatively low clay concentrations that do not assess the impact of the clay concentration on overall stability. This study examined the effects of two electrolyte salts on the stability of high concentration sodium bentonite dispersions (prepared with solid content of 4%, 6%, 8%, and 10% by mass) of commercially available brands. At these high concentrations, classical theories and more standard approaches (e.g., optical) to determine dispersion stability become non-applicable. Each clay concentration was mixed with CaCl2 (in concentrations from 0.75–12 g CaCl2/L H2O) or NaCl (in concentrations from 8–90 g NaCl/L H2O) and the degree of dispersion was recorded over time as agglomeration and eventually sedimentation occurred in the samples. A quantitative method was developed to determine the minimum amount of electrolyte to cause colloid sedimentation, the critical coagulation concentration (CCC). For the high bentonite concentrations tested, CCC is almost a linear function of solids concentration for both mono- and divalent electrolytes. Relatively low levels of CaCl2, for example at levels commonly found in public water sources, were identified as the CCC where sedimentation is initiated; however, bentonite concentration, composition, and type can be optimized to control dispersion stability.