Effect of pH, calcium, and temperature on the adsorption of phosphonate inhibitor onto consolidated and crushed sandstone

In this paper, results are presented from an extensive series of phosphonate inhibitor adsorption experiments using both consolidated and crushed clean (clay free) sandstone core material. A near complete account of phosphonate adsorption in highly quartzitic systems is developed from the results presented. Although some details are known in the literature, we believe that this is the most complete analysis of these effects on phosphonate adsorption that has been assembled to date. The effects on inhibitor adsorption of pH, calcium ion concentration, temperature and inhibitor concentration are investigated in some detail. The appropriate adsorption mechanisms which operate under the various conditions are elucidated from the inhibitor adsorption experiments and from the additional work on the ?-potential measurement for quartz. In particular, the respective roles of hydrogen bonding at low pH (approximately 2) and calcium binding at higher pH (approximately 6) are clearly demonstrated and explained. At intermediate pH values (approximately 4) at room temperature, the adsorption of inhibitor is found to be lower than at both pH 2 and pH 6 due to a relative weakening of both the hydrogen bonding and calcium binding mechanisms. Mechanistic results from the crushed rock and core adsorption experiments are consistent although the level of adsorption is rather different for each medium. At elevated temperatures, inhibitor adsorption is higher under all conditions and, when Ca²⁺ is present, the effect of calcium enhanced surface condensation (or surface precipitation) is clearly shown. The importance of this enhanced adsorption mechanism is discussed in the context of field scale inhibitor adsorption/desorption and `precipitation' type squeeze treatments. This work should be viewed in the context of: (a) previous work on the effect of pH on phosphonate adsorption and; (b) forthcoming work on adsorption onto pure clay mineral substrates and in clay-containing reservoir cores.