This paper presents results and analysis for several cycles of low concentration phosphonate (DETPMP) inhibitor floods in highly reproducible sandpacks. The central objective of this work is to present a detailed study of the effects of solution pH on the adsorption behaviour of phosphonate inhibitor. Displacements at constant near neutral and low pH conditions and in pH gradients have been carried out The latter type of flood is analysed in terms of coupled adsorption where the adsorption isotherm is a function of both inhibitor concentration and [H+] (or pH). The main finding is that phosphonate inhibitor adsorption level is higher at lower pH values. Non-equilibrium adsorption is also evident in the low pH floods. The floods are simulated and analysed using a non-equilibrium, coupled adsorption numerical model which reproduced most of the main features observed in the experiments. The modelling also leads to some novel interpretations of the experimental results which are then tested by further experiments. In order to put this study in a practical context, some further sensitivities, to calcium ion concentartion and pH on high concentration phosphonate adsorption, are also presented in this work. Again, it is shown that the pH and [Ca++] effects exert a coupled influence on phosphonate adsorption.

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