Phosphonate scale inhibitors (SIs) are commonly applied for barite scale prevention in oilfields. Their inhibition efficiency (IE) is affected by the barium sulphate saturation ratio (SR) of the brine mix as well as in the presence of divalent cations, Ca2+ and Mg2+. What is less well known is that the precise balance between these factors (SR and Ca2+/Mg2+) can vary significantly for different phosphonate species. This paper presents novel IE experimental results for phosphonate scale inhibitors DETPMP and HMTPMP (pentaphosphonates), OMTHP (hexaphosphonate) and HMDP (tetraphosphonate). Minimum Inhibitor Concentration (MIC) levels for each SI are established using a wide range of brine mixing ratios which changes (i) barite saturation ratio and precipitate mass; (ii) molar ratio of Ca/Mg and (iii) the ionic strength in the brine mix.
The phosphonate SIs were categorised into two types based on their IE/MIC behaviour: Type 1 (e.g. DETPMP and OMTHP) are affected principally by SR and are rather less sensitive to Ca2+/Mg2+and Type 2 (e.g. HMTPMP and HMDP) which are severely affected by brine Ca2+/Mg2+ as well as SR. To demonstrate these effects conclusively, a series of IE experiments are presented with varying brine mix but at a fixed Ca2+/Mg2+ ratio in which the MIC level using both types of phosphonate SI then correlates very well with the barite saturation ratio. In addition, we find the MICs of both types of SI are much lower in the fixed (higher Ca2+/Mg2+ratio) case experiments, compared to the base case – due to the beneficial effect of higher [Ca2+]. The effects observed are important for field application of phosphonate SIs since they show how the various species are sensitive to the changing scaling problem as the %SW increases, in terms of both SR and Ca2+/Mg2+ratio.