Solubility and Inhibition Efficiency of Phosphonate Scale Inhibitor_Calcium_Magnesium Complexes for Application in Precipitation Squeeze Treatment

Abstract

Abstract Scale inhibitor (SI) squeeze treatments are applied extensively for controlling scale formation during oil production. The current research involves phosphonate/metal precipitate studies in the context of precipitation squeeze treatments. The main focus here is on the precipitation and solubility behaviour of the SI_Ca_Mg complexes of HEDP (di-phosphonate), DETPMP (penta-phosphonate) and OMTHP (hexa-phosphonate) phosphonate Sis; these phosphonate/mixed Ca/Mg divalent precipitates are denoted as SI_Can1_Mgn2, where n1 and n2 are the stoichiometric ratios of Ca and Mg to SI. Precipitation experiments with SI_Can1_Mgn2 species were carried out over the temperature range, 20–95°C, while varying the Mg/Ca molar ratio over a wide range from all Ca to all Mg. These precipitates were formed in MgCl2·6H2O/CaCl2·6H2O brine solutions with appropriate molar ratios of metals, then separated from supernatant by filtration. Subsequently, the solubility of the collected precipitate was found in a solution of the same Mg/Ca molar composition from which it was prepared. In this type of experiment, the solubility of the SI_Can1_Mgn2 precipitate without any re-speciation is determined. In addition, another type of solubility experiment was carried out for a precipitate formed in a brine with one fixed Mg/Ca ratio and this was subsequently placed into a solution with different Mg/Ca compositions (from all Ca to all Mg). In these experiments, re-speciation of the precipitate may occur. We have been able to establish the solubility (Cs) of the precipitates of 3 SIs (HEDP, OMTHP and DETPMP) as a function of both temperature and Mg/Ca molar ratio. It has been shown, that the solubility (Cs) of precipitate is in equilibrium with Mg and Ca concentrations in solution and any change of these parameters leads to solubility (Cs) variation. All phosphonate/metal precipitates become less soluble with increasing temperature and much more soluble as the proportion of Mg increases. We have found, that any change in Mg/Ca ratio of brine does lead to a re-distribution of Ca, Mg and SI concentrations in a given precipitate and bulk solution, and hence leads to some variation in the precipitate solubility. In addition, the inhibition efficiency (IE) of precipitated and then re-dissolved HEDP, OMTHP and DETPMP SIs was tested and was compared with the IE of industrial stock products. We show that, unlike polymeric SI precipitates, the inhibition activity of phosphonate SIs does not depend very significantly on the precipitation process and the IE of precipitated and re-dissolved SI_Ca and SI_Ca_Mg complex is very close to that of the industrial stock solutions. These results can be used directly for modelling phosphonates precipitation squeeze treatments and the significance of these results for field applications is explained.