Add time:09/24/2019         Source:sciencedirect.com

Phosphonic acids are commonly used as scale and corrosion inhibitors, but little is known about their solubility and stoichiometry. Hence, questions concerning the retention mechanisms of phosphonates in reservoir systems remain unanswered. A method is described in this paper to measure the solubility and stoichiometry of the phosphonates and is applied to diethylenetriaminepenta (methylene phosphonic acid) (DTPMP). The experiments reported herein were performed in 2.0 M brine solutions at 70°C for varying Ca and DTPMP concentrations at 4.7 and 5.0 pH.The conditional equilibrium constants of the Ca-DTPMP salts formed under the conditions of the experiment are 4.34 × 10−4 and 3.04 × 10−4 at pH values of 4.7 and 5.0, respectively. The stoichiometry of the solid phase precipitated in the experiments and the dominant solution complex are Ca3.5H3PHN·3H2O and Ca3H3PHN1− where PHN refers to the deprotonated DTPMP ligand. These results suggest that retention mechanisms involved in an inhibitor squeeze are either adsorption or precipitation/coprecipitation with cations other than Ca and/or other ligands.

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