Structure, stoichiometry, and modelling of calcium phosphonate scale inhibitor complexes for application in precipitation squeeze processes

Phosphonate scale inhibitors (SIs) applied in downhole squeeze applications may be retained in the near-well formation through adsorption and/or precipitation mechanisms. In this paper, we focus on the properties of precipitated calcium phosphonate complexes formed by 9 common phosphonate species. The stoichiometry (Ca/P ratios) in various precipitates is established experimentally and the effect of solution pH on the molar ratio of Ca/P in the precipitate is investigated. All static precipitation tests were carried out in distilled water, with only Ca2+ (as CaCl2) and SI present in the system at test temperatures from 20oC to 95oC. The molar ratio of Ca/P in the solid precipitate was determined by assaying for Ca2+ and P in the supernatant liquid under each test condition by ICP spectroscopy (Cao and Po are known, but are also measured experimentally). We show experimentally that the molar ratio of precipitated Ca2+/P (or Ca2+/SI; or n in the SI_Can complex) depends on the SI itself and is a function of pH, for all phosphonates tested. It is found that, as pH increases, the molar ratio of Ca2+/P (n in the SI_Can) in the precipitate increases up to a theoretical maximum, depending on the chemical structure of the phosphonate. Our findings corroborate proposed SI-metal-complex ion structures which were presented previously (Shaw et al., 2012c), as discussed in detail in the paper. In addition, the precipitation behaviour of the various compounds is modelled theoretically by developing and solving a set of simplified equilibrium equations. We find that the precipitation behaviour can be modelled, but only if a fraction, ?, of “non-SI” of the initial phosphonate SI is taken into account. The quantity ? can be as high as 0.2 (i.e. ~20% non-SI), although there is a degree of variability in this factor from product to product. However, good quantitative agreement is shown comparing the predictions of the equilibrium solubility model with experiment. Such models can be used directly in the modelling of field phosphonate precipitation squeeze treatments.