Structure, Stoichiometry, and Modeling of Calcium Phosphonate Scale-Inhibitor Complexes for Application in Precipitation-Squeeze Processes

Abstract

Summary 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 nine common phosphonate species. The stoichiometry [calcium ion to phosphorous (Ca2+/P) ratios] in various precipitates is established experimentally, and the effect of solution pH on the molar ratio of Ca2+/P in the precipitate is investigated. All static precipitation tests were carried out in distilled water (DW), with only Ca2+ [as calcium chloride (CaCl2)] and SI present in the system at test temperatures from 20 to 95°C. The molar ratio of Ca2+/P in the solid precipitate was determined by assaying for Ca2+ and P in the supernatant liquid under each test condition by inductively coupled plasma (ICP) spectroscopy (Ca0 and P0 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 in Shaw et al. (2012c), as discussed in detail in this paper. In addition, the precipitation behavior of the various compounds is modeled theoretically by developing and solving a set of simplified equilibrium equations. We find that the precipitation behavior can be modeled, but only if a fraction (β) of “non-SI” of the initial phosphonate SI is taken into account. The quantity β can be as high as 0.2 (i.e., approximately 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 the experiment. Such models can be used directly in the modeling of field phosphonate precipitation-squeeze treatments.