Mechanistic understanding of calcium–phosphonate solid dissolution and scale inhibitor return behavior in oilfield reservoir: formation of middle phase

Phosphonates are an important class of mineral scale inhibitors used for oilfield scale control. By injecting the phosphonate into an oilfield reservoir, calcium–phosphonate precipitate will form and subsequently release the phosphonate into produced water for scale control. In this study, a systematic procedure is developed to mechanistically characterize an acidic calcium–phosphonate amorphous material that is later developed into a middle phase and eventually a crystalline phase. The phosphonate used in this study is diethylenetriamine pentakis (methylene phosphonic acid) (DTPMP). An amorphous calcium–DTPMP solid is precipitated by mixing a calcium-containing solution with a DTPMP solution. The stoichiometry of this initially formed solid can be experimentally confirmed via a static dissolution test. Following another dynamic development test, two additional Ca–DTPMP solid phases, i.e., a middle phase and a crystalline phase have been observed. Electron microscopy and X-ray diffraction were employed to characterize the morphology and crystallinity of different Ca–DTPMP solids of interest. Evidently, the dynamic brine flushing of the Ca–DTPMP solid developed the initially amorphous material into a middle phase solid with an amorphous/microcrystalline structure and eventually into a crystalline material. Furthermore, a dissolution characterization study was carried out to determine the solubility product of the middle phase solid at different conditions. The obtained mechanistic understanding of the Ca–DTPMP solid related to precipitation chemistry, dissolution behavior and phase transition is critical to elucidate oilfield DTPMP return data and more importantly, can optimize the oilfield scale squeeze design to achieve an extended squeeze lifetime.