Oilfield scale inhibitors for application in precipitation squeeze treatments: Solubility of the Ca-PPCA complex

Chemical scale inhibitors (SI) are widely applied in the oil industry to prevent downhole scale formation in squeeze treatments. Poly-Phosphino Carboxylic Acid (PPCA) is an industry standard polymeric scale inhibitors which is often applied as a precipitation treatment, where PPCA forms a sparingly complex with calcium ions, denoted Ca-PPCA. In this paper, we study the detailed solubility behaviour of the Ca-PPCA complex which plays an important role in precipitation squeeze treatments. We describe several novel findings on the solubility of (Ca-PPCA) complex system. Specifically, 1. The precipitated Ca-PPCA complex was isolated and used to determine experimentally the solubility of the species involved in a field squeeze for various compositions and temperatures. 2. Different fractions of PPCA have been analysed by both ICP (for P) and Hyamine (for polymeric content) and for their inhibition efficiency (IE) against barite scale. We show that PPCA "precipitate" is richer in the higher molecular weight polymer components. 3. The solubility of the precipitated PPCA/Ca complex becomes lower as it is exposed to successive fresh supernatant brine and the behaviour is very unlike that expected from a "solubility product" model. We believe that these results are the most detailed to be published in the literature on the PPCA system and that they are of particular significance and application for all polymeric SI precipitation squeeze treatments. The significance of these results for field precipitation squeeze treatments is also discussed.