Fundamental study of the dissolution of calcium phosphonates from porous media

Phosphonate-scale inhibitors are commonly used to prevent scale formation in many industrial processes involving high-salinity brine solutions. To effectively prevent scale formation in these industrial processes, one must have a fundamental understanding of how phosphonates are released into high-salinity brines. Because phosphonates can precipitate with divalent cations such as calcium, their release into aqueous media is often governed by many dissolution mechanisms. This study focuses on the release of calcium;phosphonate precipitates from porous media (as related to oil-field applications) and a mathematical model describing these release mechanisms based on mechanistic studies of pore-level phenomena. The phosphonate used was 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). The release of two distinct calcium-HEDP precipitates from porous media was studied: soluble, fibrous 1:1; insoluble, spherical 2:1. Visual studies using etched-glass micromodels showed that five distinct regimes govern the release of 1:1 calcium-HEDP precipitate from porous media. Conversely, the release of 2:1 calcium-HEDP precipitate was dominated by two distinct regimes. A continuum model developed describes the release of both precipitates from porous media by mathematically describing each of the distinct release regimes and defining conditions raider which the transition between release regimes occurred. Experimental data agreed excellently with model simulations for both precipitates.