FUNDAMENTAL-STUDY OF THE DISSOLUTION OF CALCIUM PHOSPHONATES FROM POROUS-MEDIA

Phosphonate-scale inhibitors are commonly used to prevent scale format ion in many industrial processes involving high-salinity brine solutio ns. To effectively prevent scale formation in these industrial process es, one must have a fundamental understanding of how phosphonates are released into high-salinity brines. Because phosphonates can precipita te with divalent cations such as calcium, their release into aqueous m edia is often governed by many dissolution mechanisms. This study focu ses on the release of calcium;phosphonate precipitates from porous med ia (as related to oil-field applications) and a mathematical model des cribing these release mechanisms based on mechanistic studies of pore- level phenomena. The phosphonate used was 1-hydroxyethylidene-1,1-diph osphonic acid (HEDP). The release of two distinct calcium-HEDP precipi tates from porous media was studied: soluble, fibrous 1:1; insoluble, spherical 2:1. Visual studies using etched-glass micromodels showed th at five distinct regimes govern the release of 1:1 calcium-HEDP precip itate from porous media. Conversely, the release of 2:1 calcium-HEDP p recipitate was dominated by two distinct regimes. A continuum model de veloped describes the release of both precipitates from porous media b y mathematically describing each of the distinct release regimes and d efining conditions raider which the transition between release regimes occurred. Experimental data agreed excellently with model simulations for both precipitates.