New acidic ferrous and calcium phosphonate crystal phases in oilfield brine

The precipitation of ferrous and calcium phosphonates was performed in batch experiments under acidic conditions for a range of temperatures and ionic strengths. The stoichiometries of the precipitates were found to be Fe_1.25H_3.5NTMP and Ca_1.25H_3.5NTMP, where NTMP is a phosphonate. Using a speciation model, concentrations of free iron and phosphonate were calculated, from which the solubility product constants were derived as a function of temperature and ionic strength. The temperature and ionic strength dependence of solubility product constants (pK_sp) were thus determined: pKSP&parl0; Fe1.25H3.5NTMP&parr0;= 53.23&parl0;±0.74&parr0;-8.35&parl0;±1.03&parr0; *IS^0.5+2.67&parl0;±0.33&parr0;*IS-4 787&parl0;±83&parr0;T pKSP &parl0;Ca1.25H3.5 NTMP&parr0;= 40.96&parl0;±1.16&parr0;-5.07&parl0;±0.79&parr0; *IS^0.5+1.79&parl0;±0.31&parr0;*IS-1 985&parl0;±375&parr0;T Simulations using field data suggested the possible formation of these acidic phases during squeeze treatment. In addition, column experiments were performed to test the presence of these phases and the effects of iron ions on the retention of NTMP.